Help braking by putting transmision in neutral

[Ian P.]

Bosch has had positive fuel shut-off since the late 70s with the mechanical fuel injection systems and later with the fully electronic systems.
I believe it is more for emissions reduction than fuel economy.
Our 03 Audi does it as did our 88-90.
In the case of turbo diesels, there may be some additional factors involved, reducing turbo lag, reducing the magnitude of engine braking and cutting the black smoke on throttle up.
Engine braking is not a simple do or don't issue. In the case of some Honda race bikes, they installed a releasing clutch to reduce engine braking as it caused rear wheel lock-up. Up until 2007 some F1 teams were using the TC system to power the engine to act as a rear wheel anti-lock system (yes it was against the "spirit" of the rules bu it was legal) there are cases where race cars can decellerate faster than the engine can loose revs, hence engine braking doesn't work.
Modern F1 engines are so light and have such low rotational mass that engine braking is likely always effective at adding to the overall braking effect.
Neutral or not, there are situations where it can work and some where it doesn't. Depends on many factors.

[ernos5]

If any of you ride motorbikes, you use the mentioned earlier technique "Hell and Toe downshifting" so a smooth deceleration can be achieved under braking and downshifting. This is ESPECIALLY necessary when cornering, I've been in situations many time when I'm in a too low gear and i want to have the RPM in a high range for the corner exit so i downshift while leaning into the corner. If the throttle was not used to match the engine revs with transmission speed, the rear wheel would jolt out and that would be the end of your ride [-X

Also being an ex-F1 Sim racer, i used engine braking a lot during my racing and qualifying. This was especially noticeable at Monza braking into the first chicane. With engine braking i could almost brake at the 100m board, but without engine braking i needed at least 20m extra in length of the braking zone to reach the same desirable corning speed.

Engine braking definitely helps decrease braking distance and if correct "Heel and Toe" is used, your gearbox life will increase as well.

[Scuderia_Russ]

Our town's nickname is Chamy Town

There'sa some doinsa transpirin' Brandine.

[Belatti]

In the beggining it was for Formula 1 but now there at automotive

Oh, sorry.

Well, I think a simple question deserves a simple answer: by putting transmision in neutral you let engine revs drop to idle. A F1 car idle engine speed must be arround 4000rpm. So, when you need power on again, the engine would have to go from 4000 to lets say 16000 in less that a tenth. Thats the opposite of saving the engine.

Try this in your manual transmision car: traveling at 100mph press the clutch, brake hard till 35 mph and engage 2nd gear. Lift the clutch. Tell me what happens...

[alelanza]

Very nice to see this graph, i had always assumed the fuel cut off to be the case in the fuel injected cars I've driven but never saw it in a chart like this.
I do realize this is no longer that much about F1, so please let me know if I should start a new topic elsewhere.
Some non scientifical observations i've made about this subject, pls correct me if i'm wrong;

- While going downhill in compression, ancillaries can be used for 'free' (not really cause you had to get the car up there in the first place, but you can get some of it back at least). In fact sometimes i turn the AC on if the hill i'm in requires more braking than say 3rd gear can afford but not as much as 2nd gear would give me.
- There's no better way to cool down an engine, even turning the car off won't drop temps as quick as if you use compression. This can be very good in very hilly countries, such as mine, where the engine can take a beating going uphill on a loaded vehicle for prolonged periods of time. In addition to what was already mentioned here regarding prevention of brakes overheating.
- On very steep inclines, going downhill on a RWD car, one has to be careful. Too much engine braking could partially lock your rear wheels, so brake pedal usage is very important there. Especially when the back of the car is very light.
- When downshifting i always make sure to apply a bit of brake pressure in order to reduce clutch wear, once engaged i let go of the brake.

And now two questions.

- Engine braking/compression, i kinda always assumed that what was slowing me down was the engine compressing the air. But thinking about it, on the intake stroke, the engine is actually working against a vacuum no? given I have the throttle closed. So now i'm thinking that in this case, intake stroke actually consumes power, compression stroke gives it back, power stroke takes power again, and exhaust stroke i guess does not really give all the power back as the valve has opened so the piston is not being sucked back into top dead center, unlike the compression stroke. That would also explain why the engine cools down so quickly. Thoughts? am i way off here?
- For the people that think fuel may still be consumed during engine braking, where would that fuel go to? what would happen to it?

Thanks

[alelanza]

Well i guess all my assumptions are correct, otherwise i would have already been told

[MegaHurts]

There's a straight hill near me where if I start out at the top at at 80mph, stick it in nuetral and roll for about 2 miles, I'll be doing 75mph as the hill levels out. If I leave it in gear, even in 5th, than I have to press the accelerator to keep up my speed because of engine breaking. A pretty extreme example I suppose, but rolling in neutral must use less fuel in some cases than leaving it in gear.

Then, assuming it does, how can you work out where it would be best to use engine breaking and where it would be best to coast in neutral purely for the best economy?

Just a question I've often wondered as I roll down that hill, but it also applies to rolling in neutral on a flat before a decline as opposed to keeping the throttle open then engine breaking down the hill or rolling in neutral up to the lights instead of keeping the throttle open longer then using engine breaking.

[alelanza]

There's a straight hill near me where if I start out at the top at at 80mph, stick it in nuetral and roll for about 2 miles, I'll be doing 75mph as the hill levels out. If I leave it in gear, even in 5th, than I have to press the accelerator to keep up my speed because of engine breaking. A pretty extreme example I suppose, but rolling in neutral must use less fuel in some cases than leaving it in gear.

Then, assuming it does, how can you work out where it would be best to use engine breaking and where it would be best to coast in neutral purely for the best economy?

Just a question I've often wondered as I roll down that hill, but it also applies to rolling in neutral on a flat before a decline as opposed to keeping the throttle open then engine breaking down the hill or rolling in neutral up to the lights instead of keeping the throttle open longer then using engine breaking.

As it turns out i have the same question, there's one particular hill where if i just let it go in neutral, i know i can travel about two miles at a pretty decent speed, whereas if i stay in fifth gear i do have to throttle it and i will still go slower. It 'feels' like traveling quicker, just feeding the engine enough for it to idle, spends less gas than traveling slower and giving it enough throttle to get to the same point. It's the only place where it applies as the hill is not too steep, elsewhere engine braking is a necessity/replacement for regular brakes.