Most of the large manufacturers are now developing plug-in hybrids. There is a lot of variation in basic design and, combined with likely improvements in battery technology, the next decade might produce some interesting surprises.
A press released from Ford mentions using an Atkinson cycle internal combustion engine as part of their power train. You probably know about the otto-cycle four stroke engine where four distinct strokes - intake, compression, power and exhaust - take place in a cycle that sees two revolutions of the crankshaft. Atkinson developed a novel design to get around otto-cycle patents in the early 1880s.. The idea was to use a special crankshaft that creates a power stroke that is longer than the compression stroke It turns out this design can have a greater thermal efficiency than otto-cycle engines. Unfortunately the power density is reduced and the original design was something of a curiosity.
In recent years it has been possible to alter the timing of the values on an engine and otto-cycle engines have been modified to effectively reduce the compression ratio in intake stroke. The engine can be more efficient than a conventional engine. This can be used in hybrid vehicles where a small power output internal combustion engine is acceptable and, in fact, several hybrids have gone this way including the Toyota Prius. A giveaway in the specs is to look at the compression ratio. The effective compression ratio for the compression stroke is somewhere between 9 and 10:1, but the quoted ratio from TDC to BDC is often 13:1 - and that is the quoted number.
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