Hybrid Car Mpg
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Why do some hybrid cars have better city mpg than at highway speed?
I'm looking for an in depth answer, not an opinion. Thanks!
In general a gasoline vehicle is more efficient when it is moving slower. This is because of the operating characteristics of a gasoline engine and the vehicle suffers from more wind resistance at higher speeds. But it is also true that it costs more energy to start and accelerate a vehicle than to cruise at a constant speed. This is because a vehicle starting up has to overcome inertia. Many starts and stops ie, city driving, in a normal vehicle gives less gas mileage than highway driving because of the starts and stops even though the speed is slower. But mileage would be even better at a constant lower speed of around 35 to 40 mph.
A normal gasoline engine is most efficient only at a fairly narrow operating range. It only has a good amount of ability to move the car when it is racing along. This is why drag racers will rev up the engine before starting. A gasoline engine is at most about 30% efficient.
An electric motor has a wider range where it does well. It is particularly good at starting a vehicle from a standstill. An electric motor is about 95% efficient.
In a hybrid vehicle the best qualities of both engines are used when possible. From a start the 95% efficient electric motor overcomes inertia to get the car rolling. Once there is sufficient speed the gasoline engine also helps move the car when it is also more efficient. (In a parallel hybrid like the Toyota Prius but not in a series hybrid.)
Hybrid cars also incorporate regenerative breaking in their systems. This is a way to recover energy from the inertia of the vehicle. In the city there are many starts and stops. This is usually very wasteful, but for a hybrid vehicle with regenerative breaking the starts are offset by the slow downs. Energy that is normally lost is recovered and this means that the gasoline engine will not have to run to charge the batteries by the amount recovered. This is another savings.
A hybrid vehicle uses its efficient electric system mostly for starts, stops and moving slowly. There is not enough stored battery power for highway driving. But for highway driving the hybrid can use a smaller gasoline engine because it does not have to be over-sized for starts and stops. The smaller engine is more efficient than a normal car's necessary larger engine. So the hybrid can get good highway mileage. But this is still not as good as the very efficient electric motor combined with regenerative braking that is used in the city.
