>when automatic transmissions are consistently outperforming humans at fuel efficiency
Do you have data to back this up? Unless you're driving a dual clutch "dry" transmission or a CVT, because of Physics, an automatic transmission will always be less efficient. The Automatic transmission Torque converter [1] will waste some of the energy provided by the engine, especially with city driving.
Also, most automatic cars tend to shift gear up too early, and can't switch to gear n+2. Because of this, you spend more time accelerating to your cruise speed. It's well known that accelerating rather quickly to your cruise speed is the optimal strategy for fuel efficiency, hence the reason to switch from 3rd to 5th gear directly.
With a manual, you can also disengage the clutch when you consider it safe (and not slowing down traffic) and you know you're going to have to stop at the red light.
In modern cars, disengaging the clutch (or using neutral) when coasting (toward a stop or down a hill) is less efficient than staying in gear. This is because it takes some fuel to keep the engine idling, where staying in gear allows the computer to completely cut fuel flow, as long as the forward motion is keeping the engine turning.
Two days ago my friend and I were driving a rented car and we've noticed that when coasting, the computer said the fuel consumption is 0, but when the friend disengaged the clutch, the fuel consumption actually went up (!) to some value. We've been wondering what's going on, and we actually suspected a software bug in the fuel consumption indicator. Thanks!
This is called Deceleration (or Direct) Fuel Cut Off (DFCO). When the engine is turning at > 1500 RPM, and your foot is off the accelerator, fuel is stopped to the engine.
This shifts the engine from combustion, where gas explosions cause motion, to motion causing the engine to turn. Because the engine is still in motion, when the RPM drops below 1500, then engine can resume fuel and begin combustion again.
When you disengage the clutch, the engine is disconnected from the wheels, and requires combustion again to keep running.
Not only that but staying in gear when going down hills or slowing down will reduce the wear on your breaks by using the inertia of the engine to slow the car instead of break pad friction/heat.
You maintain better control of the vehicle if you leave the clutch engaged and engine brake instead of slowing down with it disengaged and using the actual brakes. It may not seem that way to inexperienced manual drivers, but once you've driven a manual in traffic a few times you learn that engine braking is safer and more precise.
Also, on a large, heavy vehicle with a manual transmission, you can get in trouble with misjudging braking time/distance if you only rely on the brakes to slow down and stop. When I have a full load in my pickup (1982 C10) I have no choice but to engine brake as the wheel brakes lose more stopping distance the heavier the load, not to mention it keeps them from overheating and failing early over time.
Cars are designed so that you stop the car by pressing the brake pedal, so that is generally the better way to do it.
* 4-wheel braking, even if it is a 2wd car.
* ABS/Traction control systems are designed around brakes, not engine braking.
* Regenerative braking (if equipped) uses the brake pedal
* There is a smooth application of braking power from 80mph to 0mph. Engine braking is indexed (I can only choose which gear), and cannot bring the car to a complete stop.
* The limiting factor with brakes is traction, which isn't improved by using engine braking.
* Using the brake pedal is a single action. Engine braking involves at least 3 (clutch in, shift, clutch out).
* Brakes are cheaper to replace than clutch/transmission.
* Engine braking alone does not activate brake lights.
The only exception is when there is a danger that the brakes will overheat: descending a large hill or hauling a heavy load. The default instinct should be the brake pedal alone.
> Cars are designed so that you stop the car by pressing the brake pedal, so that is generally the better way to do it.
I wasn't speaking of coming to a complete stop, only of slowing down with the flow of traffic. I should have worded it better, I'm sorry.
> Using the brake pedal is a single action. Engine braking involves at least 3 (clutch in, shift, clutch out).
But if you're already downshifting while slowing down, which you should be, it's actually not extra effort. It's completely natural.
> Brakes are cheaper to replace than clutch/transmission.
That entirely depends on the vehicle and the type of clutch. In my extensive experience (I repair cars on the side, and have done so my entire adult life) the parts themselves often cost about the same. For example, when restoring my Bronco II I bought a clutch kit for $150 and a full set of front brake rotors, pads, and new calipers for about $200.
On my Crown Victoria, which was formerly a police vehicle and therefore has the upgraded heavy duty disc brakes on all four wheels, a shop quoted me $700 for a brake job. The parts alone were $350 through them. Of course I saved a ton by doing it myself, but most people can't or won't do that. The labor for a clutch swap versus a brake job is, again, dependent on the complexity of the vehicle. Most manuals still in use in the US are older model cars, with relatively easy to replace clutch parts.
> The only exception is when...hauling a heavy load.
Which I specifically spoke of. :-)
> The default instinct should be the brake pedal alone.
In an automatic, yes. In a manual, only when needing to stop suddenly or when you've downshifted to 2nd or 1st and you're stopping altogether.
This makes particular sense for a pickup. When unloaded the truck's weight balance significantly skewed toward the front, allowing for less traction in the back, and so you need to rely on the front wheels more for braking. When you're loaded, that's sitting more-or-less over the rear axle, increasing traction there. So you can afford to rely more on the rear (driven) wheels with engine braking in addition to the conventional brake balance that's biased to the front.
I tend to engine brake except in circumstances where I actually need more braking power.
Especially with hilly driving, it gives you a lot more control when slowing down, and you can accelerate straight away since your foot is still on the accelerator pedal.
There's nothing worse than being stuck behind a driver (usually a tourist) in a hilly area who is riding the brakes all the way down.
In my head, using the engine/transmission to "brake" when in gear going down hills or slowing down seems like a bad idea. Something is getting stressed in order to slow the car down.
Having to replace my brakes makes sense, they get worn out, and its relatively inexpensive (especially if I do it myself). I have always been wary of staying in gear to brake. And, as it so happens, my manual has 130k miles on it, stock engine, transmission, and clutch. Never had any problems.
I would love to understand the science behind why using the engine/transmission/clutch to brake is better for the car than just using the brakes.
Your engine is designed to withstand fuel-air explosions. When engine braking, all it does is compress and expand air, at much lower pressures, so revolutions during engine braking are easier on your car than those during regular driving.
Clutches get less wear during engine braking because it doesn't slip. When you disengage and reengage the clutch because you're using the regular brakes to slow down, the clutch slips and damages itself.
In the end, the reason for engine braking isn't to reduce the cost of brakes--it's a safety measure that keeps the brakes cool for emergencies & miscalculations, since overheated brakes don't work.
> Clutches get less wear during engine braking because it doesn't slip. When you disengage and reengage the clutch because
> you're using the regular brakes to slow down, the clutch slips and damages itself.
And when you have to change down, while engine braking? (I have an odd learning style. Please don't mistake this for "You're wrong because..." It's a case of "I don't understand why, so...")
A friend once told me that the UK required you to half-engage the clutch while using the brakes and the engine together. I've also read something to the effect of modern brakes are much more efficient so (at least one) UK "advanced driving school" teaches that it doesn't really matter.
I have an older car (1998 Mazda 626 wagon). It's a manual, and when I drive downhill I put it into third and let it coast, braking to reduce speed. Should I be putting it into second, and letting it run up to 3500-4000 revs? Should I leave it in third, and use the brakes? Does it consume more fuel to do this, in a car of that era?
I've often meant to ask my local mechanic about this, but never think about it when I'm there.
Drive with clutch disengaged. Use clutch only while you quickly shift into an appropriate gear. The longest I use clutch is when I am starting of from a stop in first gear. Once I have momentum, dip clutch fully and shift and release clutch as soon as gear us shifted. When clutch is half pressed, clutch plate gets highest wearing as plates are half pressed against each other, not enough friction is there to stop clutch disc slipping (as it is while clutch is not engaged) against each other thus causing wear. When clutch is fully pressed, the clutch plates are seperated so they dont rub against each other and wear.
