In 1967 the German car firm NSU released the Ro 80. Equipped with four-wheel disc brakes, a semi-automatic gearbox, and a peculiar clutch which was actuated by pressing down on the gear knob. The Ro 80 was believed to be ahead of its time, though its features are quirky by modern standards. It was even fitted with a warning buzzer to alert the driver that the 9000RPM redline was approaching. With agile handling and a quiet and smooth engine, the Ro 80 was voted car of the year in 1968. But what truly made the Ro 80 unique was its 84 kW or 113 bhp, 995 cc twin-rotor Wankel engine.
For the time, the new engine design was regarded as a technologically advanced power plant. Leading up to the release of the Ro 80, the advent of the Wankel rotary engine was considered the next big leap in engine design amongst automakers. Spearheading its introduction was the joint partnership between NSU and Curtiss-Wright; a US based diversified product and services manufacturer.
The NSU and Curtiss-Wright venture lead to license agreements for developing the engine for American, German, and Japanese car manufacturers. Its initial attraction to automakers was the Wankel's smooth, quiet, and uncomplicated design. It positioned itself as a promising power unit for upcoming models. Back in the United States, American Motors Corporation or AMC was convinced that the Wankel engine would play an important role as a power-plant for cars and trucks in the future, eventually replacing the conventional internal combustion engine within a decade. So much so that the AMC Pacer was designed originally for the rotary engine in which units were supposed to be purchased from Curtiss-Wright. But later, AMC decided instead to purchase the engines from General Motors who were developing them for use in their own cars. General Motors later cancelled development of their rotary engine, leaving AMC to find an alternative for the Pacer.
GMC's cancellation was primarily attributed to policy changes caused by the 1973 oil crisis. Thereafter, consumer awareness placed a greater emphasis on fuel economy. Proposed US emissions legislation and minimum fuel economy standard further frustrated any uptake of the rotary engine for American automakers. Despite its eventual demise, the rotary engine possessed an upper hand over the piston engine layout in a few regards.
Let's explore the inspiration and mechanics of a rotary engine and its notable differences over a traditional engine. The inspiration for the rotary engine was derived from the geometric principle that when a circle is rolled on its own circumference along another circle, that has double the radius, a point within the circle generates a curve known as an epitrochoid. This curve forms the shape of the inner walls of the rotor housing. The rotor housing hosts all stages of the rotary's power stroke, much like a cylinder bore in a conventional engine. As for the shape of the rotor, a triangle was ideal because it yielded the most effective configuration within the housing. As the three apexes of the triangular-shaped rotor move uniformly along the inside walls of the rotor housing, the cavity between the rotor and the interior walls of the housing are divided into three continually changing areas of volume. This is because rotary engines are variable-volume, progressing-cavity systems. As each rotor has three faces and each face has three cavities of volume per housing. In effect, each face of the rotor "sweeps" its own volume as the rotor moves in an eccentric orbit within its housing. Each side of the rotor is brought closer to and then further away from the wall of the internal housing; compressing and expanding the combustion chamber.
A rotor is effectively akin to a piston. But where the cylinder volume changes as the piston travels up and down in a cylinder. The volume, configuration, and position of the operating cavity changes as the rotor orbits in an eccentric fashion. Generally, a reciprocating piston engine requires valves for the air/fuel intake and exhaust cycles. The air and fuel mixture is introduced through the intake port when the intake valve opens. This happens while the piston begins the intake stroke. When the intake valve closes and the piston is brought upward, the gas is then compressed. As the spark plug ignites, the piston is forced downward by the combustion. The exhaust valve then opens and burnt gas is forced out by the rising piston. Air and fuel are brought in again and the cycle is repeated. It's easily observed that while the crankshaft turns twice, the explosion occurs only once. On the other hand, the rotary engine has no valves. Instead, the rotor itself behaves like a valve as it orbits. Air and fuel is introduced through the intake port located on the side of the housing, previously closed off by the rotor.