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 of from ·700 to ·739 specific gravity showed that 98% of the lighter sample distilled over below 120° C. while only 88% of the heavier came over within the same temperature range. The heavier petrol is not so easily converted into vapour. The great modern development of the motor car gives the light oil engine a most important place as one of the leading sources of motive power in the world. The total petrol power now applied to cars on land and to vessels on sea amounts to at least two million H.P. The petrol engine has also enabled aeroplanes to be used in practice.

The earliest proposal to use oil as a means to produce motive power was made by an English inventor—Street—in 1794, but the first practical petroleum engine was that of Julius Hock of Vienna, produced in 1870. This engine, like Lenoir’s gas engine, operated without compression. The piston took in a charge of air and light petroleum spray which was ignited by a flame jet and produced a low-pressure explosion. Like all non-compression engines, Hock’s machine was very cumbrous and gave little power. In 1873, Brayton, an English engineer, who had settled in America, produced a light oil engine working on the constant pressure system without explosion. This appears to have been the earliest compression engine to use oil fuel instead of gas.

Shortly after the introduction of the “Otto” gas engine in 1876, a motor of this type was operated by an inflammable vapour produced by passing air on its way to the cylinder through the light oil then known as gasolene. A further air supply was drawn into the cylinder to form the required explosive mixture, which was subsequently compressed and ignited in the usual way. The Spiel petroleum engine was the first Otto cycle motor introduced into practice which dispensed with an independent vaporizing apparatus. Light hydrocarbon of a specific gravity of not greater than 0·725 was injected directly into the cylinder on the suction stroke by means of a pump. In entering it formed spray mixed with the air, was vaporized, and on compression an explosion was obtained just as in the gas engine.

Until the year 1883 the different gas and oil engines constructed were of a heavy type rotating at about 150 to 250 revolutions per minute. In that year Daimler conceived the idea of constructing very small engines with light moving parts, in order to enable them to be rotated at such high speeds as 800 and 1000 revolutions per minute. At that time engineers did not consider it practicable to run engines at such speeds; it was supposed that low speed was necessary to durability and smooth running. Daimler showed this idea to be wrong by producing his first small engine in 1883. In 1886 he made his first experiment with a motor bicycle, and on the 4th of March 1887 he ran for the first time a motor car propelled by a petrol engine. Daimler deserves great credit for realizing the possibility of producing durable and effective engines rotating at such unusually high speeds; and, further, for proving that his ideas were right in actual practice. His little engines contained nothing new in their cycles of operation, but they provided the first step in the startlingly rapid development of petrol motive power which we have seen in the last twenty years. The high speed of rotation enabled motors to be constructed giving a very large power for a very small weight.

Fig. 1 is a diagrammatic section of an early Daimler motor. A is the cylinder, B the piston, C the connecting rod, and D the crank, which is entirely enclosed in a casing. A small fly-wheel is carried by the crank-shaft, and it serves the double purpose of a flywheel and a clutch, a is the combustion space, E the single port, which serves both for inlet of the charge and for discharge of exhaust. W is the exhaust valve, F the charge inlet valve, which is automatic in its action, and is held closed by a spring f, G the carburettor, H the igniter tube, I the igniter tube lamp, K the charge inlet passage, L the air filter chamber, and M an adjustable air inlet cap for regulating the air inlet area. The light oil—or petrol, as it is commonly called—is supplied to the float chamber N of the vaporizer by means of the valve O. So long as the level of the petrol is high, the float n, acting by levers about it, holds the valve O closed against oil forced by air pressure along the pipe P When the level falls, however, the valve opens and more petrol is admitted. When the piston B makes its suction stroke, air passes from the atmosphere by the passage K through the valve F, which it opens automatically. The pressure falls within the passage K, and a spurt of petrol passes by the jet G¹, separate air at the same time passing by the passage K¹ round the jet. The petrol breaks up into spray by impact against the walls of the passage K, and then it vaporizes and passes into the cylinder A as an inflammable mixture. When the piston B returns it compresses the charge into a, and upon compression the incandescent igniter tube H fires the charge. H is a short platinum tube, which is always open to the compression space. It is rendered incandescent by the burner I, fed with petrol from the pipe supplying the vaporizer. The open incandescent tube is found to act well for small engines, and it does not ignite the charge until the compression takes place, because the inflammable mixture cannot come into contact with the hot part till it is forced up the tube by the compression. The engine is started by giving the crank-shaft a smart turn round by means of a detachable handle. The exhaust is alone actuated from the valve shaft. The shaft Q is operated by pinion and a spur-wheel Q² at half the rate of the crank-shaft. The governing is accomplished by cutting out explosions as with the gas engine, but the governor operates by preventing the exhaust valve from opening, so that no charge is discharged from the cylinder, and therefore no charge is drawn in. The cam R operates the exhaust valve, the levers shown are so controlled by the governor (not shown) that the knife edge S is pressed out when speed is too high, and cannot engage the recess T until it falls. The engine has a water jacket V, through which water is circulated. Cooling devices are used to economize water.

Benz of Mannheim followed close on the work of Daimler, and in France Panhard and Levassor, Peugeot, De Dion, Delahaye and Renault all contributed to the development of the petrol engine, while Napier, Lanchester, Royce and Austin were the most prominent among the many English designers.

The modern petrol engine differs in many respects from the Daimler engine just described both as to general design, method of carburetting, igniting and controlling the power and speed. The carburettor now used is usually of the float and jet type shown in fig. 1, but alterations have been made to