The way I shift is as I approach a downhill I keep driving it in gear rather than coast. If its steep downhill, you can brake ahead to lower your speed to something you are comfortable at. Shift down to the gear for that speed and then back off the gas if you need engine braking. You can use brake in combination with engine braking gear if you needs to slow down more than using engine braking alone. I shift down when speed gets lower than in current gear.
The Car Talk guys addressed this and agreed with you. It's better to use the brakes to slow down since they're ablative and designed to be replaced with wear. Causing the engine to do extra revolutions means extra wear on something that's quite difficult to replace/repair...
That said, I drive a manual and use the engine for braking...
Growing up in the mountains, in Colorado, it was drilled incessantly into me how important it was to use engine/transmission braking on long hills, etc.
I know it, I understand it ... and it always irked me.
I never liked the idea that I would save wear and tear on a consumable that is meant to be regularly worn and repaired and instead transfer that wear and tear to an integral part of the car that is non-trivial to replace or repair.
I would think very carefully about this if I were towing a boat or a horse trailer, but in a normal car it just seems backwards.
It's not about wear and tear. Brakes heat up when they're used, and descending an entire mountain on the brakes is likely to overheat them to the point where you have no braking power left at all. Not a good situation to be in on a steep slope. Using engine braking to keep your speed under control prevents this.
Engine braking imposes negligible wear and tear on the engine. Instead of fuel-air explosions driving the pistons up and down, the pistons inefficiently compress and expand air at ambient temperature and pressure. Sure, extended use might take 100 miles of life off a 300,000 mile part, but are you really going to worry about that?
I believe the reasoning here is that braking all the way down a steep grade can cause your brakes to overheat and become temporarily inoperable. I don't know whether this is a real effect or not, but I have heard it given as a justification for engine-braking.
It's definitely a real thing. It's usually referred to as "brake fade" in car enthusiast communities. It’s usually more of an issue when racing, but it can happen if you have to do many hard stops during regular driving as well. I think it has to do with the pads releasing a gas when they get hot which creates a thin layer of “air” between the pad and the rotor.
It's the reason that sports cars generally come with bigger brakes. Sure, a Toyota Corolla might be able to stop in a similar distance, but things change when you have to do it 5 times in a row.
Oh yeah its real. Back 20 years ago I came down from Guenella Pass to Georgetown in Colorado and just about lost my brakes on those switchbacks just above Georgetown. One of the scariest situations I've been in.
This happened to a friend of mine driving through the rocky mountains in Canada; brake system overheated and he had no breaking ability until it cooled down.
unkeljoe: "braking all the way down a steep grade can cause your brakes to overheat and become temporarily inoperable. "
That condition is known as brake fade/fading and occurs mostly with drum brakes and can occur on bicycles, motorbikes and automobiles.
Braking heats the brake drums, which then expand. The drum may expand in diameter to the extent that the brake pads no longer contact the drum sufficiently enough to slow the vehicle. Your brakes "fade" away.
Many vehicles have disc brakes in front and drums on the rear, an arrangement usually sufficient even in hilly country. Of course when pulling a boat or trailer having all disk brakes would be a safer bet.
Brake drum fade is by far the most common manifestation, but there are other types of "fade", e.g., brake fluid can get heated up enought to boil and reduce braking system pressure, brake pads can "slip" more at extremely high temperatures:
Last ditch efforts: shift into a lower gear, use any emergency brake (which is mechanical and bypasses the braking fluid system and so won't fade due to boiling fluid), and finally, use the inside of a hill/mountain as a giant brake pad by sliding your car's body into it as gently(!) as possible.
Finally I feel compelled to warn anyone who ever pulls a trailer, boat or RV about a potentially fatal phenomenon they may encounter on even gently sloped roads: undamped driven harmonic oscillation between towed and towing vehicle. My first experience:
VW Beetle towing a U-Haul trailer on a Pennsylvania turnpike mountainside. Traffic moderate in both directions.
On a long downhill segment the trailer hitch began to move to-and-fro left and right, gently at first but, as I attempted to correct with steering, rapidly growing in amplitude. My steering reaction time and corrections were unfortunately timed precisely so as to _increase_ the amplitude of the oscillation. In a flash the rear end of the Beetle was hopping right and left! Insight - I need to dampen the oscillation. I gripped the steering wheel, braced both forearms against my legs and reduced all steering corrections to a minimum (I just kept the car on the road and in the proper lane). The car's front end skidded left and right as I kept the wheels as straight forward as possible. Then I slowly applied the brakes. This brought the oscillations under control and the speed down. I continued the trip at a much slower speed despite the honking of frustrated drivers behind me.
That first experience, enhanced by both impending ignominious death on a mountainside and the sudden realization of the utility of my mathematical physics class* [1], was exhilarating.
In the years since I have myself seen this occur several more times, which makes me think it must be a not-uncommon event that requires some warning.
It once again occurred on IH 10 between Houston and New Orleans, one of the flattest pieces of land in the USA. A heavy-duty six-wheeler pickup was hauling a trailer full of goods on an extremely gentle slope at near 70-mph when his trailer hitch began to oscillate left and right. I had been following and observing his truck and noticed that the system seemed to be periodically oscillating, so I stayed well back and did not attempt to pass even on a four-lane highway. Finally things took a turn for the worse and, within 6 seconds of back-and-forth oscillation and attempts at correction, both truck and trailer were driven off the road into the grassy median. Luckily the median was wide flat grass and no harm occurred to driver, truck or trailer. I stopped and crossed the road as the driver took off his cowboy hat, waved it at his truck and trailer as if dismissing an unruly horse, bent his back, and put his hands on his knees in amazement.
I spoke to him awhile, reassured him and gave as best an explanation to him of what I saw and what he might do to prevent further mishaps. He was quite out of sorts and I'm not certain he was fully able to absorb the lesson. He was definitely astonished to find himself on the median with his truck and trailer turned around 270 degrees from the direction he intended.
Had this happened on a strip of highway without such a wide median the truck would have driven at ~60 mph into ongoing traffic at 70 mph. Had this happened on a hillside, I would estimate a 50% chance of both vehicle and trailer plummeting downhill. For these and other reasons, I think this phenomenon must kill more than a few people each year.
Very similar but there was an additional degree-of-freedom, the driver's steering (and braking), which can either improve or worsen the situation. Initially I tried to countersteer to correct the oscillation, but I was too slow - the oscillation amplitude increased! Changing tactics to damping the oscillation by keeping the wheels as straight ahead as possible (and slowing) did the trick or I likely wouldn't be posting this!
I needed to get to the Northeast for a summer job. One of my professors had a Beetle and a locked U-Haul that she wanted moved to Boston. Now that I look back I probably should have asked to see the contents of the trailer!8-))
It was a fun trip with only a minute or so of harrowing possible death-down-the-mountainside. I'd recommend it to anyone!
The theory is if you go down a long mountain the brakes will be very hot when you are getting towards the bottom which means the performance is not as great. So if you then suddenly need the full power of the brakes it is not available anymore.
Also if the mountain is tall enough the brakes might not make it all the way down which has happened a few times in Norway with catastrophic results for large vehicles like busses.
I just listened to Click and Clack a couple weeks ago, and a guy called in asking why his brakes caught on fire after he went down a long hill.
Suffice to say that they do not recommend against engine braking--quite the contrary. Think about it: the engine is already spinning, so spinning it at a higher RPM for a few tens of seconds does no more wear than accelerating.
The issue is not brake wear--the issue is brake fade, and potentially boiling your brake fluid!
