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How Cruise Control Systems Work

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Cruise control is an invaluable feature on Ameri­can ­cars. Without cruise control, long road trips would be more tiring, for the driver at least, and those of us suffering from lead-foot syndrome would probably get a lot more speeding tickets.

­Cruise control is far more common on American cars than European cars, because the roads in America are generally bigger and straighter, and destinations are farther apart. With traffic continually increasing, basic cruise control is becoming less useful, but instead of becoming obsolete, cruise control systems are adapting to this new reality -- soon, cars will be equipped with adaptive cruise control, which will allow your ­car to follow the car in front of it while continually adjusting speed to maintain a safe distance.

In this article, we'll learn how a conventional cruise control system works, and then we'll take a look at adaptive cruise control systems that are under development.

What Cruise Control Does

Cruise control acceleration and deceleration, controlling the cruise control, adaptive cruise control.

The cruise control system actually has a lot of functions other than controlling the speed of your car. For instance, the cruise control pictured below can accelerate or decelerate the car by 1 mph with the tap of a button. Hit the button five times to go 5 mph faster. There are also several important safety features -- the cruise control will disengage as soon as you hit the brake pedal, and it won't engage at speeds less than 25 mph (40 kph).

The system pictured below has five buttons: On, Off, Set/Accel, Resume and Coast. It also has a sixth control -- the brake pedal, and if your car has a manual transmission the clutch pedal is also hooked up to the cruise control.

• The on and off buttons don't actually do much. Hitting the on button does not do anything except tell the car that you might be hitting another button soon. The off button turns the cruise control off even if it is engaged. Some cruise controls don't have these buttons; instead, they turn off when the driver hits the brakes, and turn on when the driver hits the set button.
• The set/accel button tells the car to maintain the speed you are currently driving. If you hit the set button at 45 mph, the car will maintain your speed at 45 mph. Holding down the set/accel button will make the car accelerate; and on this car, tapping it once will make the car go 1 mph faster.
• If you recently disengaged the cruise control by hitting the brake pedal, hitting the resume button will command the car to accelerate back to the most recent speed setting.
• Holding down the coast button will cause the car to decelerate, just as if you took your foot completely off the gas. On this car, tapping the coast button once will cause the car to slow down by 1 mph.
• The brake pedal and clutch pedal each have a switch that disengages the cruise control as soon as the pedal is pressed, so you can shut off the cruise control with a light tap on the brake or clutch.

The cruise control system controls the speed of your car the same way you do -- by adjusting the throttle position . But cruise control actuates the throttle valve by a cable connected to an actuator , instead of by pressing a pedal. The throttle valve controls the power and speed of the engine by limiting how much air the engine takes in (see How Fuel Injection Systems Work for more details).

In the picture above, you can see two cables connected to a pivot that moves the throttle valve. One cable comes from the accelerator pedal, and one from the actuator. When the cruise control is engaged, the actuator moves the cable connected to the pivot, which adjusts the throttle; but it also pulls on the cable that is connected to the gas pedal -- this is why your pedal moves up and down when the cruise control is engaged.

Many cars use actuators powered by engine vacuum to open and close the throttle. These systems use a small, electronically-controlled valve to regulate the vacuum in a diaphragm. This works in a similar way to the brake booster , which provides power to your brake system.

The brain of a cruise control system is a small computer that is normally found under the hood or behind the dashboard. It connects to the throttle control seen in the previous section, as well as several sensors. The diagram below shows the inputs and outputs of a typical cruise control system.

A good cruise control system accelerates aggressively to the desired speed without overshooting, and then maintains that speed with little deviation no matter how much weight is in the car, or how steep the hill you drive up. Controlling the speed of a car is a classic application of control system theory . The cruise control system controls the speed of the car by adjusting the throttle position, so it needs sensors to tell it the speed and throttle position. It also needs to monitor the controls so it can tell what the desired speed is and when to disengage.

The most important input is the speed signal; the cruise control system does a lot with this signal. First, let's start with one of the most basic control systems you could have -- a proportional control .

In a proportional control system, the cruise control adjusts the throttle proportional to the error, the error being the difference between the desired speed and the actual speed. So, if the cruise control is set at 60 mph and the car is going 50 mph, the throttle position will be open quite far. When the car is going 55 mph, the throttle position opening will be only half of what it was before. The result is that the closer the car gets to the desired speed, the slower it accelerates. Also, if you were on a steep enough hill, the car might not accelerate at all.

Most cruise control systems use a control scheme called proportional-integral-derivative control (a.k.a. PID control). Don't worry, you don't need to know any calculus to make it through this explanation -- just remember that:

• The integral of speed is distance.
• The derivative of speed is acceleration.

A PID control system uses these three factors -- proportional, integral and derivative, calculating each individually and adding them to get the throttle position.

