A Rational Basis for Correlating Data on COMPRESSION-IGNITION ENGINE PERFORMANCE at Different Intake and Exhaust Conditions1

THIS paper discusses first the theoretical relation between thermal efficiency and fuel-air ratio for assumed limited-pressure fuel-air cycles. Data on actual cycles are then presented showing the relation of indicated efficiency to fuel-air ratio. This and other relationships are then used in deriving equations for relating the power output of engines operated at constant speed and throttle setting to ambient conditions.

The fundamental nature of the relation between thermal efficiency and fuel-air ratio is emphasized by showing the variation with fuel-air ratio of computed efficiency of assumed limited-pressure fuel-air cycles. This computed efficiency for a given cycle and compression ratio is determined solely by the thermodynamic properties of the fuel-air medium and is the maximum efficiency theoretically possible for the assumed conditions.

The paper also includes an analysis of published data on compression-ignition engine performance showing the relation between actual indicated efficiency and fuel-air ratio. The data available indicate that this relation is not affected significantly by changes in ambient conditions, provided that the pressure at the intake equals the pressure at the final exhaust. Because this basic relation is independent of ambient conditions, it may be used in predicting the effect of changes in ambient conditions on the power output of compression-ignition engines operated at constant speed and throttle setting. This can be done only if the effect of changes in ambient conditions on fuel-air ratio is known or can be computed. Inasmuch as the quantity of fuel supplied to a compression-ignition engine depends only on the speed and throttle setting and is not affected by changes in ambient conditions, the effect on fuel-air ratio of such changes can be determined when the quantity of air entering the engine at each condition is known. This latter factor is determined by the density of the air and by the volumetric efficiency; both of these factors depend on ambient temperature and pressure. Equations relating all of these factors are presented herein. These equations furnish a rational basis for computing the power output of a compression-ignition engine at a given set of conditions from the power output at some other set of conditions. The precision with which the equations represent test results is shown.

The effect of changes in ambient conditions on the power output of engines in which the fuel-air ratio is constant for operation at constant speed and throttle setting is discussed briefly. This discussion applies to the gasoline or more precisely the spark-ignition engine equipped with a carburetor. It is pointed out that knowledge of the relation between fuel-air ratio and indicated efficiency is not required in formulating equations for computing the effect of ambient conditions on the power output of gasoline engines because fuel-air ratio and, consequently, indicated efficiency are more or less constant, depending on the operating characteristics of the carburetor.