Lesson 5 - Energy recovery in hybrid systems

Under certain conditions, when the combustion engine does not have to be assisted, the electronic control unit switches the IMG unit into a generator operating mode generating electricity during driving and braking. The kinetic energy of vehicle braking is used to drive the generator - it is called regenerative braking. During normal braking, the kinetic energy of the vehicle turns into heat (friction) and is lost. During recuperation (recovery of kinetic energy of braking) the IMG unit acts on the one hand as engine brakes, reducing the speed of the car, and on the other like a generator, generating electricity accumulated in the battery.

 

A parallel hybrid system was used in the Toyota Prius. In order to obtain the optimal use of the driving force from an internal combustion engine and an electric motor, the system automatically changes to one of the following operating states depending on the conditions and load:

-      starting, reversing, slow driving;

-      driving in normal conditions;

-      rapid acceleration;

-      braking;

-      idle;

During braking, the combustion engine is switched off; the MG2 engine works as a generator, recovers the kinetic energy of the rotating wheels and converts it into electricity to charge the battery.

Pic 1. Energy flow in a hybrid system during braking (Toyota Prius)

Pic 2. Energy flow at different phases of the Toyota Prius