When you're going down a long hill, downshift and save your brakes! It could save your life!
>Something is getting stressed in order to slow the car down.
The exact same components that are stressed in order to speed your car up.
Since your engine doesn't have combustion raising cylinder pressures, the total power 'absorption?' under engine braking is necessarily less than the full power output of the engine which the drive train was designed to handle.
The exception is if you have rear wheel drive and enough weight transfer to the front to get wheel hop. But, afaik, this is a problem unique to motorcycles.
When breaking with engine, the kinetic energy of movement is converted to heat not via friction (like breaks do) but via compressing air in cylinders which does not cause extra wear on anything.
>> Just what the previous commentor said: saves on wear and tear on the brake pads.
When I was learning how to drive a manual, my Dad was a huge fan of them and all of our family cars were manual. I was taught that it makes a huge difference when you're driving in the mountains to use engine braking instead of your brakes. He also explained the difference in braking in traffic and using engine braking to do so and how it supposedly saves on gas as well - something about idling the engine versus continuing to have the engine running?
I'd be interested to hear from people who regularly drive in the mountains and the advantages/disadvantages of engine braking versus using your brakes and if that was some myth or actually real.
The point about gas is certainly true, thought I can't speak to the rest as much.
While you're accelerating, you're giving the engine gas to keep it moving and prevent a stall. While braking you aren't accelerating, but you still need the engine to keep going at speed.
If you depress the clutch, it disconnects the engine from the wheels. The engine will then get a trickle of gas (even if you don't accelerate) to keep it moving while the car breaks.
If you don't depress the clutch, the engine stays in connection with the wheels. Their motion drives the engine - the reverse of normal - keeping it moving without burning any gasoline at all.
It's not generally a big difference, but if you're in the hills of Colorado you might care more.
Sure, but it will cause increased wear on the clutch plates. However, idling, keeping the clutch disengaged in neutral for longer periods of time increases wear on the dual-mass flywheel. So, whatever you do, you are damaging something :) . Personally I try to balance it out, but on longer down-hill slopes or when slowly coming to a traffic light I try to keep it in gear. It takes a bit of effort to train, but I really think that you can come close to hybrid economy figures (your mileage may vary :)) ) with thinking while driving. Remember Clarkson from Top Gear driving a 4.0 liter Audi for more than 1200 kilometers with one tank (which is around 82-85 liters) but with a lot of "work" behind the wheel. He got around 6.5 l/100km which is around 36-37 US MPG (or whatever freakin' unit you guys over the pond are using :) ).
Are you sure? I am pretty sure the clutch does not wear when fully engaged. It is only during engagement and disengagement where there is chance of wear.
The transmission on the other hand...
This is true, but the clutch bearings and spring may wear faster if the clutch is kept disengaged for too long, instead of switching the gearbox to neutral.
Some ECUs also cut fuel on braking. I remember reading a review of a car where the reviewer (obviously an "enthusiastic" driver) was disappointed that he couldn't do left-foot braking because it kept cutting the fuel.
(Left-foot braking is a technique where you keep the power down in a bend but dab the brake to adjust the fore-aft balance of the car. It is common in rallying - there is some awesome footage somewhere on YouTube of the driver's feet in an 80s rally car. His feet move so fast it looks like he is dancing on the pedals.)
This is one of my pet peeves. Sometimes I use left foot braking to transfer chassis weight from one side to tve other. Having a throttle cut off sucks. I won't buy a car with it anymore.
I can't think of any situation outside of a race course where this would be a necessary maneuver. If you're going so fast on public roads where you need to left foot brake to transfer weight to avoid skidding off the road, it sounds like something is wrong. Where are you doing this enough where having a car with throttle cut off is enough of an annoyance where you won't buy a car with it anymore?
For reference, I got pretty good at this technique while driving autox, which for those unfamiliar is basically a race course around cones in a giant stadium parking lot, one car at a time. But in my 10 years of driving on public roads, I've never felt the need to use it even once.
Different driving styles. I focus on safe driving. Being able to transfer weight around bad bumpy roads helps me drive safe. Roads are not great in Puerto Rico and I drive a FWD car.
Edit: Downvotes? For driving safely on shitty roads? I hope people don't assume Im being a jackass driver just because I use my left foot.
Could you give a specific example of how this makes you safer? I really don't understand how braking could cause an appreciable weight transfer at speeds I consider appropriate for "bad bumpy roads".
I have never been to Puerto Rico, but in Nicaragua and Honduras is highly advantageous for your health and safety to flow with the rest of traffic. He is completely right, this comes in handy because so many roads (especially in mountain areas) are very inconsistent and messy.
I'm not downvoting you, but I disagree with your line of reasoning.
Just because YOU don't see why a feature might be useful does not mean that it isn't useful. It just means you don't see a use for it.
Similarly, while cutting the fuel on break application might be useful or even beneficial in the case of a normal / particularly bad driver (slamming down all the peddles in an emergency) it does mean a drastic change in the human/machine interface. I would say that just like ABS is a good safety feature and airbags / restraint belts are good safety features, the vehicle should CLEARLY MARK these features. Their automation may (EG as in the cases of baby seats) be counter-intuitively less safe in circumstances.
I agree. It's also incredibly difficult for those who haven't practiced as you have. The few times I have tried it (in safe conditions) I have ended up braking far too hard. My left foot somehow doesn't have the "feeling" of my right foot on the pedal.
One racing driver technique that can be useful in normal conditions (in a manual) is heel-and-toe braking. I use it to avoid excessive engine braking while downshifting, and sometimes for hill starts without using the handbrake.
It's much safer if that how you learned to drive from day one. Unfortunately most existing drivers don't have the skills to safely left-foot brake so that's how we teach new drivers.
You are right! Its not a common skill. Im left handed so left foot breaking feels natural to me. Plus I drove manual cars for years and am used to feel the clutch/brake bite with the left foot.
It doesn't take long to get smooth with a left foot.
Also left foot braking is much more useful day to day than heel-toe. Especially if you ever deal with suboptimal traction. Plus, faster reaction times.
Heel-toe is also dependant on the pedal placement and clutch weight. But I don't see it being useful in day to day. Rev matching is not something you do on daily driving. You usually tend to short shift at lower rpm.
This used to be the case with fuel injected engines from 80's to 00's but because of new regulations current ECUs might, again, inject fuel while coasting just to keep the catalytic converter warm and operational.
The amount is obviously negligible but it's quite crazy to spend more fuel in order to reduce emissions :)
When you coast with engaged clutch, you will engine brake. Energy doesn't go to the engine for free. Adding the fuel to counter the engine braking is less efficient than letting the engine run idle while coasting due to losses in the power-train. Plus, more efficient cars still have to inject fuel. The electro-hydraulically controlled conventional gearbox of the Lupo 3L disengages the dry clutch while coasting for a reason.
Of course, engine braking is obviously more efficient than friction braking, but that's not coasting, that's engine braking.
Another funny note is that "conventional" automatic gearboxes have the highest gear (usually the only overdrive gear) have a coasting clutch on it, so it's not even technically possible to coast with engine engaged. You're always idling if you let go of the speeder, losing energy to the torque converter that doubles as oil pump for the transmission.
When you coast with engaged clutch, you will engine brake
True
Energy doesn't go to the engine for free.
Adding the fuel to counter the engine braking is less
efficient than letting the engine run idle while coasting
due to losses in the power-train
It is a bit hard to understand where you're trying to go here but assuming that you state it takes fuel to 'counter the engine braking' you're wrong. Engine braking is just that, using the engine as a brake. The engine is used as a pump, air (and only air, no fuel is added) is compressed, the air heats up, this heat is dissipated by the cooling system. That is where the energy actually goes, as heat. Losses in the power train only add to the braking effect.
