The Wankel: How Big and How Heavy?
If you've been reading about the Wankel engine in general magazines you've probably seen many uniformly glowing claims for spectacular advantages in size and weight. Said one, “Wankel-type power plants are only half the size of normal ones...” Wrote Prof. David Cole in Scientific American, “For equivalent horsepower a Wankel engine is only about half the size and weight of a conventional engine.” There doesn't seem to be much disagreement on the subject. Said Fortune of the Wankel, “It has 40 percent fewer parts, weighs anywhere from a third to half as much and is half the size.
Fortune and others seem to be picking up this statement from Jan Norbye's book, The Wankel Engine: “One of the main advantages of the Wankel engine is that it is about half the size and weight of a piston engine of comparable power output.” Is that really all there is to be said on the subject? I haven't thought so for some time. If a detailed and impartial analysis of Wankel engine weight and size has been made, it hasn’t come to my attention. So we'll take a look at it.
Obviously the car makers have studied the relative dimensions of the Wankel, but they're not telling us everything they've learned. General Motors Wankel developer Bob Templin has released some of GM's findings, however, observing that the Wankel rotary “has about 40 percent fewer parts than a comparable 6-cylinder engine. (Now we see where Fortune got its 40 percent figure.) The reduction is due mainly to elimination of the valve train...” Templin is much more conservative about the weight of the Wankel than those quoted above, describing a single-rotor GMRE that's “comparable to our production 6-cylinder engine but weighs 30 percent less.”
Relative bulk and weight of the Wankel are of vital importance because they're the fundamental reasons most often quoted in favor of the wide adoption of the engine in production cars. The smaller and lighter Wankel, its advocates say, will make room under the hood for the emissions-control devices needed in 1975 and after, and will also allow the weight of the car to be reduced. The reasoning on the latter point is that a lighter engine needs less metal in the body and the suspension to support it. And when weight is reduced, goes the Detroit axiom, cost is cut down also. The reported lightness and compactness of the Wankel engine can be called the main reasons behind the interest that has led GM to decide to tool up for limited production of its GMRE. Relatively minor significance attaches to other qualities of emissions and smoothness.
Trying to compare Wankel with reciprocating engines is enough without taking in all possible versions of each. For this reason I'll leave out reciprocating two-strokes and air-cooled versions of both engines. I'll also pass over engines built solely for racing, considering only those that are either in production or designed with passenger-car production in mind.
In order to make a comparison of size and weight it's necessary first to find engines of both types that are comparable in power. This isn't as easy as it might seem to be because the Wankel and “recip” power characteristics differ in a typical manner, the Wankel having less torque for its cylinder capacity through the lower part of the power range and then coming on strong at the top end. Some reasonable comparisons are possible, however. Among smaller engines a surprisingly close match is provided by the 115 bhp two-rotor Wankel engine of the NSU Ro80 with the 110 hp of SAAB's fuel-injected EMS model. Both are net DIN power ratings at a peak of 5,500 rpm, the SAAB engine having a margin of five horsepower over the NSU at speeds below 3,500 rpm.
At higher power levels the choice is more difficult because no large Wankels are yet in production. The logical comparison is with the two-rotor RC2-60U5 engine designed for automotive use in the early 1960s by Curtiss-Wright. It makes extensive use of aluminum in its housings and accessories, so a fair comparison can be made with a reciprocating engine designed at about the same time, the 1963 Buick Special V-8 with its aluminum cylinder block and heads. The Wankel's peak power is higher but its published rating is by gross rather than net standards so the two are fairly comparable.
Taking a look at relative bulk, first of all, we have to estimate this by imagining a box formed by the engine's main dimensions of length, height width, a box containing all its main engine elements and accessories. Because an engine isn’t shaped like a shoebox this isn’t an exact yardstick, but it’s the best we can do without actually measuring the external displacement of each engine. I grouped power units for which such dimensions are available and which lend themselves to a comparison.
My analysis showed that the Wankel unit has a small but significant edge at the 110-bhp level and a much larger advantage—thanks also to exceptionally compact design by Curtiss-Wright— in the 180-bhp class. This supports the assertion by Mazda’s Kenichi Yamamoto that “the rotary engine tends to become increasingly compact for larger output.”
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