I was wondering if anyone knows :-
if a diesel engine was at 2000 revs and then the accelerator is released, while the engine is returning to idle is any fuel injected then ?
Obviously fuel will be injected to keep the engine running at idle or at load (under acceleration) but what happens with no accelerator ?
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I understood modern diesels do not inject fuel when the throttle is closed.
If you roll down a hill on the over-run in gear at, say, 2,500rpm, you are using no more fuel than if you knock the car into neutral and coast at idle speed.
You might even be using more fuel by coasting, because some is being injected at idle.
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I believe all modern cars (diesel and petrol) have no reason to inject fuel on the overrun.
My seven-year-old petrol has a definite feel to it when the fuelling is re-instated at around 1200-1500 RPM, on the overrun. It suddenly stops decelerating and gives a little lurch.
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>> I understood modern diesels do not inject fuel when the throttle is closed.
So do I.
>> If you roll down a hill on the over-run in gear at, say, 2,500rpm, you
>> are using no more fuel than if you knock the car into neutral and coast
>> at idle speed.
You will, I thnik, use less fuel if you're "overrunning" but *for the duration of the descent only*.
>> You might even be using more fuel by coasting, because some is being injected at
>> idle.
But you will use less "coasting" overall, because your car will roll further.
However, you're not supposed to "coast" because of the "lack of control". Hm.
Last edited by: FotheringtonTomas on Tue 5 Oct 10 at 12:38
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I agree, no fuel injected during overrun as the engine is being driven by the wheels, as the revs drop to about 1,100 fuel is injected to bring the engine down smoothly to idle speed.
Last edited by: Old Navy on Tue 5 Oct 10 at 14:25
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It will inject some diesel on the overrun otherwise the engine stalls - older diesels had an anti-stall adjustment on the injection pump to prevent this, stopping the ramp down of inection happening too fast and stalling the engine.
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How can an engine stall if it is being driven by the wheels on the overrun?
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I'm pretty sure my Skoda cut the fuel completely on the overrun. The aftermarket radio would only pick up a steady hum on AM (a common VAG problem) which seemed to correspond to the running of the injection solenoids. When driving on the overrun the hum cut out completely until the engine speed dropped to 900rpm when it reappeared.
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>> I'm pretty sure my Skoda cut the fuel completely on the overrun.>>
>> until the engine speed dropped to 900rpm when it reappeared.
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As did my 2.0 tdci Focus, don't know about my Cee'd, (yet), but the instant fuel readout goes to 99.9 mpg on the overrun.
Last edited by: Old Navy on Tue 5 Oct 10 at 22:08
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Diesel engines inject fuel - when ? - Iffy
