The Extra Mile

The Extra Mile

We often refer on this page to the relatively poor fuel economy of a cold engine, mainly in terms of reduced combustion efficiency, not to mention the significantly higher emissions produced by a cold engine. A significant amount of energy and fuel is used to bring engine components, the transmission, and fluids like engine coolant and oil, all up to working temperature. The same goes for the tyres, which take several miles to stabilise at their elevated running temperature, around 20 to 30 degrees Celsius above ambient, at which their rolling resistance is significantly lower than when they are cold, and when they consequently use less fuel.

It’s hard to arrive at precise figures for this cold start loss of fuel economy, but taking a typical engine and transmission that might weigh 200 to 250kgs, you can arrive at a figure of the order of 0.3 to 0.5 litres of extra diesel used in this period, to warm up these items and the tyres. It’s obviously dependent on engine size and weight how efficiently and quickly the engine warms up by heat conduction through the engine, and the ambient temperatures involved, and it obviously takes longer in winter temperatures. But do these estimated figures convert into the lower fuel economy that we may observe in real life during the warm-up period? If we take 10 miles as the distance for an engine to reach optimum working temperature, and a car that typically averages 45mpg when hot, and use the lower speculative figure above of 0.3 litres of diesel to warm up the engine and transmission, it produces a cold start fuel economy figure of just 35mpg ñ a drop of 10mpg. This seems reasonably correct, and owners making a 10-mile trip to and from work every day will average as much as 10mpg less than they might achieve on longer journeys with a hot engine, which again is reasonable.

If there’s any doubt about how thirsty a car can be when its engine is cold, some figures from a true cold start on the MPG Marathon fuel economy event a few years back, in a Jaguar XF 2.2D automatic, support the case. On a very cold and frosty morning, and with a freshly zeroed trip computer, the mpg display took nearly 20 miles to climb above 40mpg. Compare this with the average figure for the two-day event, over nearly 400 miles in two days, of 58.5mpg over the same sort of terrain, with fairly brief stops for coffee and lunch, during which the engine will have cooled down a little.

Where does such speculation about cold start fuel economy take us? Well, there’s no real reason at all why UK market cars could not be offered with engine block heaters that are used to preheat the coolant, and thus the engine. These are routinely specified in Scandinavia and other countries with extreme winters, allowing owners to plug in mains electricity engine heaters (typically of around 1.5kW power) programmed to warm up an engine for maybe an hour or two before the car is needed, or for longer periods in extreme cold. In the UK, they would probably use around 3 to 4kWh of electricity, costing perhaps 50 to 60p, to give almost instant cabin heating, easy clearance of frosted-up windows and, by virtue of better oil circulation, reduced engine wear. This, in effect, is precisely the sort of pre-heating facility now offered on today’s electric cars, in their case specifically to pre-heat the cabin and thus to extend their battery range. But manufacturers apparently don’t see any reason to offer British motorists with conventional cars what has long been offered in other countries, and the many benefits, even in Britain’s relatively mild climate, are ignored. They offer heated seats, heated screens, heated mirrors, and heated washer nozzles, but we’re not offered the option of engine pre-heating, something that would save fuel, make winter cold starts less of a challenge, and also cut engine emissions when they’re at their very highest.

Victor Harman 

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