What makes hybrid vehicles popular in the automobile industry?

A hybrid automobile is one that uses both an internal-combustion engine and an electric motor for propulsion, but the only fuel you’ll put in is gasoline. (Diesel hybrids do exist, but they’re mainly found in locomotives and other extreme heavy-duty applications.) Hybrids have small high-voltage batteries to power their electric motors, but you don’t plug them in.

With no external power source, electricity for the motor is scavenged under braking in a process called regenerative braking—regen for short. If you’re an F1 racing fan, you may have heard it called a Kinetic Energy Recovery System (KERS). The regen system does not replace the traditional brakes but instead works as a crucial alternative. Electrical energy collected via this process is saved in the battery for immediate reuse the next time you accelerate. When you leave a stoplight, the saved energy gets the car going again and delays the restart of the gasoline engine—in some cases until you reach 25 mph. When you stop again, the cycle repeats. This makes a hybrid’s city fuel economy much higher than a nonhybrid’s, not to mention its own highway economy.