We've already discussed the proportional factor. The integral factor is based on the time integral of the vehicle speed error . Translation: the difference between the distance your car actually traveled and the distance it would have traveled if it were going at the desired speed, calculated over a set period of time. This factor helps the car deal with hills, and also helps it settle into the correct speed and stay there. Let's say your car starts to go up a hill and slows down. The proportional control increases the throttle a little, but you may still slow down. After a little while, the integral control will start to increase the throttle, opening it more and more, because the longer the car maintains a speed slower than the desired speed, the larger the distance error gets.

Now let's add in the final factor, the derivative . Remember that the derivative of speed is acceleration. This factor helps the cruise control respond quickly to changes, such as hills. If the car starts to slow down, the cruise control can see this acceleration (slowing down and speeding up are both acceleration) before the speed can actually change much, and respond by increasing the throttle position.

Two companies are developing a more advanced cruise control that can automatically adjust a car's speed to maintain a safe following distance. This new technology, called adaptive cruise control , uses forward-looking radar , installed behind the grill of a vehicle, to detect the speed and distance of the vehicle ahead of it.

Adaptive cruise control is similar to conventional cruise control in that it maintains the vehicle's pre-set speed. However, unlike conventional cruise control, this new system can automatically adjust speed in order to maintain a proper distance between vehicles in the same lane. This is achieved through a radar headway sensor , digital signal processor and longitudinal controller . If the lead vehicle slows down, or if another object is detected, the system sends a signal to the engine or braking system to decelerate. Then, when the road is clear, the system will re-accelerate the vehicle back to the set speed.

The 77-GHz Autocruise radar system made by TRW has a forward-looking range of up to 492 feet (150 meters), and operates at vehicle speeds ranging from 18.6 miles per hour (30 kph) to 111 mph (180 kph). Delphi's 76-GHz system can also detect objects as far away as 492 feet, and operates at speeds as low as 20 mph (32 kph).

Adaptive cruise control is just a preview of the technology being developed by both companies. These systems are being enhanced to include collision warning capabilities that will warn drivers through visual and/or audio signals that a collision is imminent and that braking or evasive steering is needed.

For more information on cruise control, check out the links below.

Cruise Control FAQ

How does cruise control work, how does adaptive cruise control work, will adaptive cruise control stop the vehicle, when would you use cruise control, how useful is cruise control, lots more information, related articles.

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When the system is set, cruise control will maintain a steady speed for your vehicle

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Your vehicle’s cruise control system unlocks numerous benefits at the touch of a button — but according to web search data, drivers still have plenty of questions about what it is, what it does, and how it works.

What is Cruise Control? A Comprehensive Guide Back to video

Below, we’ll answer some of the most popular Internet search questions about cruise control, so you can make the best use of this important system.

What is Cruise Control?

With cruise control, your vehicle will maintain a steady speed when the system is set.

What is adaptive cruise control?

With adaptive cruise control, the vehicle will also automatically slow down and speed up to maintain a safe position in traffic.

How does cruise control work?

Cruise control work with a manual transmission just the same way it does with an automatic.  Drivers set their cruising speed with a button press, and the system works the throttle automatically to maintain the desired cruising speed as evenly as possible. In some vehicles with a manual transmission, pressing the clutch pedal to shift gears turns the cruise control off, requiring an additional button press to reengage it after a gear change.

In other cars with a manual transmission, pressing the clutch pedal simply pauses the cruise control system a moment, allowing drivers to complete their gearshift. When they’ve released the clutch, the cruise control picks up where it left off — no additional button press required.

Is cruise control more fuel efficient?

Yes.  Increasing your vehicle’s speed uses fuel. While cruising, even a highly competent driver who isn’t using cruise control will tend to slow down and speed up in a repeated cycle, possibly several times per minute. Though fluctuations in speed may be slight, they do cause your engine to use more fuel than required. Point is, the more time you spend at a steady speed, the less fuel your engine needs.

For most drivers, using cruise control on the highway at 80 km/h can reduce fuel consumption by about 20 per cent. For drivers who find difficulty in maintaining a steady speed and frequently experience big fluctuations, using cruise control can cut fuel use by over 40 per cent.

Depending on what you drive and how fast you drive it, using cruise control could save you between $4 and $20 per hour — based on information from Natural Resources Canada that shows most drivers who don’t use cruise control on the highway will tend to experience a 10 km/h speed fluctuation about three times per minute.

Drivers who have the most difficulty managing their cruising speed could be using 60 per cent more fuel than they need to.

Is cruise control bad for your transmission?

No.  Your vehicle’s engine, transmission and other components are designed to work hand-in-hand with its cruise control system and are extensively tested and integrated with one another for trouble-free performance.

Using cruise control also reduces wear and tear on both your engine and transmission by running things more smoothly and steadily, and reducing workload on both components.

Can you add cruise control to a car?

Yes.  Depending on the year, make and model, cruise control may be available for add-on or retrofit. Professional installation is recommended, and you’ll want to talk to a professional about the specific availability of parts and integration for your vehicle.

