A motor in a hybrid currently available assists in moving the car and charging the battery pack. Next generation cars like the Volt use the gas/diesel motor only to charge the batteries, the electric motor(s) move the car. In this case the gas or diesel motor runs at a set speed, and it is either on or off. Idling is not an issue, speed and smoothness of restarting and stopping the motor become important factors. People in the car don’t want to hear, feel, or notice when the gas/diesel motor starts and stops. This should be possible in either gas or diesel, but if a gas engine is stopped at just the right point it fires up again very quickly. At this point advantage gas.
Perhaps we may see the wankel type rotary engine again someday. They are very smooth, low weight, and great power to weight ratio. There are technical issues to perfect with them but they could be worked out. A smaller gas engine allows more room in the engine bay for electric motors and battery packs so the passenger and trunk space can be maximized.