Investigations indicate that detonation may be controlled by retarding the rate of combustion by chemicals added to the mixture, which serve to increase its specific heat and prevent excessive temperature, and by reducing the temperature of the walls of the combustion-chamber, so that the temperature of the charge previous to ignition will be lower and thus insure a normal rate of combustion. The present discussion is devoted to methods of controlling the temperature of the charge before and after the mixture enters the combustion-chamber, and before normal ignition occurs. Tests previously made on a poppet-valve engine and on a sleeve-valve engine revealed the impracticability of applying the laboratory methods used at that time to commercial practice and the need of eliminating some of the difficulties inherent in those methods of detonation control. The various changes made in the engine are described, including the specially designed spark-plugs. The conclusions drawn from the results of tests covering a period of 2 years are that, when reasonable care is exercised in maintaining the mixture, the spark-plugs, the valves and the combustion-chamber at the proper temperature, a compression-pressure of 125 lb. per sq. in. can be used without detonation by the addition of a small amount of anti-knock compound to the fuel, and that enough increase in the efficiency of the engine will be produced to warrant the additional expense.