Cruise control (speed control or auto cruise) is a system that automatically controls the speed of a motor vehicle. The system takes over the throttle of the car to maintain a steady speed as set by the driver
Theory of operation
In modern designs, the cruise control may need to be turned on before use â€ in some designs it is always "on" but not always enabled, others have a separate "on/off" switch, while still others just have an "on" switch that must be pressed after the vehicle has been started. Most designs have buttons for "set", "resume", "accelerate", and "coast" functions. Some also have a "cancel" button. Alternatively, tapping the brake or clutch pedal will disable the system so the driver can change the speed without resistance from the system. The system is operated with controls easily within the driver's reach, usually with two or more buttons on the steering wheel spokes or on the edge of the hub like those on Honda vehicles, on the turn signal stalk like in some General Motors vehicles or on a dedicated stalk like those found in Toyota and Mercedes-Benz vehicles. Early designs used a dial to set speed choice.
The driver must bring the car up to speed manually and use a button to set the cruise control to the current speed. The cruise control takes its speed signal from a rotating drive shaft, speedometer cable, wheel speed sensor or from the engine's RPM. Most systems do not allow the use of the cruise control below a certain speed (normally 35 mph/55 km/h) to discourage use in city driving. The car will maintain that speed by pulling the throttle cable with a solenoid or a vacuum driven servomechanism.
All systems must be turned off both explicitly and automatically, when the driver hits the brake or clutch. Cruise control often includes a memory feature to resume the set speed after braking and a coast feature to reset the speed lower without braking. When the cruise control is in effect, the throttle can still be used to accelerate the car, but once the accelerator is released the car will then slow down until it reaches the previously set speed.
On the latest vehicles fitted with electronic throttle control, cruise control can be easily integrated into the vehicle's engine management system. Cruise controls currently being developed include the ability to automatically reduce speed when the distance to a car in front, or the speed limit decreases. This is an advantage for those driving in unfamiliar areas.
see more http://www.howstuffworks.com/cruise-control.htm