142 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Figure 6-36. The motor-generator is the key to modern hybrid power sources. It can be used as a motor to assist the engine in propelling the vehicle. It can also be used as a starting motor to spin the engine crankshaft for engine start-up or as an electric generator to help slow the vehicle while recharging the battery pack. (Honda) A B C Figure 6-37. Study the operation of a hybrid power plant. A—When cruising at a steady speed, the gasoline engine powers the vehicle normally. B—In the charging mode, the motor-generator acts as an electrical generator to recharge the battery pack. The load of turning the generator also helps slow the vehicle during braking. C—In assist mode, a large flow of current is directed to the motor-generator to help spin the crankshaft. (Honda) A power control unit is used to operate the motor- generator and gasoline engine. It is usually located between the passenger compartment and the trunk. The power control unit can internally switch the electrical connections on the motor-generator from the generator mode (charge batteries) to the motor mode (assist engine). The batteries and electric motor supply power assist to the gasoline engine when the car accelerates. This greatly reduces the consumption of gasoline since the stored electri- cal energy and not engine power alone are used to propel the vehicle. Once cruising speeds are reached, the gasoline engine takes over and the motor is used as a generator to recharge the batteries. Most hybrids use a continuous variable transmission (CVT). This type transmission allows the engine to run at an almost constant speed. The engine does not have to increase or decrease its speed (rpm) as much during vehicle speed changes. This allows the engine to be run at its most efficient speed, thus eliminating the need for varying other engine operating parameters. The motor-generator is the key to modern hybrid technology. The motor-generator is an ultra-thin, brush- less motor-generator. Refer to Figure 6-36. It can produce electricity (generator mode) and convert electricity into rotary motion (motor assist mode). The thin, but large-diameter motor-generator is normally located between the engine and the transmission. Large electrical leads connect the motor-generator with its array of batteries. For high electrical efficiency, a series of electrical coils are placed around the motor armature. The coil windings can produce a very powerful magnetic field to spin the motor armature or they can have current induced into them to recharge the battery array. The motor-generator can also replace the conventional engine starting motor. When you turn the key to start the engine, the large motor can easily spin the engine crankshaft until the engine fires and runs on its own power. The battery array or battery pack is a series of high- power, nickel-metal hydride cells. The batteries are initially recharged through regenerative braking and then fully charged by engine and generator operation. The energy stored in the batteries is discharged into the motor-generator on acceleration or when added power is needed, as when climbing a hill for example. Since the motor armature is fastened to the engine crankshaft, the armature helps rotate the transmission input shaft and driveline. Figure 6-37 shows the three modes of operation for a hybrid vehicle. Regenerative braking uses the energy generated by slowing the vehicle to generate electricity, which is stored in the batteries. When the brakes are applied, the