A UK-based company called Ilmor Engineering is the creator of a concept for a very innovative new kind of internal combustion petrol engine that has five strokes per cycle. Basically, it reuses the expanding gases that exit the cylinders and passes them through another low pressure cylinder before they go out the exhaust.
According to the official video published three years ago, the setup not only uses up more of the stored energy in the fuel, but is apparently also cleaner and more powerful. The video shows the prototype engine on the test bench, and we can see it uses a turbocharger like any modern gasoline engine.
The design was first shown in 2011, but such designs date back all the way to the beginning of the Otto-cycle engine. Apparently, there was even a six-stroke engine in development in the late 1890s, but that was just a regular four-stroke with two power strokes (one using gasoline, and another using steam).
This design by Ilmor is obviously more advanced than that and its promises are undeniable. However, in the four or so years since it was revealed we haven’t yet heard of a real-life test.