Quick Dips

Sterling Stirling Research

A former engineering post-graduate at Canterbury University, Dr Donald Clucas, has received international and local recognition for a quiet Stirling engine-driven electricity generator which he developed under the supervision of Dr John Raine.

The Stirling engine is well known for its clean, quiet, efficient, low vibration, low maintenance and multi-fuel operation. Several technical and commercial problems, however, have prevented mass production in the past. It is believed that the new system, which has been manufactured and successfully tested, overcomes these problems and could initiate a significant commercial operation in New Zealand.

The Stirling Engine operates on a closed thermodynamic cycle which repeatedly expands and compresses one mass of gas at different temperatures. It has a number of advantages over conventional engines. Because it operates as a closed cycle and has no valve gear, the engine can run as quietly as a domestic refrigerator. Theoretically the Stirling cycle has the highest thermodynamic efficiency of heat engines, but with current technology overall efficiencies comparable to the diesel engine can be achieved.

Unlike the internal combustion engine, fuel combustion does not occur in the cylinder. This allows virtually any fuel or heat source to be used and enables long engine life. Solar radiation, LPG, CNG, diesel, vegetable oil and biogas are all potential fuels. As the heat source is generally a continuous flame, the combustion process can be accurately controlled to minimise the emission of pollutants. Approximately twice as much heat is directed to the radiator or cooling system, instead of the exhaust, compared to a similar-sized diesel engine. This heat can be utilised in domestic co-generation systems where hot water heating is a useful by-product of generating electricity, offering high overall efficiency.

Dynamically balanced mechanisms for Stirling engines can be produced giving virtually no engine vibration. Driving a Stirling engine with a motor causes it to act as a refrigerator or heat pump. As the working fluid is normally air or helium, ozone-friendly CFC-free heat pumps and refrigerators can be produced -- a popular commercial application internationally. Many Stirling engine designs do not require oil lubricants, hence very clean and often maintenance-free operation is possible.

A demand exists for quieter, user-friendly, omni-fuel and weather-independent means of generating electricity in yachts, caravans and remote homes. The first prototype of the new engine has been developed for this application. Other markets for the device include co-generation units operating on locally available fuels, solar generators, refrigeration and heat pump equipment, and hybrid electric vehicles.

The invention of a new mechanism called a wobble yoke was the most significant development in the project. This mechanism transfers the linear piston motion into rotary motion of the shaft. The wobble yoke offers many benefits, and patent protection for the mechanism is in progress. Other applications for the new wobble yoke include its use in compressors, internal combustion engines, hydraulic pumps and motors, positioning equipment.

A pre-production prototype 200-watt battery charging unit has been successfully designed, manufactured and demonstrated. The engine, which is designed to be scaled up, is now undergoing endurance tests. This first prototype is considered to be the smallest in a range of engines up to 20 kW. A 3-kW electric generator set is currently under development. This system, with waste heat recovery, will be ideally suited to domestic power generation from clean burning natural gas or LPG.

With increasing awareness of the environment, decreasing fossil fuel reserves and the trend towards maintenance-free and quiet operation, it was considered that early development of this technology could have substantial financial gains for local manufacturers. Two companies are currently being incorporated to patent, protect, further develop and market the new technology. Significant international markets have been identified and investigated. Rather than license the design direct to international manufacturers, the owners decided to continue the development for local manufacture.

Throughout the engine design phase, local manufacture was a high priority, so all parts can either be locally purchased or manufactured using conventional engineering machinery. New Zealand companies have been sought to add value to the intellectual property by manufacturing and exporting the product as "turn key" functional units. In taking this approach the designers have increased the potential revenue to New Zealand and assisted employment creation. Continuing research and development of this project at Canterbury University is being funded by Southpower.