I understand that, in an internal combustion engine, efficiency can often (up to a point) be improved by increasing the compression ratio, this being a major reason why diesel engines (which do not need to limit compression to avoid knock) can be more efficient than their spark-ignition counterparts.
In most engines, compression and expansion are symmetric. However, Atkinson-cycle engines allow the expansion ratio of an engine to differ from its compression ratio, typically by allowing the piston to rise substantially above BDC before the intake valve is closed and compression is allowed to begin, such that the effective displacement of the engine is reduced and the expansion ratio (which still uses the full stroke of the piston) exceeds the now-limited compression ratio.
Atkinson-cycle engines can achieve higher efficiency (lower specific fuel consumption) than Otto-cycle counterparts, usually at the cost of reduced specific output (since they use only a fraction of their displacement during compression). My question, however, is how this (low compression, high expansion) compares to the alternative - high compression and high expansion.
All else being equal, and assuming knock can be avoided in all cases, I would expect an Atkinson-cycle engine with an expansion ratio of 20 and a compression ratio of 10, and an Otto-cycle engine with a compression ratio of 20, to both exceed the efficiency of an Otto-cycle engine with a compression ratio of 10. However, how would the efficiency of the Atkinson cycle engine compare to its high-compression Otto cycle counterpart? Is it the expansion (extracting more heat from the same amount of combustion gases before what's left goes out the exhaust) that provides the efficiency improvement, or does the high compression itself also contribute (perhaps by allowing a higher peak combustion temperature)? If high compression does contribute, how does it compare to the contribution of high expansion? Does it account for half the improvement? A small fraction of it? Most of it?
TL;DR does the Atkinson cycle allow you to gain all or most of the efficiency of a higher-compression Otto-cycle engine without risking knock (at the cost of reduced power output), or does a higher compression ratio contribute efficiency of its own which is lost when increasing only expansion ratio?