Quote:
Originally Posted by JohnnyGrey
Oh, I did, but the streets weren't empty enough to really take any meaningful measurements.
So for engine-off, closed throttle:
1. - The intake stroke works against the vacuum
2. + The vacuum returns the power on the compression stroke.
3. - Vacuum works against the the combustion stroke.
4. 0 Exhaust valve opens when chamber contents are about equal to what the manifold pressure was, air rushes in through the exhaust valve and gets pumped back out on the exhaust stroke.
Net effect: -1
For engine-off WOT:
1. 0 Intake stroke freely sucks in air.
2. - Compression stroke compresses the air.
3. + Combustion stroke, pushed down by compressed air.
4. 0 Exhaust stroke, atmospheric air is evacuated.
Net effect: 0
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Consider this: If you have a cylinder and piston with no intake or exhaust valves there would be no gain or loss in pumping the piston, other than heat generated and the dissippated away, and metal to metal friction.
It you then put a small hole in the cylinder head there would be a lot of loss as air is drawn into the cylinder and then pushed out of it. In fact, this is what is often used on some racing 2-stroke motorcycles for an engine brake. A lever on the handlebar is pushed which opens a single valve in the cylinder head causing air to rush into and then back out of the cylinder with every piston stroke.
Therefore my intuitive guess regarding your original experiment is that with engine OFF, car coasting, at WOT, the motor has to pull a maximum amount of air into the cylinder (intake stroke) and then ultimately push it back out of the cylinder (exhaust stroke), regardless of the two self canceling strokes that occured in between. This should cause engine braking.
With engine OFF, car coasting, closed throttle, the piston will pull a small amount of air into the cylinder through the exhaust valve when it opens but it wont even fill the cylinder with air because the piston will already be starting up to push it out just as quickly as it started to come in. This should cause a smaller amount of engine braking than the scenario above.
Still, I am eager to hear of yours or anyone else's experience doing the experiment. I may do it myself if I can get a good opportunity.