Rotary Motoring

Michael Austin
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Piston-driven engines are easy to figure out. Intake, compression, ignition and exhaust all occur in the same space above the undulating piston head. Valves open and shut to move air in and out, and the up and down motion of the piston is transferred to the rotating crank.

The rotary engine, now that's something else. Unfamiliar to the general public, most major automakers gave up on the concept a long time ago, except for Mazda. The 250-horsepower Renesis engine in the new Madza RX-8 shows that the rotary still has life left in it. For those unfamiliar with Dr. Wankel's design, here's a primer on how the engine actually works.

0304 Renesis 1

For starters, the rotary is made of two main parts: the rotor and the housing. There are no valves, the intake and exhaust ports are effectively opened and closed as the rotor turns. The housing is oval-shaped with a slight pinch in at the sides - think of a fat peanut. The rotor is a big triangle, and each side of the triangle is called a flank.

Inside the rotor is a ring of gears that matches up with the gears on the crank at a 3:2 ratio, so the rotor follows an eccentric orbit, allowing all three edges to remain in constant contact with the housing. In effect, one flank of the rotor is like an individual piston, sweeping the air through the four phases of combustion as it travels around its path.

Due to the gear reduction, the rotor only makes one revolution for every three turns of the crank. This has a few consequences. When you consider one flank, the rotary only fires two-thirds as often as a piston engine (a typical four-stroke fires every other revolution). But the rotor has 3 flanks, so a rotary engine actually has twice as many power pulses as a piston engine. There is also longer time for the combustion phase to occur in the rotary, which is good for burning fuel, but bad for heat transfer. Rotaries tend to lose more efficiency through heat loss than piston engines, but have a smoother power delivery from the long combustion.

Wankel's engine does offer some advantages over the more popular piston powerplant. The rotary has a higher power-to-weight ratio and specific output, mainly because one rotor effectively acts as three pistons. Case in point: the new RX-8 displaces 1.3 liters, but produces horsepower in a league with the 3.3-liter BMW 330I and 3.5-liter Nissan 350Z. Rotaries tend to be smooth runners as well because using two rotors offsets the primary up and down vibration. The rotary is far from perfect, though. Early engines had trouble with the rotor seals, emissions have always been a problem, and engine repairs quickly become costly.

Mazda continues to sing the virtues of the rotary long after Citoen, Mercedes, Ford, and GM (to name a few) have given up. The new Renesis benefits from these years of development and holds its own for power, efficiency, and emissions. With the RX-8, you can reap the rewards of Mazda's hard work.

0304 Renesis 2

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