Appraising Diesel Engines by Air Capacity and Speed 320054
BRAKE mean effective pressure is held by the author to be inadequate as a measure of Diesel-engine capacity, because it fails to distinguish between losses in capacity that are chargeable to low volumetric efficiency and losses resulting from imperfections in combustion. A study of air capacities assists in making this essential distinction and is advocated as a better basis for appraisal. Variation in effective air supply with speed is regarded as especially important, as acceleration and peak power depend on it.
Experimental work cited had to do mainly with the determination of the percentage of carbon dioxide in the exhaust gas, and this is used as a basis for calculating the quantity of air passing through the engine. Conclusions are drawn also from some experiments in which the intake air was measured directly by an orifice-type meter.
An excess-air coefficient is set up as the ratio between the quantity of air actually passing through the engine and that theoretically required for the work done, on the assumption that combustion conforms to the theoretical mixed-pressure cycle. This coefficient is said to register directly the engine's ability to produce power from fuel, and its variation with engine speed is regarded as of paramount importance in evaluating an engine for automotive propulsion and thus reducing the commercial hazard of launching a new type of engine.
Discussers give more attention to engine design than to the method of evaluation, some advocating the undivided combustion chamber, rather than the divided which was used in two of the tests reported, and others questioning the author's assumptions on stroke-bore ratio. One discusser presents a scale for measuring the smokiness of the exhaust and another reports an arrangement of injection valves that is said to make the turbulence in an antechamber engine more uniform.