From a laboratory examination of the controlling relationships between carburetion and engine performance still in progress, the general conclusions so far reached include fuel metering characteristics, the physical structure of the charge, fuel combustion factors and details of engine design and manufacture.
In every throttle-controlled engine, the variation in fuel metering for best utilization is inversely functional with the relative loading and with the compression ratio, but the nature of the fuel leaves these general relationships undisturbed. The physical structure of the charge influences largely the net engine performance and the order of variation of the best metering with change in load. Perfect homogeneity in the charge is theoretically desirable but entails losses in performance. With the least perfectly formed charge that will operate an engine, increased outputs result from a more nearly uniform distribution of fuel in the charge, even though accompanied by a reduction in charge weight. Considering available petroleum fuels, those requiring the higher charge temperatures for maximum utilization also possess the ability to ignite at relatively lower temperatures when mixtures suitable for engine use are made.
It should be possible to operate an engine continuously at open throttle, without detonation, on a properly constructed charge of any petroleum fuel proposed to date, with a compression ratio resulting in gage pressures including 75 lb. per sq. in., but an analysis of the failure to do this shows that preignition is the direct result of local overheating in the charge. Augmenting turbulence decreases local overheating and so appreciable inequalities of combustion-chamber surfaces must exist, and high piston-head and exhaust-valve temperatures contribute to a condition favoring detonation. Piston design, exhaust-valve and spark-plug electrode cooling are then considered, mention being made also of piston gas-tightness as being very important. Net engine performance is the square root of the brake mean effective pressure divided by the brake specific consumption, combining the cost of unit power and the amount of power obtained. Its value attains a maximum with mixtures only slightly richer than those resulting in the true minimum specific consumption. With manual throttle control, steady and consistent operation obtains throughout the throttling range when realizing the minimum possible specific fuel consumption but, so far, possible maximum utilizations cannot be realized in operation under governor control. These considerations are illustrated by charts, showing curves resulting from the experiments made.
