Rather than further hijacking a GP thread, I’m putting this here.
Imagine a normally aspirated gasoline engine turning over, but out of gas…say it’s coasting down a hill with a manual transmission in gear.
On the compression, and expansion strokes, the air in the cylinder behaves as a slightly lossy spring…absorbs a little power, but mostly balances out.
The other two strokes of the cycle are another matter though:
Over the intake stroke, the piston is being acted upon * by the intake manifold pressure (vacuum) minus (even more vacuum) pressure drop across the intake valve/port.
On the exhaust stroke it is being acted upon * by the exhaust manifold pressure PLUS pressure drop across the exhaust valve/port.
Both of these strokes are thus resisting the motion of the crank, and therefore absorbing power. This power goes toward heating the air that is pumped through the engine.
This is “pumping loss”. Since exhaust manifold pressure (assuming a decent exhaust system) is typically near ambient, pumping loss is mostly a function of intake vacuum.
So our out of gas driver can adjust his speed down the hill by using the gas pedal to vary the intake vacuum. Open the throttle -> less vacuum -> less resistance.**
If we now add fuel to the situation, the power produced by the expansion stroke can more than offset the pumping loss, but this power is still not available at the ouput shaft. None the less, fuel had to be burned to create it.
Note however that while the intake stroke loss is the same as the no-fuel case, the exhaust stroke losses have increased, because the pressure at the start of the exhaust stroke is the blowdown pressure you mentioned, and the hotter exhaust gasses must be pushed through the exhaust port at higher velocity owing to needing to move the same mass of a less dense gas.
*The crankcase pressure is kept slightly below ambient by the PCV system. The is relivent because that is what is acting on the underside of the piston, and therefore the net force on the piston is modified. however this is the same for all four strokes of the cycle, so was ignored in above discription.
**This is VERY obvious on a motorcycle, which typically have HUGE displacement/weight ratios…and run out of gas more often than cars.
