Despite the complaints we often hear about the “outdated” technology of opposed-piston engines, the industry has found nothing decisively better with which to propel smaller airplanes. Not for lack of trying: The history of small aircraft engines—in fact, of engines in general—is a freaks’ graveyard. There have been barrel-shaped engines, spherical engines, cubical engines and disk-shaped engines, steam engines, perpetual-motion engines, engines with flaps and vanes and sliding and spinning parts in every arrangement that human ingenuity could devise and spatial visualization conceive. Invariably, their inventors asserted that they would be superior to existing engines; almost as invariably, they died in infancy.
But hope springs eternal.
I received from a friend a copy of a 1997 patent for a novel type of engine. The inventor of the device was a John Whurr—a name so onomatopoetic as to arouse suspicion that it is itself an invention—and the assignee was the mighty Rolls-Royce corporation.
In cutaway profile, the engine appears to be a medium-bypass turbofan, with the characteristically large shroud surrounding a fan and, behind it, a schematic turbojet, with a multistage axial compressor, a combustor and an exhaust-driven turbine. On closer inspection, however, the confusing cluster of rectangles where the combustors ought to be turns out to represent a three-rotor Wankel rotary engine.
Paddling our way through the quaint jargon of patent attorneys, we learn that this is a two-shaft engine, with one shaft “drivingly connecting” the turbine and the fan, and the second shaft “drivingly interconnecting said at least one rotary internal combustion engine and said air compressor.” In other words, the Wankel engine drives the compressor, which supercharges it; the exhaust from the Wankel drives the turbine, which in turn drives the fan, propelling the airplane.
This document arrived at the beginning of April, and at first I took it for a joke. How could the Wankel possibly have the required mass flow? It’s not for nothing that intermittent combustion engines have small air intakes and jets have huge ones. For that matter, how could the entire output of the rotary engine be required to compress its own induction air? But further investigation led to a NASA technical memorandum from 1976 comparing just such an engine with a turbofan as potential powerplants for a subsonic transport. The main parameters of the comparison—which was theoretical; no metal was harmed in its creation—were limited to fuel consumption and engine weight. The idea underlying both this study and the Rolls-Royce patent is that the intermittent-combustion engine is more efficient than the burner cans of a conventional turbofan, and so the rotary engine would need less fuel to spin the fan and compressor than the turbofan does. On the other hand, the rotary engine is heavier than a bunch of burner cans, and so the fuel saved on a given trip must make up for the cost of hauling around the extra engine weight.
NASA’s 1976 conclusion—it should be emphasized that turbofans have evolved considerably since then—was that the compound engine’s advantage depended heavily upon limiting the rotary’s cooling losses. In no case was the compound engine less efficient than the turbofan, but only with special insulation and high-temperature materials did the Wankel reduce fuel consumption by as much as 8 percent.
Strange as this contraption appears to be, it actually has a lot in common with your ordinary turbocharged piston engine. The relative sizes of the components are different, and so is the arrangement of what is “drivingly connected” to what, but the building blocks are the same. A turbocharger consists of a turbine and a compressor on a common shaft; the turbine captures energy from the exhaust stream and uses it to compress the induction air. In this case, the engine drives the propeller; in the rotary-turbofan hybrid, the turbine drives the propeller (called “fan,” but it’s the same thing: a bunch of spinning wings producing forward lift) while the engine drives the compressor. But it doesn’t really matter what drives what, any more than it matters, when you’re building a house, who hammers, who saws and who cleans up. A certain amount of work needs to be done, and three workers are there to do it.
In the aftermath of World War II, Curtiss-Wright set out to adapt its R-3350 engine, which had powered the B-29 bomber, to airline use by reducing its fuel consumption. Rearranging the same three basic components as we have already seen, they placed turbines in the three exhaust collectors and “drivingly connected” these turbines to the crankshaft, which itself drove a mechanical supercharger. The arrangement is called “turbo-compound,” and has been used ever since on other engines as well, notably in diesel trucks, although with a turbocharger rather than a mechanical supercharger.
The results of the R-3350 modification are variously reported. Several hundred horsepower were recaptured from the exhaust stream that would have otherwise simply disappeared overboard, and the specific fuel consumption dropped at least to 0.4 pounds per horsepower per hour, or maybe 0.38, depending whom you ask. Anyway, it was a notable improvement, although at some cost in reliability. Jesting mechanics translated the initials PRT, by which Curtiss-Wright meant “power recovery turbine,” as “parts recovery turbine,” because of the number of valve bits that turned up in them. I don’t know why the PRTs would have affected valve life—maybe back pressure, but I doubt it—but it is easy to see how the PRTs themselves could have been affected by flying chunks of Inconel.
Early in the 1990s, NASA took an interest in rotary engines and commissioned several studies of a high-performance multifuel engine design called Score, for “Stratified Charge Omnivorous Rotary Engine.” The target output was in the 350 to 400 hp range, with a very high critical altitude. Such an engine would compete with a small turboprop, with much lower acquisition cost and fuel consumption.
The specific fuel consumption of rotaries is unfortunately about 10 percent higher than that of comparable reciprocating engines, and one of the aims of the Score project was to see if it could be brought down to a diesel-like 0.34 pounds per horsepower-hour on jet-A. John Deere, which then held the U.S. rights to the Wankel engine, did a theoretical study to find the optimum combination of elements for a hypothetical twin-engine airplane operating above FL 300. Turbo-compounding, with the power recovery turbine placed upstream of the turbocharger, was found to be essential, contributing from 40 to as much as 80 additional horsepower for the same fuel flow.
A year later, John Deere washed its hands of the rotary business.
Well, that’s how it goes with engines. An idea may have great appeal to mechanical engineers, but it must enthrall the corporation’s MBAs if it is to survive in the thin air of the top floor. As the poet Thomas Gray said:
