Hybrid Turbine

A novel but not new kind of engine is figuring more and more often in engineering literature and bull sessions. Last week the Cleveland Diesel Engine Division of General Motors Corp. gave details about the large (6,000 h.p.) free piston engine that it has built to repower the Liberty Ship William Patterson. Smaller free piston engines are under development by General Motors for passenger cars and trucks. The Ford Motor Co. is deep in free pistons, and is trying them out for autos and farm tractors.

Just Like Steam. The charm of the free piston engine is that it has many of the advantages of the straight gas turbine with none of the main disadvantages. The glamorous turbojet that flies through the air with such wonderful ease is as helpless on a highway as a bat or a hummingbird. Even the workaday turboprop (a gas turbine that delivers power through a shaft, not through a jet of gas) is hard to adapt to ground uses. Chief failings: 1) poor fuel economy, especially at low speed. 2) cost of heat-resistant parts, 3) sluggish response when power is called for.

The free piston engine is a hybrid: a simplified piston engine that develops no direct mechanical power, but delivers high pressure exhaust gas that can be made to run a turbine, just like steam from a boiler (see diagram).

In each horizontal cylinder or "gasifier" (there may be any number of them feeding the same turbine) are two pistons that slide back and forth. When they move together in the center, they compress a charge of air and heat it so hot that fuel sprayed into it burns immediately, as it does in a conventional diesel engine. The explosion heats the air still hotter, raises the pressure and forces the pistons apart. As they move away from each other, they do three things: 1) the large disks on their outer end draw fresh air from the atmosphere into chambers behind them; 2) they compress captive air in a "bounce chamber" at each end of the cylinder; 3) they uncover two sets of ports in the wall of the combustion chamber.

Compressed air previously stored in the compartment surrounding the combustion chamber rushes through the open ports, scavenging out of it the burned gases and pushing them, still at high pressure, into the pipe that leads to the turbine. At last the increasing pressure in the bounce chambers stops the pistons and pushes them back toward the middle of the combustion chamber. Their end disks act as air pumps, raising the pressure of the air in the storage compartment. When the pistons have covered the ports, the air in the combustion chamber is compressed and heated, and the cycle starts over again. The net result of this operation, which may be repeated thousands of times per minute, is a large volume of gas forced through the spinning turbine to create mechanical energy.

Jack-Rabbit Start. Free piston engines are still full of bugs, but their seemingly indirect way of generating power has its points. Since the combustion gases start their work cycle at extremely high pressure and temperature, the thermal efficiency of the engine (the amount of mechanical energy that it gets out of the fuel) can be very good. It has no flywheel, crankshaft or connecting rods. It has many valves to shunt air through the various chambers, but they are all self-operating, and none are exposed to high temperature. The engine can be made to run on almost any combustible liquid, even thick black bunker oil. Since the gases that spin the turbine have been mixed with scavenging air, they are not very hot (800-900DEG F.), so the turbine need not be made of expensive, heat-resistant metals.

For use in automobiles, trucks, etc. the free piston engine has special advantages. The gasifier, its heaviest part, can be placed under the hood while the turbine can be in the rear at the end of a gas pipe. This eliminates the drive shaft that clutters low-slung cars, and it distributes the engine's weight in a desirable way. Unlike straight gas turbines, free piston engines have quick response. The man whose self-confidence is supported by making jackrabbit starts when the traffic light turns green will not suffer deflation if his dream-car of the future has free pistons in it.

This file is automatically generated by a robot program, so reader's discretion is required.