How does a car with the cruise control on maintain its speed while going down hill?


A cruise control system would apply the brakes to maintain speed. This would be accurate for vehicles with electronic braking.

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Land transport
Cruise control

Cruise control (sometimes known as speed control or autocruise, or tempomat in some countries) 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.

Railway brake

Brakes are used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them standing when parked. While the basic principle is familiar from road vehicle usage, operational features are more complex because of the need to control multiple linked carriages and to be effective on vehicles left without a prime mover. Clasp brakes are one type of brakes historically used on trains.

In the earliest days of railways, braking technology was primitive. The first trains had brakes operative on the locomotive tender and on vehicles in the train, where "porters" or, in the United States brakemen, travelling for the purpose on those vehicles operated the brakes. Some railways fitted a special deep-noted brake whistle to locomotives to indicate to the porters the necessity to apply the brakes. All the brakes at this stage of development were applied by operation of a screw and linkage to brake blocks applied to wheel treads, and these brakes could be used when vehicles were parked. In the earliest times, the porters travelled in crude shelters outside the vehicles, but "assistant guards" who travelled inside passenger vehicles, and who had access to a brake wheel at their posts supplanted them.


Hill Descent Control (HDC) allows a smooth and controlled hill descent in rough terrain without the driver needing to touch the brake pedal. When on, the vehicle will descend using the ABS brake system to control each wheel's speed. If the vehicle accelerates without driver input, the system will automatically apply the brakes to slow down to the desired vehicle speed. Cruise control buttons can adjust the speed to a comfortable level. Applying pressure to the accelerator or brake pedal will override the HDC system when the driver requires. The other name for this is Hill Mode Descent Control.

With Hill Descent Control drivers can be confident that even the ride down hills with slippery or rough terrain will be smooth and controlled, and that they will be able to maintain control as long as sufficient traction exists. Four-wheel-drive (4WD) and All Wheel Drive (AWD) vehicles, such as Ford Territory, may have a Hill Descent Control system installed, using the ABS braking to control the car's motion downhill, initially developed by Bosch for Land Rover. The system can be controlled, usually by the Cruise Control buttons near or on the steering wheel.

Autonomous cruise control (also called adaptive or radar cruise control) is an optional cruise control system for road vehicles that automatically adjusts the vehicle speed to maintain a safe distance from vehicles ahead. It makes no use of satellite or roadside infrastructures nor of any cooperative support from other vehicles. Hence control is imposed based on sensor information from on-board sensors only. The extension to cooperative cruise control requires either fixed infrastructure as with satellites, roadside beacons or mobile infrastructures as reflectors or transmitters on the back of other vehicles ahead.]citation needed[

Such systems go under many different trade names according to the manufacturer. These systems use either a radar or laser sensor setup allowing the vehicle to slow when approaching another vehicle ahead and accelerate again to the preset speed when traffic allows - example video. ACC technology is widely regarded as a key component of any future generations of intelligent cars. The impact is equally on driver safety as on economising capacity]disambiguation needed[ of roads by adjusting the distance between vehicles according to the conditions.

Mechanical engineering

Mechanical engineering is a discipline of engineering that applies the principles of engineering, physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the design, production, and operation of machines and tools. It is one of the oldest and broadest engineering disciplines.

The engineering field requires an understanding of core concepts including mechanics, kinematics, thermodynamics, materials science, structural analysis, and electricity. Mechanical engineers use these core principles along with tools like computer-aided engineering, and product lifecycle management to design and analyze manufacturing plants, industrial equipment and machinery, heating and cooling systems, transport systems, aircraft, watercraft, robotics, medical devices, weapons, and others.

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