[bdman]

Originally Posted by sagarpadaki

Just to confirm you that fuel supply is totally cut off while coasting in gear. Have verified it using torque android app and a obd on my I20 diesel

In which case aren't we hurting our gas mileage coasting in gear due to engine braking? In the sense, one could coast further in neutral than in "gear"?

[Gansan]

"When coasting with the engine running and manual transmission in neutral, or clutch depressed, there will still be some fuel consumption due to the engine needing to maintain idle engine speed. While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag, but has the added safety benefit of being able to react in any sudden change in a potential dangerous traffic situation, and being in the right gear when acceleration is required."


Source : http://en.wikipedia.org/wiki/Energy-efficient_driving

[kaushik51094]

Originally Posted by SS-Traveller

'D' or 'Drive' automatically selects the best ratio for road speed and engine rpm combination when the engine is revved. And it's actually P-R-N-D, not D-N-P-R.

Oh, so, P-R-N-D is how you people call it? I'm a noob at auto transmission. I just saw these four things in the picture and typed it in whichever way i was able to remember Anyways, in such a gearbox, while driving downhill, if you are in D, it coasts, right? If yes, then how do you prevent coasting under such conditions?

[saket77]

Originally Posted by Gansan

"While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. "

Source : http://en.wikipedia.org/wiki/Energy-efficient_driving

While I agree that one should always coast only in an appropriate gear, I doubt that if MPFI systems cut off the fuel supply while doing this. If that's the case, the engine shouldn't turn off by itself? If you say that engine is driven by the wheels in motion, then shouldn't it be the same as turning off the engine manually by the key (and consequently juddering to a halt!).

I think, the engine will be supplied with some fuel to keep it running, like when idling or may be a little lesser than what is required for idling.

Regards,
Saket

[DerAlte]

Originally Posted by kaushik51094

... Anyways, in such a gearbox, while driving downhill, if you are in D, it coasts, right? If yes, then how do you prevent coasting under such conditions?

Either brake to a halt (preferably at the side of the road) or accelerate. Your question has a possible interpretation that there is a confusion in your mind regarding "what is coasting". Well, "coasting" is simply allowing the car's momentum to take the car forward, without using any power or reducing momentum by braking. On an up-slope or level road the speed gradually reduces, on a down-slope it increases.

And no, in D (Drive, the other positions being Park, Reverse, and Neutral) it doesn't coast by itself unless the driver allows it to.

Originally Posted by saket77

... I doubt that if MPFI systems cut off the fuel supply while doing this. If that's the case, the engine shouldn't turn off by itself? ...

No, it really stops fuel injection. Why should the engine stop rotating? Since it is coupled to the wheels via gearbox, during coasting the vehicle's momentum is supplying energy to the crankshaft to keep it rotating, and the energy is used up in compressing the air in the cylinder (heat, taken away by the cooling system). That is why even on a level road, a vehicle will coast to a stop - ultimately.

[SS-Traveller]

Originally Posted by kaushik51094

Oh, so, P-R-N-D is how you people call it? I'm a noob at auto transmission.

No, that's not how WE call it! It's the standardized arrangement of the shift lever - as DerAlte pointed out, the letters stand for D=Drive, the other positions being Park, Reverse, and Neutral. 1 & 2 means you slot into 1st gear and 2nd gear and retain the gear at that, without the automatic box trying to shift to another gear or allowing coasting when the A-pedal is lifted off.

[saket77]

Originally Posted by DerAlte

No, it really stops fuel injection. Why should the engine stop rotating? Since it is coupled to the wheels via gearbox, during coasting the vehicle's momentum is supplying energy to the crankshaft to keep it rotating, and the energy is used up in compressing the air in the cylinder (heat, taken away by the cooling system). That is why even on a level road, a vehicle will coast to a stop - ultimately.

Thanks for the explanation. However, I am still confused.

Please improve my knowledge on this- I agree that while coasting in a gear, the engine will rotate via wheels in motion-gearbox-crankshaft-pistons> hence the engine. So, when the MPFI system cuts out total fuel supply, means that no combustion is taking place in the cylinders, right? Then why should it be at all different from a scenario where say one is driving at 50KMPH in the 4th gear. Then the driver switches off the engine manually by turning off the ignition key without stopping the car, (& without pressing the clutch pedal or shifting to neutral) i.e..while doing 50kmph in 4th gear. The judder would start immediately the engine is turned off - Am I right, Sir?


In this case of manually turning off the engine, the only difference in my opinion would be that spark plugs (in a petrol engine) would have no current supply from the distributor to fire the cylinders.

Now comparing with the coasting scenario, the spark plugs would keep firing while coasting in a gear since the ignition power is not cut-off, but since the fuel supply is, there would be no fuel in the cylinder, hence no combustion would take place. Now shouldn't the car start juddering immediately once the accelerator pedal is let-off if the fuel supply is cut off & there is no combustion taking place? Isn't it like a scenario where the car behaves just like when it is pushed by a few blokes while in a gear?

And I am sure that when we are talking about old carbureted cars which had mechanical AC pumps mostly, the fuel supply will not be cut-off until the engine is rotating, either by itself, or via wheels.

Thanks in anticipation of your reply,

Regards,
Saket

[bblost]

Switch off means electric connection has been broken.

Put switch back into on and the engine will switch back on after maybe one judder.

[Ricci]

Originally Posted by ramzsys

When coasting in a slope, the power flow is from the wheels to the engine. So the engine won't/can't stall if the rpm is enough to overcome the compression. And as SS-Traveller mentions, when nearing idling rpm, ECU starts injecting fuel.

