Why are hybrid cars more fuel efficient?


I don’t mean plug-in hybrids, I mean the once that charge batter with engine and regenerative braking. Assuming mixed city/highway cycle regenerative braking should add as little as 5% range and I would imagine that energy would be lost in the process of transfer from engine to battery. Factoring in extra battery weight it seems to me that “MPG” on hybrids should be basically comparable to ICE vehicles, yet it’s advertised as significantly higher MPG and benefits to the envioronment.

Where am I wrong?

In: 1

Gasoline engines can actually hit 35% efficiency without too much trouble (and diesels can hit 45%), but only within a specific RPM and load range within their powerband. You tend not to operate there, because the engine has to rev all over the place during normal operation.

On a hybrid you don’t have that problem; the electric motor can handle the speed changes, at which point the internal combustion engine can just sit at the optimal speed/torque ranges and make power as efficiently as possible.

That efficiency translates into higher MPG.

Hybrids are mostly better at city driving. Once a car is cruising, in a high gear, and not at a speed where wind resistance is a major economy reduction, it takes very little fuel to keep the car moving.

Getting 2 tons of steel from a dead stop to 40mph is where the fuel is slurped down fast. A hybrid can capture “free” energy from regenerative braking when slowing down instead of just using friction brakes and expending that kinetic energy as heat. It can then take that “free” electricity to power the electric motor and take load off of the gasoline engine. The gasoline engine itself is also tuned down in power and focused on economy since the electric power can make up the power loss when flooring it.

Another major factor is the overall design of a hybrid car. Just throwing a hybrid engine in a vehicle with no other consideration doesn’t do a whole lot. The old Chevy Tahoe hybrid only got a few more mpg. The Jeep Wrangler hybrid gets effectively no mpg boost without having the battery plugged in and fully charged. A dedicated hybrid platform is aiming for fuel economy at the expense of other characteristics. They’ll prioritize aerodynamics over style. Very narrow, low rolling resistance tires instead of wide tires for better grip and braking performance.

There’s more that goes into it but that’s most of the major points.