APPLICATION OF SUPERCHARGERS TO AUTOMOBILE ENGINES 260013
Progress has been made steadily in the application of superchargers to racing automobiles, and the author of the following paper has no doubt that the supercharger will be adopted as standard equipment on passenger automobiles of the better class within the next few years. Builders of the Mercedes car have placed on the market two models regularly equipped with a supercharger of the Roots blower type, and the increased power, flexibility and speed of these cars have shown the public the great possibilities obtainable by the supercharger.
In addition to the foregoing advantages, the supercharger gives greater mechanical efficiency and fuel economy. Moreover, the engine can be made smaller, the compression can be well below that at which detonation occurs and still afford a surplus of power, heat losses in the water-jacket are reduced because of the larger charges of mixture in the cylinders, expansion of the gas can be utilized more fully by opening the exhaust-valves later, the engine can be made quieter by lifting the inlet-valves more gradually, and wear can be reduced by using lighter valves and valve-operating mechanism.
The smaller engine develops sufficient power at nearly full load to drive a car at constant speed on a level road and the supercharger is the means of providing excess power for acceleration and hill-climbing. It is shown that the average American automobile engine can develop power greatly in excess of that needed to drive it at constant speed on the level road and that fuel consumption increases rapidly as the load on the engine is decreased from three-quarter load, as when throttled. The mechanical efficiency also is much lower at partial load.
The problem involved in designing an automobile supercharger is to introduce a larger charge of mixture in the time allotted by the timing diagram, thereby maintaining full volumetric efficiency at the higher engine-speeds and increasing the power output. Area of valve opening cannot be increased materially, hence it is necessary to force the larger charge into the cylinders under pressure. To avoid too high a compression-ratio, the ratio can be decreased and the engine supercharged only enough to give the original final pressure.
The Mercedes supercharged engine is described briefly and the performance of the car with and without the supercharger is analyzed. The analysis and comparison of power curves show that acceleration from 10 to 30 m.p.h. is much more rapid with the supercharger, in both high and intermediate gear the maximum speed is increased from 40 to 50 per cent, the unsupercharged engine has the characteristics of a low-speed engine and the supercharged engine those of a high-speed engine, the size of the manifolds and valves has been reduced, the supercharger maintains approximately 100-per cent volumetric efficiency at maximum engine speed, and the thermal efficiency is increased. Power required to drive the supercharger is calculated to be 10.1 hp.
As the Mercedes supercharger does not begin to function at the lower car-speeds, no increase of engine torque due to the supercharger can be expected at these speeds, hence the car is provided with full change-speed gearing. It may be that sometime a supercharger will be developed which will supply at low speeds the additional torque that is now provided by the change-speed gears and thereby make it possible to dispense with the transmission, as some authorities believe can be done when a supercharger is installed.