SIDEBAR: Inside the Rotary Engine
(Regular readers will note that this text is borrowed from the sidebar in part 1 of the Mazda rotary article; we repeat it here for ease of reference.)
A rotary combustion engine — commonly known as a Wankel engine — is a type of four-stroke internal combustion engine in which the movement of a three-lobed (trochoidal) rotor within a peanut-shaped (epitrochoidal) housing completes the four stages of the combustion cycle. The rotor drives an eccentric shaft through cycloidal gears and the eccentric shaft in turn drives the output shaft, which rotates three times for each revolution of the rotor. The following animation illustrates the process:
The basic combustion process of a rotary engine. Note that the hypothetical engine illustrated above has its intake and exhaust ports in the rotor housing (peripheral ports). The position of the ports has an effect comparable to the camshaft profile of a reciprocating engine; peripheral ports improve high-speed breathing and power at the expense of low-end torque while side ports (which in this view would be mounted ‘behind’ the rotor in the upper part of the chamber) have the opposite effect. Most NSU rotaries had peripheral ports, but Mazda opted for side intake and peripheral exhaust ports for better idle quality and low-speed response. Dual spark plugs were common but not universal on production rotary engines. On Mazda rotaries, one plug fired 5 to 15 degrees after the other to promote more complete combustion. (Animation: “Wankel Cycle anim en” © 2005 User:Y_tambe; used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)
The rotary is dramatically simpler than a reciprocating engine. While a rotary engine does have counterweights at each end of the eccentric shaft to balance the wobble caused by the rotor’s eccentric motion, a Wankel engine has no connecting rods, no crankshaft, and no valves or valve gear. Intake and exhaust are through fixed ports, either in the side plates or in the rotor housing. (Of course, a rotary engine still requires the same accessories as a piston engine: water and oil pumps, alternator or generator, et al.)
Compared to piston engines, rotary engines have a number of advantages and several serious disadvantages:
- Fewer parts: A rotary engine has fewer than half as many parts as a piston engine, which reduces manufacturing costs and (at least in theory) repair and overhaul costs.
- Light weight and compactness: With no valvetrain, connecting rods, or bulky crankshaft, a rotary engine takes up less space than a comparable reciprocating engine and usually weighs less, benefiting packaging (and often performance and handling as well).
- Smoothness: Unlike the pistons of a reciprocating engine, the rotors in a rotary engine never change direction, and each power cycle has a longer duration than that of a reciprocating engine; both factors greatly reduce vibration. Because of the eccentric motion of the rotor, the rotary is not quite as smooth as is a turbine engine, but rotaries have little of the shake inherent to many piston engine configurations.
- Rev potential: With excellent volumetric efficiency (which at some speeds can exceed 100%) and relatively low rotational inertia, a rotary engine can rev quickly and reach very high engine speeds.
- High specific output: A rotary engine can produce more power than a reciprocating engine of the same swept volume (geometric displacement). With the advent of variable valve timing and fuel injection, the difference is no longer as great as it once was, but for many years a rotary engine was considered comparable to a piston engine of two times the rotary engine’s total geometric displacement. For example, the output of a 995 cc (61 cu. in.) rotary engine was roughly equivalent to that of a 1,990 cc (121 cu. in.) piston engine.
- Modest octane requirements: Because of the size and shape of their combustion chambers, rotary engines generally have lower octane requirements than do piston engines.
- Low NOx emissions: Rotary engines tend to have lower combustion temperatures than do piston engines, substantially reducing nitrogen oxide (NOx) emissions.
- Ease of manufacture: A rotary engine may have fewer parts than a piston engine, but some components, such as the rotor housing, are complicated or difficult to produce, driving up manufacturing costs.
- Difficult sealing: With their mathematically complex curves, adequate sealing is challenging and often problematic for rotary engines, from the corner and apex seals at the tips of the rotors to the O-rings between the rotor housings and side plates. Oil sealing is also more complicated than the piston rings of a reciprocating engine.
- Fuel consumption: Compared to OHV or OHC piston engines, the thermal efficiency of a rotary engine is poor, and a certain amount of fuel mixture clings to the chamber surfaces and rotor, where it is eventually forced out the exhaust ports without being burned. As a result, rotary engines tend to be thirsty for their size and output, with high specific fuel consumption (units of fuel burned per unit of power produced per hour). Based on the Society for Automotive Engineers thermal equivalency formula, a rotary engine has thermal efficiency (and thus fuel economy) comparable to a reciprocating engine of three times its geometric displacement. By that formula, for example, a 995 cc (61 cu. in.) rotary engine would be about as thermally efficient — and thus about as thirsty — as a 2,985 cc (182 cu. in.) piston engine!
- Oil consumption: Even with effective oil sealing, rotary engines consume some oil for rotor lubrication, much like a two-stroke engine. Many production rotaries have used metering systems to inject small amounts of oil either into the carburetor or (in later engines) directly into the rotor chamber itself.
- High HC and CO emissions: The same factors that cause the rotary engine’s high fuel consumption and low nitrogen oxide emissions contribute to higher levels of unburned hydrocarbon (HC) and carbon monoxide emissions.
- Higher cooling requirements: The rotary engine’s low thermal efficiency means that more of the energy of combustion is lost as heat than in most modern reciprocating engines. That waste heat puts a heavier load on both the cooling and oil systems of a rotary engine than with a piston engine of comparable output, requiring greater radiator capacity and sometimes the use of an engine oil cooler.