<div class="section abstract"><div class="htmlview paragraph">THE acute operational problem of spark-plug fouling in both the aviation and automobile industries is discussed in this paper, with special reference to the use of a tricresyl phosphate scavenger mix, which has been found to permit the laboratory aviation engine to run for 62 hr without fouling.</div><div class="htmlview paragraph">The mechanism by which tcp relieves spark-plug fouling is believed to be the prevention of formation of metallic lead, but additional factors include the formation of lead salts having low electrical conductivity; the reduction in deposition of free carbon, and the production of soft, powdery deposits, which slough off easily, compared with the usual hard, glazed form.</div><div class="htmlview paragraph">The successful use of tcp, as indicated in both laboratory and field tests, points to a future in which spark-plug fouling may become one of the minor causes of ignition failure.</div></div>