[Jeroen]

Originally Posted by Kumar R

Hi Jeroen, I have a technical question

While coasting in neutral, the engine would be idling at idle speed (say 1000 rpm).

While engine braking (or slowing down at a lower gear) the engine would always be in 1400-2200 rpm.

Will not the higher rev account for a (relatively) greater fuel consumption?

NO, as essentially the momentum moves the car forward the engine is just being turned over through the transmission/cltuch etc. In essence, the ECU could shut of the fuel supply completely. (And some older ECU's do). On most cars the ECU still commands a tiny amount of fuel to be injected. Much less then the amount for normal idling. There are several reasons, one is to keep a bit of real (hot) exhaust gasses to flow to the cat convertor.

So while engine braking the RPM don't matter for fuel consumption. The only thing that matters is how the ECU is set up and what amount of fuel gets injected during these specific condition. Again, its a tiny amount.

Jeroen

Jeroen

[im_srini]

Originally Posted by a4anurag

Neutral will use fuel as it has to keep the engine running whereas in gear deceleration will not use fuel as the ECU will cut off supply irrespective of the RPM it is.

Originally Posted by Aroy

While coasting in gear, the ECU will supply fuel just enough to stop stalling.
As the wheels are running the engine, the fuel supplied will be extremely low, as no fuel is required to maintain the car's momentum.
Once the speed is low enough to stall the car, the ECU will start increase the fuel to prevent stalling.

Originally Posted by Jeroen

In essence, the ECU could shut of the fuel supply completely (and some older ECU's do).
On most cars the ECU still commands a tiny amount of fuel to be injected. Much less then the amount for normal idling.
There are several reasons, one is to keep a bit of real (hot) exhaust gasses to flow to the cat convertor.

Always had a some questions about this.
On my older 2001 Alto, when durng engine-braking, the ECU is in 'Fuel Cut-Off' mode till the engine RPM drops to ~1,700 RPM.
On my newer 2010 K10, when during engine-braking, the ECU is in 'Fuel Cut-Off' mode till the engine RPM drops to ~1,100 RPM.
In both cases, the Injector-Timing is shown as 0 ms, which I guess means the fuel is fully cut-off - after the RPM drops below this Minimum value, fuelling is resumed.
That said, during engine-braking, during this 'Fuel Cut-Off' mode, you can distinctly hear that the engine is running ( I have a pretty loud FFE so I'm sure I'm not not hearing things ).
Is what I'm hearing the sound of the engine working as a pump ( just compression 'pops' ) or is the ECU actually fuelling as Joroen has mentioned ?
A more basic question, how does an ECU figure out that engine-braking is taking place anyway - TPS indicates OFF ( no throttle ), MAP indicates HIGH, engine RPM indicates high ( greater than Idling RPM ) & vehicle speed is non-zero ?

[sgiitk]

Originally Posted by Kumar R

Hi Jeroen, I have a technical question

While coasting in neutral, the engine would be idling at idle speed (say 1000 rpm).
While engine braking (or slowing down at a lower gear) the engine would always be in 1400-2200 rpm.

Will not the higher rev account for a (relatively) greater fuel consumption?

Try this in the UK and if you are giving a driving test you FAIL.

[Jeroen]

Originally Posted by im_srini

Always had a some questions about this.
On my older 2001 Alto, when durng engine-braking, the ECU is in 'Fuel Cut-Off' mode till the engine RPM drops to ~1,700 RPM.
On my newer 2010 K10, when during engine-braking, the ECU is in 'Fuel Cut-Off' mode till the engine RPM drops to ~1,100 RPM.
In both cases, the Injector-Timing is shown as 0 ms, which I guess means the fuel is fully cut-off - after the RPM drops below this Minimum value, fuelling is resumed.
That said, during engine-braking, during this 'Fuel Cut-Off' mode, you can distinctly hear that the engine is running ( I have a pretty loud FFE so I'm sure I'm not not hearing things ).
Is what I'm hearing the sound of the engine working as a pump ( just compression 'pops' ) or is the ECU actually fuelling as Joroen has mentioned ?
A more basic question, how does an ECU figure out that engine-braking is taking place anyway - TPS indicates OFF ( no throttle ), MAP indicates HIGH, engine RPM indicates high ( greater than Idling RPM ) & vehicle speed is non-zero ?

The ECU in the most basic form knows you are 'engine braking if the throttle position doesnt match the RPM. So if the throttle position is for idle and the RPM is still high, it knows the engine is "driven". In practice it might be a little more complex.

How do you know your engine is in cutoff mode, I assume you are using some OBD device? On the 0ms, I know that on some ECU apparently the ECU commands intermittently a tiny fuel injection. So it might be case that your read out just doesnt have the resolution/accuracy to show this. But maybe somebody else can offer further insights?

Jeroen

[Sutripta]

Originally Posted by Jeroen

On most cars the ECU still commands a tiny amount of fuel to be injected. Much less then the amount for normal idling.

Originally Posted by Jeroen

I know that on some ECU apparently the ECU commands intermittently a tiny fuel injection.

Tiny amounts, or not so tiny amounts, but injected very very infrequently? (Talking of petrols.)

Regards
Sutripta

[fordday]

Thanks Jeroen and others for clearing my doubts.

Now I understood. Pottering around town without touching the accelerator is same as idling. When you are cruising fast and then remove the accelerator input, the ECU senses this and cuts off the fuel supply (almost totally) till the engine rpm drops to near stalling level.

Fordday.

[AjayJoshuaN]

Originally Posted by Jeroen

4) Momentum is a combination of speed and weight nothing else.

Jeroen

No, you have not understood momentum. it is not "a combination of speed and weight nothing else" !
Of course, momentum is the product of velocity and mass. But what does that mean ?
It is how hard it is to get something to stop (or to change directions).
For example, in an EMU, momentum is used by coasting after turning off the motors. So the heavier something is and/or the faster it’s moving the more momentum it has .

