Page 398 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Vehicle motion control C HAPTER 13.1

Fig. 13.1-10a Cruise control electronics (analog).

required deceleration can be achieved by a throttle re- An ACC system has a means of automatic brake ap-
duction (i.e., reduced engine power). The engine has plication whenever deceleration with throttle input alone
limited braking capability with a closed throttle, and this is inadequate. A somewhat simplified block diagram of an
braking in combination with aerodynamic drag and tire- ACC is shown in Fig. 13.1-11 emphasizing the automatic
rolling resistance may not provide sufficient deceleration braking portion.
to maintain the set speed. For example, a car entering This system consists of a conventional brake system
a long, relatively steep downgrade may accelerate due to with master cylinder wheel cylinders, vacuum boost
gravity even with the throttle closed. (power brakes), and various brake lines. Fig. 13.1-11
For this driving condition, vehicle speed can be shows only a single-wheel cylinder, although there are
maintained only by application of the brakes. For cars four in actual practice. In addition, proportioning
equipped with a conventional cruise control system, the valves are present to regulate the front/rear brake
driver has to apply braking to hold speed. force ratio.

Fig. 13.1-10b Typical sample-and-hold circuit.

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