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An Engine that Runs on Air

14th January 1949
Page 51
Page 51, 14th January 1949 — An Engine that Runs on Air
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ARE we on the eve of a revolution in prime movers? By the success' which has already been recorded by the Philips organization in

Holland with a new form of hot-air engine, it would seem that the time is not far distant when vehicles will be powered by a unit that is practically noiseless and i:lossesses such excellent torque characteristics that the conventional gearbox will no longer be required.

Hot-air engines as generally known, depend for their working on the alternative heating and cooling of the air admitted to the power cylinder, the power output being governed by the temperature difference between minimum and maximum, and in the size of the unit. Although many attempts have been made to produce wider extremes in this temperature change, resort was made to the easier way of obtaining increased power by building engines of enormous size and weight.

A typical unit, producing about 2 h.p., would have a swept volume of about 25 litres with a weight of about 13 cwt. Even at their best, hot-air engines were most uneconomical, and at no stage in their existence did they attain any position of popularity. Although the Philips engine depends upon the alternate heating and cooling of the air 'admitted to the power cylinder, it in no sense represents an improvement on the older type, being, as it is, a complete departure in all its design features.

The success of the Philips engine is bound up in the design and construction of the heater, cooler and regenerator, particularly the last-named. The regenerator consists of a sponge-like "fabric" built up of extremely fine wire, through which the air passes on its way from the hot area to the cold one, and vice versa.

Each time the hot air is expanded it gives off the greater part of its heat to the regenerator, which, in turn. transmits 95 per ccnt, of this absorbed heat to the cold air on its return through the regenerator after compression. The regenerator may thus he likened to a heat sponge. in that it rapidly absorbs a large quantity of heat, which it is able to give back again.

The heat change at every cycle is from about 100 degrees C. to 650 degrees C., and this can be achieved so rapidly that engine speeds in excess of 3,000 r.p.m. are readily obtainable. It is claimed that the power developed per litre of swept volume, and per lb. weight, is greater than that obtainable from either a petrol or oil engine.

Fuel consumption works out at something between that obtained with a good petrol engine and an oil engine, but the advantage lies in the fact that petrol, paraffin, oil fuel. methyl alcohol or town gas can be used with equal facility. The general running characteristics of the Philips hot-air engine are similar to those of a steam engine, and a 15 h.p unit makes no more noise than a sewing machine. An important feature is that torque is not dependent upon engine revolutions, and this high degree_ of flexibility means that a gearbox of a much simplified type may be used or. in certain circumstances, dispensed with. Whilst constructional details are not available, the accompanying diagram will serve to show the general arrangement of a four-cylindered engine, a laboratory example of which is shown in the half-tone reproduction. This particular unit has swash-plate mechanism which accounts for its compact construction. Units as small as 1-10th h.p. are" practicable. and there is. apparently, no limit to total power output.

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