Lodge Plugs, Ltd., have recently conducted some valuable experiments at their laboratories at Rugby in connection with the temperatures of sparking-plug insulators and electrodes. Measurement of temperatures was made by incorporating very fine thermocouples in the plugs themselves. Thermocouple plugs enable the correct type of sparking plug for a given engine working under a particular set of conditions to be chosen accurately without further experiment. Lodge Plugs, Ltd. have developed thermocouple plugs which enable temperature measurements of the electrode to be made with an accuracy of +2° C., and, as the thermocouple, set 3 1/2 in. from the end of the electrode, is only 10 thousandths of an inch in diameter, the characteristics of the plug remain unaffected. The hottest part of a standard-type sparking plug proved to be the tip of the central electrode. It was found that when this part of the plug reached approx. 1,100° to 1,150° C. pre-ignition would occur and that after-running or auto-ignition happened at an electrode-tip temperature of approx. 1,000° to 1,050° C. The exact temperature at which these troubles arose was, naturally, dependent on fuel, compression ratio, engine design, etc. The insulator, if made of sintered alumina, as in Lodge plugs, normally remained at a temperature slightly lower than that of the electrode, but deposits on the insulator would become incandescent appreciably before the electrode reached this state.
Tests with a popular 11-h.p. car showed a plug electrode temperature of 545° C. at a steady speed of 30 m.p.h., rising to 775° C. at 70 m.p.h., and 745° C. at maximum speed in third gear. When testing a 6-cylinder car on the road the electrodes of the plugs in cylinders No. 3 and No. 6 remained identical, rising from 480° C. at 30 m.p.h. to 700° C. at 70 m.p.h., but the plug in cylinder No. 1 showed an electrode temperature of 520° C. at 30 m.p.h. and 830° C. at 70 m.p.h. and higher temperatures at intermediate speeds, thus revealing a cooling or distribution malady affecting No. 1 cylinder. Experiments with a Rolls-Royce single-cylinder test engine of “Merlin” type, using Lodge RS5/4 plugs, showed an electrode temperature rise of approx. 250° C. between a variation in b.m.e.p. from approx. 85 to 383. It was found that shortly before a plug reaches pre-ignition point the rate of temperature rise decreases, due to auto-ignition taking place, and that thereafter there is a sudden and appreciable temperature increase to pre-ignition temperature. The thermocouple plug enables plug design to be adjusted to obviate these temperatures from being reached without courting other troubles due to too low an electrode temperature. Its application to racing car engine research after the war is obvious.