Road Test

Cruise Control Explained

By Bernd Felsche

Copyright © 1998,1999 Bernd Felsche, Perth, Western Australia
All Rights Reserved
This Document may not be copied without the author's written permission.

Today's cruise controls are very simple gadgets compared to the average digital watch. They maintain a preset vehicle speed to the best abilities of engine and transmission; helping to reduce driver fatigue, as well as the frequency of inadvertent speeding tickets.

They are in fact, only a speed control. The Germans call it a "Tempostat" which is a far better name.


A long time ago, when all cars were very basic transport, and roads were often little more than horse-carriage tracks, I'm told that there was no such thing as cruise control fitted to cars.

History tells us though, that there were chauffeurs for the more wealthy owners, which was something more a matter of necessity than luxury as early luxury cars were large, unwieldy, heavy to operate and much more difficult to drive than cars of the last 50 years. Very few were fun to drive, so what one could call the ultimate cruise control was appropriate; a chauffeur, if one could afford one.

As cars grew more sophisticated, more attainable and numerous, roads improved; speeds increased and more owners drove. It wasn't long though before there was a hankering to relieve the constant pressure under the right foot, especially on long drives so the more adventurous would resort to bits of string to hold the throttle open at a fixed position, or place the proverbial brick on the accelerator. It should go without saying that this sometimes resulted in unfortunate consequences, and it was less than adequate at maintaining constant speed over hills and even on the flat as road surfaces and other conditions varied.

More sophisticated devices, using fly-weights, similar to steam engine governors could have been used, as they were in trains, but complexity and cost prevented them from becoming widespread in automotive use.

It was the dawn of consumer electronics in the late 1970s that made cruise control practical to be installed in volume-production motor cars. Early versions were built from transistors, resistors, capacitors and bits of wire; so-called "discrete components". Reliability was poor by today's standards as the components were susceptible to heat and mechanical stresses, as well as the inevitable variability due to a large number of individual parts. And it was still quite expensive.

By 1980, most of the electronic components had been integrated into a single silicon chip, which made a world of difference in production cost and reliability.

Now, as we head towards the end of the millennium in our cruise-controlled, modern day cars, the cruise control function may be just one of many others performed by one of the computer hiding in some recess of the vehicle.

How it Works

Of course, controlling the speed involves more than a black box. It needs to connect to the throttle linkage to open or close the throttle to control engine speed. It also needs many other "inputs" such as road speed, brake and clutch pedal position, as well as gear lever position.

All cruise controls use some sort of speed-sensing device. Nowadays, this is often a Hall-effect sensor; a solid-state semi-conductor that senses magnetic fields. The magnetic field it senses is usually that from a rotating magnet on the speedometer cable or similar. as the magnet rotates, the magnetic field alternates. The brains of the cruise control measures the rate at which the field changes using the Hall effect sensor, the rate of field reversals being proportional to the vehicle's speed.

The brain compares the measured rate to that which corresponds to the rate preset by the driver at the desired speed and, if the cruise control is engaged, it will alter the throttle opening according to the difference between the two rates.

For safety reasons, the cruise control also needs to know when the brakes are applied, the existing brake light switch providing a convenient tell-tale signal. It is also possible for a separate, "dedicated" sensor to be used by the cruise control to enhance safety.

There are also mechanical considerations to take into account, such as making sure that the car is in a gear where the preset speed can be attained without blowing up the engine. With a manual transmission, this often requires that the car be in one of the two highest gears, and with an automatic, that "drive" be engaged.

Other limits are also placed on the system, such as a minimum preset speed, and in the more rapid mobiles; a maximum preset speed.

Operating Characteristics

A cruise control has certain operating characteristics; the most obvious being the speed error allowed. Most systems keep the speed within 1 kmh or better taking into account limitations of engine power. When the inclines get too steep however, then vehicle speed will drop more, especially in a manual, whereas the automatic will (hopefully) kick down to a lower ratio to maintain a constant speed.

On the downhill though, it could be a different story. A very steep hill can lead to speed increasing well above that desired, even with the throttle valve shut completely. Advanced cruise controls can call upon the computerised braking system to not only slow down to the desired speed, but also to maintain traction and vehicle stability.

The other main characteristic of cruise controls is the rate at which they resume the preset speed. Older systems tended to operate lethargically, using only gentle throttle openings until the required speed was reached.

With electronic fuel injection and computerised cruise control, more vigorous acceleration is possible, with no appreciable penalty in fuel consumption due to the more rapid acceleration. It makes cruise control more traffic-friendly and improves its versatility.

Fuel Consumption

Keeping a constant speed when driving has long been one of the guidelines for obtaining the best economy figures. However; doggedly maintaining the same speed over hills is a costly exercise. With the engine straining to keep the car going uphill at the preset speed, perhaps kicking down to a lower gear in an automatic, and then throttling back fully when going down the other side of the hill, perhaps even applying brakes, is not part of the formula for good fuel economy.

The problem with cruise control is that it doesn't (yet) have the ability to look ahead and plan for an appropriate speed. Allowing the cruise control a wider margin of error (i.e. making it more "relaxed" about speed changes).

A moderately-good driver can consistently achieve better fuel consumption figures than the current cruise controls because the driver has the ability to perceive what's ahead and to plan accordingly.

The Future

Cruise controls will become more integrated into the rest of the vehicle. Some cars already have the cruise functions built into the engine control computer, and using "drive-by-wire" technology, where there's no physical connection between the accelerator pedal and the throttle, these systems promise higher reliability thanks to less mechanical complexity.

As mentioned before, the cruise control can be connected to the computer-controlled braking system as is present in ABS-braked vehicles, to apply brakes when necessary.

Mercedes is already connecting the engine controls to the ABS and using another computer to enhance vehicle stability and handling. Cruise control can only add to this, perhaps adapting speed automatically without the driver having to adjust or to disengage the system. Additional sensors throughout the car give the stability control computer essential information about what the car is doing; if it's sliding, braking, going up-hill, etc. The controller then adjusts throttle and brake pressure at each wheel to minimise skidding; enough so that a bad driver can look competent. One could say that this is a big selling point for Mercedes!

Another future prospect for cruise control is the ability to adapt to conditions before they are encountered, instead of just reacting to what the car is doing at any instant.

Volkswagen has been testing various technologies for automatic convoy operation at high speeds and at high traffic density. You basically programme your destination and join a high-speed (perhaps 150kmh or faster) stream of traffic and travel along with much-reduced following distance which is only safe as the cars in front of yours are constantly informing each other and yours of what they are doing.

If the lead driver drops anchor suddenly, then it's a matter of thousandths of a second before brakes in all the cars of the convoy are applied with equal insistence. Similar communications about what the cars in front are doing provide real cruise facilities because they supply the necessary data to steer your car as well, so it's a "hands off" all controls until the warning sounds for you to leave the convoy.

The ability for your car to read what the cars in front are experiencing will allow future cruise controls to plan ahead, giving them the potential to be better drivers than most humans claiming that title.

One can only hope that cruise controls of the future won't take all the fun out of driving; just the tedium.

Originally Published in ViaWest, April 1998
Copyright © 1998 Bernd Felsche, Perth, Western Australia
All Rights Reserved