HSD engine: 13.5 : 1 expansion ratio (because it can use Atkinson cycle).
IMA engine: 10.8 : 1 expansion ratio (because it can't use Atkinson cycle).
Which is the major reason why...
HSD engine: 36% peak thermal efficiency
IMA engine: 30% peak thermal efficiency
HSD engine can always run at full throttle and hence close to peak efficiency.
IMA engine is almost always throttled during normal driving, so running at substantially less than peak efficiency.
So HSD can extract at least 20% more energy from each gallon of fuel than IMA, hence it is the more advanced.
What can Honda do to catch up? Leapfrog Toyota and go series hybrid with a proper Atkinson cycle engine. (not simulated Atkinson cycle as used in the Prius engine)
