[Aquinasfan]

Most the power of an automobile is for acceleration. In a standard auto this requires that the engine be sized for such. In a hybrid, the engine provides cruising speed and battery charging. The acceleration power is provided by the addition of electric power. The overall engine size is reduced and therefore consumes less energy. This is why they have diesel-electric trains, (where the idea came from.)

And braking energy, that's usually dissipated in friction and heat in a gas-powered car, is used to recharge the batteries in a hybrid. That's the "free lunch" benefit of hybrids.

[D Rider]

One of the benefits of electric engines, it that they can also be generators as when used for braking.