Hotair Engine Competitions

The 5 cm3 (0.3 in3) engines of Urwick and Collins were built for the first hot-air engine competition, held at the 1977 Model Engineer Exhibition in London. This competition was sponsored by A. N. Clark and promoted by Model Engineer magazine. In light of the increasing interest in model Stirling engines, it was correctly believed that the time was right for an international competition (Chaddock 1976).

Since originality was to be encouraged, the rules were few. The only significant restriction was that power piston displacement be limited to 5 cm (0.3 in3), and the only aim was to produce as much power with that volume as possible.

Seventeen people entered the competition, and the winner. F.. F. Clapham of Bristol, produced an engine beyond the sponsors' wildest expectations. It was pressurized with air at over 6.7 MN/nr (1000 lb per sq in), and it produced 39.4 watts at 900 rpm. This machine was designed and built in 600 hours, and it was the first Stirling engine Clapham had ever built.

C'lapham's engine, shown in Fig. 20.6, was a co-axial piston-displacer design with a pressurized crankcase. Tubular heat exchangers provided almost I22.6cnr (19 sq in) of external heater surface area, and over 45.2 cm' (7 sq in) of cooler area. The crankshaft mechanism was a modified bell-crank type, designed to operate without oil lubrication. The crankcase was run dry to prevent oil contamination and any danger of explosion. A separate crosshead was used, with PTFE pads to absorb side

Fig. 20.6. Model Stirling engine by Clapham (1977). This high-pressure high-performance engine won the first hot-air engine competition for engines with 5 cm1 piston displacement. Output was 95 watts at 2000 revolutions per minute with helium at 80 bar mean pressure.

loads. The piston rings were carbon, backed by 0-rings. A commercial carbon face-seal of the pressure-balanced type was used to seal the crankshaft as it emerged from the crankcase.

Clapham (1977) has described how, after the competition, further modifications were made to this most impressive engine, to increase the power to 55 watts at about 1350 rpm on air at 6.7 MN/nr (1000 lb per sq in) and 95 watts at about 2000 rpm on helium at S MN/nr (1200 lb per sq in).

It is interesting lo compare Gapham's design with the 65 cm1 (4 in3) rhombic of Ross. Both are high-speed engines and both have produced 55 watts on air, yet Clapham's machine has only 1/13 the volume, but 34 times the pressure, of Ross's machine. The Clapham engine has a much higher dead-volume ratio, as would be expected of an engine of such high output per unit volume. The overall dimensions arc comparable, the 5 cm3 (0.3 in3) engine standing about 24.1 cm (9.5 in) tall, the 65 cm3 (4in3) engine being 29.2cm (11.5 in) tall.

Fhe second place engine at the Model Engineer competition was a beautiful rhombic-drive design of Dr. James Senft (1977), of Minot, North Dakota. He had previously built a number of small Stirling engines and (Senft 1976) had earlier written a detailed and extensive article for model engineers on Stirling engine theory. Senft is unusual among model engineers in that he also works professionally on Stirling engines. His engine, shown in Fig. 20.7, was designed to be a reliable and practical low-pressure machine suitable for model purposes. It employed a compact version of the rhombic-drive, with the connecting rod centre length only 2.5 times the crankpin radius. A machined teflon cup with a spring expander sealed the piston: a similar seal was used on the one shaft that emerges from the pressurized crankcase. The displacer was of the clearance type, with no regenerator.

This engine made 4.7 watts at 1260 rpm on air at 0.3 MN/nr (451b per sq in) during the competition. Subsequent tests on helium at 0.4 MN/nr (601b per sq in) produced 12.9 watts at 2920 rpm. The highest speed recorded on the engine is 3860 rmp. This was not the 'no load' speed, as the load then was still S.6 watts.

Other interesting engines in this first competition were described in an article by Chaddock (1977) who, as technical consultant to the Model Engineer, conducted the power tests.

The subsequent 1978 competition was encumbered by considerably more complicated rules (Model Engineer 1977). To enable testing to be carried out within the exhibition hall, various restrictions on fuel tanks, supply pipes, gas fittings, etc. were specified. External flame guards were required so no external flame would be visible while the engines ran. Some sort of rapid extinguishment device was necessary. Pressure was

Pig. 20.7. Mode! Stirling engine by J. Scnft (19771. This engine was sccond in first hot-air engines competition.

limiled to 0.76 MN/rn2 (115 lb per sq in). Helium was permitted, and many potential competitors no doubt felt it would be essential to be competitive.

