SPARKING PLUGS

Press, Volume LXXII, Issue 21939, 13 November 1936, Page 6

SPARKING PLUGS

NEED FOR CAREFUL CHOOSING CORRECT OPERATING TEMPERATURE Lack of knowledge among car owners about the characteristics of different sparking plugs is more common than might be supposed. But many motorists pay little attention to plug charts when they need replacements. It is necessary to understand that a sparking plug must operate at a certain heat. If its design is such that it has a strong resistance to the heat in the combustion chamber, and remains cooler than is desirable, it will probably oil up, as its electrodes are not hot enough to burn away the oil which may settle on them. At the other extreme, if its resistance to heat is not high enough, the electrodes will become incandescent, causing pre-igni-tion, while the insulator of the plug will probably crack under the strain of excessive heat. A happy mean must be achieved; the plug should become hot enough to keep the electrodes free of oil, but not so hot that the electrodes become incandescent. As the conditions of heat in the combustion chambers of different en-

gines vary greatly, it should be obvious that one type of sparking plug will not suit every one of them. Take, for example, a car which has a compression ratio of 7 to 1, which, though nigh, is used in several standard productions. It is only reasonable to assume that the heat in the combustion chambers of this engine will be much higher than it is in the combustion chambers of a power unit whose compression ratio is 6 to 1. which is about the average to-day. ft follows that the first engine will need a sparking plug with a higher resistance to heat than the second, a type of plug in fact that, used in the second engine would probably ojl up, though giving perfectly satisfactory results in the high compression power unitWith very highly efficient racing engines, some of which have a compression ratio as high as 12 to 1, and which, when once warmed up, attain terrifically high combustion chamber temperatures, it is customary to start from cold on a set of plugs designed to heat up quickly, then, when the engine has reached normal operating temperature, to switch off and change to a set of plugs having the quite different characteristic of high resistance to heat, the car then being ready for racing. The correct operating temperature for a sparking plug is between 1000 and 1300 degrees F„ and so irrespective of whether the threads are the .■ame or not. the design of sparking plugs is varied a good deal in order that a correct type may be available for every engine. The grading of plugs is generally accomplished by varying the length and thickness of that portion of the insulator exposed to the hot gases of the combustion chamber.

For an engine that does not attain a very high combustion chamber temperature, the correct type of plug is one that will heat up quickly, and it has a considerable amount of insulator exposed to the hot gases so as to acquire sufficient heat to provide what is termed a self-cleaning temperature. Use of this sort of plug in a hot engine, however, would be fatal, and, instead, a plug in which very little of the insulator is exposed to the gases must bo employed. This plug will stand the additional heat, whereas the other one will not. From this it should be very clear that a considerable amount of care must be exercised in buying new sparking plugs. The motorist should not take Ute first one offered that happens to have a suitable thread, but must be careful to see that the insulator extends into the combustion chamber the required depth. The best thing to do is to study the chart of some recognised sparking plug manufacturer, and to adhere strictly to its recommendations. NEW SMALL CAB Details have just been received from England of a miniature motor-car which puts in the shade any previous efforts to produce ultra-economical motor vehicles. Although very tiny, the car provides accommodation for two people within a streamlined body. Apparently no provision is made ( there is not even a windscreen t to protect the occupants from the weather. It is powered by a 250 c.c. motor-cycle two-stroke engine in the streamlined tail, ha® a three-speed gearbox, and will, it is claimed, achieve 45 miles au hour with a petrol consumption of 80 miles a gallop. The tipy road wheels are fitted with very fat balloon tyres. The little car the product of a very well-known Pritiah manufacturing organisation, and or its pppcjflcpmdns it appears to be thoroughly practical and roadworthy. It remains to be seen, however, whether the public, so bound by convention, will buy it in any numbers '