| Hot Air and other External Combustion Engines |
| Transferator Engines - An Idea from the 1860s reworked for the 21st Century. |
| Every so often an interesting design for a hot-air engine comes along and catches my imagination. This is a variation of the Classic Hot-Air or Stirling Cycle Engine - given an update by members of the Stirling Engine Society in 2003/2004. The use of thin gauge stainless steel containers for the hot-end heat exchanger allows this compact engine to be re-created successfully using modern materials. It is expected that more prototypes will appear by the end of 2004 including those with up to 5" / 127 mm diameter pistons. These engines are scalable for use in anything from model boats to full size converted rowing skiffs. Simple to construct and incorporating improved heat exchangers to make it more efficient on lower temperature wood or biomass fires. The transferator engine lends itself to being easily constructed from stainless steel cooking containers and may well have applications for small scale water pumping and electricity generation in the Developing World. |
| Transferator Engines New August 2004 I am currently working on a variant of the hot-air engine - which is called the Transferator Engine. It was first proposed in 1861 by M. Laubereau of Paris, then later patented in the USA and appeared commercially in the form of an electrically heated engine - made by Hotpoint in the 1920s used for animated shop-window displays. More recently it has applied to the propulsion of model boats by John Bourne of Hampshire. John's Article can be found from the link on the right. In 2003/2004 several members of the Stirling Engine Society have built Transferator engines, and are now characterising their performance. Transferator engines are by their construction a low compression ratio engine, and thus fall into the category of Medium Temperature difference engines. They will run well on the lower temperratures found from wood fires - and do not require red heat for them to run. The transferator engine is similar to a conventional beta type hot-air engine, but uses an open cup to transfer the internal air from the hot end of the engine to the cold end rather than the conventional displacer. The power cylinder and cooler are packaged into the open cup, and this helps to reduce the height of the engine - making it very compact. - see photo opposite. |
| A 2 cylinder experimental Transferator Engine made by Julian Wood of Sterling Stirling - shown at the Thames Traditional Boat Rally July 2004. The hot cap and transferator are made from low cost stainless steel storage jars. The hot cap is about 100mm diameter and the power cylinder is 50mm bore. |
| Twenty Five Years of the Transferator Engine - a Word article by John Bourne |
| John Steele's Experimental Transferator Engine This engine will run at 330rpm from the heat from a 200W electric bulb. |
| How the Transferator Engine works With reference to the photo above of Julian Wood's engine - which is an experimental engine with a 4" diameter transferator, a 2" diameter power piston with a 1.5" stroke. The transferator is an open ended stainless steel cup made from a food storace tin, which is connected to a small diameter rod which passes through an air tight gland down the centre of the power piston the to the outside of the engine. This rod is connected to the crankshaft via a linkage. The engine above uses a simple bellcrank linkage. The power cylinder and cooler form a cylindrical assembly which is sized such that it fits neatly within the transferator cup with a small annular clearance gap - about 1.6mm or 1/16". Transferator and power cylinder are then enclosed in a top-hat shaped outer hot cap. Again there is a small clearance gap between the outer surface of the transferator and the inner dimension of the hot cap. As the transferator descends over the power cylinder it causes the air between the cup and the cylinder to be displaced down the annular gap and up the gap between the transferator and hot cap. The result is that the relative ly cold air is transferrred to the hot end of the engine where it quickly expands and causes the internal pressure of the engine to rise. The top of the power cylinder is open to the underside of the transferator, and so the piston experiences a force caused by the increase in internal pressure. This is the expansive power stroke. The transferator is connected to the crankshaft of the engine in such a way that it is phased 90 degrees ahead of the power piston. As the power piston descends, the transferator is already starting to rise towards the hot end of the engine. The heated air is transferred from the hot cap space, around the skirt of the transferator back into the space surrounding the cold power cylinder. As a result it cools and contracts and the internal air pressure of the engine falls below atmospheric. Atmospheric pressure acting on the underside of the power piston, exerts a force on the piston causing it to move inwards to the heart of the engine. This is the compression stroke. The transferator engine is characterised by its squat appearance and the manner in which the power cylinder is neatly packaged within the transferator space. Typically the diameter of the transferator is aranged to be approximately twice the diameter of the power piston. Some Key Points about Transferator Engines. 1. Transferator engines are a variation of the hot-air engine first proposed by Laubereau in 1861. 2. The word Transferator was coined by John Bourne , a Hampshire Model Engineer in his 1980 "Model Engineer" article - who used them in model boats. 3. Transferator engines have a low compression ratio and are thus suited to running on temperatures of about 350 C / 600F - ideal for wood stoves - They belong to a sub-class of hot-air engines known as Medium Temperature Difference (MTD) engines. 4. The transferator cup does not need to withstand pressure differences (unlike a displacer) It can be scaled up to a large diameter with no ill-effects. 5. The large surface area of the end of the hot cap is the main heat exchange surface. It scales up to larger sizes quite acceptably. 6. Transferator engines lend themselves to be made from stainless steel cooking containers and pans. 7. It is believed that for power applications under 100W that the trannsferator engine offers the best solution based on ease of construction and material availability. 8. The cooler can be made from a coil of annealed copper brake tubing wound around the power cylinder. 9. Transferator engines have gained new interest in 2003/2004 as a result of members of the "Stirling Engine Society" Several prototypes have been built. 10. The unique packaging of the power cylinder within the transferator makes this a compact engine compare to the equivalen classic hot air engine. 11. The use of modern stainless steels allows engines of this type to be made with acceptable heat transfer characteristics. 12. The use of the transferator allows an economical engine to be made which can run on the lower temperature found when burning wood or biomass. This may provide a low cost form of power for Developing countries, with applications in battery charging, low-energy lighting and water pumping. 14. The waste heat rejected from the cooler in the frm of hot-water can be used for washing and cleaning purposes. This could lead to greater levels of hygene in areas normaly associated with poor sanitation. |
| Alternative Hot Air Engines. These usually use the expansion of a heated gas working against a piston or turbine to produce mechanical work. Air expands when heated, and if constrained by a piston, it will do work on that piston causing it to move. This is a simple fact and it doesen't matter if you are in Monson Road or on the Moon, a heat engine can be devised around this basic physical principle. Stirling engines operate on the basis of having a fixed amount of air sealed within the engine and are known as closed cycle hot air engines. The air is alternately heated then cooled by moving it from the hot side of the engine to the cold side by means of a displacer and the resulting rises and falls in internal pressure do work against a piston. The reciprocating motion of the piston is converted by means of a connecting rod and crankshaft produce rotary motion. The displacer is driven by an additional linkage from the crankshaft and generally produces no work itself. Other types of engine exist, where the air is not trapped inside the engine but is discharged at the end of each stroke and a fresh charge taken in. These are known as open-cycle engines. Open cycle hot air engines have been proposed and built by many engineers over the years. One of the most prolific was John Ericsson who's name has been perpetuated in the Ericsson cycle engine. For a potted history of John Ericsson's life see Bob Sier's excellent short biography. |
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