most hybrids us a gas engine miller or atkins cycle engine rather than the typical otto cycle. The advantage is much higher thermal efficiency, nearing that of a diesel. The disadvantage is that miller or atkins cycle engines have very poor low rpm torque. But couple that with an electric motor that has very high low end torque and you end up with a great combination. High thermal efficiency and very good low rpm performance.
I suspect that the gains from going to a diesel are mitigated by the cost and higher weight.
For maximum thermal effiency you want the exhaust temperature to be at ambient and cylinder pressure at the bottom of the power stroke to be at atmospheric pressure. The atkins and miller cycle reduce both compared to a typical otto cycle engine.