This is also why it is a bad idea to use engine braking with a 2-stroke petrol engine: 2-strokes rely on oil mixed in with fuel for lubrication. When using the engine as a brake no - or hardly any - fuel is added to the incoming air while maintaining high engine speeds, starving the engine of lubrication.
It's a bit hard to understand where you're trying to go here but assuming that you state it doesn't take fuel to "counter engine braking", you're wrong.
Engine braking is just that, using the engine as a brake. If no fuel is added, pumping losses will make it act as a (relatively weak) brake. If you want to coast without braking, with the clutch engaged, you will need to inject fuel to counter the pumping losses of the engine, losses which are higher if the engine is not idle. Losses in the power train add to the braking effect, which must further be counted with additional fuel.
If you're going downhill with clutch disengaged and maintaining your speed, engaging the clutch will very likely increase your fuel consumption for the same task (maintaining speed). Automatics have a coasting gear (usually the last or only overdrive gear) for this very reason.
(Sorry, I had to do that.)
Side notes:
1. Coasting is not braking. People seem to be mixing "coasting to a stop" and "coasting" in general. Engine braking, when your intention is to reduce your speed, is more efficient than friction brakes, but coasting does not mean that you intend to reduce your speed.
2. Who drives a car with a 2-stroke petrol engine? :)
"In modern cars, disengaging the clutch (or using neutral) when coasting (toward a stop or down a hill) is less efficient than staying in gear. This is because it takes some fuel to keep the engine idling, where staying in gear allows the computer to completely cut fuel flow, as long as the forward motion is keeping the engine turning."
This is very interesting - thank you.
I shift cars into neutral while going down hills because I strongly dislike the subtle lurching of the hill descent control ...
I never thought I was saving any appreciable fuel.
However, what does happen is that (provided you don't mind speeding) you exit the bottom of the hill with much more stored energy, allowing you to coast (in or out of gear) much further before it is necessary to apply the throttle again.
So it's interesting that the direct effect of coasting in neutral does not save fuel - however I still think the net effect (again, provided you don't mind speeding when exiting a hill) is fuel-saving ...
Coasting down a hill in neutral definitely saves fuel compared to coasting in gear if you can utilize the kinetic energy. If you're coasting down a hill and then need to stop quickly it's better to coast in gear.
The fact that you use extra fuel when coasting isn't enough to make it less efficient, you'd have to be using less extra fuel than the extra fuel you would otherwise have to use to replace the benefit you extract from your potential energy by coasting.
There is nothing preventing cars with manual clutches from doing that. People going down hills with manual transmission also keep the car in gear (they teach you that). And it doesn't matter as it is not much fuel compared to the fuel used when accelerating, where automatic transmissions are less efficient.
If that is the only reason for automatic transmission to be more efficient, then i doubt it is.
A counter point: In my previous car the screen showed 0.0 when idling, which obviously was false (as the engine was still running). It should probably show infinite instead. (km/l for 0 km).
Of course it works that way, but just for the note: what the car computer reports in dash is manipulated by software and it can be anything. It's not necessarily completely true.
Not so simple. Until recently, a standard ICE couldn't stop injecting fuel while coasting (unless you turned the car off, which brings other problems...).
I remember when I was learning to drive being taught about engine braking (with an automatic) and the tradeoff between using up the brakes or burning fuel.
For every fuel-injected engine the ECU should cut injection when coasting. So "recently" in your case applies to carburettors which will behave as if the engine were idle and thus still feed fuel to the engine. I'm not sure 25 years ago counts as "recently".
Well, I own a 911 that says otherwise. It's an older fuel injection system (Bosch K-Jetronic), but it's most certainly fuel injection. Porsche wasn't the only maker to use Bosch fuel injection.
I just don't think DFCO (deceleration fuel cutoff) was a feature of most of the early electronic fuel injection systems. Perhaps it was standard by the time EFI made it to economy cars. I'm fighting a massive urge to go down this historical rabbit hole and read about it, but there's work to be done today.
How modern is modern? I could believe this of my 2009 Honda Fit, but I'm fairly certain that my dad's '92 Civic, '99 Civic, and '92 Accord got better mileage when coasting with the clutch disengaged.
Think about what is required to turn the engine over and prevent stalling.
-When the clutch is engaged whilst going down hill the wheels turn the engine. No Fuel Required.
-When the clutch is disengaged the engine has to turn itself over - which needs fuel.
*Disclaimer, i'm sure carbourettas (sp?) could well be doing weird ass things so whilst conceptually the above holds they may well be different.
It's not the fuel required by the engine, it's the engine braking. Going down a hill with clutch disengaged, the Accord would accelerate. With clutch engaged, it would slightly decelerate. That's fuel that the engine doesn't need to burn to accelerate it back up to cruising speed.
Ditto when coasting to a stoplight - with clutch disengaged (or in neutral), it would coast for a longer distance, which - if you're careful and know your vehicle - means you can let up on the gas a lot sooner.
Anecdotal, but I did some tests in my previous car (a 2014 Mercedes, automatic) and it coasted really, really, really well. In drive, downhill, on the freeway, feet off all pedals, it would accelerate as much as if I put it in neutral, so I think its software and transmission allowed it to perform as good as a manual in that situation.
Not calling you out specifically, but a lot of people arguing for manual transmissions in this thread seem to base their experiences on ~10 year old cars or older, and weirdly assuming that nothing has happened since, that software and technology in cars has been magically frozen in time, despite enormous incentives for car manufacturers to improve it.
Automatics don't engine brake the same way, the torque converter is only fully engaged when it has some pressure buildup, so it will slip (and not maintain fuel cut) when coasting whenever the wheel speed won't maintain the pressure.
To test, watch your rpm when you left off the gas, if it drops to ~750-1000 rpm, your torque converter is slipping (as designed) and not maintaining fuel cut.
To agree with you, I think the comments in this thread started from a manual transmission standpoint and then everybody thought it applied to automatics.
It's not that simple, since everybody has had lockup torque converters (based on a computer-controlled clutch) for a long time now. As long as the TC is locked up, you have full engine braking. As you slow, it eventually drops out of lockup, you see the rpm drop to idle or close to it, and you feel the engine braking diminish (though you do still have a small amount, as the the transmission does have pumping and drag losses).
In a manual you are burning fuel when the clutch is disengaged to keep the engine turning over. With the engine engaged you steal from momentum which lowers fuel use.
The mid point of keeping the engine engaged and adding enough fuel to reach your desired stopping point once the RPM enters the same range as an idling engine is actually most efficient.
I'm pretty far out of my element here, but as I understand even that isn't universal. BMW's Valvetronic for instance doesn't engine break the same way as other engines, because they don't have a throttle valve and don't have a vacuum outside the intake valves:
Pretty much any engine that has an ECU I would suspect.
The oldest car I have that has an ECU is from 1987 (although judging the fuel efficiency on that car when it was new is a bit hard since the engine has been completely reworked since then -- including an aftermarket ECU, new injectors, new ignition etc which makes the car a tad more fuel efficient than my 2012 car)
Come to think of it: the car I drove in 1988 cut fuel when coasting and it had a rather unrefined ECU mapping that would cause somewhat hard transitions from the fuel-cut state to feeding the engine go-juice.
I also have an 09 Fit. I use the clutch to coast all the time. I think the only gauge that shows fuel usage is the mpg one? when I coast it just goes over to infinite. I don't think it does that when I don't put the clutch in.
Oh, it isn't on the road, my dad totaled it in 2008. It was, however, what I drove for my first 3 years in the workforce, so I got a pretty good feel for how to drive it economically.
