What are typical compression ratios for small IC engines? Does it vary between two-stroke and four-stroke engines?
This one opens a can of worms. First let’s dispose of the 4-stroke versus 2-stroke part; the answer being “not at all, sort of”. The “not at all” bit is because the compression ratio required is related to the fuel used, not how it is inducted, burnt, and exhausted. The “sort of” is because some folk, having thought about all the holes in a two-stroke cylinder, perceive that a difference in compression ratio must exist between the two types of engine. With both valves closed, a four-stroke will begin compressing the mixture as soon as the piston rises from BDC. With a two-stroke, actual compression won’t start until the exhaust port closes, which can be as late as 70 degrees ABDC. So what’s the deal? Well, as it turns out, engine designers and reviewers, when dealing with two-strokes, generally qualify “compression ratio”, saying “Effective Compression Ratio” when they are taking port closure into account.
