Also, engine compression readings can vary depending on altitude and other factors, so it is more important to compare the cylinders to each other rather than the assumed compression number.

Late-model engine designs have come a long way in the engineering of pistons and cylinder heads to improve combustion efficiency, allowing higher-compression engines to run on regular gas.

Some engines use a spark to ignite their fuel, and others rely on compression. What does all of that mean? And how do the two kinds of engines differ?

HCCI engines have a compression ratio of 12:1 (similar to that of a conventional direct-injected gas engine), so they can run on regular pump gas and E85.