Of course, gravity induced wheel rotation will prop up the engine rotation, we are speaking of engine braking and coasting under level roads. I always thought the ECU will reduce fuel flow than completely shut it, under the latter scenario the engine braking will be pretty severe. I haven't been driving cars much and of that haven't really driven to test engine braking, my experience is thus more on FI motorcycles, and it could be that motorcycles are programmed differently, to be more progressive in fuel flow metering than abruptly shutting fuel flow in order to prevent upsetting the balance.

[Guna]

Originally Posted by DerAlte

No, it really stops fuel injection. Why should the engine stop rotating? Since it is coupled to the wheels via gearbox, during coasting the vehicle's momentum is supplying energy to the crankshaft to keep it rotating, and the energy is used up in compressing the air in the cylinder (heat, taken away by the cooling system). That is why even on a level road, a vehicle will coast to a stop - ultimately.

Do you mean to say that engines stops firing (no ignition) altogether? In that case the engine sound should change and when the throttle is given it should come to life (again sudden change in engine note). I thought some minimal fuel is supplied to keep the ignition cycle going.

[ramzsys]

Originally Posted by Ricci

I always thought the ECU will reduce fuel flow than completely shut it

Originally Posted by Guna

I thought some minimal fuel is supplied to keep the ignition cycle going.

I have checked this in the 1.4 tdci.

The diagnostics say that Fuel flow is stopped totally('fuel volume' data remains constant once i lift off the right foot), and the count continues once it reaches lower rpms.

[Jeroen]

It's been mentioned before, and the relevant links are mentioned to threads on this forum, that keeping the gear in idle and coasting shuts down the fuel to the engine, safe the occasional burst. And if you coast in neutral the engine will still use fuel to keep it idling.

So here's an interesting youtube video to prove that exact point. The engines uses less fuel in gear than in idle going down the hill. Anybody with an on board computer that give you continuous actual fuel use age can do this experiment for themselves.

Jeroen

http://www.youtube.com/watch?v=rgXGzAiszak

[saket77]

Originally Posted by ramzsys

I have checked this in the 1.4 tdci.

The diagnostics say that Fuel flow is stopped totally('fuel volume' data remains constant once i lift off the right foot), and the count continues once it reaches lower rpms.

I have looked on Google and they seem to support the view that no fuel is used. Pardon me for still questioning & arguing but my thinking is that very small quantity of fuel might be used. And the ECU may register this 'very small quantity' as Zero. This is why your OBD may not register change in fuel volume. Also, IMO, tdci engines can be quite different from normal petrol engines, but still would like to ask that what do you think the OBD would say about fuel volume and DTE if you descend a long downhill of say 5-6 kms?

Also, while a long descend of a few kms, one may not use accelerator at all and use engine braking through out. At times, the engine braking is so high that the rpm shoots too high. It sounds the same I guess as when accelerated(?). An engine running without fuel & one running of kinetic energy should sound significantly different?

Also, if no fuel is burnt, what about the effect on catalytic converter? Wouldn't a small fuel supply to engine would be required to keep catalytic converter safe and at optimal operating temperature?

@Jeroen: I am in agreement of the fact that coasting in gear would use lesser fuel, but not so convinced that its literally ZERO fuel, of course before reaching the idling rpm while coasting.

[SS-Traveller]

Originally Posted by saket77

Pardon me for still questioning & arguing but my thinking is that very small quantity of fuel might be used. And the ECU may register this 'very small quantity' as Zero.
I am in agreement of the fact that coasting in gear would use lesser fuel, but not so convinced that its literally ZERO fuel, of course before reaching the idling rpm while coasting.

It would completely depend on the ECU's software, whether you get zero fuel or a micro-quantity of fuel past the injectors when the engine is overrunning. AFAIK, older cars (pre-BS-III, pre-2005) did not have a complete fuel cut-off program written into the ECU software.

Another issue (applicable in petrols): If fuel supply is maintained into the cylinders while the engine overruns and A-pedal is not applied, is that the air supply is also cut off (throttle shut). In such a situation there is incomplete fuel burn in the cylinders, and a fuel-rich exhaust goes out and can cause detonation in the exhaust system, esp. the red-hot catcon - this can lead to catcon damage. Hence the necessity of programming to have zero fuel feed when an engine is in overrun mode.

[saket77]

Originally Posted by SS-Traveller

Another issue (applicable in petrols): If fuel supply is maintained into the cylinders while the engine overruns and A-pedal is not applied, is that the air supply is also cut off (throttle shut). In such a situation there is incomplete fuel burn in the cylinders, and a fuel-rich exhaust goes out and can cause detonation in the exhaust system, esp. the red-hot catcon - this can lead to catcon damage. Hence the necessity of programming to have zero fuel feed when an engine is in overrun mode.

Thanks for adding one more dimension to the discussion, but I would rule out cat-con damage.
In any case, I would believe that coasting in gear would save fuel. This means that fuel being fed in the cylinders by the system would be lesser than used at idling. I think this will not result in incomplete combustion as even with no throttle input, there will be sufficient air intake for the fuel to be burnt completely. If fuel can burnt effectively even at idle & with no throttle input, then certainly it can, with even less quantity while coasting in gear.

Also, if going by your argument, i.e.. no fuel & no air intake while coasting, there will be literally Zero emission by the car. Isn't it?

Just my 2 cents!