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The silent Stirling will run on salad oil
Page 50
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by Paul Brockington • A Stirling external-combustion engine was demonstrated on Monday at Stretford Technical College, Manchester. It operated without vibration on a variety of fuels and its noise level could be compared with that of an electric motor. First patented by Robert Stirling in 1816, the engine has been developed in its modern form by Philips of Eindhoven, Holland, and according to Philips it could eventually prove superior to any other prime mover in terms of specific power and efficiency.
The engine was demonstrated at a meeting of the Institute of the Motor Industry, East Lancashire and Cheshire region, by Mr. R. A. J. O. van Witteveen, of the Philips research laboratory.
Stirling engines can be run equally well on diesel oil, petrol, Kerosene, benzole, gas oil, lubricating oil, crude oil, natural gas, salad oil, olive oil and alcohol. It could also be operated by the heat of radio-active isotopes or from a nuclear reactor. In slightly modified form it can act as a refrigerator, heat pump, gas engine or cold gas engine.
Although the price and weight of a production automotive engine cannot be accurately assessed at this stage, it could well be comparable to that of a typical diesel in due course. An experimental fourcylinder automotive Stirling engine, developing 200 bhp at 1,400 rpm, has been fitted to a stage carriage bus. On full load it operates at an efficiency of 41.8 per cent while the average efficiency is 35 per cent. Its lowspeed torque compares favourably with the torque of a diesel engine. Space required is rather more than that occupied by a diesel because of the bulk of the burner equipment.
The unit is basically a hot-air engine. It has two pistons in each cylinder and the rod of each piston is linked to a rhombic contra-rotating mechanism that is fully balanced, the two output shafts being geared together. The working medium is hydrogen or helium and the medium is displaced in a closed circuit. The pistons have a dif ferential action.
Heat is applied by the burner to the hot space above the upper piston and the gas expands, causing the piston to move downwards. The pressure falls with expansion of the gas and as the lower piston moves upwards hot gas is directed through an external regenerative heat exchanger to the space between the pistons, which creates a separating force on the pistons that is converted into useful power. When the pistons separate, pressure falls in the space between them and another cycle is initiated.
The engine has an overload capacity of 50/100 per cent over a relatively long period. Continuous overload capacity is determined by the creep of the hot parts, which is not affected by the normal overload of an automotive engine. Air pollution by the exhaust gas compares favourably with the pollution created by a typical petrol or diesel engine. Virtually no oil is consumed and overrun braking is obtainable equivalent to 40 per cent of the maximum output. A four-cylinder unit has no flywheel.
The rhombic drive, developed by Philips, has been a major contribution to the success of the Stirling engine. A moving hermetical seal is fitted which is hydraulically loaded and has a long life. A great deal of development work has been concentrated on eliminating the high thermal stresses resulting from a working temperature of 700deg. C. The maximum operating pressure of the engine is about 1,500 p.s.i. but the rate of pressure rise is relatively tow and this promotes smooth running and a long life.