Is the I.C. Engine a HEAT WASTER?
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A Simple Analysis of Thermal Efficiency, Showing the Sources of Heat Losses in Petrol and Oil Engines DURING a discussion on engine design an operator said that he wondered why the 1.C. engine was called a heat engine and that a heat waster would be mare accurate. He thought, too, that it was time that designers did something about it. ThTre is certainly some truth in his first remark, but as regards designers, they have, of course. been trying to do something about it ever since James Watt decided he could make a less wasteful engine than Newcomen. On this question of heat utilization, the internal combustion engine shows to a greater advantage than even the best type of steam engine. 35 per cent. Thermal Efficiency For example, whilst the thermal efficiency of an oil engine as used on commercial vehicles reaches about 35 per cent., a reciprocating steam engine with a condenser attains about 15 per cent., whilst without a condenser, as on most locomotives, an efficiency of 10 per cent is nearer the mark. Nevertheless, designers of 1.C. engines are not complacent. They are quite aware that even small gains in efficiency are possible only by careful research, and that there must be no sacrifice of reliability or longevity for any such gain. It is first necessary to ascertain how the heat supplied to the engine in the form of fuel is distributed, and for this purpose a heat balance is prepared as shown diagrammatically in Fig. 1. The method of calculating how the heat is used is interesting, but calls • for rather elaborate apparatus if a detailed analysis is to be made. The useful heat is easily found from the fuel consumption figures while the a6 engine is running on the test bed. Fuel consumption is generally measured in pints, and this must be converted into pounds, as the calorific or heat value of the fuel is generally stated in heat units (B.Th.U. per pound). An example will make the method clear*:—suppeSe an engine uses 0.4 lb. of fuel, of a calorific value of 19,000 B.Th.U. per lb. per b.h.p. per hour. Then beat in the fuel is 0.4 x 19,000-7,600 B.Th.U. 33,000 x 60 1 b.h.p. per hour — 33.5 per cent. 7,600 As regards the heat passing to the cooling water, this necessitates measuring the quantity of water passing through the cylinder jackets and noting the inlet and outlet temperatures The quantity of water is usually found by allowing the outlet water to pass through a measuring tank, the quantity collected in a given time being noted. The weight of water, multiplied by the rise in temperature, gives the number of heat units lost to the cooling water. The heat lost to the exhaust is often found by subtracting the heat used plus cooling losses, from the heat supplied: If a more accurate result be required, the quantity of air passing to the engine must be measured and this, plus the weight of fuel, will be equal to the weight of the exhaust gases. The frictional losses are most readily found by motoring the engine on the test bed, that is finding the amount of power required to turn the engine at a given speed. It is now possible to draw up a heat balance which will give a true picture of the sources of heat loss. The next point is, how much of this loss is unavoidable? To answer this question one must make a closer study of what happens inside a cylinder. During combustion, the heat liberated by the burning of the fuel raises the temperature of the air and increases. its pressure. The term air may be used, as the amount of fuel present is relatively small. There is a definite connection between the volume of air and its temperature and pressure. Dealing first with temperature, one would expect a gas to expand in proportion to the rise in temperature. Thus, Fig. 2 shows a piston enclosing, say, 1 cubic ft. of air. If the temperature be doubled the volume will be doubled as shown in the broken line. The point arises, however, as to what scale of temperature is used? Ask the average person what temperature reading indicates twice 60 degrees Fahrenheit, and he would reply 120 degrees Fahrenheit, but the chemist would call the latter temperature 49 degrees Centigrade, and on the Continent the answer would probably be 39 degrees Reaumur. To avoid confusion, a scale called the absolute scale is used in which freezing point, 32 degrees F., corresponds to 492 degrees absolute. The advantage of this scale is that zero absolute, 492 degrees F., is the temperature at which a gas would, theoretically, have no volume. Thus, 60 degrees F. now becomes 520 degrees absolute, which, if doubled, equals 1,040 degrees .absolute—somewhat different to 120 degrees F. Thus the volume of a gas varies as the absolute temperature if the pressure be kept constant.
