Luke,
There are many assumptions and subtleties baked into what is seemingly a pretty simple question on the surface. I’m going to start off with some assumptions, attempt some definitions, and then try to answer your questions.
Assumptions:
The main assumption here is that we are talking about wide-open throttle operation. The purpose of a throttle is to force the engine to operate at less than 100% efficiency, so it doesn’t make much sense to talk in terms of volumetric efficiency when running at part throttle (even though that’s where your engine operates 99% of the time).
Also, since you didn’t specify an engine speed, I’ll assume that you speaking in terms of the engine’s peak power RPM. Note that Volumetric Efficiency is a strong function of RPM.
Definitions:
It’s not really proper to speak in terms of “Volumetric Efficiency” for a two-stroke engine because there is so much more going on during the gas exchange process. In a four-stroke, the intake and exhaust processes are mostly decoupled – a separate stroke of the piston is reserved for each operation. In a two-stroke, on the other hand, the intake and exhaust processes are tightly coupled, and this makes the measurement of efficiency more complex. Because the intake air/fuel stream coming through the transfer ports must displace the spent gasses in the cylinder, there is a large opportunity for these two sets of gasses to mix – something that you don’t want to happen (when trying to achieve maximum power anyway – there are emissions benefits, but that’s a different topic). Because of this mixing opportunity, an added measure of efficiency must be introduced to keep track of how much the fresh charge is diluted by the remaining exhaust gases from the previous cycle. The terminology that the two-stroke community has settled on is Scavenging Efficiency. In words, Scavenging Efficiency is the volume of fresh charge that is in the cylinder with respect to the overall cylinder volume. Trapping Efficiency, the term that is most closely related to Volumetric Efficiency, is the volume of trapped gasses in the cylinder with respect to overall cylinder volume. Note that these trapped gasses are a mixture of fresh charge and combusted gasses from the previous cycle.
So....what's the answer?
All good two-stroke engine designs attempt to maximize both Trapping Efficiency and Scavenging Efficiency. The right combination of crankcase compression ratio, port geometry, port timing, and exhaust tuning can easily create a ‘supercharge’ effect in a two-stroke engine (i.e. the pressure in the cylinder at the time of exhaust port closing is higher than atmospheric pressure). This supercharging, however, only relates to Trapping Efficiency. Worst case, the porting configuration could encourage a lot of mixing between the fresh charge and the combusted gasses resulting in low Scavenging Efficiency and low peak power (even though the engine was supercharged by four-stroke standards). That is why all high performance two-stroke engine designs ‘flush’ a lot more fresh charge through the cylinder than what it can actually hold (this flushing process, by the way, is what gives two-strokes high hydrocarbon emissions compared to the four-stroke). The goal, for peak performance, is to flush the cylinder so that almost no exhaust gas remains, and to have a nice lump of fresh charge waiting just outside the exhaust port near to when the piston closes the port off. If the exhaust system is properly designed, a reversion wave (emanating from the converging cone of the pipe) will meet up with that fresh charge and push it back through the exhaust port just before the port closes – thus supercharging the engine. In this way, both Trapping Efficiency and Scavenging Efficiency are maximized.
Hope that helped…
Sean