In some cases coasting in neutral will save fuel, in others it will not. This depends on the specific situation.
If you're coasting down a hill to a stop, then it's better to coast in gear to use approximately no fuel. If you're coasting down a hill to long level stretch of road it's better to coast in neutral to obtain kinetic energy. If one wants to follow speed limits it's sometimes necessary to first coast in neutral then use engine braking at the bottom of the hill.
The optimal strategy in all cases is actually coasting in neutral with the engine turned off, and then starting the engine when you need to increase your speed. (This advanced technique may be illegal, though).
Slight tangent but my car could fail the test then :) It automatically coasts (yep, engine back to idle and everything) when in it's most economical automatic mode.
Haha, yes, that's why I specified it as a tangent. I did think it was a weird feature though as I knew the controversy over coasting :) And yep, a red AMG A45!
Apparently Mercedes call it "glide mode" or "gliding mode" depending which bit of marketing you look at. I couldn't find a technical description though (beyond what you already said).
I've seen it called "sailing" (and the icon on the dashboard is of a sailboat). See https://www.youtube.com/watch?v=S4N8BlXro9w (see 48 seconds in) - but this could be a language difference between territories.
But if you're coasting down a hill and not braking, your engine is idling and you're making use of gravity. The engine will be turning over at ~1000rpm instead of 2000rpm in my diesel.
Understandably it is dangerous as you are not in control of the transmission and therefore the wheels, hence the failure in the UK. But I got through some incredibly scarce times by coasting a bit on the commute to work...
With automatics you would need to brake down the hill to stop the car "running away" as it wouldn't engine brake automatically to compensate for gravity's effect.
With automatics you would need to brake down the hill to stop the car "running away" as it wouldn't engine brake automatically to compensate for gravity's effect.
Most automatic transmissions (or at least the ones I've driven, which arguably is not more recent than late 90s) have low ranges where there is engine braking. That's the exact use case for them. Automatic doesn't (or didn't) mean that you must never shift manually.
Automatic transmissions in large vehicles like trucks and buses usually don't have freewheels either, so there is always engine braking.
> The engine will be turning over at ~1000rpm instead of 2000rpm in my diesel.
Right, but when you're coasting you're burning fuel to turn over, unless you have a perpetual motion engine. Whereas engine braking (on a car with an ECU i.e. one made since 1996 or so) you could be doing 2000rpm but burning no fuel (gravity is turning the wheels which are turning the engine).
Sure. Turning the engine is going to take energy one way or the other. But you can use energy that would otherwise be wasted heating up the brake pads rather than energy from burning fuel.
So if you're coasting you're spending fuel to increase your speed (relative to what you'd do by staying in gear). In the situation where you're considering coasting that's probably not a tradeoff you want to be making.
Every automatic car I've ever seen has had the ability to downshift to engage engine braking. (Even the CVT in cars like the Prius has an engine braking mode.) I've seen advocates of manual transmissions cite a lack of engine braking as a reason to switch in multiple conversations, so I've gotten the impression that a lot of people just aren't taught how to drive an automatic properly. (Do they not talk about this in driver's ed? I don't even remember anymore.)
> Even the CVT in cars like the Prius has an engine braking mode
Well, of course a hybrid has engine braking - that's kind of pivotal to the whole hybrid design. It freaked my wife out the first time we went down a 7% grade in ours in 'B(raking)' - once the battery charged holding down the brake pedal caused the second MG to disengage from the transaxle and spin at max RPM's to bleed off the excess energy.
But yes, every automatic car I've ever been in has had engine braking as well - going down the same grade by mother will shift her Chevy Impala into '3' and can easily coast down the same hill only applying the brake to turn corners.
Would be nice if our Prius didn't require holding down the brake pedal, but in a way I think it plays into the natural instincts of a driver to do so - but it's a habit that can get you killed driving a manual or standard automatic transmission by overheating the brakes, so hopefully my wife doesn't go down any grades in a car other than our own without me.
The e-brake in my automatic Volvo is shot, so my wife and I were practicing how to engine brake down from 50 km/h to a crawl. She was surprised that she was never made to learn this in driver's ed.
My current car has "regenerative breaking" that charges the battery(not a hybrid, just a normal BMW). I haven't tried any testing without cruise control on, but with cruise on it will definitely do a little breaking on highway downhills to maintain speed.
Every generation of car is getting better at all these things, so statements about efficiency almost all need a "cars built prior to 20XX".
Honda Civic 06 i-Drive (CVT) will shift into lower gear, thus engine brake, if you brake hard enough to trigger the shift threshold. And you can go directly into manual override whenever you hit the wheel paddles or push/pull the stick.
I don’t have data on hand, but there are some issues with your claims. They're accurate when comparing a manual transmission to an automatic from the 70's, but newer automatics have essentially closed any efficiency gap.
1. On most automatic transmissions (probably anything newer than the early 90's), the torque converter has clutches inside of it which allow the pump to be directly locked to the turbine. This makes it behave basically the same as a clutch in a manual transmission - no constant slipping. The converter will usually “lock up” once you’ve reached a cruising speed. So, it is less efficient during acceleration, but once the converter locks up, it’s much more comparable to a regular manual transmission. The only thing that still hurts the automatic at that point is the fact that there’s a hydraulic pump which must be driven at all times to maintain pressure to keep the various clutches clamped. So yes, this is where city driving can show some efficiency differences. However, ...
2. On newer automatic transmissions (i.e., transmissions from probably around 2006 and up, such as the ZF 6HPxx series which is found on many Fords, BMWs, and some other brands), the torque converter actually behaves even more like the clutch in a manual transmission in the sense that it’s far more “aggressive” regarding when it locks up. For example, my 2011 BMW will usually lock the torque converter as soon as you take off in first gear (it even sort of feels like when you’re taking off in a car with a manual transmission). Then, it remains locked the entire time after that (even during shifts and wide-open-throttle conditions).
3. Newer automatic transmissions (such as the ZF mentioned above) do in fact have the ability to jump to any gear. I believe this is one of the main things that separate transmissions like the ZF 6HPxx from older style electronic automatics like the 4L80E which use a sprag system. The newer transmissions have a clutch pack for every gear which allows them to jump to any gear rather than following the sequence.
4.I believe that many CVT transmissions still use a torque converter anyway.
Those things have not been true of automatics for the last two or three generations. They have clutches inside the torque converter, they can block shift. But it is only the latest generation of gearboxes that return better economy figures than manuals. Until very very recently auto boxes were indeed much worse for economy.
These can be pretty inaccurate in my experience. It's annecdata, but my brother's Corolla is rated at 6.7 L/100km on highway for the manual, but he had no problem getting 5.6 L/100km in reality. 16% less than claimed on a 10 hour drive.
I think they may be vastly underestimating manual drivers ability to optimize this.
EDIT: Actually on the US site the Auto is rated for 6.9 L/100km, 7.1 L/100km on the manual. The Canadian NRCAN rated it 6.7 L/100km.
Everyone feels like they are a better-than-average driver.
The EPA has a standard testing regime, and modern automatic transmissions nearly always score at parity or slightly better than the manual option mounted to the same vehicle.
Interestingly, this is also true for things like 1/4 mile times. "Muscle car" guys hate this, but if you look at the numbers, a modern automatic transmission is both more efficient and more performant than the manual option.
> It's well known that accelerating rather quickly to your cruise speed is the optimal strategy for fuel efficiency, hence the reason to switch from 3rd to 5th gear directly.