Although these rules were not unreasonable, they dampened enthusiasm to such a degree that only two builders entered the competition. The winner was F. R. Wilkinson, whose double-acting piston-displacer

engine was similar to Robert Stirling's engine of 1843 for the Dundee foundry. It made 8 watts when pressurized to 0.76 MN/m2 (115 lb per sq in). One interesting feature was a form of isothermalizer in the hot end which is said to have increased power by 2 watts.

The other contestant was Dr. Brian Thomas, who has built many small Stirling engines of imaginative and unusual design, including several with cam activated displacers. His competition engine was a rhombic-drive with an 'external piston'; that is. a piston with a skirt sealing against the outside of the cylinder. This line engine had taken third place in the prior year's competition by producing 2.5 watts at 2000 rpm. For this year. Dr. Thomas had developed a self-pressurizing pump, but various difficulties kept power down to 3.7 watts.

FUTURE ACTIVITY: MODELS AND LARGER ENGINES

Regardless of the future of the competition it seems certain that development of the model Stirling will continue. Ross has recently completed a practical die to produce wax patterns for externally and internally finned heater heads; these have been successfully investment-cast in brass, and in Type 316 stainless steel. Senft and others are planning to incorporate regenerators in their small engines. Thomas is making progress on his self-pressurizing machines, and is also experimenting with a 'stulled' heater, whereby a simple pressed heater can provide considerably extended heat transfer area. New designs continue to appear, such as the Henshall (1977) crank geometry linking two cylinders, each containing a co-axial piston and displaces It seems at least possible that the history of the model engineers in developing flash steam hydroplane engines, or miniature two stroke internal combusion engines, could be repeated with the small Stirlings.

Many of these builders are also very interested in building practical engines of 100 to 800 W (1/8 to I hp) suitable for powering small skiffs or bicycles, perhaps on solid fuel. One builder who has already completed such a project is Morris Bomford < 1975), shown with his Stirling-powered boat in Fig. 20.8. I lis large-volume, slow-turning engine propels the skiff at 1.3 to 1.8 m/s (3 to 4 mph). It is expected there will be much more amateur activity in this area, too, in the near future.

On all these matters, there exists an active written correspondence, not only among the model engineers themselves, but also between them and a number of the independent professionals in the field. Amateurs and professionals alike have in common both the rewards and the frustrations of working in a lield where there are more questions than answers.

GLOSSARY (Terms are defined as they are used in the context of Stirling engines)

Adiabatic compression and expansion: Thermodynamic process of volume, pressure, temperature change, and also adiabatic process change that occurs without heat transfer to or from the system. Beale free-piston Stirling engine: A type of Stirling engine in which the piston and displacer move entirely under the action of fluid forces. There are no connecting mechanisms between the piston and displacer. The load is direct-coupled to the piston.

Clearance space: The minimum volume of the compression and expansion spaces.

Coefficient of performance: The ratio of heat transferred to input work. For a refrigerator the COP = Meat lifted (refrigeration cffcct)/Work supplied. For a heat pump the COP= Meat rejected/Work supplied (i.e. the inverse of thermal efficiency).

Compound working fluid: The working fluid of a Stirling engine that consists of two or more components and which may exist as a liquid, gas. vapour, or dissociated elements.

Compression space: The part of the working space in a Stirling engine where the working fluid is principally concentrated when the total system volume is decreased, the pressure rises, and heat is rejected to the cooling water. In a prime mover the compression space is cooler than the expansion space. In a refrigerator or heat pump the compression space is warmer than the expansion space.

ComfiJnf pressure process: Thermodynamic heating or cooling process that occurs at constant pressure. lTiis may or may not be regenerative. Constant temperature process: Thermodynamic heating or cooling process that occurs at constant temperature.

Constant volume process: Thermodynamic heating or cooling process that occurs at constant volume. This may or may not be regenerative. Cooler: The heat exchanger provided to facilitate the transfer of thermal energy from the working fluid to the cooling medium, water, air. or some other fluid.

Crank drive: One form of kinematic drive consisting of a crank and connecting rod used to convert reciprocating to rotary motion and to convey power between pistons and drive shaft.

Cryogcnerator: A cooling engine capable of achieving refrigeration at cryogenic temperatures (less than 100 K or 180°R).

Dead-volume ratio: That part of the total working spacc not included in the variable volumes of the expansion and compression spaces, expressed in terms of the variable volume of the expansion space. Direct /leafing; A system in which the hot products of combustion pass directly over the heater tubes in which the working fluid flows so that heat is transferred directly from the combustion products to the heater tube walls and hence to the working fluid.

