Peter Lynn ARCHIVE
Stirling engines operate on a closed cycle where the energy is supplied as a continuous flame that heats hot end heat exchangers (top finned components). The energy is transferred to the internal pressurised gas (working fluid) in the engine, raising the gas pressure. (Internal combustion engines intermittently draw air and fuel into the cylinder, burns it, and then exhausts the combustion products.)
For the WhisperGenTM the piston motion moves the hot gas to the cold heat exchangers (finned aluminium component) where the waste heat is transferred to water that meets the home heating needs.
To increase the engine power the working fluid is pressurised at manufacture (nitrogen at 28bar in this case) and the pistons cycle the gas causing cyclic heating and cooling. The electricity grid tied AC WhisperGenTM moves the gas at around 1500 cycles per minute or 1500 rpm of the generator shaft.
The equivalent total swept volume (capacity) of the final model WhisperGenTM is 0.17 litre. Running at between 1500rpm the AC WhisperGen produced 1kw of electricity and the DC WhisperGen 800 watts. The water heating capacity is 8kw from the engine cooling plus an additional 5kw from a boost burner.
The generic WhisperGenTM design uses four axially disposed cylinders with pistons acting on the ‘wobble yoke’ mechanism that converts the linear piston motion/force to rotation/torque of the crankshaft with integral generator. The wobble yoke was Don's patented invention and is mechanically efficient, has a long life, doesn’t need oil sump lubrication, and enables almost perfect balance, all essential for a domestic appliance.
There are good reasons why Stirling engines are not mass produced and most relate to almost insurmountable technical challenges. Here are just a few that the Whisper Tech Ltd. team overcame...
The domestic microCHP market requires a minimum system life of ten years or 30,000 hours. Considering there are only 8,600 hours in a year, proving the wobble yoke life in just a couple of years, to meet production deadlines, was the most difficult challenge. As an aside, this required running more than 50 test engines, making Whisper Tech Ltd. Christchurch’s largest user of LPG resulting in LOTS of heat to dissipate. The company was located on Armagh St. and the Centennial swimming pool was just across the road. A pipe was laid and heat from our engines heated the pool! With the engines feeding the electricity grid its a good example of Combined Heat and Power.
Hot end heat exchangers: four per engine intricately (and expensively) machined from 253MA high-temperature resistant stainless-steel alloy. The internal fins were cut using a newly developed slitting saw process. These components are glowing red hot (~800 C) when the engine is operating and have to last 30,000 hours at more than 28 bar! Good temperature control from the gas fuelled flame was essential!
Free oil in the engine would block the heat exchangers so the piston rings and guides were made from a specially developed carbon filled Teflon that ran dry against a cast iron cylinder with a prescribed honed finish. The wobble yoke caused piston reciprocation with the connecting rod lower bearing following near straight line motion this meant no pivot (wrist pin) is required at the piston end and just a flexing of the connecting rod is sufficient.
By no means the least of the team’s challenges was obtaining EU approval for a kitchen or basement located gas fired system with 28 bar pressurisation back feeding electricity to the national grid!
Peter Lynn for the Roger Mahan Heritage Centre, July 2025