Volume and Pressure
As regards volume and pressure, the relationship again is a simple one. If you double the pressure on a gas you halve the volume (Fig 3). The pressure, however, is always added to that of the atmosphere, approximately 15 lb. per sq. in. To make this clear, suppose I cubic ft. of air be under a pressure of 10 lb. per sq. in., according to a pressure gauge. Now if the pressure be raised to 20 lb. per sq. in. by compressing the air, the volume is not halved. Thus, 10 lb. per sq. in. equals 25 lb. per sq. in. total or absolute pressure, and 20 lh per sq. in. will be 35 lb. per sq. in. The pressure will have to be increased to 50 lb. per sq. in. or 35 lb. per sq. in. by gauge reading to halve the volume.
A clear understanding of the foregoing is necessary if we are to appreciate the changes in volume and pressure which take place in the cylinder. Thus, during combustion, there is a rapid rise in temperature, resulting in a corresponding rise in pressure. As the piston is forced down, the volume increases almost in proportion to the fall in pressure. The word "almost" is used as the conditions in an engine cylinder are not the same as when a gas is compressed or expanded slowly.
It will be realized that the charge is only temporarily changed in temperature and pressure, and that once the heat has done its work, the quicker the original conditions are reached the better. For instance, the designer takes all possible steps to extract the maximum amount of heat from the fuel by using a high compression ratio, providing an easy flow for the gases and, especially with oil engines, providing for the right degree of turbulence. He then proceeds to fit adequate water jackets and aluminium pistons, both of which are designed to disperse the heat as efficiently as possible.
This apparent contradiction is often a nuzzle to the uninitiated, but the purpose becomes clear if one distinguishes clearly between useful and waste heat. During combustion every additional heat unit increases the pressure rise, and consequently the power, but once the heat has entered the walls and head of the engine it is waste and must be disposed of rapidly if troubles such as a condition suggestive of excessive ignition advance in the case of a petrol engine, and a smoky exhaust in the case of an oil engine, are to be avoided. This, then, accounts for most of the heat loss to the cooling water in the balance sheet. It must be remembered, however, that any overcooling will lead to a greater heat flow to the cooling water than that shown. Hence the importance of thermostats, heated garages and reasonably rapid warming-up of the engine if heat losses are to be reduced.
Can these cooling losses serve any useful purpose? Some petrol engines use a water-heated induction system, and on some buses and coaches some of this heat is employed for interior heating. The main difficulty, however, is to store, such heat so that it can he used as and when required.
Turning now to the exhaust system, it will be seen that this is the worst offender. The explanation is brought out clearly by a glance at an average valve-timing diagram (Fig. 4). This varies but little for either a petrol or oil engine, although the exhaust losses are less in the case of the latter. The fact that the exhaust valve opens some 45 degrees before bottom dead centre means that the heat energy possessed by the gases passes down the exhaust pipe. Arranging for a later.opening of the exhaust would mean that this energy could be expended on the piston, but any such gain would be offset on the next stroke due to imperfect scavenging. Power output depends on the weight of charge induced per stroke, and any exhaust gas remaining in the cylinder means a reduced charge during induction.
At slow speeds the opening point could be delayed, owing to the increased time available for charging and discharging, but it would have to be advanced with an increase in engine speed. No one has yet devised a practical form of variable camshaft so that designers have been forced to adopt the best compromise.
Using the Exhaust Gases
As in the case of the cooling water, exhaust gases are used to assist vaporization of the petrol-air mixture and, in some instances, for interior heating. Apart from such uses, an attractive possibility is an exhaust-driven turbo supercharger (Fig 5), but until supercharging finds wider acceptance, such a use for the exhaust gases must remain in abeyance.
It would appear, therefore, that with our present type of engine, no great gains in thermal efficiency can be expected. In fact, in comparison with what many consider to be its ultimate successor, namely, the gas turbine, its efficiency is much superior. However, the piston engine as we know it has had some 50 years' development, so that it is hardly fair to make comparisons on this score. It is certain, however, that although designers are ever seeking to improve engine efficiency, they will not do it at the expense of such essential features as reliability, long life, ease of starting, and flexibility.