The Extra Mile

The Extra Mile

We often get reader feedback regarding the use of cruise control, as to whether or not it is beneficial for fuel economy. Many seem to think it is beneficial ñ possibly more than those who have experimented and found it has no benefits. So it’s worth digging a bit deeper and analysing precisely how cruise control works, and whether all of its operations and responses are beneficial. Most cruise control systems only operate above a specified minimum operating speed, and thus only in the higher gears on the open road, as inferred by its name.

In older cars, a vehicle would maintain a set speed by using a vacuum or electric servo mechanism acting on the throttle cable, but today they operate electronically, using the car’s ìdrive-by-wireî throttle system. The precise response to a call for more power to maintain speed, or to accelerate the car to a higher set speed, depends on individual set-up, and many systems use ìproportional integral derivativeî controls. With these, any minor deviation from a set speed produces a response proportional to the speed differential, and generally demands only small increases in power to maintain a set speed. When cruise control is resumed after being disengaged, the set speed may be much higher than the existing speed. The system then demands higher engine power, for strong acceleration, which then tapers off as the speed differential is progressively reduced. This high acceleration rate is not always a comfortable experience, or appropriate in fairly heavy traffic, and it may often be wiser to use the accelerator to reach your chosen cruising speed, before then re-engaging the cruise control.

How the system may affect your fuel economy depends very much upon personal driving style. If you tend to drift up and down in speed, or if you tend to accelerate strongly, and use your brakes frequently, then using cruise control may well save you significant amounts of fuel. If your speed maintenance and variance is generally smooth though, you may see little in the way of gains. In fact, by allowing your car to slow down a little on rising gradients, and speed up on downhill stretches, as it will on a steady throttle setting, you may save fuel compared with using cruise control, which increases engine power to climb at a set speed, and cuts engine power on downhill stretches. In seriously undulating country though, with steep uphill and downhill gradients, it is considered best practice to disengage cruise control totally, and maintain direct control of the car.

But what of cruise control with an automatic transmission? They will, in theory, choose the best gear for any situation, and with today’s eight- and nine-speed automatic transmissions this is a great benefit, as cruise control stays in continued operation in any ratio selected. Manual gearboxes drop out of cruise control when you change gear though, and require the ìresumeî button to re-activate them, and it’s probably not advisable to mix frequent manual gear changes with use of cruise control. Sometimes though, such as on a long stretch of motorway with a temporary 40 or 50mph limit, setting cruise control at the limit in fifth gear, rather than sixth, may be an appropriate ticket-avoiding action.

But we’ve ignored ìadaptiveî or ìactiveî cruise control systems (ACC), which maintain a constant time interval between you and the vehicle ahead ñ the distance itself being automatically varied with speed. These systems should respond smoothly when a vehicle in front slows unexpectedly, by reducing engine power, and without unnecessary braking, and thus wastage of fuel. In practice though, they can brake unexpectedly at times, such as when you might close up on a vehicle ahead, change lanes, and accelerate for an overtake. Also, selecting even the closest of the ACC following distance settings may offer a tempting invitation for other drivers to slot into the ìsafe distanceî gap created ahead. When another car does, the ACC system cuts engine power, and may possibly even brake, to re-establish the ìsafe distanceî, and then re-applies engine power to return to the previous speed setting. Such uneven progress is not a good scenario for economy, and ACC may be best avoided if you’re looking to save fuel, or at least until these systems are further refined.

You should, of course, always stay well behind the vehicle ahead, allowing ample time for smooth and undramatic speed adjustments, and a simple cruise control system can help you achieve this, and save fuel, if you learn when and how to use it to best advantage on any given roadway.

Victor Harman 

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