Discontinuous piston motion: The non-sinusoidal motion of the piston and displacers required to achieve the necessary volume variations of the idealized thermodynamic cycles.

Displaccr: A lightweight structural reciprocating element in a Stirling engine characterized by a large temperature difference but a negligible pressure difference between the upper arid lower transverse faces. Double-acting engines: A family of Stirling engines having a single reciprocating element per thermodynamic system. There is a minimum number of two cylinders but no maximum number. Duplex Stirling engine: Two Stirling engines arranged so that one operating as a prime mover receives heat at a high temperature and produces work to drive the second Stirling engine acting as a cooling engine refrigerator or heat pump.

Emission products: The constituents of the exhaust products of an engine. With fossil fuel combustion these are principally water vapour, unburned hydrocarbon, carbon monoxide, nitrogen, and oxides of nitrogen. Ericsson cycle: An idealized thermodynamic cycle consisting of isothermal compression and expansion processes at different temperatures bounded by constant pressure regenerative processes. Exhaust-gfl.v heat exchanger: See Regenerative cycle. Exhaust-gas recirculation: A system whereby a sizeable fraction of the exhaust gas is circulated back through the combustion system. Used to minimize the quantity of oxides of nitrogen in the exhaust emissions. Expansion space: The variable volume of the working space in a Stirling engine where the working fluid is principally concentrated when the total system volume is increased, the pressure falls and heat is absorbed. In a prime mover, the expansion space is hotter than the compression space. In a refrigerator or heat pump the expansion space is cooler than the compression space.

Finkelstein adiabatic cycle: An idealized thermodynamic cycle for Stirling engines with no heat transfer in the compression and expansion spaces and infinite rates of heat transfer in the heat exchangers. Free-displacer engines: A form of Ericsson regenerative engine (Bush type) where the displaccr moves under the action of fluid forces. Used principally as a pressure generator or pump.

Freezer: The heat exchanger provided in a refrigerator or heat pump to facilitate the transfer of heat lo Ihe working fluid from an external low temperature source.

Harmonic piston motion: The near sinusoidal motion of the pistons and displacers used in practical Stirling engines.

Heat pipe: A device used in an indirect heating system in which an intermediate fluid is used to transfer heat from an external energy source to the working fluid. Usually the intermediate fluid (a liquid metal, usually sodium) is evaporated at the thermal inlet and condenses at the thermal outlet. Large rates of heat transfer can be effected with minimal temperature differences. Furthermore, large differences in the rates of heat transfer can be achieved. This provides the possibility for large areas for heat transfer from the combustion products and small areas for heat transfer to heat tubes. 'Hot spots' on the heater tubes are avoided and a significant improvement in heater temperature and cycle efficiency can be gained.

Heat pump: A machine driven from external power supply absorbing heat at ambient temperature and rejecting the heat at some higher temperature.

Heater: The heat exchanger provided in a prime mover to facilitate the transfer of thermal energy from an external source to the working fluid. Hybrid free - displacerf crank - con trolled piston engine: A form of Stirling engine where the reciprocating piston has kinematic coupling to a rotating shaft but the displacer is oscillated under the action of fluid forces. Indirect heating: A system in which thermal energy from an outside source heats an intermediate fluid (sodium, say) which conveys the energy to the heater tubes and hence to the working fluid (see Heat pipe), fsentropic process: Thermodynamic process of volume, pressure, and temperature change that takes place at constant entropy. Isobaric process: See Constant pressure process. Isometric process: See Constant volume process.

Isothermal compression and expansion: The process of volume and pressure change that occurs without change in the temperature of the system. Isothermal process: See Constant temperature process. Kinematic drive: A system of cranks, connecting rods, levers or swash-plates used to regulate and control the reciprocating motion of pistons or displacers and to convey power between the pistons and drive shafts. Metallurgical limit: The maximum temperature of operation for the materials used in the hot spaces of the engine.

Multifuel capacity: The ability of an engine to operate on various fuels or energy sources.

Phase angle: The angle by which volume variations in the expansion space lead those in the compression space.

Piston: A heavy structural reciprocating element of a Stirling engine characterized by a large pressure difference but a negligible temperature difference between the upper and lower transverse faces. Porosity: The total void volume expressed as a fraction of the volume envelope of the porous solid (frequently expressed also as a percentage).

