Mechanics: The Science of Machinery

by A. Russell Bond

In one type of engine, known as the semi-Diesel, a lower compression is used in the cylinder. This is not high enough to ignite the oil that is sprayed in, and so a hot tube or bulb is employed at the top of the cylinder against which the fuel jet is directed. This bulb is heated to a dark cherry red by means of a torch until the engine has developed enough heat to keep the bulb at the requisite temperature.

THE DOUBLE-ACTING JUNKER ENGINE

FIG. 52.—THE JUNKER ENGINE IN WHICH TWO PISTONS RECIPROCATE TOWARD AND AWAY FROM EACH OTHER

An interesting modification of the Diesel engine is the Junker engine (Figure 52) in which the cylinder consists of a tube open at each end. In this there are two pistons which reciprocate toward and away from each other. Air is compressed between them as they approach each other and fuel is injected into this air, ignites and forces them apart. Both pistons are connected to the same crank shaft, one pushing down and the other pulling up. The pulling piston has a yoke on the end of the piston rod from which a pair of connecting rods run down at either side of the cylinder to a pair of cranks on the crank shaft. Between these cranks on the opposite side of the shaft is the crank to which the pushing piston is connected. The advantages of this arrangement are that the moving masses are perfectly balanced, the construction of the cylinder is very simple and especially adapted to high pressures, and the reaction of the gases, instead of being directed against a fixed part of the engine, is directed against a moving piston, thus reducing the strain on the structure. Of course the power is not doubled or increased, because each piston moves only half as far as it would for a given expansion of gas were it operating in a cylinder closed by a cylinder head.

The efficiency of Diesel engines, although greater than that of the gas and gasoline engines, is still very low. The semi-Diesel will yield about 30 per cent of the energy in the fuel, the Junker engine about 34 per cent, and the best Diesel, four-cycle engine in large units, about 36 per cent. This is a wonderfully high efficiency compared with that of a locomotive, and yet it seems pitiably low when we consider that nearly two-thirds of the energy stored in the fuel is thrown away.

SILENCING THE NOISY EXHAUST

The waste of energy is clearly evident in the exhaust pipe of an internal combustion engine. The gases after doing their work on the piston rush out with such velocity as to produce a sharp explosive sound. The noise of the exhaust is highly objectionable in automobiles and must be overcome, but instead of utilizing the boisterous energy of the escaping gases and getting a little more useful work out of them, means are provided for hushing their noise. This is done by passing them through a series of baffles which reduces the pressure of the gases before they are discharged into the atmosphere. By letting the gases expand gradually instead of bursting suddenly into the atmosphere the noise of the discharge is reduced. However, this must not be done at the expense of the engine power. Unless the gases pass quickly through the muffler they will choke and retard the exhaust and make the engine do useless work in driving them through. Even the best of mufflers will use up nearly 5 per cent of the engine power. The energy that escapes at the exhaust is not the only loss. All the heat that radiates from the engine represents just so much wasted energy. In our water and air-cooled systems we deliberately abstract heat from the burning gases and throw it away. If we had materials that would stand the intense heat of burning gases and enable us to conserve all the heat developed in the cylinder we could use our fuels much more economically.

Many attempts have been made to utilize the wasted heat of internal combustion engines, but they have not met with any considerable degree of success, with the exception of the invention of William Joseph Still. This engine, which was the result of many years of patient work, was first made public in a paper read before the Royal Society of Arts in London, May 26, 1919, and when it was shown that the new engine had developed an efficiency of 38 to 41 per cent it was realized that here was a remarkable advance over any other machine for turning heat into power.