The compression pressure of the mixture before admission is, however, steadily reduced as the load is reduced, and at very light loads the engine is running practically as a non-compression engine.
A test by Professor Unwin of a 412 nominal horse-power Priestman engine, cylinder 8·5 in, diameter, 12 in. stroke, normal speed 180 revolutions per minute, showed the consumption of oil per indicated horse-power hour to be 1·066 ℔ and per brake horse-power hour 1·243 ℔. The oil used was that known as Broxburn Lighthouse, a Scottish paraffin oil produced by the destructive distillation of shale, having a density of ·81 and a flashing-point about 152° F. With a 5 H.P. engine of the same dimensions, the volume swept by the piston per stroke being ·395 cub. ft. and the clearance space in the cylinder at the end of the stroke ·210 cub. ft., the principal results were:—
| Daylight Oil. | Russoline Oil. | |
| Indicated horse-power | 9·369 | 7·408 |
| Brake horse-power | 7·722 | 6·765 |
| Mean speed (revolutions per minute) | 204·33 | 207·73 |
| Mean available pressure (revolutions per minute) | 53·2 | 41·38 |
| Oil consumed per indicated horse-power per hour | ·694 ℔ | ·864 ℔ |
| Oil consumed per brake horse-power per hour | ·842 ℔ | ·946 ℔ |
With daylight oil the explosion pressure was 151·4 ℔ per square inch above atmosphere, and with Russoline 134·3 ℔. The terminal pressure at the moment of opening the exhaust valve with daylight oil was 35·4 ℔ and with Russoline 33·7 per square inch. The compression pressure with daylight oil was 35 ℔, and with Russoline 27·6 ℔ pressure above atmosphere. Professor Unwin calculated the amount of heat accounted for by the indicator as 18·8% in the case of daylight oil and 15·2 in the case of Russoline oil.
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| Fig. 12.—Hornsby-Ackroyd | Fig. 13.—Hornsby-Ackroyd | |
| Engine (section through | Engine (section through valves, | |
| vaporizer and cylinder). | vaporizer and cylinder). |
The Hornsby-Ackroyd engine is an example of the class in which the oil is injected into the cylinder and there vaporized. Fig. 12 is a section through the vaporizer and cylinder of this engine, and fig. 13 shows the inlet and exhaust valves also in section placed in front of the vaporizer and cylinder section. Vaporizing is conducted in the interior of the combustion chamber, which is so arranged that the heat of each explosion maintains it at a temperature sufficiently high to enable the oil to be vaporized by mere injection upon the hot surfaces. The vaporizer A is heated up by a separate lamp, the oil is injected at the oil inlet B, and the engine is rotated by hand. The piston then takes in a charge of air by the air-inlet valve into the cylinder, the air passing by the port directly into the cylinder without passing through the vaporizer chamber. While the piston is moving forward, taking in the charge of air, the oil thrown into the vaporizer is vaporizing and diffusing itself through the vaporizer chamber, mixing, however, only with the hot products of combustion left by the preceding explosion. During the charging stroke the air enters through the cylinder, and the vapour formed from the oil is almost entirely confined to the combustion chamber. On the return stroke of the piston air is forced through the somewhat narrow neck a into the combustion chamber, and is there mixed with the vapour contained in it. At first, however, the mixture is too rich in inflammable vapour to be capable of ignition. As the compression
proceeds, however, more and more air is forced into the vaporizer chamber, and just as compression is completed the mixture attains proper explosive proportions. The sides of the chamber are sufficiently hot to cause explosion, under the pressure of which the piston moves forward. As the vaporizer A is not water-jacketed, and is connected to the metal of the back cover only by the small section or area of cast-iron forming the metal neck a, the heat given to the surface by each explosion is sufficient to keep its temperature at about 700-800° C. Oil vapour mixed with air will explode by contact with a metal surface at a comparatively low temperature; this accounts for the explosion of the compressed mixture in the combustion chamber A, which is never really raised to a red heat. It has long been known that under certain conditions of internal surface a gas engine may be made to run with very great regularity, without incandescent tube or any other form of igniter, if some portion of the interior surfaces of the cylinder or combustion space be so arranged that the temperature can rise moderately; then, although the temperature may be too low to ignite the mixture at atmospheric temperature, yet when compression is completed the mixture will often ignite in a perfectly regular manner. It is a curious fact that with heavy oils ignition is more easily accomplished at a low temperature than with light oils. The explanation seems to be that, while in the case of light oils the hydrocarbon vapours formed are tolerably stable from a chemical point of view, the heavy oils very easily decompose by heat, and separate out their carbons, liberating the combined hydrogen, and at the moment of liberation the hydrogen, being in what chemists know as the nascent state, very readily enters into combination with the oxygen beside it. To start the engine the vaporizer is heated by a separate heating lamp, which is supplied with an air blast by means of a hand-operated fan. This operation should take about nine minutes. The engine is then moved round by hand, and starts in the usual manner. The oil tank is placed in the bed plate of the engine. The air and exhaust valves are driven by cams on a valve shaft. The governing is effected by a centrifugal governor which operates a by-pass valve, opening it when the speed is too high, and causes the oil pump to return the oil to the oil tank. At a test of one of these engines, which weighed 40 cwt. and was given as of 8 brake horse-power, with cylinder 10 in. in diameter and 15 in. stroke, according to Professor Capper’s report, the revolutions were very constant, and the power developed did not vary one quarter of a brake horse-power from day to day. The oil consumed, reckoned on the average of the three days over which the trial extended, was ·919 ℔ per brake horse-power per hour, the mean power exerted being 8·35 brake horse. At another full-power trial of the same engine a brake horse-power of 8·57 was obtained, the mean speed being 239·66 revolutions per minute and the test lasting for two hours; the indicated power was 10·3 horse, the explosions per minute 119·83, the mean effective pressure 28·9 per sq. in., the oil used per indicated horse-power per hour was ·81 ℔, and per brake horse-power per hour —·977 ℔. In a test at half power, the brake horse-power developed was 4·57 at 235·9 revolutions per minute, and the oil used per brake horse-power was 1·48 ℔. On a four hours’ test, without a load, at 240 revolutions per minute, the consumption of oil was 4·23 ℔ per hour. Engines of this class are those manufactured by Messrs Crossley Bros., Ltd., and the National Gas Engine Co., Ltd.
Fig. 14.—Crossley Oil Engine.
Figs. 14 and 15 show a longitudinal section and detail views of the operative parts of the Crossley oil engine. On the suction stroke, air is drawn into the cylinder by the piston A through the automatic inlet valve D, and oil is then pumped into the heated vaporizer C through the oil sprayer G, as seen in section at fig. 15. The vaporizer C is bolted to the water-jacketed part B; and, like the Hornsby, this vaporizer is first heated by lamp and then the heat of the explosions keeps up its temperature to a sufficiently high point to vaporize the oil when sprayed against it. On the compression stroke of the piston A the charge of air is forced into the combustion chamber B and the vaporizer chamber C, where it mixes with the oil vapour, and the mixture is ignited at the termination of the stroke by the ignition tube H. This tube is isolated to some extent from the vaporizer chamber C, and so it becomes hotter than the chamber C and is relied upon to ignite the mixture when formed at times when C
would be too cold for the purpose. E is the exhaust valve, which