Collier's New Encyclopedia (1921)/Diesel Engine
DIESEL ENGINE, a special type of internal combustion engine. The principle on which it works differs from that of the ordinary gas engine, in which an explosive effect is produced by drawing the combustible charge into the cylinder and igniting instantaneously. In the Diesel engine, air is drawn into the cylinder and compressed, and then oil is injected as a fine spray and burned gradually. The engine is made in two types, the four-stroke and the two-stroke. In the four-stroke engine, air is drawn into the cylinder on the first stroke, and is compressed on the second stroke to a pressure of 450 pounds per square inch. This sudden increase in pressure causes a rise in temperature to about 550 degrees C, and during part of the third stroke, oil is injected, and, owing to the high temperature, ignites. The gases thus produced expand, and during the fourth stroke of the piston the products of combustion are expelled. In the two-stroke engine, the general procedure is the same, but differs in details. Air, instead of being drawn into the engine by the stroke of the piston, is forced in under slight pressure, and is then further compressed to the same pressure as in the case of the four-stroke engine. Fuel is injected, and ignites, the gases expand, and are finally expelled by the incoming charge of air.
Among the advantages claimed for the Diesel engine are: firstly, the fact that it will burn any class of oil, refined or crude; secondly, the facility with which it can be started; thirdly, its low fuel consumption, and finally the small space occupied by it. The makers claim for it a mechanical efficiency of upward of 70 per cent., and provided the fuel has a calorific value of not less than 18,000 B. T. U's per pound they guarantee that the consumption at full load will not exceed 0.4 lbs. per b. h. p. hour in the larger sizes and 0.5 lbs. per b. h. p. hour in the smaller sizes. Moreover, the engine runs quietly and as the flash-point of the oil fuel is high there is no danger of explosion.
These many advantageous features render the engine of value for marine service, and during recent years it has been installed on a number of passenger and war vessels, particularly in ships of the British navy. The first passenger vessel propelled by Diesel engines was the “Selandia” belonging to the East Asiatic Company, plying between Copenhagen and Bangkok. Its gross tonnage was 4,964, its length 370 feet, and beam 53 feet. It had twin screws, each driven at 140 revolutions per minute by an eight-cylinder, four-cycle Diesel engine. Its speed was 12 knots and its indicated horse power 2,500. A feature of the boat was that it had no funnels, the exhaust gases being carried away up the mizzen mast.
The great advantage of the Diesel engine over the steam turbine for marine service is the tremendous reduction which can be effected in the weight of fuel. It is estimated that this amounts to only one-fourth to one-fifth of that consumed by a vessel equipped with steam turbines. It follows from this that there is an actual saving in the cost of the fuel where the price of oil is not more than four times that of coal, but it must also be remembered that a vessel equipped with Diesel engines has a cruising radius at least four times as great as a steamship having the same bunker capacity. The latter point is of particular value when considered in connection with war vessels. In the case of passenger and cargo boats, the reduced space occupied by the machinery and its smaller weight are equally important. Allowing for the same bunker space, with its consequent increased cruising radius, a vessel equipped with Diesel engines has 15 per cent, more cargo space than a boat fitted with a steam engine, and with the same cruising radius the reduction in bunker space renders still more room available for cargo. Stokers, moreover, are entirely dispensed with, and the number of men required in the engine room is usually about two-thirds of the number needed in the engine room of a steam vessel.
Although the principles underlying the Diesel engine are simple, its design and construction demand the highest engineering skill and its present state of perfection has been reached only after many years of experiment and investigation. It follows that the engine, although economical to run, is costly to install, and as a result, many attempts have been made to produce a modified form of the engine which should be equal to the original as regards fuel economy, but which should be easier, and cheaper to construct. Most of these modifications seek to avoid the high compression and high pressure air blast which are needed in the Diesel engine, and several very successful types have been designed. These engines are commonly known as “semi-Diesels”. Since the compression of the charge is comparatively low (varying from 125 to 250 lbs. per square inch), some auxiliary igniting device is necessary. The one most commonly adopted is the so-called “hot bulb.” This consists of a bulb-shaped chamber, communicating with the combustion chamber. A portion of the oil fuel is sprayed into this bulb, the rest being delivered into the combustion chamber. The oil in the bulb is heated, at starting, by a lamp, and is thus ignited. The flame produced impinges upon and ignites the oil spray in the main combustion chamber. Once the engine is running, the lamp is no longer needed, as the ignition bulb is kept hot by the combustion of the fuel within it.