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 t

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.