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GM Stir-Lec. Hybrid prototype with the body of the Opel Kadett-B Coupé

ER Classics Desktop 2022

During the XNUMXs, General Motors developed many hybrid prototypes. At the end of the Sixties, this resulted in the special GM Stir-Lec I and II. Thanks in part to the application of the Stirling engine, they were unique in their technical kind. The Stir-Lecs did not make it to the production stage. However, the principles of that time still offer perspective in a modern sense for the issues of today and tomorrow. 

As the basis for the Stir-Lec hybrid, GM took the Kadett-B Coupé (Type 95, specially composed for the American market, based on the Coupé F). And although there was a variety of (also smaller) combustion engines on the Opel shelves alone, the engineers opted for the use of a small, less than 10 HP, rear-mounted and environmentally friendly Stirling engine.


Stirling engine

Put very simply: a Stirling engine - a concept that has existed since 1816 - works by expanding and contracting gases within a closed circuit. This is heated from an external combustion chamber, which is suitable for various heat sources due to its placement. A regenerator cools heated air or gas and reheats the cold air. A displacement mechanism ensures that the air or gas is transported to a cold and a hot space. When the air or gas expands, pressure is created, causing a power piston to move and the engine to do its work. Finally, broadly speaking, there are two main types of Stirling engines. The Alpha variant consists of two cylinders (one hot and one cold), the Beta variant consists of one cylinder with a hot and cold compartment.

Multiple external heat sources, closed circuit hydrogen

When developing the Stir-Lec, GM's engineers opted for the Beta-type Stirling engine, which includes a cold and warm compartment. The cooling of the cold compartment took place via water cooling channels with water that was cooled via a traditional cooling cycle with a radiator (and possibly fan). The external combustion chamber was responsible for the heat generation within the warm section. Kerosene, unleaded petrol or diesel was heated in it (optionally). A hot air fan supplied warm air to the external combustion chamber. There the combustion took place thanks to a balanced fuel/air ratio and that heat therefore ended up in the hot part of the closed circuit.

Closed circuit suitable for helium

The working fluid in the closed circuit was initially helium, which was stored in a reservoir above the rear axle. The expansion of the working fluid caused pressure. And that enabled the GM Stir-Lec engine to charge the fourteen lead-acid batteries via an alternator. These 12 Volt batteries found a place in the front of the Kadett body.

Other applications in the GM Stir-Lec

The engineers also installed a small electric motor (a three-phase induction motor). This drew the power from the batteries and provided the drive. This was transferred to the rear wheels via an automatic transmission (gear 3:1) and a differential. In addition, the necessary other items - such as a DC/AC converter and control systems - were given a place in the prototype. The Americans chose this set-up for a reason. The Stirling engine was suitable for various heat sources/fuels because of its external combustion chamber. In addition, the engine relieved the environment, because, partly due to the closed gas/air circuit, less external fuel was needed. As a result, fewer emissions were released.

Some cons

However, it was expensive to build a good Stirling engine (applied peripherals) and the all-important sealing of the closed circuit showed vulnerabilities. The practical impossibility of developing rapid acceleration and keeping the power modest due to the prevention of overheating also spoke less in favor of this ECE engine type.

GM proudly presented an advertisement in August 1969, with a cross-section of the GM Stir-Lec 1 ©GM Company

Nice operation, nice range at lower speeds.

But the operation was virtually silent, and the efficiency of this technique was high. Converted, the Stir-Lec was more economical than a comparable car with an internal combustion engine. The Stir-Lec engine ran at a constant speed of 2.800 revolutions per minute. The fourteen batteries, in combination with the maximum fuel/heat source supply of 18 liters, provided a range of 250 to 350 kilometers at a speed of 50 kilometers per hour. When the Stir-Lec was driven at top speed (90 kilometers per hour), the range decreased considerably to a value of 50 to 60 kilometers. The range based on fully electric driving (so only on the batteries) was about 40 kilometers. In addition, the car - partly due to the placement of the batteries - carried a weight of 1.550 kilograms.

Proud, but never in series production

General Motors was proud of the GM Stir-Lec I. The American concern developed an advertisement for the first Stir-Lec with the title “An electric car that makes its own electricity”. But General Motors had not yet finished developing the Stir-Lec.

The GM Stir-Lec II

There was a Stir-Lec II, which relied more on peripherals that worked according to the direct current principle and more compact, simpler operating electronics. The reduction gearbox disappeared in favor of a friction reduction gearbox, and the three-phase induction motor was replaced by a direct current electric motor. And the closed circuit of the Stirling engine was now filled with hydrogen, instead of helium. The Stir-Lec II was slightly heavier, and slightly faster. In terms of range, it kept pace with the Stir-Lec I. The Stir-Lec prototypes never went into series production. The high costs of production and the associated potential high purchase price reasons and the demand for practical space associated with the technology were important reasons. In the latter case, the applications were much less compact than today.

Provident concept

But the usefulness of the GM Stir-Lec was foreseeable. Early on, the concept formulated an insight into environmental considerations, multiple use of heat sources and the realization of fuel savings. Decades later, engineers can still use that insight in modernized variants as a starting point for low-emission technology and fuel economy. It indicates that more than fifty years ago there was little wrong with the basic thinking behind the GM Stir-Lec prototypes.

3 Comments

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  1. All this at a time when the electric motor was not nearly as economical as it is today, and batteries were not a fraction of the li-ion batteries of today.
    Not to mention the driving regulations.
    At that time, many electric vehicles, such as those of the baker who drove his Spijkstaal steel dog through the street, consumed just as much at low speeds as they did at high speeds.
    Because at low speeds, part of the power was simply consumed by a kind of incandescent coil that converts excess power into heat.

    Google a HaWe electric car, they still use the same principle

  2. If this technique had received as much attention over 50 years as its two-stroke, four-stroke petrol and diesel siblings, then we probably all drove today with this piece of beautiful technology instead of the politically pressed e-cars.

  3. In any case, looks much nicer than today's Prius pudding buns. And related items 🤢 And with only 1550 kg, thanks to the lead-acid batteries, not even significantly heavier.

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