Coasting (in neutral) dissipates this stored energy against aerodynamic drag and rolling resistance which must always be overcome by the vehicle.

"Foot-off accelerator in gear" dissipates this stored energy against aerodynamic drag and rolling resistance and additionally engine braking which must always be overcome by the vehicle.

Engine braking is a waste of momentum. Car drivers are pretty much oblivious to the energy that engine braking wastes because driving doesn't require any real, physical effort like a bicycle. Keeping the engine at higher RPM's most of the time is not good.

[Jeroen]

Originally Posted by AjayJoshuaN

No, you have not understood momentum. it is not "a combination of speed and weight nothing else" !
Of course, momentum is the product of velocity and mass.
.

Actually, that was the only thing I said and we seem to agree on that. So it means the faster you go or the heavier an object is the more moment it carries. Si I'm not sure what I don't understand, but maybe thats the problem.

This thread is not about how to gain or loose momentum perse, but about whether coasting saves fuel. Which as you go through the thread, in essence comes down to understanding the behaviour of modern electronic engine management system during coasting conditions. Coasting in most cases will allow you to reduce the momentum of the car. (unless of course you go down a steep hill, have very strong wind from the back, or are being pushed at the same speed)

But if you like to discuss the design of say brake systems as an example on how to reduce momentum of an object; I have designed and tested various brake systems for various industries so it would be a topic I would be happy to give some momentum to.

Jeroen

[AjayJoshuaN]

Originally Posted by Jeroen

This thread is not about how to gain or loose momentum perse, but about whether coasting saves fuel. Which as you go through the thread, in essence comes down to understanding the behavior of modern electronic engine management system during coasting conditions.
Jeroen

Now I understand what you are saying. The time period you are considering is "during coasting in gear/neutral". But I was not considering this alone as the time period.
I agree, during the "coasting in gear" time period, fuel supply is near zero, since vehicle momentum turns the engine. So fuel is saved during the "coasting in gear" time period.

But I do not agree with the general statement that coasting saves fuel, because here the time period is ambiguous.


So given a scenario where you have to reach from a source to a destination, coasting in neutral at the right times will save more fuel compared to coasting in gear.
That is because, when coasting in gear, it is vehicle momentum against aerodynamic drag + rolling resistance + engine braking.
When coasting in neutral, it is vehicle momentum against aerodynamic drag + rolling resistance.

So the distance traveled when coasting in gear would be lesser compared to the distance traveled when coasting in neutral.
Fuel used when coasting in gear would be lesser compared to the fuel used when coasting in neutral.

However, the fuel used to make up for traveling the same distance would be more, compared to coasting in neutral, because of the increased drag due to engine braking.

[Jeroen]

It would be really helpfull if you would read the whole thread first. You need to see the statement "does coasting save fuel" in context of the discussion here. Again, as I stated earlier its ,ore about how the fuel and engine management system deals with coasting, e.g. Shutting down completely or not.

Jeroen

[Aroy]

Coasting in neutral to standstill, will save fuel (if you switch the engine off). Otherwise coasting in neutral will waste fuel as you will be using more fuel to accelerate after coasting.

If your aim is to slow down and ultimately stop, coasting in gear will save more fuel than coasting in neutral with engine on.

[arunmichael]

I too was thinking that idling in neutral is better to save fuel compared to coasting in gear. IMO most modern cars gets best mileage somewhere around 70 - 90 kmph. Suppose 2 people are driving at 80kmph. One decides to shift to neutral and the other decides to coast. For the first driver the rpm suddenly reaches idle rpm and for the second driver rpm slowly comes down and also his speed. I am sure that the distance covered by the first driver will be more by the time the rpm of second driver reaches idle rpm and hence mileage clocked by first driver to cover same distance should be high.

Now from what i understand from this thread, the fuel supply is cut off as soon as driver takes his foot off the accelerator pedal. If that is the case then i am definitely wrong. . But why do we still hear the engine grunt if its not working?

[a4anurag]

Originally Posted by arunmichael

Now from what i understand from this thread, the fuel supply is cut off as soon as driver takes his foot off the accelerator pedal. If that is the case then i am definitely wrong. But why do we still hear the engine grunt if its not working?

Hey, to get you clear your confusion, I used Torque App and OBD reader to get the data from the ECU. Here you go:


Fuel flow at 80 kmph:



Fuel flow at 100 kmph:



Coasting in Neutral (Note the yellow arrow in the fuel flow tab):



Coasting in Gear (Note the yellow arrow in the fuel flow tab):



In this I have attached three consecutive shots to show the deceleration (Notice the RPM dropping) but the fuel flow is '0'). Engine load too is 0.

To summarize:

[arunmichael]

Thanks Anurag. That is definitely an eye opener. I know at least half a dozen guys who, just like me, thinks that going neutral is better than coasting in gears. The proof that you have submitted is more than enough for me to convince them.

[a4anurag]

Originally Posted by arunmichael

Thanks Anurag. That is definitely an eye opener. I know at least half a dozen guys who, just like me, thinks that going neutral is better than coasting in gears. The proof that you have submitted is more than enough for me to convince them.

And I have witnessed this with many drivers especially the driver that my has. He used to do this every time and when I ask him why he does, he says to save fuel. I made him sit in my car, showed him the live OBD readings. Though he isn't technical at all but that '0' made him realise what he was doing is wrong.

Moreover coasting in Neutral is dangerous as one doesn't have control on the car and IF there is an emergency situation requiring instant throttle response, the instinct will stamp the accelerator pedal but due to being in Neutral the engine will only revv whereas the same case if one is in gear then pulling out of that emergency is easier.