Hi Oggers.
Yes, you are correct that there are other factors at play in a real engine.
Volumetric efficiency impacts on thermal efficiency though. If the VE is poor then you are suffering pumping losses and using more energy just to keep the engine running. Idealised heat engine models are all well and good up to a point but you have to move beyond a PV diagram to understand the constraints on the efficiency of a real engine.
The loss of efficiency due to pumping losses (ie the engine having to suck air in and blow it out) are comparatively small. The biggest loss of efficiency in an engine with poor VE is because of a drop in effective compression ratio. For example, if an engine can only suck in half a cylinder full of air because of intake/exhaust restrictions or throttle position - a 10:1 compression ratio engine is actually only effectively producing a 5:1 compression against atmospheric pressure. I still don't think VE is important in the question of the OP.
Waste heat in the exhaust is simply that which you are unable to turn into work before the power stroke ends.
Exactly. This is exactly what I said. A higher compression ratio adds the heat to a hotter air charge, so once the engine reaches BDC the higher compression engine "fluid" will be cooler. By "absorb less heat", I meant at the end of the cycle the fluid has absorbed less total heat during the cycle (not saying it's cooler at the point of ignition - it is, in fact, hotter as you said).
You are barking up the wrong tree. The significance of running the compression ratio high is to get more heat into the charge (without going too far) in order to promote better combustion. It is NOT to get more heat into the charge so the charge then absorbs less heat from combustion because that is not going to work:
Sorry, disagree. I do agree higher compression engines have better mixing and better burn, but this is not the main reason they are more efficient.
You need to think again about the scenario I posed earlier. Consider two engines, one high and one low compression ratio, burning 1L of fuel per minute. I guarantee the higher compression ratio will have a lower exhaust gas temperature once back at atmospheric pressure.
Your argument is that higher compression engine will have hotter exhaust gas and that better and more complete combustion is the only reason it will be more efficient. You can't get more "umph", plus hotter exhaust gases too - where is all this extra energy coming from? The difference in percentage of unburnt fuel between a low and high compression engine is quite small - trust me - I've done it on a lab dyno at university.