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