Variable Compression Ratio Engine

Indeed, a longer expansion stroke enables efficiency gains to be made. Another way of doing it is to keep the inlet valve open until part way up the compression stroke, so that the effective compression ratio is less than the geometric ratio; the power stroke uses the full length. Some engines in current production actually use this approach.

Double or triple expansion is essential in a steam engine to obtain reasonable efficiency, but difficult to arrange in an internal combustion engine. The hot, high-pressure gases would tend to lose much of their energy in the process of transferring them from the small cylinder to the large one. A better idea is to extract the residual energy in the exhaust gases by means of a turbocharger. Many designs for doing this exist, including current Formula 1 car engines. Napier were among the first.

 

Basic, simple engines originally had fixed ignition timing, fixed valve timing, fixed carburation jetting, and fixed compression ratio. Each setting represented a compromise, since the optimum figure for each parameter varies according to needs and circumstances.

As the state of the art has developed, automatically varying ignition timing came first, followed by fuel injection with engine management. Then variable valve timing was introduced, even in bikes at last. The obvious next step is variable compression ratio, which promises to open up new options.

 

Here's a link to an article in an American magazine about the same topic, with diagrams of the latest version of the mechanical layout:
Variable Compression Ratio Coming to New Infiniti Four-Cylinder Turbo – News – Car and Driver | Car and Driver Blog

 

The Nissan Infiniti car with this variable compression ratio engine is just about to go on sale to the public at last.