2 ms·

There are limits to compression. The fuel-air mix will combust spontaneously if you compress it too much. So that's why max compression ratio is something like

There are limits to compression. The fuel-air mix will combust spontaneously if you compress it too much. So that's why max compression ratio is something like ten. So a normal engine has compression of ten and expansion of ten. A more efficient engine with a different cycle might still have compression of ten but expansion goes up to say eleven. This means it's possible to extract more power at the end of the power stroke. There are some tricks to achieve that. Toyota does it by creating an engine with a geometry that would otherwise have very high compression and expansion ratio, but they can avoid the high compression ratio: they don't close the intake valves when the cylinder has been sucked in full of fresh air, like in a normal engine. They keep them open so at the start of the compression cycle, fresh air flows backwards out of the cylinder for a while, before they close the valves.

ThenAsNow 10y ago
Actually, the advantage of the Atkinson or Miller cycles is not due to avoiding autoignition limits. Rather it is because of, as you pointed out, the greater than unity ratio of (expansion ratio)/(compression ratio). So you are improving the ratio of work that can be extracted from the power cycle as compared to the work that you are doing on the mixture. For the same engine displacement as a standard Otto cycle engine, the maximum power you can develop with the Atkinson-type cycle is decreased, hence you trade efficiency for power density. The Miller cycle approach is a way to shift that balance back toward the Otto range by doing some precompression on the air.