Air-cooled Wankel Rotary Engine for General Use (III)

Factors Influencing Power Performance (2)

Abstract

Principal factors influencing the power performances of an air-and charge-cooled Wankel rotary engine for general use are gas seal clearances, the number of side seals, etc. which were discussed in the previous report. Besides these, there are many other factors, such as 1) constructions of the main induction mixture passage, 2) shapes and locations of a recess provided on the rotor flank, 3) positions of a spark plug, etc.
Experimental and theoretical investigations were done on the relationship between these factors and power performance. The results may be summarized as follows:—
(1) Air-and charge-cooled Wankel single rotor engines can be classified as 1) engines in which three holes provided just under the apex slots of the rotor are used as the main induction mixture passage, and 2) ones in which three holes provided on the rotor journal of an eccentric shaft are used as the main passage. As far as the engine power output is concerned, the former has an advantage of 8% over the latter (Fig. 1, 2).
(2) The optimum shape and location of the recess by which higher engine power output can be produced are the circular arc shape recess in which the deepest arc bottom provided on the rotor flank is located at a short distance to the leading side in the direction of rotation from the minor axis. From the results of the experiment using the RX67H 67cc engine, the power output of the engine equipped with the rotor having circular arc recess had the advantage of 7-8% over that having straight recess. (Fig. 3).
(3) In a single spark plug engine, the spark plug should be installed on the leading side in the direction of rotation from the minor axis so that higher power output may be produced. The power output of the engine in which the spark plug was located E=7mm from the axis had the advantage of about 7% over that of E=10.5mm (Fig. 4).
(4) Effective measures for transfering the maximum torque point to the low speed side are, 1) decreasing of the venturi-tube diameter of carburettor, 2) advancing of inlet port closing, and 3) delaying of ignition timing, etc. However, these measures will substantially decrease the power output in high speed range, and the improvement of the torque characteristics in low speed range cannot be expected so much. For this reason, the optimum measures for improving the torque characteristics in low speed range are, 1) reducing of the friction loss of gas seals and 2) minimizing of the quantity of escape gases past the sealing devices out of working chambers (Fig. 5).
5) Due to the high concentration of thermal stress occuring around a transfer hole connecting epitrochoidal surface with the electrode chamber of a spark plug, there is a possibility of thermal deformation occurrence within a certain range around the transfer hole. It was found from the experiment that the deformation was stopped when it reached to t=4.5mm. As the electrode part, however, is apart from the epitrochoidal surface of the combustion chamber when the thickness t is increased too much, the gas exchange between the combustion chamber and the electrode chamber is not smoothly done. Therefore, difficult problem will arise such as the deterioration of combustion characteristics at part throttle load (Fig. 6, 7).