In this question I’m comparing non-plug-in hybrid fuel/electric vehicles with fuel-only vehicles. Also, I’m not disputing that hybrid cars are more fuel efficient, I’m just trying to understand how.
If I remember two things from high school physics, it’s:
- Creating energy from nothing is against the law.
- Converting energy from kinetic to stored energy and back again is always less that 100% efficient - you end up with less kinetic energy than you put in, the rest being “lost” to the environment in the form of heat or whatever.
Both a fuel-only car and a hybrid derive all their locomotive energy from the fuel (ok, and gravity, if you’re going downhill, but let’s ignore that). But the hybrid involves an extra conversion to/from stored energy on the way, i.e. charging and discharging the battery, which according to the second principle above should make it less efficient.
Furthermore, the battery entails additional mass, which, if it were dead weight, would reduce the power/weight ratio of the car, reducing efficiency.
So to achieve greater efficiency, the hybrid engine must somehow make use of energy that would ordinarily be lost in the engine of the fuel-only car. In order for the hybrid to be more fuel efficient than the fuel-only engine, it needs to do this in a way that overcomes the above inefficiencies and then some.
So my question is, what broad physical/mechanical techniques does a hybrid engine use to achieve this, and (in rough numbers) how much of the efficiency gains come from the different techniques?
I know there’s regenerative braking, but how efficient is this in practice? I.e. how much of a car’s kinetic energy that would otherwise be lost through braking is actually captured and re-used as locomotive energy? And is this basically it, or are there other techniques that significantly contribute to the efficiency of a hybrid?