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GM's Amazing New 2-Piston Engine!
March 1965 Science & Mechanics

June 1941 Popular Science
June 1941 Science Popular Science - RF Cafe[Table of Contents]

Wax nostalgic over early technology. See articles from Popular Science, published 1872 - 2021. All copyrights are hereby acknowledged.


GM's Amazing New 2-Piston Engine!

This whisper-quiet power plant burns any kind of fuel and tops the efficiency of a diesel engine!

By Paul Weissler

It's a laboratory engine today, still undergoing development, but tomorrow it may power cars, trucks, boats, and spaceships!

Theoretical Stirling combustion cycle. See text for explanation of each of four steps.

A new whisper-quiet engine, more efficient than any in use today, and capable of running on anything that burns, has been developed by General Motors and proved reliable by the U.S. Army. The engine, called the Stirling Thermal, is as new as tomorrow, but its operating principles were virtually ancient history when Otto was designing the gasoline engine.

Robert Stirling, a Scottish minister, designed the engine 140 years ago. Although the engine was fantastically efficient "on paper," the low state of the engineering art made it a disappointment in practice, and it never really made the grade.

But a modern understanding of thermodynamics (the relationship of heat to work) and increased knowledge of metallurgy have created a brand-new version. GM envisions the modem Stirling in military land vehicles, submarines and even space ships. Other uses include trucks and boats, and a late report from Detroit indicates that serious consideration is being given to using the Stirling in a production passenger car.

Actual combustion cycle after engineering compromises have produced a practical engine.

The term "efficiency" referred to is "thermal efficiency" - the ability of an engine to convert the heat energy in the fuel into mechanical work ... more simply, rate of fuel consumption.

The Stirling is an external combustion engine. Unlike a gasoline or diesel engine, the fuel is burned outside the cylinder, in a burner, which means it could be designed to run on anything from coal to Farmer Brown's applejack. The heat of combustion is carried from the burner to heating tubes, and from the heating tubes to the cylinder. The cylinder is a sealed section containing a twin-piston arrangement plus a gas (usually hydrogen or helium) under pressure.

The heat from the burner combines with stored heat in a regenerator (heat exchanger) to push down on one of the cylinder's pistons to develop power. The same hydrogen or helium gas is used over and over, being heated and cooled in different parts of the cylinder. Because the gas is re-used the engine is referred to as a "closed cycle" type.

Working Parts of the Stirling. Top piston is "displacer"; lower is the "power" piston.


The Stirling story starts with the theoretical, or ideal, cycle, and then proceeds to the actual cycle, which represents the result after engineering compromises are made.

Let's look at the Stirling ideal cycle. (See the photos on the second page of this article.) The cylinder contains two pistons, the upper one called a "displacer" and the lower one the "power" piston. The displacer (piston) rod runs through the center of the power piston and its rod, which is hollow. The fit is close, carefully sealed to prevent leakage.

Because the heating elements are at the top of the cylinder, the displacer's motion controls the temperature of the gas. At top dead center (TDC) the displacer has forced the gas between the two pistons, where the temperature is lowest. At bottom dead center on its stroke (BDC), the displacer has forced the gas to the top of the cylinder, where the heating elements adjoin.

Note there are no valves or lifters to make noise, and no ignition system to fail and cause misfire. These two factors help account for the remarkably quiet performance of the Stirling.

In position I the power piston is at BDC and the displacer is at TDC. The gas is in the cold space between the two pistons, where it is being cooled.

From position I to II, the power piston moves up to compress the gas without raising its temperature (the heat generated by compression is rejected to a cooling system to keep the temperature constant).

In the next step, II to III, the displacer moves from TDC to BDC, while the power piston is held still. The downward movement of the displacer forces the gas to the top of the cylinder, where it absorbs heat from the regenerator.

From III to IV, the regenerator-heated gas absorbs additional heat from the burner and expands, forcing the power piston down. (The temperature again remains constant. Heat from the burner is absorbed at a rate that converts all of the heat energy into work on the top of the power piston.)

From IV back to I, the displacer moves upward, forcing the gas into the space be-tween the pistons for cooling. The heat in the gas is rejected to the regenerator for temporary storage.

Rhombic Drive transmits power from reciprocating piston to crankshaft assembly and then to a pair of timing gears to convert the oscillating motion of the drive to a rotary.