I don't mean to be pedantic, but I think your logic is backwards here, ie. It's well known that 5th (top) gear is the most fuel efficient, hence the reason to accelerate to it rather quickly (the optimal rate would vary between cars), I think would be more reasonable.
I don't think the cruise speed factors into it, and neither does switching from 3rd to 5th, unless it's part of the strategy to get to 5th faster.
But then again, I never realised that accelerating harder to get to 5th faster is actually more fuel efficient [1], so thanks for the heads up!
Low gear is best for acceleration and overall control, high gear best for cruising. So accelerate in 3rd (or 2nd if your car is fun) to cruising speed, put straight in 5th/6th and activate cruise control.
It's a waste of energy and clutch/gearbox actuation to use the in between gears and there's never anything wrong with being in a low gear at any point unless your engine hits the red line.
On most cars fourth gear is really for cruising at a lower speed than is comfortable (or safe) in fifth.
You can also very easily downshift sequentially through the gearbox. Downshifting through several gears takes much more practice to do properly and isn't very useful on the road.
I learned to tease my 4-speed automatic TDI into shifting from 2nd directly into 4th, by accelerating strongly to 40-50 and then abruptly lifting throttle.
Disengaging the clutch is not recommended, since it modifies the balance of the car. When you are accelerating, the rear wheels are supposed to be under more pressure, then have better grip. The opposite when using the brakes. Not having that balance (for which the car is designed/tested) can lead to dangerous situations in certain conditions. Also, the gain in gas consumption is very debatable.
In another subthread to this I posted numbers for various models of BMW, Audi, and Mercedes. I could find one Mercedes model where they report worse fuel economy and slower 0-100km/h acceleration time for the version with an automatic gearbox, in all the other models I looked at, the manufacturers rate the automatics as more efficient and having better acceleration than the manuals.
These are of course high-end current model year vehicles, if the tech they have hasn't already trickled down into cheaper cars, it will do so very shortly, making my statement even more broadly true.
Please bear in mind that manufacturers currently self-police and that they test under artificial conditions. Hence, there have already been several scandals with Honda, Ford, Hyundai and Volkswagen either manipulating, or falsifying results; not even the onboard computer can be fully trusted: the only precise measure is noting how many liters were put in and how many kilometers were driven on that quantity, and that only on the spec tire size and with the assumption that the odometer is correct.
You're right, however those only work at higher speeds, hence the "especially with city driving" :)
From wikipedia : A Lock-up clutch is used in some automatic transmissions for motor vehicles. Above a certain speed (usually 60 km/h) it locks the torque converter to minimise power loss and improve fuel efficiency.
It's typical to omit locking while the transmission is cold, some transmissions may not lock the first gear, or may not lock at lower revs, or at slower speeds, but generally the lock is there and it's just software that controls locking. You can rather safely assume that on most modern cars and in general conditions the torque converter will lock starting from the first gear.
The lock-up only locks up in very limited conditions, especially older ones. There's basically a few specific speeds with a 10km/h margin that locks up.
"We’ve combined the best attributes of conventional automatic, continuously variable and dual clutch transmissions. The transmission locks up to couple directly with the engine over a far wider range than other automatics. This delivers a significant improvement in fuel economy and a more direct, manual-like power delivery and driving feel. Shifts are quick, accurate and smooth across the six speeds."
So, as they mention, this transmission is unique. It does, however, not lock up in all conditions - it remains unlocked until you hit 8mph. It sounds like an interesting transmission, but this is not the "norm". All conventional automatic transmissions should be able to do this, although it results in very different wear patterns (brake rings and clutches in the transmission will see much greater wear). It is not quite as interesting as a dual shift transmission, though.
(Having looked at a picture of the transmission, ignore the "combined the best attributes of..." - it appears to be an entirely conventional automatic transmission with a very different operating profile.)
It locks up after 8 mph probably because if it locked up at any lower of a speed, the engine would be too close to stalling.
Also, the ZF 6HPxx (and newer) transmissions behave the same way. My 2011 335i will lock up in first gear (probably around 10 mph, but it depends on the load). This transmission has been used by BMW since at least 2006. A derivative of it is also used in the Ford Mustang now.
I should try a car with one of those transmissions, then. They sound quite interesting. While I have driven quite a lot of cars, I have never dealt with such a transmission, but it sounds both nice and efficient. How's the comfort tradeoff? I believe the lock-up was initially avoided due to comfort, as people liked the "gooey" transition between gears.
I'll keep that transmission in mind next time I go shopping.
I also drive a CX-5. When I test drove it, one of the things that stood out to me was how it felt "heavy" when taking your foot off the gas. I guess that was just the engine breaking that I wasn't used to on my previous vehicle. Over the last year I've averaged 30-31mpg (mostly highway), which I don't consider too bad for an AWD compact SUV.
Yup, and it is also nice that if you just touch the brakes when going downhill the AT downshifts to engine-brake stronger. I can also feel when the converter locks up when accelerating. Particularly when the engine / AT are cold the acceleration is not ideally smooth because of this (well, it is still much smoother than I can do with my other car with MT).
Well, engine braking to the red light would have been more efficient. :)
But indeed, automatic cars are less efficient. They do not know what you will do next, so they often make erroneous gear shifts that waste fuel and time. They don't know when to coast or when to engine brake (For older transmissions, the overdrive gear had permanent coasting, and could not engine brake at all), often have lossy pumps, etc.
That was certainly true for older automatics. However, the tranny in my 2 yrs old bmw 3 seems to be freaking prescient. It seems to always be in the right gear at the right time and transitions are buttery smooth. I tried playing with manual gear control, via stick or shift paddles for old times sakes. But the result is noticeably messier. And btw, it's not a dual clutch.
I am from Brazil, here the population hates "hydramatic" cars as they call automatic cars.
I saw one accident that the cause was lack of transmission knowing the future:
road crossing, bad road signs didn't made clear who had priority, so both drivers went forward.
Both cars tried then to stop, but the automatic car instead had just shifted up, then as the driver tried to stop, it lurched forward and hit the other car, then it spun and crashed on the wall of a school.
The automatic car was a sports car from the 70s though, so I guess the transmission was even more dangerous.
Automatic transmissions back then were quite the hydraulic maze, as they operated entirely on hydraulics. These days, they're electronically controlled. I don't think the failure rate has changed.
In a manual, you can of course clutch out as long as your clutch cable (or hose for hydraulic clutched cars) doesn't fail, which is not an option you have for automatics. These kinds of failures do occur today, though. See the famous Toyota "unintentional acceleration" scandal, where a software error read "0% throttle" as "100% THROTTLE GO GO GO GO", causing death in a few cases. There's also a case of a person that was stuck in a car that could not stop, driving at constantly increasing speed on the highways of France, IIRC. He couldn't turn the car off or get it out of gear. You can disengage a manual if you're quick with an intact clutch cable, but you're a bit lost in an automatic, especially if it's too powerful to hold on the brake.
Up here, manuals are normal (with automatics slowly getting more and more popular), mostly because everyone knows just fine how to drive them, and automatics are much more expensive. I, personally, can't decide which I prefer. I'd much rather drive fully manual than a "retrofitted" automatic like the one in a Lupo 3L, and I still have to try a dual clutch transmission, for which I might end up having similar opinions. They always seem to change wrong, the changes are rough and slow, and the clutch engagement is very binary, leading to odd engage/disengage.
I like conventional automatics in cars that haul heavy loads (pushing a 4 metric tonne trailer over a curb in reverse at low speeds without the possibility of gaining some speed first make you worry a lot about your clutch in a manual), and our busses and trucks are generally fully automatic, with more gears than you would bother counting.
> You can disengage a manual if you're quick with an intact clutch cable, but you're a bit lost in an automatic, especially if it's too powerful to hold on the brake.
