Abstract The term “quiescent” is originated as describing more accurately than “non-turbulent” the condition of the combustion chamber in some fuel-injection engines at the time of injection of the fuel. Such a combustion chamber is considered quiescent if the movement of the air has no appreciable effect on the distribution of the fuel to the air with which it must unite during the process of combustion. Performance tests of a single-cylinder unit furnished the material for the paper, in the general program of the National Advisory Committee for Aeronautics in investigating the compression-ignition, fuel-injection engine as a source of power for aircraft. The simplest combustion chamber is the quiescent one, and the simplest injection orifice is the round hole; and the main tests were directed toward obtaining the optimum combination of these two elements.