does coasting save petrol

tbourner

EazyDuz wrote: »

If i did the same thing but in gear, the engine breaking would be so bad i'd be constantly adding throttle on the way down the hill, therefore using much more fuel.

How do you know you'd be using much more fuel? Why can't it be using a lot LESS fuel because the mass of the car is turning the engine over, meaning just enough fuel is required to supplement that movement, rather than a set amount of fuel as is required to maintain idle with no helpful forces coming through the gearbox.

mikey72

tbourner wrote: »

How do you know you'd be using much more fuel? Why can't it be using a lot LESS fuel because the mass of the car is turning the engine over, meaning just enough fuel is required to supplement that movement, rather than a set amount of fuel as is required to maintain idle with no helpful forces coming through the gearbox.

Because on idle it only has to move the pistons at idle rpm.
Which is the lowest amount of force possible, at the lowest revs.
When you accelerate, it still keeps the same mixture, but at a higher rpm.
It won't run lean.
The ecu won't allow a mixture out of spec.
So it must use more fuel.
That's why you need to keep using the throttle in gear, or it'll slow.

tbourner

mikey72 wrote: »

Because on idle it only has to move the pistons at idle rpm.
Which is the lowest amountforce possible, at the lowest revs.
When you accelerate, it still keeps the same mixture, but at a higher rpm.
It won't run lean.
The ecu won't allow a mixture out of spec.
So it must use more fuel.
Tjat's why you need to keep using the throttle in gear, or it'll slow.

But the mass of the car is turning the wheels, which turn the gearbox, which turn the crankshaft, which push the pistons up and down. So you apply throttle to get to the set speed you want and the car puts in the amount of fuel you require for that speed, regardless of the RPM it will be LESS fuel than at idle because it needs less power due to being supplemented by the mass of the car helping it along.

//edit: the car is capable of putting more or less fuel in for a set RPM.

mikey72

tbourner wrote: »

But the mass of the car is turning the wheels, which turn the gearbox, which turn the crankshaft, which push the pistons up and down. So you apply throttle to get to the set speed you want and the car puts in the amount of fuel you require for that speed, regardless of the RPM it will be LESS fuel than at idle because it needs less power due to being supplemented by the mass of the car helping it along.

//edit: the car is capable of putting more or less fuel in for a set RPM.

But the lambda sensor can never let it underfuel.
So for a set rpm, it'll maintain the fuel to give the correct burn.
So, with the butterfly closed, it has to depend on rpm.
Faster rpm, more fuel.
Or no fuel at all, but then engine braking etc.

mkirkby

tbourner wrote: »

But the mass of the car is turning the wheels, which turn the gearbox, which turn the crankshaft, which push the pistons up and down. So you apply throttle to get to the set speed you want and the car puts in the amount of fuel you require for that speed, regardless of the RPM it will be LESS fuel than at idle because it needs less power due to being supplemented by the mass of the car helping it along.

//edit: the car is capable of putting more or less fuel in for a set RPM.

Exactly. Simple physics.

energy cannot be destroyed, only converted, there are two ways to make the engine turn as you go down the hill:-
1) potential energy from the car
2) the chemical energy from burning fuel.

Now I wonder which is more efficient baring in mind that energy losses by transmission are usually a max of around 25% whereas energy losses in a normal petrol engine (even when idling) are about 60-65%???

EazyDuz wrote: »

I agree with above, my car will cut fuel with no throttle but the engine braking is ridiculous.

Yes it might be *ridiculous*, but that engine braking is down to the compression of the engine which has to be overcome whether you're coasting in gear or the engine is at idle. Now think about it, bearing in mind the above energy losses, would you rather use the energy of the 1.5 tonne vehicle + a bit of fuel to overcome it, or would you rather use only the fuel to overcome the engine braking?

The idea that you are using more fuel if you have to keep your foot slightly on the accelerator downhill to maintain speed is just plain wrong. It's psychological simply because you're not having to put your foot on the accelerator if the engine is idling. It's still using fuel to overcome that ridiculous engine braking and a damn sight more than would be needed to keep the engine turning using the potential energy of the car with a tiny bit of help from your right foot.

