Effect of Air Movement on Combustion Rates in Spark-Ignition Engines

Abstract

A movement within the combustion chamber of an internal-combustion engine can be divided into two general classifications: (1) air swirl and (2) air turbulence. Air swirl is defined as a regular rotation, as a whole, of the charge within the cylinder or the combustion chamber. Air turbulence is defined as irregular movement of the charge within the cylinder or the combustion chamber; turbulence may be either small scale or large scale. The air movement may be produced during the induction period, in which case it is called * 'induced'' air movement; or it may be produced by forcing the charge through a restricting passage within the cylinder or the combustion chamber during the compression stroke, in which case it is called ''forced air movement. In spark-ignition engines for aircraft, the necessity of a simple combustion-chamber shape to provide high volumetric efficiencies and adequate cooling has resulted in little attention being paid to forced airflow. Since, however, the air or the charge enters the cylinder at high velocities, a certain amount of induced air movement is always produced in the cylinder and the combustion chamber. This movement, once started, persists to a varying degree during the entire compression stroke. Air movement within the engine cylinder has several different effects on the combustion of the charge. It may effect the homogeneity of the mixture, the rate of heat transfer from the mixture to the combustion chamber walls, and the rate of combustion when the movement persists through the combustion period. The first two effects may, in themselves, affect the combustion rate. It is the purpose of this paper to present data obtained at the Langley Memorial Aeronautical Laboratory illustrating the two different classifications of air movement and to present data that show some of the effects of these movements on the rates and regularity of combustion.