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a I run a fleet of petrol-engined vans and have been
Page 135
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advised that increasing the spark-plug gap would improve fuel consumption. What is your view?
A Increasing the spark-plug gap can give an appreciable improvement in part-load fuel consumption, but unless a high-output coil is employed the gain may not be realized and engine missing may be experienced at higher loads. There is little or no improvement if the vehicle is running continuously under high load, as for example at high speed on a motorway.
It is usually recommended that the plug gap be increased from the normal 0.022/0,025in. to say 0.040/0.045in. given that the coil is designed to provide a good spark with a gap of this order. At low loads the effective compression ratio of the engine is reduced and therefore the rate of combustion and the peak cylinder pressure. A spark spanning a wide gap ignites a larger volume of gas in the process and this increases the rate of combustion if the compression pressure is relatively low.
Incidentally, it is preferable to use a sparking plug in which the points are extended into the combustion space if maximum fuel economy is to be obtained. In a plug of this type, however, the lengthened heat-dissipation path may promote more rapid burning of the points.