In many cases with modern cars, it’s generally easier and more cost effective to just opt for a unit equipped with cruise control from the factory. Cruise control is widely available as standard equipment on most modern vehicles.

Can cruise control get stuck?

Yes , but it’s extremely rare. Older cable-based cruise control systems seem more prone to this rare problem, in which the cable can slip or bind, making the throttle stick into position. More modern vehicles with electronic throttle and monitoring systems make this problem even less likely.

If the cruise control on your car fails to disengage when you want to slow down, slip the vehicle into neutral to disconnect drive power from the wheels, pull over, and address the situation.

However, chances are, you’ll never experience this problem.

When should cruise control be used?

Any time you’re trying to drive at a constant speed.  Whether around town at 60 km/h, in a residential area at 40 km/h, or out on the highway at 105 km/h, switching the cruise control on makes for a smoother and more fuel efficient drive that’s easier on your engine and transmission. it can also help you do your part to maintain the steady and efficient flow of traffic.

Most cruise control systems can be engaged at speeds above 30 km/h.

When should cruise control not be used?

Any time you can’t safely drive at a steady speed.  If the road surface is snowy, icy, slushy or very wet, you’ll want to think twice about switching your cruise control on.

In older cars, having your cruise control engaged on a wet or icy road could allow the vehicle to accelerate or experience wheelspin when driven wheels encounter a low-traction surface. Here, the cruise control system maintains throttle pressure, even in situations where it should be released instead. This could lead to a loss of control.

In more modern vehicles, electronic monitoring is used to automatically disengage cruise control when one of the following happens:

• one or more wheels slip
• one or more wheels leave the surface of the road after a major bump or dip
• the wipers are set to the maximum speed setting in heavy rain

Use your judgement. Safely navigating certain slippery or dangerous driving conditions requires careful manual control of your vehicle’s throttle, and in these situations, you’re best to leave the cruise control off.

What causes cruise control to kick off?

If the cruise control in your vehicle suddenly kicks off, a few things may be to blame.

Sometimes, cruise control disengages automatically when the vehicle’s automatic wipers detect heavy rainfall, or when a wheel spins or (briefly) leaves the surface of the road after a big bump or dip. In other situations, there may be a problem with one or more sensors or switches, including the brake pedal switch, throttle position sensor, or one or more wheel speed sensors.

Your modern cruise control system relies on various sensors and switches to do its job properly. As a failsafe, any problem with these sensors can cause the cruise control system to go offline. If you notice this happening regularly, have your vehicle diagnosed by a professional.

Will the cruise control work with an ABS light on?

Probably not.  Your car’s Antilock Braking System works hand-in-hand with your cruise control system, and any fault with the ABS system will typically take your cruise control offline until it’s fixed.

An ABS warning light in your instrument cluster can indicate one of several major problems or malfunctions, so be sure to have a professional investigate as quickly as possible.

Will cruise control apply the brakes?

No, but adaptive cruise control will.  Standard cruise control (simply called cruise control) holds your vehicle’s speed as constant as possible once set. More advanced adaptive cruise control systems use camera or radar-based sensing to determine the traffic situation in front of your vehicle, and can automatically apply the brakes to maintain a pre-set following distance.

With cruise control, drivers need to brake to slow down as they close in on a slower vehicle in traffic. With adaptive cruise control, the system makes these braking inputs automatically, and the vehicle speeds back up to its pre-set cruising speed once traffic clears.

Before you buy, be sure to determine which type of cruise control system is fitted to the car you’re considering.

What is Super Cruise?

Super Cruise is a next-level driver assistance feature available on certain GM vehicles.

With Super Cruise, the vehicle uses special GPS hardware and sensors to accurately compare its position to a 3D map of pre-qualified highways .

On these pre-qualified highways, Super Cruise allows drivers to go hands-free for extended periods as the vehicle automatically maintains its cruising speed, adjusts that speed for changing traffic conditions, and even precisely follows the curves in the road. It can even make hands-free lane changes.

Super Cruise is not an autonomous system, and in order for it to work, drivers need to keep their eyes on the road — there’s even a camera that monitors the position of the driver’s eyes to make sure they’re focused on the road ahead.

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Justin Pritchard is an experienced motoring expert whose work is read and watched by Canadians across the country on a weekly basis. Starting his career at Auto123.com back in 2005 (while finishing his final year of studies at Laurentian University in Sudbury, Ontario), Justin quickly applied his passion for writing, presenting, and photography, working under some of the most recognized editors in the Canadian motoring scene.

Justin has written one of the largest collections of used car buyer guides on the internet, and his TV program, AutoPilot, has aired over 600 episodes across 16 seasons. Presently, AutoPilot is the only English-language motoring program on Canadian cable TV, though he's lent his informative style and easy-to-identify voice to video features for Youtube, Driving.ca, Autotrader.ca, Motoring TV, and elsewhere. With 4 years as co-chair of the Canadian Car of the Year Awards (CCOTY) program, a passion for vehicle testing shines though in all of his work.

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