"Ye canna change the laws of physics, Cap'n!"

mkirkby

Tobster86 wrote: »

Half a second is a hell of a lot of time in terms of computing. The weighing machines I work with, that use 22Mhz Zilog derived processor does about 50 'ticks' in that time (full processing of a single raw sample). This looks more like a programmed time delay.

Or a crap programmer (probably not though, probably what you said).

22Mhz Zilog though; beats the ZX81 I started out on but then if it isn't broken even after 30+ years...

mikey72

mkirkby wrote: »

Exactly. Simple physics.

energy cannot be destroyed, only converted, there are two ways to make the engine turn as you go down the hill:-
1) potential energy from the car
2) the chemical energy from burning fuel.

Now I wonder which is more efficient baring in mind that energy losses by transmission are usually a max of around 25% whereas energy losses in a normal petrol engine (even when idling) are about 60-65%???



Yes it might be *ridiculous*, but that engine braking is down to the compression of the engine which has to be overcome whether you're coasting in gear or the engine is at idle. Now think about it, bearing in mind the above energy losses, would you rather use the energy of the 1.5 tonne vehicle + a bit of fuel to overcome it, or would you rather use only the fuel to overcome the engine braking?

The idea that you are using more fuel if you have to keep your foot slightly on the accelerator downhill to maintain speed is just plain wrong. It's psychological simply because you're not having to put your foot on the accelerator if the engine is idling. It's still using fuel to overcome that ridiculous engine braking and a damn sight more than would be needed to keep the engine turning using the potential energy of the car with a tiny bit of help from your right foot.

"Ye canna change the laws of physics, Cap'n!"

Petrol burns at a ratio of about 14:1
It doesn't change if the pistons push the engine, or if the engine pushes the piston.
It just burns 14:1.

tbourner

Yep but more air is going in at idle than there is when rolling down a hill in gear, the engine closes the butterfly because it knows it needs less power.
Are you honestly saying that driving at a constant speed and then putting your foot down uses the same amount of fuel? No cos when you put your foot down more air is shoved in so the ecu ramps up the duty cycle to 100% to cope with the demand for power, just cruising the duty cycle might only have been 20% so using 80% less fuel at the same rpm.

mikey72

tbourner wrote: »

Yep but more air is going in at idle than there is when rolling down a hill in gear, the engine closes the butterfly because it knows it needs less power.
Are you honestly saying that driving at a constant speed and then putting your foot down uses the same amount of fuel? No cos when you put your foot down more air is shoved in so the ecu ramps up the duty cycle to 100% to cope with the demand for power, just cruising the duty cycle might only have been 20% so using 80% less fuel at the same rpm.

The engine doesn't control the butterfly.
The driver does, by lifting off, even on drive by wire.
Lift off, the butterfly closes.

So either coasting in neutral, or on overrun in gear, the butterfly is closed, if your foot is off the accelerator.

The only thing that pulls in more air per minute, is more rpm.

So, overrun, then accelerating, as you say, pulls up the duty cycle, and uses even more fuel.

Constant idle is just that. Constant. Low duty cycle.

Tobster86

mikey72 wrote: »

Petrol burns at a ratio of about 14:1
It doesn't change if the pistons push the engine, or if the engine pushes the piston.
It just burns 14:1.

You're assuming that airflow is proportional to engine speed, which it is not.

Yes, the ECU will try to maintain as near as possible the stoichiometric ratio of petrol, 14:1 as you said; but this can still be achieved with a small initial air mass; and there's no reason why that can't occur in high-rpm, low throttle situations.

Next test; and my BMW 323i is an ideal candidate with it's analog fuel consumption gauge. It will also prove that EazyDuz was wrong to coast in the described situation.

Find a downhill slope which can be coasted, or requires some throttle in fifth gear to maintain constant speed. Observe the mpg when in gear with throttle, and observe it while coasting.

The only problem I can see with this test is that in this situation it's very likely that the aerodynamic drag forces would cause the car to slow down from the said 'steady speed' when coasting.

Which leads on to another consideration; there would have to be undesirably too much engine braking from the vehicles top gear, yet sufficient slope for the vehicle to maintain speed whilst coasting. I can't imagine any vehicle which that would be the case for, except maybe a Massey Ferguson 135 going really REALLY fast!