Once thought too polluting, the two-stroke engine makes a comeback in advance of stricter fuel efficiency standards.

In Two Stroke Spark Ignition engines the thermal efficiency increases with increase in compression ratio. Also, with higher compression ratios the lean burning limit increases because of the reduction ...

Two-stroke? Forced induction? Compression-ignition-capable? That is many unexpected engine technologies combined into a single engine.

"This two-stroke engine could solve these practical difficulties and simultaneously permits a much larger range of compression ratio adjustment," added the official.

A compression test is to determine how much pressure the engine makes, and a leakdown test measures its ability to hold pressure.

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?