The efficiency of internal-combustion engines increases with the pressure of the charge at the time of ignition. Therefore, a compression at full load just below that of premature ignition is ordinarily maintained. But when such an engine is controlled by throttling, the efficiency drops as the compression is reduced, and as automobile engines use less than one-quarter of their available power the greater part of the time, the fuel consumption is necessarily high for the horsepower output.
On account, also, of the rarefaction due to throttling, more power must be developed than is necessary to drive the car; automobile engines in which the fuel is introduced during the induction stroke, would be more efficient, therefore, if the maximum compression were constant during all ranges of load. In the design under consideration this feature is accomplished by allowing free air, or the products of combustion, when the engine is running at less than full load, to enter the cylinder first, and the gas charge last so that it will be adjacent to the spark-plugs at the ignition time. By designing the combustion-chamber so that it will maintain a stratified charge during compression, ignition is effective, irrespective of the number of heat units introduced or the mean effective pressure desired. In the gasoline fuel at present available the induced charge is composed mostly of air with liquid spheres floating in it, so that violent agitation during the heat of compression is necessary to convert the liquid particles into gas; but the time is so short that, even with the maximum amount of perturbation, a considerable part of the fuel content is unconsumed.
While turbulence seems necessary for ordinary variable-compression prime-movers, it would be fatal to a constant-compression engine the cycle of which depends upon stratification. To operate such an engine successfully, the fuel, before admission, must be converted into a fixed gas without raising the temperature of the air above the dew-point.
When accomplished in the manner shown, a constant-compression engine is entirely feasible. It may be constructed with a distributing-valve that varies the time during which air and gas are admitted or with poppet-valves that control the intake gases in the same way. While such an engine should have practically the same efficiency at full load as engines of standard construction, the fuel consumption per car-mile on less than full load should be considerably less.


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