Posted on November 29th, 2011 by Rob Marshall

Regular followers of these blogs will be aware that I view the fuel economy figures, quoted in vehicle manufacturers’ literature, with scepticism. Many members have made use of GEM Motoring Assist’s one-to-one technical service, expressing concern that their car might have a fault, due to its inability to match the Official Combined fuel figure as quoted by its maker, often by a considerable percentage. Yet, I do not blame the car manufacturers, as the laboratory tests are performed independently. Just as certain factions of our education system have been teaching children how to pass academic examinations, rather than using their knowledge to excel in the ‘real world’, carmakers adapt their cars to excel in the European tests.

The advent of ‘Stop-Start’ is one of the technologies that has emerged in recent years. The premise is simple. With the car’s engine at its optimum temperature, the footbrake depressed and the gear lever in neutral, the engine will cut out. When either the clutch or throttle pedals are depressed, the engine restarts promptly, in readiness to move off. While this automatic system is ideal for a beneficial result in a laboratory, because neither fuel is consumed nor carbon-dioxide exhaled during the traffic simulation test, I am concerned by both the safety and longevity aspects of an engine being stopped and restarted automatically over many years of everyday wear and tear.

Although the technology has been present for several years, Jaguar/Land Rover has refined the system to work in conjunction with an automatic gearbox and it was while testing the latest 2.2-litre Jaguar XF model, with a colleague, that prompted this blog. While stationary, awaiting an opportunity to take a filter right at a busy junction, the engine cut out as intended. However, the steering’s power assistance vanished simultaneously and, had a fast-moving car approached our rear, the safety margin would have reduced substantially by the time delay needed for the engine to restart and the car to select drive again, prior to taking avoiding action. We both felt highly vulnerable in two tonnes-worth of luxury.

While Stop-Start can be turned off manually, determining theunfamiliar location of its button on the XF’s centre console was not intuitive. It was something we had not checked, prior to setting off. The situation was not helped by the ‘eco’ (‘Stop-Start’) mode being the car’s default setting.

I also feel that certain Stop-Start systems are not particularly considerate for following motorists. The Jaguar sampled is not unique in that its engine only restarts, once the brake pedal is released. To maintain the stall in traffic, the brake pedal has to remain depressed, thus firing three sets of high-intensity brake lights into the retinas of following drivers. While this situation is annoying enough in daylight, it becomes worse nocturnally and especially during rain and snow fall, when the windscreen wipers exacerbate visibility issues.

Additionally, in the long term, battery, alternator, starter motor and maybe turbocharger longevity could be curtailed. While the overall eco-benefits can be called into question, I see Stop-Start as a system that can cause more problems than it may resolve. Should your car be fitted with it, ensure that you are familiar with how and when to disengage and re-engage it, to maximise the safety of you and those around you, as well as your car’s fuel economy.