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01-09-2018, 12:48 PM
71

Re: The Multi Million Pound Speeding Business

Originally Posted by Besoeker ->
Coasting in neutral with the engine idling uses fuel.
Obviously. But you're using the minimum amount of fuel because the engine is only idling, which is typically under 1000rpm.

Originally Posted by Besoeker ->
Stay in gear with engine braking cuts off the fuel in most modern cars thus saving fuel.
If you cut off the fuel the car engine would stop so something is off in what you say. I'm not referring to those cars with "Stop/Start" technology here.

If the engine is running at all then it IS burning fuel. The engine cannot run without fuel. That being the case the smallest amount of fuel one can use is the minimum to simply keep the engine ticking over.
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01-09-2018, 01:54 PM
72

Re: The Multi Million Pound Speeding Business

If you are coasting in neutral the engine still req fuel to be sent to the combustion chambers to maintain idling speed, typically around 1000rpm, if you leave it in gear and lift off the accelerator the computer shuts down fuel so no fuel is being used, the car won’t stall because you are in gear..
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04-09-2018, 07:51 PM
73

Re: The Multi Million Pound Speeding Business

Originally Posted by Primus1 ->
if you leave it in gear and lift off the accelerator the computer shuts down fuel so no fuel is being used, the car won’t stall because you are in gear..
Sorry but I'm not buying that for an instant. It is the nature and physical design of an engine to use fuel in order to work.
Cut off the fuel and you cut off the engine. It can no longer be ticking over at that point.

If you believe otherwise you'll have to provide a source reference.
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04-09-2018, 08:29 PM
74

Re: The Multi Million Pound Speeding Business

Originally Posted by Realist ->
Sorry but I'm not buying that for an instant. It is the nature and physical design of an engine to use fuel in order to work.
Cut off the fuel and you cut off the engine. It can no longer be ticking over at that point.

If you believe otherwise you'll have to provide a source reference.
Let's suppose you were descending a hill with a low gear selected and no accelerator being used, would there still be fuel injected into the cylinders? I would assume that with today's engine management systems the fuel would have been cut off because the engine was being used for braking and still under load conditions. Had fuel still been injected into the engine, it would accelerate and not brake. If the vehicle was in neutral though, fuel would be allowed in the cylinders to prevent stalling because there would be no load on the engine. When you push start a vehicle, the accelerator must be used to allow fuel into the cylinders, otherwise, no fuel would be present in the cylinders and the vehicle would fail to start...
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05-09-2018, 05:55 PM
75

Re: The Multi Million Pound Speeding Business

Originally Posted by OldGreyFox ->
Let's suppose you were descending a hill with a low gear selected and no accelerator being used, would there still be fuel injected into the cylinders? I would assume that with today's engine management systems the fuel would have been cut off because the engine was being used for braking and still under load conditions.
I wouldn't assume that at all. If the fuel is cut off then what you are saying is that all the pistons and valves are still going up and down but with no fuel in them. That surely wouldn't be good. They are designed to have fuel in them and have the gasses expelled during that movement cycle.

Originally Posted by OldGreyFox ->
Had fuel still been injected into the engine, it would accelerate and not brake.
No. If the engine is simply working at idle speed then the accelleration is minimal. Try going into 1st gear on a flat surface, lift the clutch and stay off the accellerator. The car will either stall or slowly creep along the road.
The effort of Idle speed will make little difference going down a hill. The force of gravity will be the issue.

The entire idea of cutting fuel off whilst the engine is still running is flawed.

Stop/Start cars may well cut fuel off, but at the same time they cut the engine off too. Why don't they keep the engine running and just cut the fuel off instead ? Answer because cutting the fuel off would cut the engine off obviously.

If you had a moving car on a downward hill with engine running, ignition on and you cut off the fuel, you would essentially be putting the car into "bump" mode, the thing you do when your battery is flat, bump start.

Running your car in "bump start" mode for any significant period of time isn't going to do it any good imo.
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06-09-2018, 07:23 AM
76

Re: The Multi Million Pound Speeding Business

A quick Google search would show you that most computers cut off fuel to the engine when the car is in overrun (ie driven by the transmission) There is even an overrun cut off valve.

End of silliness?
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06-09-2018, 10:00 AM
77

Re: The Multi Million Pound Speeding Business

Originally Posted by Bruce ->
A quick Google search would show you that most computers cut off fuel to the engine when the car is in overrun (ie driven by the transmission) There is even an overrun cut off valve.

End of silliness?
I thought that was the case Bruce, but you've explained it better than I ever could........I just know it works...
 
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