Car Advice

What is cruise control and how does it work?

09 Nov 2016

Isaac Bober 1

Just about every single new car is fitted with cruise control or its clever new brother, active cruise control, so what is it and how does it work?

THE VERY FIRST car to get a speed governing system was a Wilson-Pilcher car in the early 1900s, see the picture below of what’s believed to be the last-surviving example of a Wilson-Pilcher car. The speed governor, which was a very early form of cruise control sat in the middle of the camshaft, both four- and six-cylinder engines were available, and via a lever on the steering column the driver could set the speed to be maintained.

What is cruise control?

According to the The Macquarie Dictionary of Motoring, cruise control is: “[A] device which enables the driver to maintain a fixed speed. The vehicle will keep to the chosen speed regardless of road undulations unless the brake pedal or accelerator is touched. It is mainly used to reduce fatigue and comply with speed regulations on long trips. Fuel savings are claimed. Also called ASCD (Automatic Speed Control Device)”.

The birth of cruise control

They say necessity is the mother of all invention and so it was with cruise control. In the 1940s, Ralph Teetor came up with the concept for cruise control after travelling with his lawyer who would slow down when listening and speed up when talking. Teetor filed his first patent for cruise control in 1948.

Now, Teeter, it’s worth mentioning, was completely blind but was widely considered to be one of the best automotive engineers of his day. After a few tweaks to his device to automatically control a car’s speed, cruise control had been refined and was introduced on the Chrysler Imperial, New Yorker and Imperial (in 1958), and was standard fitment on all Cadillacs of the period. Teeter was eventually inducted into the Automotive Hall of Fame.

How does cruise control actually control the car’s speed?

Cruise control controls your car’s speed (try saying that five times fast) in exactly the same way you do: via the throttle. In a car with a cable driven throttle there will usually be two cables side by side, one being the throttle cable proper and the other being the cable that the cruise control uses, which comes from an actuator via a vacuum instead of the throttle pedal. When cruise control is engaged the ‘second’ cable from the actuator moves, moving the throttle cable with it.

In a car with a fly-by-wire throttle then the system will be, obviously, an electronic arrangement. But, in both scenarios the system is designed to increase or decrease the engine’s power, which speeds up or slows the car down. The cruise control doesn’t usually touch the car’s brakes, it works on the throttle only. And if you’ve got a car with an automatic gearbox then the gears will change without affecting the cruise control.

The whole thing, no matter the mechanics of the system, is run through a cruise control computer which tells the cruise control when the car has reached the desired speed or when it’s falling away from the desired speed. It’s also able to tell the system when the brake pedal has been pressed so that it doesn’t try and fight against the driver, and instead deactives the cruise system. Clever.

How to use a cruise control system?

Cruise control has moved on since Teeter’s day and is now an electronic feature, rather than one that relies on cables and vacuums. There are subtle differences between systems, but predominantly the cruise control stalk or steering wheel-mounted buttons will perform a similar function. See the main picture.

All systems feature an On/Off switch but don’t think that simply by pressing this your car’s cruise control will automatically know what speed you want it to hold. Nope, all this does is turn the system on; you then have to set (usually by pressing a button on the stalk, or by moving the stalk) the speed you want to hold. Once you’ve pressed the Set button your car will maintain that speed.

Then there’s a Cancel button which, when pressed, deactives cruise control but does not switch it off – there’s the off switch for that . Also, cruise control systems will deactivate when you press either the brake or in a manual car, the clutch. You’ll be in control of the speed until you Set the speed again with the cruise control. If you accelerate the cruise control will stay active, and if you stop accelerating the car naturally slow down and not let the speed drop once the target speed is met.

A Resume button can be used to revert to the previously Set speed, provided you haven’t already switched it off. There are also + and – buttons that allow you, once cruise control is activated, to creep up or down in speed by one or two kilometres per hour at a time. You’ll need to check your owner’s manual for a description of how your car’s system works but some allow you to hold the + or – button to accelerate by a few kilometres per hour in a single jump, or hold them down for a gradual increase or decrease in speed. Releasing the button will then see the system hold that speed. Some systems also have two buttons, one for an incremental increase or decrease in the speed and another one for a jump of 5km/h. Pressing the Resume button should see the system return to the previously Set speed, provided the system has not been switched off.

You should consult your owner’s manual to make sure you know exactly how your car’s cruise control works.

Why does my car speed up when driving down hill with cruise control on?

Because cruise control usually just relies on the throttle to control speed and not the brakes, there’s very little way the system can do to prevent the car from ‘running away’ down a hill. So, if the engine is not strong enough to hold the car at the set speed then it will speed up (because of a lack of engine braking – cruise control can not downshift an automatic to control speed just release the throttle) when travelling down a hill, even if the cruise control has released the throttle totally.

Most modern automatic transmissions have something called ‘grade logic control’ which allows them to sync with cruise control and, if the speed is exceeding the pre-set speed, like when you’re driving down a hill, then the system will downshift the gearbox to provide more engine braking and attempt to keep the car travelling at the pre-set speed. Some advanced cruise control systems do apply the brakes to keep speed in check when travelling downhill, and some even detect corners and slow down.

The myth about cruise control and aquaplaning

Cruise control systems don’t know to slow down for a corner and they don’t know when its raining. So, while there are plenty of theories out there on the Interweb about how cruise control will cause a car to aquaplane, none of them are true. Rather it’s how a driver reacts when the cruise control is on and the car starts to aquaplane that can turn a bad situation into a terrible one. You can read more about aquaplaning here.

The moral to the story is pay attention when you’re driving and remember that cruise control is simply a way of maintaining a set speed, and that the system has no way of working out what’s on the road, what the corner’s like or whether you need to brake before the bend in the road. That’s the driver’s responsibility.

What is adaptive cruise control?

Adaptive cruise control (ACC), or active cruise control as its sometimes known, is the next stage of cruise control and adds in either a radar or cameras to help your car use another car’s speed to regulate its own based on your own pre-set speed.

The idea with ACC is that you turn it on and set a maximum speed, say, 100km/h, then, once you’re in traffic and the system locks onto the car in front of you the ACC will ensure you travel at that car’s speed or the maximum you have set, whichever is the lesser, following behind with a gap that can be varied from about 2 to 4 seconds. As long as that car doesn’t over-speed or slow down all of a sudden then your car, using ACC, will manage the speed, up or down (carefully) as necessary.

An ACC system is always linked to autonomous emergency braking systems and will warn the driver if a slower vehicle has pulled in front and will initiate autonomous braking to reduce the collision impact if necessary.

You can read more about adaptive cruise control here.