THE WASTE OF POWER.
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All Engines Waste More Power Than They Usefully Employ. How This Loss May Be Reduced.
IF A BUSINESS MAN spent one hundred pounds every time he wanted thirty pounds' worth of goods, he would rightly be deemed extravagant —and something worse. Yet no better than this is the prodigal misuse of our power resources in the engines that we employ at the present day, for even the best of these gives a, useful output of only about 33 per cent, of the fuel capacity we put into it. An engine iz simply a mechanical device for converting into power the heat energy of the various substances generally known as fuels, and a fuel is a substance of which the chemical composition is such that it will burn.
Naturehas endowed the world with fuels on its usual lavish scale, but the geographical distribution of these fuels does not happen to be always just as convenient as mankind would like to have it. When this question is considered from the national point of view—as it must necessarily be in the present state of evolution of human society—the matter becomes serious. We are faced with the fact that our resources of the two fuels of cardinal importance, coal and oil, ate limited in extent. And although we may have sufficient of these fuels to last for some generations from now, it is only in accordance with the principles of common forethought—sometimes termed wise statesmanship—that we should take measures to prevent, so far as possible, waste in any shape or form.
Every fuel has a definite calorific value, which is a numerical expression denoting the number of heat units that will be evolved when that fuel is completely burnt. The fundamental relationship between heat units and work units, first enunciated by Dr. Joule, is that one heat unit (B.Th.U.) equals 778 ft.-lbs. of work. From this it follows that a pound of any given fuel, if burnt in such a way that all of the heat generated shall be converted into useful work, will produce a certain definite number of ft.-lbs. of work, proportional to the calorific value of the fuel. And, further, if this burning under ideal oonditions be performed within a certain definite unit of time, then the result will be a certain output of horsepower.
The Thermal Efficiency of Various Engines.
In any engine, the relationship between the output of work in a certain time on the one hand, and the calorific value of the fuel consumed in that time on the other hand, gives a numerical quantity known as the thermal efficiency of the engine. Thermal efficiency, then, is simply an expression denoting the extent to which the engine uses our natural resources of fuel, and the following figures may be taken as typical of fair, average thermal efficiencies of engines working under normal conditions :— Gas engines ... ... 28 per cent. Petrol engines ... ... 25 per cent.* Diesel engines ... ... 33 per cent.
The-se figures in themselves are illuminating, for they show how prodigal of fuel resources are the best engines that human ingenuity has so far been able to evolve. There is, however, no reason for mankind to be ashamed of its efforts, because an inquiry into the development of engines for converting heat into work will show that, since the time when Hero some two thousand years ago caused the steam, issuing from a vessel something like a kettle, to impinge on -to the blades of a fan so as to impart motion to the fan, there has always been manifest a tendency
gradually to improve upon this primitive engine, the improvement always showing increments, -however
in the thermal efficiency. Two or three generations ago, the very best steam engines wasted C30
94 per cent. of the heat energy of their fuelt; yet to-day there are Diesel engines wasting only about 67 per cent. Truly this progress may appropriately be described as satisfactory, but the point to remember is that there is no half-way between progress and retrogression. We must go forward, and going forward in this case means that we must make more comPlete use of such fuel resources as are in the world.
Where the Heat Goes.
Where, then, does the heat go in these engines that are so wasteful of it? In internal-combustion engines the calorifto value of the fuel is expended in five different ways. The following table shows the percentage of heat expended in these respective directions in a typical Diesel engine; the Diesel engine has directions, selected because, so far, it represents
the most efficient prime mover we have :— 1.—Heat lost to cooling water, 35 per cent. 2.—Heat lost with exhaust, 16 per cent. 3.—Heat absorbed in engine friction, 9 per cent. 4. —Heat utilized as work,. 33 per cent. 5.—Heat radiated, 7 per cent. Of these items only No. 4 represents a useful return for our expenditure of the fuel's heat value. All
the other items represent heat energy which is lost, being the price that we have to pay for our lack of 'knowledge with respect to the science of thermodynamics as related to engine design. These facts are useful in that they give us an indication of the direc tions in which we have to set to work to obviate the losses, and a brief examination of them will serve to suggest to us the directions in which the pi ogress of science will be able, in the nearer or -later future, to effect economy. The cooling water accounts for most of the heat lost in internal-combustion engines. That this is so is due to the fact of there being a water jacket through which passes a constant stream of more or less cold water ; and this water carries away with it heat units that are thus rendered unavailable for transformation into work. Users of motor vehicles are accustomed to water-cooling and they know that the engine will seize up if it becomes overheated; but probably, in many cases, they -do not know that indispensable though water-cooling may be in prac tice, it is at best a necessary evil, the use of which is imposed upon us by the limitations of the internalcombustion cycle. Steam engines are jacketed for precisely the opposite reason: with steam cylinders the jacket is filled with steam to keep the inside of the cylinder warm, and the outside of the jacket is lagged to prevent radiation.
Brains Busy on Heat Recovery.
So far as item No. 5 is concerned, the loss is very small and the prospect of recovering it even smaller. But in the ease of item No. 2 (the loss due to heat being carried away with exhaust gases) something tangible is being done which may afford the basis for almost immediate development. During the past few years much strenuous and systematized work has been applied to this problem ; and there is every reason to hope that practical results will accrue from this research in the immediate future, although it is too early as yet to indicate the exact form of mechanical device that will be used to convert heat of exhaust gases into useful work.