Mechman-
Some people that know better than me or you insist that compression ratio DOES make a difference in the efficiency in the diesel cycle. See
The Diesel Engine
Even though the compression cycle is an "air spring" that regains it's energy in expansion, the expansion includes the added heat of the fuel burning... so they are not equal.
College was a long time ago, but I remember being sursprised that the diesel cycle is actually less efficient than the otto cycle also- the greater efficiencies of diesel engines come from their higher compression ratios, mainly, and secondarily from the higher heat content of the fuel.