The Rotary-Engine Motorcycle—Some Pros And Cons
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THE ROTARY-ENGINE MOTORCYCLE -SOME PROS AND CONS
THE QUESTION OF who will market the first rotary engined motorcycle has been answered, and that honor belongs to Suzuki. Theirs, called the RE5, should be available in dealers' showrooms across the country by the time you read this.
But can the bike compete with conventional designs in terms of reliability, economy and performance? What are the pros and cons? Read on, because after spending a day in class, learning rotary terminology and design, and after riding an RE5 some 300 miles, I have some answers.
Paramount in everyone’s mind is the question of reliability, mostly because of widely publicized strengths and shortcomings of the rotary-engined Mazda automobile.
The major problem has been poor rotor tip seal life. There are two main causes. More heat than is customary in internal combustion engines is one. The other concerns velocity. The rotor tip seal travels much faster along the chamber wall than conventional piston rings do.
One solution is to go to a twin-rotor design. That allows both a large displacement and a smaller rotor. The smaller rotor in turn has less tip velocity at any given rpm than a larger one. Consequently, less sophisticated and less expensive materials can be used in construction of the engine.
The other solution, or if you will, the Suzuki solution, is to dive into research and develop materials that have good wear characteristics at both high speed and temperature. Suzuki began by plating the rotor chamber. Then, complementing this, they made the apex seals from “ferro-tic,” a combination of standard cast-iron alloy and titanium. Add to this a modified apex seal tip radius, which moves the seal in and out in its groove (to prevent carbon build-up and subsequent sticking), and you have a cure for seal life.
Temperature differences around the rotary chamber, severe enough to cause warpage of the engine cases, have been another stumbling block. You see, in a rotary, intake, compression, combustion and exhaust occur in separate areas of the rotor chamber. Consequently, the combustion side is continually heated at the same time the opposite or intake side is cooled by the incoming fuel charge. In a short time, the temperature difference becomes too severe, cases warp, and engines fail.
What Suzuki decided upon is an ingenious system of water cooling/heating. Water from a radiator passes first to the area of combustion, where it carries away heat and itself becomes heated. Next, it passes to the area of exhaust, the second hottest spot in the rotor chamber. Again, cooling is the intended function.
At this point in the cycle, the water is hot, hot enough for heating, and it is passed to the intake side to raise the temperature. Once in the area, a portion is bled off to the
radiator. The remainder of the water is held over in complicated passageways long enough for maximum heat transfer. Then it returns to the radiator and the cycle is complete.
Lastly, in the interest of reliability, there are two independent oil systems. The primary, or crankcase, system pressure feeds the main rotor bearing (a plain bearing), the eccentric shaft bearings, etc. Oil pressure is 100 psi. Oil from this system also passes through the rotor itself for additional cooling.
In addition, there is a total-loss oiling system to lubricate the rotor tips. It is similar to a two-stroke autolube system in that it injects fresh oil into the intake tract where it mixes with gasoline at a ratio of 100 parts gas to one part oil.
So far, the systems I’ve discussed have proven more than adequate. Test engines, run for 50,000 miles, have remained well within the tolerance limits, and that is more than can be said for most conventional bike power plants!
As far as economy goes, the rotary uses more gas than 750-900cc four-strokes and less gas than 500-750 twostrokes. On the 300-mile ride I mentioned earlier, speed was kept high to test reliability. The approximate norm was 80 mph with frequent bursts up to 100. Under these abnormal conditions, a low of 31, and a high of 36 miles per gallon, was recorded. Dropping the speed to the 55 to 70 mph range should yield 40 mpg, and that’s about average for a bike of the RE5’s size and performance.
Performance of the RE5 will evoke mixed emotions, depending on whether you value torque or horsepower. And before you even begin to make a judgment, forget everything you have heard about the Mazda, it needs considerable rpm to produce any power.
The RE5 is exactly the opposite. Maximum torque (54.9 ft.-lb.) is developed at 3500 rpm, and from that point on the motor pulls like a tractor. Consider this. Place two rotary Suzukis side by side on a straight stretch of road with riders of equal weight. Have one rider grab full throttle from 3500 rpm in fifth gear. Let the other rider start in third and shift at maximum rpm. The difference in acceleration is only one bike length with the nod going to the rider who was allowed to shift. Try this on any other brand of bike and the non-shifting rider wouldn’t have a chance!
Compare the acceleration of the RE5 with that of other brands; it is not the equal of a Kawasaki Zl. But it will stay with a 900 BMW T and a 750 Honda Four. Its top speed of 110 mph (indicated on flat ground), is slightly less than that of all the opposition mentioned.
On winding roads, the RE5 will surprise you. It has good stability until pushed to the limit; and it has better ground clearance (in turns) than any big-bore roadster we’ve tested recently. It also has the right kind of power for coming out of corners with the gas all the way on. In this category then, it’s more stable than a Honda 750, is easier to ride than a Zl Kawasaki, but doesn’t offer the control on rough roads that a BMW with softer suspension does.
One thing the RE5 is, though, is unique in terms of smoothness. Out on the highway, there is simply nothing that can equal its lack of vibration. And for that reason alone, the rotary Suzuki must be rated one of the most promising designs to reach motorcycle production in quite some time.
