PARIS. — After the abortive presentation of his two-stroke, free-piston engine 3½ years ago (AUTOMOTIVE NEWS Oct. 10, 1977), Frank Stelzer began rethinking the construction principles and gas flow pattern.

The result is a superbly simple machine, with clear logic in its functional sequence, symmetrical and balanced. None of the attractions of the older version have been lost, and some have been enhanced. It offers multi-fuel capability low specific fuel consumption, low production cost, simplified maintenance and long life. High power density, lightweight, vibrationless running, low noise levels and a compact installation package are also part of the Stelzer equation. This combination of qualities has aroused interest in the Stelzer engine, not only from makers of industrial engines, but also from the automobile industry. Getting useful power for vehicle propulsion from a free-piston engine, however, imposes particular demands on the transmission system.

Free-piston engines have no rotation and no mechanical drivetrain. The basic patents were taken out in 1934 by Raul Pateras Pescara, a Spanish engineer living in Paris. The leading producers today are Sigma, in France, and Alan Muntz & Co., in Great Britain. Their engines are mainly used as gas generators for turbines, in ships or stationary installations, or as air compressors. When Arthur F. Underwood and a team of engineers at the General Motors Research Laboratories put a free-piston engine in an experimental car in 1956 (the 250 HP XP 500), it was used strictly as a gas-producer for the gas turbine that provided the motive power.

Stelzer, on the other hand, is planning two different automobile installations with his engine. Since mechanical transmission would be the most difficult to arrange, he is investigating hybrid-electric drive and a hydrostatic transmission.

Both alternatives pose problems in terms of efficiency, installation space, weight and cost. But the feasibility for either type is not in doubt.

Like the former model the new Stelzer engine has a single moving part, a common piston assembly, which reciprocates at maximum frequencies far beyond the Iimits of rods and cranks. A test engine has been run at 20,000 cycles per minute, and 30,000 may be attainable. Working pistons are attached at each end, protruding beyond the block for easy integration with electric generators or hydraulic pumps. A flatter, bigger-bore piston is mounted in the middle, where a precompression chamber occupies the space between the two working cylinders. The central piston is double-acting, forcing fresh mixture under a slight supercharge into each combustion chamber in alternation. Plug-type valves on the piston crowns open and close the connecting channels, while the piston travel covers and uncovers the exhaust ports which are grouped radially around the cylinders near bottom dead center, giving one-way scavenging. Due to the big bore (5.5 inches) and ultra-short stroke (1.57 inches), each cylinder is fitted with two spark plugs fired by a magneto ignition system. Each cycle (one stroke each way) gives two firing impulses.

When tested by undergraduate engineers at a West German technical university, SteIzer's new 1,231-cc (75 CID) prototype delivered 100 HP at 5,000 cycles per minute. In Stelzer's extrapolation, it would generate about 200 HP at perhaps 12,000 cycles, and more if the frequency can be increased further. The ultimate limit to reciprocating frequency is thought to lie in aerodynamic stall in the inlet ducts and porting. According to Otto-Peter A. BuhIer, an independent engineer who has examined the Stelzer engine, it has a theoretical range from 25 HP to over 1,000 HP. The smallest would be about a foot long, and four inches in width and height; the biggest, about the size of a turbojet of 5,000 pounds thrust.

No production problems are apparent. The working parts are cylindrical and easily machined. No complicated castings are needed, and the total number of parts is small. The piston assembly could even be made as a single piece if the housing were built up from "slices" as in a multi-rotor Wankel engine. No exotic metal parts are needed. The housing couId be made of aluminum when lightness is important, and its form could be adapted to air- or water-cooling, as desired. Experiments with ceramic pistons (by Rosenthal) will be undertaken shortly. They can reduce friction losses, since the pistons themselves serve as bearings - the only bearings in the whole engine. They save weight and offer extended resistance to high temperatures, thereby encouraging exploration of the uppermost frequency range.

For cars, hybrid-elecric drive would require generators oil both sides of the block, with electric motors in the wheel hubs (or in an inboard position on the driveshafts, where they would be carried as sprung weight). Hybrid-electric drive offers the possibility of constant-speed operation (at its most efficient load vs. frequency combination), with surplus power stored in a battery pack. The stored power could be used for operation in emission-critical urban areas with the engine switched off, or as booster-power for faster acceleration. But there are drawbacks, too. Adding two energy-conversion processes would involve severe power losses. And the size and weight of the battery pack makes small-car installation difficult. With regard to hydrostatic transmission, the basic problem is finding or developing hydraulic motors capable of meeting the speed-range requirements of a car. Hydrostatic drive systems are best suited for low-speed, high-torque applications, such as bulldozers and other construction equipment. Mechanical gearing couId perhaps be the final step but, of course, that further complicates the system and adds to the cost.

Despite the difficulties, Stelzer and his backers will go ahead with the car installations. First, however, Stelzer hopes to sell licenses for his engine to manufacturers of heat pumps for housing, irrigation and pipeline pumps and electric generator sets - applications in which no transmission problems are encountered.