The Life Of The Plugs

Northern Advocate, 1 October 1938, Page 6 (Supplement)

The Life Of The Plugs

The term “life” as applied to sparking plugs means the period during which they give, in relation to the voltage passed, a full output of spark power to ignite the gases introduced into each cylinder. A sparking plug is consumable in somewhat the same manner as is lubricating oil, inasmuch that its efficiency falls off after a considerable period of service. The length of service, however, varies, and the following are two reasons for this; —

The first is the normal burning action of the spark on the electrodes. The erosion-resisting properties of the modern electrodes ’will be readily ap-

preciated when it is realised that each plug during average service in an ordinary four-cylinder engine is called upon to pass some two million sparks in a thousand miles’ running. The other factor is the conditions under which the plug operates, being subjected to rapidly alternating periods of intense heat at the moment of firing, followed by sudden quenching when the damp mixture is introduced. These operating conditions vary considerably in different types of engine—the head design, location of the plug and valves, and compression ratio all affecting plug life. These variations explain to a large extent why the same type of plug in two different engines (for which it is equally suitable) gives in one many thousands nfote miles of satisfactory service than in the other. Thus it is impossible to lay down any hard-and-fast rules as to what constitutes the normal “life” of a sparking plug. When it has become universally accepted that to change oil at a given milage is essential for the well-being of the engine, it is equally true that “plug life” can in most cases be prolonged if plugs are removed at regular intervals, efficiently cleaned, and the gaps reset. This is ■ particularly important after the running-in period, during which slow running and abnormal use of the choke cause excessive deposits of carbon, while the plug does not get a chance to operate at temperatures high enough to burn off the deposits formed. There comes a time, however, when cleaning is only a temporary measure. When this stage is reached, which is called “ageing,” deterioration becomes rapid and the intensity of the spark falls off, with resultant loss of power, wasted petrol, and poor starting. It is then advisable to replace with new plugs, of the type recommended.