Crankshaft power stroke

[riff_raff]

...Also, as previously mentioned, a higher compression ratio confines the combustion products to a smaller volume thus providing higher pressures on the piston throughout the power stroke....

All other things being equal, in a conventional N/A recip piston engine a higher compression ratio equates to a higher expansion ratio. I would agree that lower rpms usually result in better BTE, mostly due to the fact that friction and pumping losses tend to increase exponentially with speed.

[johnny comelately]

compensating for that lazy part of the power stroke is the number of cylinders that pick up the slack in regard to the crankshaft turn.
some of the important peripheral issues are side thrust, piston tilt affecting ring seal.
some other interesting things are crankshaft centreline to bore offset and nitro injection to extend the power stroke; simple turbocharging does the same with all the angular losses its still a hoot

[Edis]

Honda uses offset cranks (crank center to piston center) in their L series engines, and I think in their R series. This is so at TDC there is leverage.

Also, the RR is often optimized for lower piston to cylinder forces at high RPMs.

Yes, Honda uses offset cranks in some engines, other manufacturers do too. But you have misunderstood the reason why it is used. The main reason to offset a crank is not to "increase leverage at TDC", but to reduce the rod angle during combustion. The purpose if this is to reduce the frictional losses and wear caused by minimizing the piston side thrust force, which is affected by the angle of the rod. If the rod angle is smaller at peak pressure, the side forces are smaller and the frictional losses are reduced.

An additional benefit of the offset crankshaft is the increased dwell at TDC which will increase the combustion pressure.

There are (at least) two ways to optimize work from the power stroke. Low RPM allows for fairly complete fuel charge combustion to take place near TDC thus avoiding the increase in pressure during the compression stroke from early ignition timing and loss of pressure during the early power stroke due to combustion kinetics. Low RPM allows both late ignition timing and near complete combustion at TDC by slowing the compression/expansion events. Lower RPM also allows for EVO later in the power stroke thus avoiding blow down losses. Again this is due to the longer time period per unit piston travel afforded for the event at low RPM.

Also, as previously mentioned, a higher compression ratio confines the combustion products to a smaller volume thus providing higher pressures on the piston throughout the power stroke.

The combustion takes place in pretty much the same time, in crankshaft degrees, regardless of engine speed. It's mainly the time from ignition to start of heat release that increases (in crankshaft degrees) with increased engine speed.

You are also missing the fact that the heat losses increase with lower engine speed, so does piston blow by. The real issue with higher engine speed is the loss of mechanical efficiency due to friction, pumping, windage and churning.

[johnny comelately]

The combustion takes place in pretty much the same time, in crankshaft degrees, regardless of engine speed. It's mainly the time from ignition to start of heat release that increases (in crankshaft degrees) with increased engine speed.

You are also missing the fact that the heat losses increase with lower engine speed, so does piston blow by. The real issue with higher engine speed is the loss of mechanical efficiency due to friction, pumping, windage and churning.[/quote]

just on that point about low engine speed heat, do you think that maybe just an analytical point, looking at one combustion comparatively. i say this because i wonder if the heat load is accumulative and similar (if all other things are equal) over a period of time