COMBUSTION roughness is defined as the ratio of the rise in pressure to the time required for the burning. It is pointed out that evidence shows that the time required to burn a charge in an engine is directly proportional to the volume of the charge.
A measure of the “natural roughness” of a combustion chamber is given by the ratio of the compression pressure to the clearance volume.
The question of how flame speed varies during the burning is taken up and it is brought out that flame speed goes through a maximum when half the volume of the charge is burned.
In designing a combustion chamber for smoothness of burning, it is necessary to consider the variations in the speed of flame propagation, and to attempt to bring the maximum speed early in the burning so as to get the flame off to a good start. In this manner it is possible to avoid unduly rapid increases in pressure at a late stage of the burning. This is accomplished by considering the “volume distribution” of the combustion chamber around the point of ignition, a procedure introduced by Janeway. It is thus possible for the maximum flame velocity to occur at any stage of the burning simply by varying the distribution of volume to produce the desired result.
Experience has shown that the point at which maximum flame speed occurs must come early in the burning in order to obtain smooth combustion. The exact location of the point of maximum flame speed will depend on the clearance volume and the compression pressure.