An Experimental Investigation into Pre-ignition in the Spark-ignition Engine

Abstract

Pre-ignition and auto-ignition are uncontrolled ignition of combustible mixture in an engine by a hot surface. Pre-ignition is distinct from “knock” or “detonation” which is caused by the rapid combustion of the last part of the mixture following the initiation of flame by a spark. More work has been carried out on knock than pre-ignition because knock sets the primary limit to the possible power-output and fuel economy of an engine. Owing to increases in engine power in recent years, however, interest in pre-ignition has been revived, and this paper describes an investigation which was primarily directed to finding the pre-ignition ratings of various fuels. Two methods of obtaining pre-ignition ratings have been developed; with one a 500 c.c. single-cylinder water-cooled unit is used, having an electrically heated hot-spot to induce pre-ignition, and with the other a single-cylinder air-cooled aircraft engine with an air-cooled pre-igniter is used. A rating scale has been constructed in which iso-octane has been given the value of 100, and cumene zero. A large number of pure fuels from the paraffin, naphthene, aromatic, and alcohol groups have been rated for pre-ignition tendency using these methods. In addition, typical commercial motor-fuels, motor-racing fuels, and water-alcohol injection fluids have been tested. The relationship between pre-ignition tendency and the temperature of the hot-spot with normal running and with pre-ignition conditions has been established. The effects of engine variables, for example, mixture strength, speed, ignition advance, and air and cylinder temperatures, on pre-ignition, have been studied, in addition to the effects of important fuel additives such as tetraethyl lead and the aromatic amines. The additive tests include substances, such as formaldehyde and nitrogen peroxide, which are of interest from the chemical aspect and, based on the results, some information on the fundamental chemical processes involved in hot-spot ignition has been obtained.