Wankel Engine
The previous discussion on internal combustion engines was restricted to reciprocating engines. The only rotary-piston engine produced in series was developed by Felix Wankel. The figure shows the geometry of the engine named after him.
The contour is achieved by superimposing two rotary motions. The traversing radius R rotates around the revolving eccentric point E. In doing so, its end points A as well as B and C, each located at an angle of 120° to R, describe a closed curve in the form of an epitrochoid.
The points A, B and C, connected by convex sides, determine the outline of the piston. Three working chambers arise whose size changes periodically. The resulting so-called chamber piston displacement is computed.
The kinematics of the piston are realized through a pinion firmly connected to the housing. At the pinion, the piston revolves over a wooden gear wheel. The ratio of radii of the wooden gear wheel to the pinion is fixed value of 3:2. The piston drives an eccentric shaft that represents the equivalent of the crankshaft in a reciprocating engine. The translational vibrations generated by this motion can be balanced without problems.
The Wankel engine works on the 4-stroke Otto process. In the course of a piston revolution (corresponds to three rotations of the eccentric shaft) the working cycles intake, compression, expansion, and exhaust take place in each of the three chambers. Mixture formation and ignition correspond to the Otto cycle.
The piston is responsible for charge exchange in the Wankel engine. While in nearly all designs the exhaust port flows into the circumference of the housing, the intake port is designed either as a peripheral or side intake. The peripheral intake, in contrast to the side intake, is always open for at least one combustion chamber. As a result, the unhindered flow of the gas column in the inlet port provides low flow losses and a good volumetric efficiency at high speeds leading in turn, to a high maximum output. At low speeds, however, the long overlap periods of the peripheral intake cause scavenging losses since it allows the exhaust gases to flow into the fresh intake air. The consequence is a loss of torque in the low speed range. This torque loss can be prevented by a side intake. In addition, by modifying the position and shape, it offers the possibility to vary the operating times within a wide range and hence influence the power output and efficiency.
In order to achieve a good torque characteristic and low fuel consumption in the part-load area with a simultaneously high maximum output at full-load, systems that consist of several intake ports per piston have to be developed on each side of the housing. The intake ports can be operated individually or in combination according to the operating condition (Mazda).