Note that heat is rejected to the cooling system only during the compression stroke, at which time the gas is at its lowest temperature (it's just gone through the cooling process IV to I.) Note also that heat is added to the gas by the fuel burner only after the regenerator has heated the gas to the maximum temperature. Theoretically all the heat supplied by the burner is converted into a force pushing down on the power piston, because the gas already is at maximum temperature, thanks to the regenerator.

In such an engine, heat from the burner is being added at the maximum temperature and rejected at the lowest temperature, and the temperature is kept constant in each situation. In thermodynamics, this is exactly what happens in the "Carnot cycle," a strictly theoretical concept for a heat engine. Although it would be practically impossible to build an engine based on the Carnot cycle, it does serve as a basis for comparison of engine efficiencies, for the Carnot cycle is the "ultimate."

A Carnot cycle engine operating between 2040° and 40° would have an efficiency rating of 80%. Because the Stirling engine has the same heat addition and rejection characteristics, it would too.

Theoretical cycles are nice, and so are theoretical efficiencies, but the average actual gasoline engine operates at less than 25% efficiency, though its typical theoretical efficiency is better than 50%. Yes, we lose something in building an actual engine, but with the Stirling, the potential is the highest, so we're left with a higher efficiency than the gasoline engine even after a host of necessary compromises.

The G M Stirling's efficiency is as high as 39%, running on cheap diesel fuel - and this engine is far from fully developed! That 39% is a lot less than the 80% of a "paper" engine, but it's even more than the 34.8% of an advanced diesel, which up to now had been our "model" of thermal efficiency.

Now let's see how the remarkable Stirling works in actual practice. First of all, the ideal cycle stops one piston while moving another. You might be able to conceive of a mechanism to do this, but no one has developed one to date, including GM and Stirling himself.

The actual GM Stirling mechanism, "Rhombic Drive," was developed by GM and the N. V. Philips factory of Holland, with which GM has a technical cooperative program (Philips did most of the early development). A rhomboid is a four-sided figure having opposite sides equal and parallel, and adjacent sides of unequal length. As you can see in the photo, Rhombic Drive is more of a square that gets squashed.

Because engine output in a Stirling engine is controlled by the amount of gas in the cylinder, a high-pressure bottle forces hydrogen into the cylinder to increase output, and a valve bleeds hydrogen out to reduce performance. Philips and GM also have developed a pump-and-reservoir setup to provide "accelerator-pedal type response" for use in vehicles. An automatic fuel control keeps maximum temperature constant, regardless of the amount of hydrogen in the cylinder.

This view of the Stirling can be seen at the GM exhibit at the World's Fair in New York. The tank at right is for heat storage.

In the ideal cycle we started with the displacer at TDC (or very near it) and the power piston at BDC. In GM's actual cycle, the power piston is at the bottom, but the displacer is only part of the way up. The displacer moves upward, forcing the gas from the hot roof to the cool spot between the pistons, where the gas gives up heat to both the cooling system and the regenerator.

At the same time, the power piston is coming up on compression, and by the time the displacer is at TDC, the power piston is almost two-thirds of the way up to its own TDC.

Before the power piston can finish compression by reaching its own TDC, the displacer is just starting down again, and on its way down blends into the expansion (power) stroke. During the expansion stroke, both pistons are coming down together. (Compare this with the ideal cycle, in which the displacer was already near the bottom of its stroke.)

The displacer reaches the bottom of its stroke before the power piston does, and starts upward to begin the cooling process (by forcing the gas out of the hot roof). So in effect, the displacer starts the cooling process before the power piston has finished the power stroke. This means that the power stroke is partly dissipated by the beginning of a cooling process.

We encounter "short of ideal" situations throughout the engine's cycle. By stopping the pistons in a theoretical cycle, we could assume perfection in the heating, power and regenerative processes. Because the actual engine can't achieve perfection in these processes, and suffers combustion and friction losses too, efficiency can never reach the ideal.

The Stirling engine has its disadvantages, such as extra cost because of its complex Rhombic Drive and heat exchangers. The engine's weight also is considerable, running about 10 lbs. per horsepower (about the same as the less expensive diesels), but the Stirling still is a laboratory engine that hasn't undergone production-style reduction. And any good production engineer can cut weight better than Metrecal.

An engine that has both GM and the Army on its side has a bright future. And right now someone high in Detroit's executive class is probably saying, "Let's try one of those Stirlings in a car!"




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