Hi to all!! I was having a discussion with a friend about the fuel consumption of cars.. the question is when a car is slowing down from say 100km/h, engaged in low gear (3rd?) using engine brake without depressing the throttle. Would there be fuel consumed?

My argument is that the kinetic energy is powering the car so there should be zero fuel consumption at this time. But my friend says if there is no fuel injected then the engine would damaged... Who is right?? And is there any difference in turbo cars?? Cheers!

 

Yes there is still fuel consumption. Even when you are off thottle, fuel is still being injected and ignited.

 

No, when you lift completely off the throttle above 1500 RPM there is no fuel being used. Some with Evo 9's have noted that it the ECU does not cut fuel above either 3000 or 3500 RPM, but in my 03 whenever I let off the throttle above 1500 RPM my wideband goes full lean.

-Paul

 

I remember reading from some motoring article that the idling mode (fuel consumed) is to keep the other function of the car running. Such as the aircon, hydraulic systems, etc. But when the car is slowing down using engine braking, the kinectic energy will run these functions so there will be zero fuel consumed. So to say putting the car to neutral while slowing down is consuming more fuel than letting it run in gear... Dunno how true is this though...

 

I will agree with PVD04, you can do a simple test, accelerate to whatever speed you want and let go off the throtle you will notice when it gets as close to 1500 rpms the computer will kick in and the car will be rolling and it will never die.
even if you are in traffic and driving on the frist gear, once the clutch is disangaged it will be rolling.

 

Im pretty sure this is correct. If you coast downhill in a lower gear the injectors essentially turn off.

 

I find this quite interesting and I believe there is at least some truth to the kinetic energy thing. My other car is an SR swapped 240sx and I've got a wideband o2 gauge on it. I've always been curious about why it will run lean (15.5:1) when engine braking and rich (12:1) when coasting down a hill or something. Maybe this has something to do with it???

 

How can you not have fuel consumption when your engine is still running? That doesn't make any sense. Even if you push in the clutch your engine will drop to idle which is still consuming fuel.

 

When the car is coasting/decelerating in gear, the momentum of the car keeps the engine turning. There is no fuel being consumed at all because it is not needed.

-Paul

 

Thats bs. Turn the ignition off and try it, see if the car still runs. It will still be turning over when its in gear, but the fuel and spark will cease to enter the cylinders.

With the ignition on, the computer in this case will still be injecting fuel/spark into the cylinders regardless if the momentum of the car is turning the engine or not. Obviously it has to be 'running' though.

Its the same idea of push starting a manual trans when your battery dies. You have to have the vehicle ON so that fuel and spark can be delivered once the alternator and fuel pump (manual or electric in any case) get moving.

 

It's true. This is how it works. Wideband goes to 21:1 when I let off the gas while cruising. This is disabled below ~1500 RPM to prevent stalling, but it does happen. If I turned off the ignition nothing would change until I got down to 1500 RPM at which point the car would not start injecting fuel and the car would continue to slow until stopped. Most newer cars work this way to improve gas mileage.

-Paul

 

^^^ 1g,2g,GVR4's are the same way.

 

So does anybody here have any proof if there's consumption? Any technical articles to share?