The electric motor in a mild system is also used to assist the gas engine—a type of electric “boost”—when accelerating from a stop.
The two functions— permitting the gas engine to be turned off for short periods and helping with acceleration after a stop—can increase fuel economy in a mild hybrid by 10% to 15% over a gasoline-only model of the same weight.
What mild hybrids don’t do is use the electric motor alone to drive the vehicle’s wheels. As a result, there is no all-electric or EV mode in a mild hybrid.
What is a Full Hybrid?
Because they use more powerful electric motors and larger-capacity batteries, full hybrids do everything mild hybrids do and more.
They can use their electric motor to enable the vehicle to run in all-electric mode for short periods at low speeds. They also use the electric motor more frequently and for much longer periods to boost the gas engine’s performance.
Full hybrid systems also can be designed to permit the engine to shut down at highway speeds, usually for only a few seconds at a time, whenever there is sufficient power in the battery to run the electric motor. Because the battery recharges while the vehicle is running, these periodic mini-shutdowns can occur dozens of times on a single trip. Typically, the sequencing of these operations is moderated by software integrated into the vehicle. Various states of hybrid operation and charging can be viewed via an infotainment screen or instrument panel.