Nothing that happens to the gas mixture inside the engine has any effect on the volume of gas passed through, measured at the inlet and exhaust valves.
This isn't right. It is an understandable mistake, and I'll try my best to explain why quickly. Volume is not conservative for compressible fluid flows. Many things happen in the engine which effect the volume flow past the intake and exhaust valves.
-The flows at the intake and exhaust ports have local mach numbers certainly over .3, probably >= .8
-Mass is very close to conserved from intake port to exhaust port*, and flow is given by mass flow=density * velocity * area
-Mass flow is related to volume flow by: mass flow=density * volume flowrate
-In compressible flows, density of fluid isn't constant. At mach numbers over .3, we can't accurately assume flows are incompressible.
-For volume flow at intake to equal volume flow at exhaust, you'd have to have two similar gases traveling through two similar ports at similar speeds and temperatures. None of this is the case in a real engine.
*New engines will be net accumulators of carbon deposits, and all engines will have some blowby gases that condense into acids, water, and other compounds in the oil. The remaining blowby gases are conserved by being reintroduced into the intake in most engines but not in aviation.
Otherwise, try and make better use of the volume you have by increasing the mass flow by keeping intake pressures up or by reducing losses by keeping exhaust back pressure down.
This was state of the art thinking at the beginning of the 20th century. Now it's just a vast oversimplification with little practical use. I only point this out because it is exactly the kind of thing that has no business anywhere near a pilot exam.
Mark1234 said:
2) if volume remained constant, the engine would not work! it's the rather large increase in volume as the mixture burns and heats the gasses that gives you the power stroke...
(well, actually, the volume increase is constrained, so that increases the pressure, which drives the piston down and the volume increases.. but you get the idea).
It is interesting to look at cylinder pressure with respect to crank angle.** The textbook definition of Otto cycle gives us a
P-V diagram like
this. Unfortunately reality is rarely so kind as the textbook. In the real world we have to account for the speed of the flamefront, limitations in rod/stroke ratio, etc, and the result is a battle to efficiently use those hot gases without detonation.
**measuring this in real time was only possible with expensive custom hardware until recently. Now it's part of control strategy on some volume produced automobiles. Instead of directly measuring pressure, you infer it by applying a voltage to the plug and measuring ionization currents through its gap...an elegant solution to the problem of knock compared to the acoustic sensors...