"A hot air engine is any heat engine which uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work. In a typical implementation, air is repeatedly heated and cooled in a cylinder and the resulting expansion and contraction is used to move a piston and produce useful mechanical work." Wikipedia - Hot air engine
The engine shown above is a 90 degree Vee twin configuration with cast iron pistons running directly in an aluminium cylinder block / crankcase.
Hot air engines are typically more efficient, cleaner and quieter than steam or internal combustion engines and can be operated by any heat source, including solar energy. This makes them extremely versatile, safe and environmentally friendly.
Originally developed in the 18th and 19th Centuries as an alternative to steam hot air engines are ideal for applications such as small generators, pumps and fans but in the 20th century they were eclipsed by the electric motor. In the 21st century increasing environmental concern and a greater willingness to embrace alternative energy technologies has led to renewed interest in hot air engines.
There are numerous theoretical models explaining the thermodynamic cycles that are employed by hot air engines. The most famous being the Sterling cycle named in honour of the 19th Century Scottish engineer Robert Stirling. Stirlings name is now synonymous with hot air engines but there are other models (also named after famous engineers) such as Ericsson, Carnot, Coleman and Stoddard.
The engine shown here is a 90 degree Vee twin configuration with cast iron pistons running directly in an aluminium cylinder block / crankcase. Enclosed spaces above the pistons are connected with small diameter stainless steel tube. One cylinder has a thin walled Stainless steel hot cap and piston extension. The other a plain piston and aluminium head. The crankshaft runs on ball races, both pistons are connected to the same crankpin. The crankcase is open so the underside of the pistons is at atmospheric pressure at all times.
It is unusual to hear of model hot air engines operating at high RPM, mainly because of mechanical friction that will place a limit. The 90 deg Vee twin configuration has quite good dynamic balance and small air cooled ones will run continuously at 4,000 RPM or more.
The diagram below shows graphically the movement of air inside a v90 twin cylinder hot air engine during one cycle. The diagrams below illustrate one full cycle from zero degrees (top left) follow the sequence in a clockwise direction.
The change in volume (hatched area) can be seen during one cycle, minimum (max compression) occurs at crank position zero degrees to dead centre (top left). Max volume (minimum compression) occurs at 180 degress (bottom right).
Monery Engines is the personal website of Peter Monery, a retired engineer and model engine enthusiast from London, England.
Peter Monery (1933-2014)
Peter Monery was born on 29th December 1933 at 378 Gander Green Lane, North Cheam, Sutton, Surrey. He won a scholarship to Wimbledon Collage at the age of 11.
At 16 he went to work at Bailey Meters and Controls Ltd in Croydon, a subsidiary of Babcock and Wilcox, and completed a 5 year apprenticeship in mechanical engineering.
Remaining at Bailey Meters Peter progressed to section leader designing control systems for power stations and large passenger liners. After 38 years he was made redundant when the Croydon works was closed in 1988.
Moving on to SLE, Specialised Laboratory Equipment Ltd, in Croydon Peter worked on the design of small ventilators for premature babies until he retired in 1998.
In retirement Peter spent his spare time designing and building the small hot air engines seen on this site.
Extract from an article submitted to Model Engineer Magazine published in vol. 190, no. 4192, April 2003.
Observations made while making and running hot air engines
I chose the 90 degree Vee twin configuration with cast iron pistons running directly in an aluminium cylinder block / crankcase. Enclosed spaces above the pistons are connected with small diameter stainless steel tube.
One cylinder has a thin walled Stainless steel hot cap and piston extension. The other a plain piston and aluminium head. The crankshaft runs on ball races, both pistons are connected to the same crankpin. The crankcase is open so the underside of the pistons is at atmospheric pressure at all times.
It is very obvious that when starting the engine, by applying heat and turning the crankshaft, some of the quantity (mass) of enclosed air must escape. The air is at normal temperature and pressure (NTP). If mean air temperature increases, air pressure will increase and push the pistons to bottom dead centre. No amount of crankshaft rotation could bring the air back to NTP because all parts of the engine are at or above ambient temperature.
Less obvious is the need for the quantity of enclosed air to vary according to the amount of heat supplied. A practical indication of this is the tendency for an engine to falter or even stop when heat is abruptly reduced during steady running with a high heat input. Fortunately the best made sliding pistons will leak. The amount of leakage must be insignificant during the period of one cycle (revolution).
When running this type of engine it becomes obvious that it is double acting. Cylinder pressures are therefore above atmospheric pressure on the downward stroke and below atmospheric pressure on the upward stroke. It seems likely that for best running, the above and below values should be similar and piston leakage will achieve this. Assume that heat is increased so that pressure rises above atmospheric, some enclosed air will leak to atmosphere for a period; if heat is decreased so that pressure falls below atmospheric, some external air will move into the enclosed space. If pressures are equal, ‘leakage in’ will equal ‘leakage out’. The quantity of enclosed air therefore adjusts automatically during running to suit the heat input.
It is unusual to hear of model hot air engines operating at high rotational speeds (RPM). I think it is mainly mechanical friction that will place a limit on running speeds. The 90 deg Vee twin configuration has quite good dynamic balance and small air cooled ones will run continuously at 4,000 RPM. One very small engine has a free running speed over l0,000 RPM but would need water cooling and continuous lubrication if run for more than a few minutes at this speed.
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