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

Performance Characteristics of Prototype Engines and Combustion Gas Leakage

Abstract

For the purpose of evaluating basic design features an air-and charge-cooled Wankel rotary engine of about 10 PS {7.5kW} for general use, two prototype single rotor engines displacing 67cc (RX 67) and 93cc (RX 93) per revolution were tested in which. side-port intake and peripheral-port exhaust system similar to that in the Fichtel & Sachs KM37 engine is adopted.
This report deals with (1) the performance chracteristics at full throttle of the two prototype engines and (2) the decrease in volumetric efficiency which restricts the specific power output of these engines. The results obtained are as follows.
(1) The principal performances of the two engines are shown in Tables 1 and 2 and Fig. 1.
There is little engine vibration except slow rotation of the engine due to gyroaction. In addition, they have excellent durability.
(2) It is evident that these features show significant advantages over conventional reciprocating engines. Admitting its high manufacturing cost by about 10%, it possesses enough desirable qualities for small size air-cooled engine for general use.
(3) The ratios of the width of rotor housing b to the eccentricity e are 3.75 for RX67L and 5.25 for RX93L, respectively, The RX93L engine designed with the larger value of b/e than the RX67L engine has an advantage over the latter in the specific power output by about 6%. The size of b, however, cannot be determined arbitrally without considering the strength of eccentric shaft, allowable bearing pressures, etc.
(4) Air-cooled Wankel rotary engines can be classified in two types: (a) low speed type, equipped with a governor for using in principle at rated speed such as 5, 000rpm and has higher torque characteristics in the low speed range, and (b) high speed type, commonly is not equipped with the governor and designed to produce higher power output in the high speed range over 5, 000rpm. The standard design values of the maximum break mean effective pressure for these engines are: (a) Pme_max=4.15kgf/cm²{407kPa} for the low speed type, but Pme_max=4.4kgf/cm²[432kPa] may be used if a little deterioration in combustion characteristics at light load is allowable, and (b) Pme_max=4.9kgf/cm²{481kPa} for the high speed type.
(5) When the running conditions of the air-cooled Wankel rotary engine are changed from motoring state to firing state, the temperature of the induction mixture rises rapidly by about 80°C. Consequently, volumetric efficiency is inevitably reduced by 20-25% and the specific power output decreases. The main reason is the leakage of high temperature combustion gases through sealing devices and the temperature rise of the induction mixture by cooling the rotor. The most difficult problem in the Wankel rotary engine is the sealing of combustion chamber against leakage without excessive friction and wear.