The typical car has brakes that can dissipate 4x the engine power, so either the driver failed to press hard enough or the brakes failed in that case.
Current VW DSG automatics are okay, butter smooth. I drove one from Germany this year. Too bad it's a VW, since I don't like their cars that much. My normal car is a manual Suzuki Jimny DDIS which drives like a tractor.
There is no such thing as a modern passenger car with an engine more powerful than the brakes. All cases of "unintentional acceleration" where the driver "could not stop" are cases of driver error: the driver refused to use the brake pedal, or was pressing the gas pedal instead.
The other thing these dummies are forgetting to do, in their panic, is to simply turn off the engine.
In case of Toyota, the error was not with the driver. Not having ninja reflexes when your car misinterprets throttle input as you are parking is not user error.
Reasons that a driver might not simply turn off the engine:
1. Twisting the key to turn off the engine engages the steering wheel lock, which might not be a good idea if you're moving fast.
2. Modern cars with start/stop buttons might end up with issues that disallow stopping the engine - physical button failure, or software issues related to the button handling.
3. Diesel runaway - diesels are quite difficult to stop, as they need very little external assistance, and can run on basically anything. Oil buildup in the intercooler (which any used car will have) that ends up sucked into the intake is a fairly normal way for a diesel engine to self-destruct if you're not quick enough to clutch in so you can keep the engine under control with some load.
Reasons a driver might not be able to hold the engine at full throttle:
1. Brakes seize up at load, and "full power" is certainly a lot of continuous load.
2. Brake failure.
3. The sheer difficulty of holding the car. I have once in my life stopped a diesel van that was idling in first gear using only the brake (long story), and that required a lot of effort. I would not have been able to do it if any throttle had been applied.
>In case of Toyota, the error was not with the driver. Not having ninja reflexes when your car misinterprets throttle input as you are parking is not user error.
That wasn't what happened. People were driving at high speeds on the highway and claiming they couldn't stop. They were wrong.
>1. Twisting the key to turn off the engine engages the steering wheel lock, which might not be a good idea if you're moving fast.
That's why you turn the engine off and then turn the key back on, or don't take the key out. Usually you have to take the key out to engage the steering lock.
Besides, which is worse, accelerating uncontrollably until you reach the car's maximum speed, or killing the engine and not being able to steer?
>2. Modern cars with start/stop buttons might end up with issues that disallow stopping the engine - physical button failure, or software issues related to the button handling.
Wrong. All such designs let you turn off the engine by either holding the button down, or by pressing it multiple times. People just were too dumb to learn how their cars worked.
>3. Diesel runaway - diesels are quite difficult to stop, as they need very little external assistance, and can run on basically anything.
Wrong and stupid. We're not talking about 1960s diesel engines with mechanical fuel pumps here, we're talking about modern vehicles with engine computers and electric pumps. Kill the power and they stop.
>1. Brakes seize up at load, and "full power" is certainly a lot of continuous load.
That's why you press them hard right away instead of trying to modulate the engine power with them....
>2. Brake failure.
Impossible. The odds of having a separate brake failure at the same time as an unintended acceleration failure are astronomical.
>3. The sheer difficulty of holding the car. I have once in my life stopped a diesel van
We're not talking about big diesel vans here, we're talking about regular cars in America which run on gasoline having unintended acceleration problems. Americans do not drive diesel cars, and certainly not diesel vans.
Do you have any more completely irrelevant anecdotes to try to disprove my assertions?
1. I may misremember, but I do seem to recall a case where the unintended acceleration happened in a Toyota as the person was parking, with the car accelerating into the wall in front of them. But, indeed, that may be misinformation, so I'll retract the statement.
However, this does not mean that people all had the time or ability to stop. Unintended acceleration can happen in many cases, such as in a case of highway driving where, as they try to brake, they notice that the throttle is held. If this happens as someone needs to perform an emergency maneuver, the braking distance will be greatly increased, and they might lose control over car as power is still transferred.
In most cases, it might be very easy to handle the situation when you're prepared, but when you have very little time to observe the problem and correct it, things are less likely to turn out well. You might not have enough time to realize "Ah! My car is not reducing throttle, so I must set the automatic transmission to Neutral!", let alone perform the action prior to collision.
2. The steering wheel lock is different from car to car. My current car (VW) engages the lock when the key is removed and the gear is in "STOP", but my previous cars engaged when the key was rotated counter-clockwise two clicks, making it easy to hit the lock instead of simply stopping the engine. Not being able to steer is quite a fatal thing - see the accidents caused when steering wheel locks were first introduced. Especially when the electric kind was introduced (Chevrolet?), which had problems with spontaneously engaging the lock. This caused people to crash as they were suddenly unable to exit, or enter a turn, with only a second or two to stop the car. On a highway, you ultimately end up crashing into the barrier if your steering wheel lock engages.
3. And how are such designs implemented, if I may ask? Oh, software? Right. Do I need to say more, or do I have to explain exactly why this makes the mechanism entirely untrustworthy?
4. Wrong - you really at least read a bit on diesel engines. First of all, modern diesel engines (That is, 2016 model cars) have two diesel pumps - a low-pressure, electric pump in the fuel tank, and a high-pressure (that is, 1-2000 bars) pump driven mechanically by the drive belt. The electric pump is needed to bring fuel up when starting the car, as cold diesel is difficult to pump, and the starter won't result in significant pumping pressure from the high-pressure pump. However, once started, the low-pressure pump is irrelevant to engine operation, providing only a slight pumping assistance. Killing the engine the "proper" way is done by not opening the injectors, causing the pumps to simply recirculate fuel.
However, this has nothing to do with diesel runaway. During runaway, the diesel engine is not running on diesel, but on engine-oil leaking into the cylinder, often through the air-intake. Modern diesel engines almost always have turbochargers, which leak engine oil into the intercooler once their sleeve bearing, through which engine oil is actively pumped to avoid metal-to-metal contact, have been worn down, increasing the gap between turbine axle and bearing. Once enough oil has built up in the intercooler (which acts as an oil reservoir in this case), you risk sucking it in - and as any diesel engine will happily run uncontrolled on engine oil, this causes engine runaway. This can of course also happen due to other leaks, such as piston or injector seals. The lack of control means you either get to engage a gear and minimize damage as you wait for engine oil to stop flowing into the intake, or if you cannot provide any load, wait for the engine to explode, leaving behind melted pistons in what is now an expensive paper-weight. Remember that a diesel engine ignites fuel with compression (with fuel injected normally injected a full compression to allow for slight ignition timing adjustments, hence the crazy fuel pressure), not spark.
While gasoline engines can also suck in engine oil, they have electric rather than mechanic ignition, and they cannot burn engine oil well enough to run on it anyway.
5. You still need to overpower the engine, even when slamming the brake with all your might. A small gasoline car is probably easy to kill, but a more torquey engine will take quite some effort to stop.
6. Brake failure impossible? What? Brake pipes can burst, caliber piston seals can leak, and regular brake disks seize.
The first two modes of failure usually happen when you apply brake pressure. The more you apply, the more likely that you will break your old brake lines and piston seals. In other words, if your brakes are going to fail this way, they're going to fail when you need them. The dual fluid system of modern cars mean that in some cases, you will get more than one braking attempt (an attempt being a press on the pedal of any kind), but not many, and not in all braking modes.
Brake seizing is of course a possibility, unless you drive around with carbon fiber brakes.
7. Going up to the parent comment - "I am from Brazil", talking about the car reputation there. Feel free to join the comments, but don't try to pretend we're talking about something else now. I don't particularly care about Americans and their poor taste in cars, or your incorrect information about them not driving diesel cars (although they are not nearly as popular as in the rest of the world). Of course, the original post was about American transmission statistics, which are quite unrelated to the engine type.
