Thermal Efficiency Simply Explained
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Why the Oil Engine Scores Over the Petrol Engine in Respect of Power Output for Fuel Consumed, The Relationship Between Heat and Mechanical Work. The Calculation of Theoretical Horse Power
DURLNIG a conversation with a haulage contractor (a keen business man but non-technical); the writer was asked if he could give some simple explanation of the rapid accept
ance and ever-increasing success of the oil engine in road transport vehicles. The satisfactory reception of this explanation, expressed by the con tractor, appeared to indicate to the writer that the subject would probably be of considerable interest to many readers who, although good business men, may not be technically minded.
It was in die year 1930 that the oil engine in commercial vehicles and p.s.v.s really started. It caught on at once, rapidly becoming popular and in great demand. That was 15 years ago and the demand still continues and• increases month by month. There are now many thousands of oil-engined vehicles runn:ng on our roads, with large numbers in the Colonies and Dominions.
Up to 1930, or thereabouts, we had been well content with our popular and conventional four-stroke petrol engine, which had been giving us good service for over 45 years, i.e., ever since the internal-combustion engine was first applied to road motor vehicles. The four-stroke cycle was first introduced by Dr. Otto in 1876, and has been most successful ever since, It is quite natural, therefore, for a non-technical man to express a desire to know how and why this rather sudden change-over, from petrol to oil, occurred. 'there are, of course, many good and sound reasons, but the most outstanding and the principal one is the great increase—almost 100 per cent.—in what is known as " thermal efficiency." This technical term must therefore be explained for the benefit of the non-technically minded.
Heat Converted into Mechanical Energy At this point we must digress somewhat. We are dealing with engines which we say " run on liquid fuel." This is not strictly correct. They run on the heat produced by the engine's method of working, and this heat is converted into mechanical work as will be shown later. Where does the heat come from? The liquid fuel in the tank is a by-product of the crude petroleum which is found by drilling deep wells in oil-producing countries.
The vehicle' does not carry "crude oil" in its tank as is sometimes supposed. Briefly, the crude petroleum from the well is conveyed, by various means, to the oil refinery, where it is put through a series of heat distillation processes which result in the production of aviation spirit, commercial petrol, kerosene, lubricating oils, and many other by-products, right down until nothing is left but a residue of coke or asphalt.
The product we are interested in— oil fuel—is known as a heavy petroleum distillate and is product obtained by continuing the distillation after the kerosene "fraction" has been collected.
At one time this product was considered as a residue of little value, and even in 1930 and 1931 was being sold in this country at tlid. per gallon, as some of our lucky hau:iers know.
When, however, it was realized that this was the ideal fuel for the oil engine and the demand rapidly increased, the authorities promptly penalized it with a tax, to get square with what they were losing on petrol. But this did not deter the rapid advance of the oil engine—it does not need any price advantage—it wins on its merits.
The Composition of Mineral Petroleums All mineral petroleums consist of a chemical combination of hydrogen and carbon—actually about 85-86 per cent. of carbon and 12-13 per cent, of hydrogen—and they are known as " hydro carbons." The odd 2 per cent, or so consists•of oxygen, nitrogen and sulphur in such small amounts as to be quite unimportant.
Nature has so provided that these hydro-carbon molecules contain a very considerable latent heat, and our oil engine has the means for destroying the liquid fuel as such and producing, in its place, the, heat we require for producing power.
The heat value of an oil is measured in British thermal units. (A British thermal unit is the quantity of heat required to raise 1 lb. of pure water, one degree Fahrenheit.) Our usual oil fuel contains approximately some 18,000 B.T.U.'s per lb. This is called the " calorific value " of the oil.
Heat and mechanical work are mutually convertible, and one unit of heat is equivalent to a definite amount of mechanical work, called the " mechanical equivalent of heat." Scientists, many years ago, found that one B.T.U. is equiva:ent to 778 ft./lb. Of mechanical work.
Thus, knowing our calorific value in B.T.U.s it is easy to calculate how many ft./lb.—and therefore how many h.p.—we ought to obtain from every pound of oil fuel consumed. This is, of course, assuming that we have a theoretically perfect engine, which we have not and never will have.
This is where we revert to the expression "thermal efficiency" mentioned earlur. It means simply, what percentage of the full theoretical heat value of the oil can we obtain from the crankshaft of our engine in the form of mechanical power?
Here, then, we have the answer for our non-technical contractor—i.e., the principal reason for the great success of the oil engine. With our petrol engine this efficiency was approximately 18-22 per cent. With our oil engine this -efficiency may be almost 40 per cent., roughly double.
Why does the oil engine extract so much more heat (power) from the fuel? It is because' the method employed by. the oil engine enables us to use compression ratios from 13 to 1 up to 16 to 1, as against the 5 or 6 to 1 ratios of the petrol engine. Even so, pre-ignition is impossible. An additional reason is that instead of a carburetter we use an injection, pump, which not only delivers an exactly equal amount of fuel to each cylinder, correctly measured to supply the power at any given moment, but correctly timed, the fuel being finely atomized by means of high-pressure sprayers.
The actual thermal efficiency of the modern oil engine is greater than that of any other form of prime mover.
The Four-stroke Cycle Principle Retained Referring once again to the " Otto " cycle—i.e., the four-stroke principle— it was indeed fortunate that the more general use of the oil engine in 1930 did not entail any need to abandon our 68-year-old four-stroke cycle to which engineers, mechanics and drivers have all been used and understand, and, indeed, on which many of them were brought up from apprenticeship days. This well-tried veteran cycle and proved principle looks like being with us until the day of the internal-combustion engine is over, and that is surely a far-away date, at least, so far as road transport is concerned. , NORTHERN ENGINEER. [Our contributor specially refers to the four-stroke cycle as the principle of operation of oil engines. Whilst there are no examples of two-stroke oilers in use in road vehicles in this country, successful units running on this principle are used in the U.S.A. It is our opinion that the post-war period will see the introduction of two-stroke oil engines to both goods and passenger vehicles in this country. A particularly successful type has been in use here for tractor work for some considerable time. .For a detailed description of such an engine we would refer our readers to the issue of " The Commercial
Motor " dated January 1944.— En.]