Hello Ted, great question but is a little out of our realm of expertise.
Hybrids-Electric vehicles (HEVs) combine the advantage of gasoline engines and electric motors. The key areas for efficiency or performance gains are regenerative braking, dual power sources, and less idling. Regenerative Braking; The drivetrain can be used to convert kinetic energy (from the moving car) into stored electrical energy (batteries). The same electric motor that powers the drivetrain is used to resist the motion of the drivetrain. This applied resistance from the electric motor causes the wheel to slow down and simultaneously recharge the batteries. Dual Power; Power can come from either the engine, motor or both depending on driving circumstances. Additional power to assist the engine in accelerating or climbing might be provided by the electric motor. Or more commonly, a smaller electric motor provides all of the power for low-speed driving conditions and is augmented by the engine at higher speeds.
OK you kinda answered both but your answer was more on how than why.
So can I assume that it is the smaller motor at low speeds that helps save energy?
How do the efficiency’s of each type of propulsion compare?
That’s an excellent question and something I’ve wondered myself for a long time now.
I can tell you that standard Induction electric motors have efficiency ratings > 80%.
Hybrid cars usually use a PM motor (Permanent Magnet) which I believe are even better than Induction types are.
The combustion engine is not nearly that good. I learned this many years ago in automotive R & D when they still used carburetors. The advent of Electronic Fuel Injection greatly improved fuel efficiency but I don’t think it approaches the efficiency of electric motors. This is probably the biggest reason why they work, not to mention the fact that when the combustion engine is off it produces no pollutants, eh?