Electromechanical Engine Valves

25.4.4 Electromechanical Engine Valves

by a spring. The rotor is coated with a special lining material to increase friction and reduce wear at the contacting surfaces. Electromagnetic actuators are finding increasing application in

A layer of piezoelectric material, such as lead zirconate titanate automotive systems. These actuators are more desirable than (PZT), is bonded to the underside of the stator. Silver elec- the other types of actuators, such as the hydraulic and pneu- trodes are printed on both sides of the piezoceramic ring. The matic actuators, because they can be more easily controlled by top electrode is segmented and the piezoceramic is polarized

a microprocessor to provide more precise control. An applica- as shown in Fig. 25.6b. The number of segments is twice the tion of electromagnetic actuators that is of particular interest order of the excited vibration mode.

is the replacement of the camshaft and tappet valve assembly

25 Automotive Applications of Power Electronics 651 the engine speed, excessive cooling occurs at highway speeds

requiring the cool air to be blended with the hot air to keep 12 V

the temperature at the desired level. Furthermore, shaft seals and rubber hoses can lead to the loss of refrigerant (CFC) and pose an environmental challenge.

In an electric air conditioner, an electric motor is used to drive the compressor [26]. The motor is usually a three-phase

Ultrasonic

brushless dc motor driven by a three-phase MOSFET bridge.

Motor

The speed of the compressor in an electric air conditioner is independent of the engine speed. As a result, the compres- sor does not have to be over-sized and excessive cooling does

FIGURE 25.7 Drive circuit for an ultrasonic motor. not occur. Also, shaft seals and hoses can be replaced with a

hermetically sealed system. Another benefit of an electric air conditioner is the flexibility in its location, since it does not have to be driven by the engine.

Solenoid

Solenoid

25.4.6 Electric and Electrohydraulic Power

Coil

Coil

Steering Systems

The hydraulic power steering system of a vehicle is another FIGURE 25.8 Power electronic circuits for driving solenoids.

(a)

(b)

example of an engine-driven accessory. This system can be replaced with an electric power steering (EPS) system in which

a brushless dc motor is used to provide the steering power by electromechanically driven engine valves [25]. The opening assist [27]. The electric power steering system is more effi- and closing of the intake and exhaust valves can be controlled cient than the hydraulic power steering system because, unlike to achieve optimum engine performance and improved fuel the engine-driven hydraulic steering pump, which is driven by economy over a wide range of conditions determined by vari- the engine all the time, the motor operates only on demand. ables such as the speed, load, altitude, and temperature. The Another system that can replace the hydraulic power steering present cam system provides a valve profile that can give opti- system is the electrohydraulic power steering (EHPS) system. mum engine performance and improved fuel economy only In this case, a brushless dc motor and inverter can be employed under certain conditions.

to drive the hydraulic steering pump. The ability of the EPHS Two power electronic circuits suitable for driving the system to drive the pump only on demand leads to energy solenoids for valve actuation are shown in Fig. 25.8. The savings of as much as 80% as compared with the conven- circuit of Fig. 25.8a is suitable for solenoids that require uni- tional hydraulic system. Challenges in implementing EPS and directional currents through their coils, while the circuit of EPHS systems include meeting the required levels of cost and Fig. 25.8b is suitable for solenoids that require bidirectional reliability for this critical vehicle subsystem. currents through their coils.