Pressure drop. Presstire loss: The difference in pressure that arises when Huid flows through a duct or heat exchanger because of aerodynamic-friction effects.

Pressure excursion: 1'he range of variation of the cyclical pressure change of the working fluid in the cylinder.

Pressure ratio: The ratio of the maximum and minimum pressures of the working fluid.

Prime mover: A Stirling engine used to produce mechanical work from heat supplied at high temperatures.

Rallis cycle: An idealized thermodynamic cycle with regenerative processes that occur partly at constant volume and partly at constant pressure. The process of compression and expansion may occur isothermally or adiabatically.

Recuperator (Recuperative heat exchanger): A form of heat exchanger (tube and shell, or tinned tube) with separate channels for the hot and cold fluids. Usually the flow is continuous and constant in the channels. Regenerative annulus: A narrow annular gap between the displacer and cylinder through which the working fluid passes en route from the expansion or compression spaces. There is a temperature difference along the length of the annulus and as the gas passes through, a measure of regenerative heat exchange is accomplished.

Regenerative cycle: A thermodynamic cycle in which some attempt is made to utilize the heat in the fluid being rejected from the cycle at low temperatures to heat the incoming fluid and so reduce the amount of 'new' heat required and hence improve the efficiency of the cycle. The regenerative action may take place periodically as in the Stirling engine or continuously as in the Brayton-cycle gas turbine. In the latter case the heat transfer unit which accomplishes the regenerative action may be cither a regenerative or a recuperative heat exchanger. Great care must be exercised to avoid confusion when discussing exhaust-gas heat exchangers for regenerative thermodynamic cycles.

Regenerative matrix: A porous volume of finely-divided material (usually metallic) contained in the working space between the compression and expansion spaces. It acts as a reservoir of thermal energy. Regenerator (Regenerative heat exchanger): A form of heat exchanger consisting of a porous solid mass with a single set of How passages through which pass periodic, alternate flows of hot and cold fluids.

Regulation: The process of temperature or power control used to regulate the output of a Stirling engine.

Reidingcr cycle: Generalized thermodynamic ideal cycle with isothermal compression and expansion processes at different temperatures bounded by regenerative processes of any nature.

Rhombic drive: A special kinematic drive for Stirling engines which regulates the motion of the piston and displacer in single-acting engines. It is possible to achieve perfect dynamic balance while operating the reciprocating elements at the required phase difference. There are no side forces on the cylinder walls.

Roll-sock seal: A rolling diaphragm seal developed by Philips for containing the working fluid in the working space.

Schmidt cycle: An idealized thermodynamic cycle for Stirling engines with sinusoidal volume variation of the isothermal compression and expansion spaces at different temperatures.

Single-acting engine: A family of Stirling engines with two reciprocating elements per thermodynamic system.

Space power system: An energy conversion device used to provide power for spacecraft.

Stirling cycle: An idealized thermodynamic cycle consisting of isothermal compression and expansion processes at different temperatures bounded by constant volume regenerative processes.

Swash-plate drive: A system used in double-acting Siemens-type Stirling engines for regulating the motion of the displacer-pistons and transmitting power to the drive shaft. The pistons are connected to an inclined disc on a rotating shaft which causes the pistons to reciprocate as the disc-rotates.

Swept-volume ratio: The volume variation in the compression space expressed in terms of the volume variation in the expansion space. Temperature ratio: The ratio of the temperatures of the working fluid in the compression and expansion space.

Thermal efficiency: The fraction of total heat supplied that is converted to useful work.

Total-energy system: An ensemble of machinery receiving a single external energy supply that is capable of providing all the utility needs of a hospital, or residential or commercial building. Total working space: See working space.

Two-phase, two-component working fluid: See Compound working fluid. Undenvater power system: An energy conversion device used to provide power for underwater purposes.

Void volume: The total volume of the void spaces in the working space of a Stirling engine including the porous volume of the regenerator and the associated heat exchangers and connecting ducts or ports.

Volume compression rutin: The ratio of the maximum and minimum volumes of the total working space. Wobble-plate drive: See Swash-plate drive.

Work done: The work done by or on the working fluid during a change in volume.

Working fluid: The gas, liquid or vapour which experiences periodic compression and expansion at different temperatures in the working space of a Stirling engine.

Working space: The ensemble of variable volumes and constant volumes comprising the Stirling engine system, including an expansion space, a compression space, void volumes of the regenerator, heater, cooler, and the volumes of clearance spaces and connecting ducts or ports.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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