General Motors is building and proving out a Stirling engine power unit as part of its contract with the U.S. Department of Energy (DOE) to develop a hybrid propulsion system by 1998.

As a result, the ‘external’ combustion Stirling engine - technology that dates back more than 175 years - could find its way onto the hybrid vehicles of the 21st century.

GM is using the Stirling engine in a series hybrid propulsion configuration to power a generator, which in turn provides electricity to propel the vehicle.

While Stirling technology may be old - the engine was first patented in 1816 by Scottish clergyman Robert Stirling - recent technical achievements have overcome past issues so that a Stirling engine is in contention for future automobiles, the advantages it offers are numerous.

"Stirling engines are very smooth, quiet and - like turbines - are continuous combustion engines so they should produce very low emissions," explained Jerry Skellenger, GM's DOE Hybrid Program technology director. "In addition they can be run on many types of fuel."

Stirlings derive their mechanical power from the expansion of confined hydrogen at high temperature through use of a process known as the ‘Stirling Cycle.’

During the cycle, hydrogen is compressed into a cooled chamber beneath the piston, and then transferred to an externally heated second chamber above an adjacent piston. The heated hydrogen expands rapidly, generating energy used to drive the pistons in sequence.

In the final step of the cycle, the expanded hot hydrogen is returned back to the cold chamber, compressed, and the cycle begins again.

Hydrogen is a working fluid, not a fuel, in the Stirling engine. It never comes in contact with a flame during the Stirling cycle, giving the powerplant the distinction of being an external combustion engine.

What, after all these years, has made the Stirling engine any more attractive than it was 10, 20 or even 50 years ago?

"In the past, the acceleration response of the Stirling was never what we needed it to be and was very complex," explained Skellenger.

"The power output was controlled by changing the overall pressure level in the engine, which made it slow to respond."

Using the Stirling engine in a series hybrid system with a swash plate drive as developed by Stirling Thermal Motors of Ann Arbor, Mich. could overcome these difficulties.

The swash plate - a rotating disc mounted at an angle and attached to the piston - replaces a traditional crank. 

However, Skellenger said, the Stirling - just like the gas turbine, fuel cell, and four-stroke compression-ignited engine - faces its share of challenges.

"Sealing in the hydrogen to keep it from leaking is something we still need to address," Skellenger said. 

"But the real unknown is cost," he said. "Right now, we're crystal balling it because there's no database on these engines in high volume production. "