Do you have any more incorrect or just plain uninteresting comments to try to disprove my assertions, or shall we let it be? :)
Not trying to be confrontational, but I have to say... I don't buy that story at all.
Automatic transmissions back then didn't even have lock-up torque converters. You could always stop the car unless the throttle got stuck. The transmission can't suddently send the car screaming forward if the person isn't on the throttle.
On another note, the main automatic transmissions of those days were probably the TH350 and TH400, which actually stands for "turbo hydramatic". That's probably where the name came from.
Browsing fueleconomy.gov looks like that's no longer the case. Looks like the newest versions of the Honda Fit, Toyota Yaris, and Toyota Corolla have better gas mileage with the automatic transmission, for example.
* Mazda 6 2.5L SkyActiv-G AT: 26/38 MPG (28/40 MPG in Grand Touring model, not available in MT)
As for the torque converter inefficiency - that is true only for some ancient transmissions. A modern torque converter will waste some energy only during shifting the gears. After synchronization, the torque converter is locked and the connection between the engine and the wheels is mechanical, just as in MT.
"It's well known that accelerating rather quickly to your cruise speed is the optimal strategy for fuel efficiency"
And now - any data to back this up? I don't have hard data, but I have some hard theory. Modern engines have variable valve timing technology which allow them to work in a much more efficient thermodynamic cycle when under light load and low RPMs. When you're not using the full torque of the engine it works in something like an Atkinson or Miller cycle, with high compression ratio and much higher thermodynamic efficiency. A bad thing about Atkinson-like operation (achieved by keeping the inlet valve open longer than normally) is that it reduces the effective capacity and hence power of the engine. So if you press the accelerator pedal fully, the ECU switches the engine to a classical Otto cycle, which is less fuel efficient but gives more power. Hence, if you accelerate fast, your mileage will be worse, not better.
Another issue is that keeping high RPM at low gear (possible with turbocharger running on full pressure) to accelerate fast makes it harder to control burning all the fuel efficiently because there is less time to do that in each cycle. And also the temperature of the cylinders goes quickly up which makes it harder to avoid detonation. So in order to not destroy the engine, the ECU will adapt the injection / spark / valve timings and lambda coefficient to avoid explosive burning of the fuel by decreasing the effective compression-ratio and increasing lambda to decrease the temperature. Lower compression-ratio and higher lambda means lower fuel efficiency.
That's why when I "eco drive", the ECU keeps the gear extremely high, so that the engine works typically in a range 1000-1500 RPM. Now this is quite hard to achieve in practice when driving an MT car, because whenever you want to accelerate faster, you'd have to gear-down by 2 or 3. That's why ATs are better for fuel efficiency.
BTW: I do own 2 cars, one with MT, one with classical hydrokinetic AT, so I have a comparison. I'd really not be able to drive my MT car using such high gears as my AT car can. And also I can't switch gears so fast as an AT can do, even though my AT is not the fastest among all ATs.
Even many of the CVTs have fake gears because people didn't like the sound and feeling of a straight up linear CVT, so they might be more comparable to automatics.
This was my general impression - especially the early shift issue. I learned to shift manuals ~500 rpm higher than where most automatics seem to, a decision that seems made to produce the smoothest possible automatic acceleration at a cost to efficiency.
Notably, this feels way less pronounced with faster, sporty automatics, which are willing to give you a feel of acceleration and shift more like manual drivers.
Having spent 12 years with a WRX, and moved a couple years to a Hyundai Genesis Coupe; while my wife still drives a Subaru Forester... Subarus are a bad example here. They've got great engines and other factors, but their automatic transmissions really suck.
Is there anywhere else where you can get what's supposed to be their high-performance car (my WRX was the year before STI came out) or the performance variant of another (my wife's Forester is an XT), where that automatic transmission is just a 4-speed? And subjectively, it's a rather balky one at that, with very slow shifts.
For comparison, my Genesis Coupe's auto tranny is an 8-speed, which maximizes the time that the engine is at its most efficient. It's also got smooth lightning-fast shifts in comparison to either Subaru.
I call bullshit. Last time I looked up the BRZ, the automatic was far better than the manual, something like 36mpg vs 30mpg (highway). Most likely, the manual has much lower gearing than the automatic.
I think this is widely known fact for modern cars. Automatics (may) have more gears & can consistently shift more optimally than average human driver, making up the loss of "wet" torque converter.
> because of Physics, an automatic transmission will always be less efficient
You making that claim based on how automatic transmissions currently work, right? And not on every way they could work.
Purely from logic, your claim has to be false: Imagine a humanoid robot sitting in your driver's seat, driving your manual transmission car, its robot arm doing the shifting exactly as a human would. Your manual car has been turned into an automatic and it is not less efficient.
I suppose you could argue that the robot, being part of the car, has an energy requirement. I would then argue that we could shrink the humanoid robot down to the
size of chip with sensor inputs, and replace the robot arm and gear shift with a solenoid or whatever, thereby saving considerable energy.
By this thought experiment alone, I don't see how anyone can make a statement about automatics always being less efficient.
> Purely from logic, your claim has to be false: Imagine a humanoid robot sitting in your driver's seat, driving your manual transmission car, its robot arm doing the shifting exactly as a human would. Your manual car has been turned into an automatic and it is not less efficient.
You're forgetting a very important factor: the automatic transmission can only respond to the drivers actions. Unlike the driver it has no knowledge of future plans. It will happily shift up 2 seconds before I hit the brakes because it has no idea I'm planning to take a left turn in 20 meters.
> It will happily shift up 2 seconds before I hit the brakes because
> it has no idea I'm planning to take a left turn in 20 meters.
Purely hypothetically, a car could be fitted with an operator control to convey that sort of intent. It could be something the driver could operate without taking their hands off the wheel — perhaps a stick on the left of the steering column?
You already have one of those. If you are driving a 4-speed automatic, you can see that you are in 3rd and sense that it might be about to upshift and a turn is coming up, you can whip it from D to 3 to hold it in 3rd for the duration.
> Surely the same data that is used for automatic braking, lane assist etc could be used to improve timings on automatic gear shifts?
I don't think that data is very useful in city driving where most of the shifting happens. No radar or lane detector can detect my intention of taking the next exit on the left.
That being said, I remember reading a piece a while back about a car manufacturer working on linking the car's navigation/GPS to the automatic transmission. So as long as you enter the destination in the GPS it can make more informed decisions.
"automatic transmission" is usually taken to specifically mean a torque-converter based hydraulic transmission, rather than a general class of self-shifting transmissions. I'm sure that's the sense in which the parent comment to yours meant it.
What you describe already exists but it doesn't get called "automatic". The car companies don't call it that, the drivers don't call it that. It'll be called DCT or the older sequential manual gearbox.
Do you have data to back this up? Unless you're driving a dual clutch "dry" transmission or a CVT, because of Physics, an automatic transmission will always be less efficient. The Automatic transmission Torque converter [1] will waste some of the energy provided by the engine, especially with city driving.
Also, most automatic cars tend to shift gear up too early, and can't switch to gear n+2. Because of this, you spend more time accelerating to your cruise speed. It's well known that accelerating rather quickly to your cruise speed is the optimal strategy for fuel efficiency, hence the reason to switch from 3rd to 5th gear directly.
With a manual, you can also disengage the clutch when you consider it safe (and not slowing down traffic) and you know you're going to have to stop at the red light.
[1] https://www.youtube.com/watch?v=z5G2zQ_3xTc