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.
