Page:EB1922 - Volume 31.djvu/552

516 VAPORISER

SHAKER CRATE 1943.

FIG. 6.

per I.H.P. hour; this statement clearly defines the state of devel- opment of the marine producer-gas engine in 1921.

Group 3 (a). Diesel Engines. The Diesel engine has now be- come established as a reliable prime-mover, having a very low consumption of fuel. A typical illustration is in fig. 7.

EXHAUST

FIG. 7.

Within a very strong water-jacketed cast-iron cylinder BB slides a long, heavy cast-iron piston AA driving a crankshaft C by means of the connecting-rod D. The cylinder head EE is a deep, well water- jacketed casting containing four valves, viz. the air inlet, fuel inlet, exhaust, and air starting valve; the latter is not shown.

On the downward stroke of the piston air only is drawn into the cylinder through the air inlet valve; during the following upward stroke this air is compressed to a pressure of 450 to 500 Ib. per sq. in. with accompanying great rise of temperature. At or near the

moment of greatest compression, and continued during the first 20 to 30 of crankshaft revolution, the necessary small charge i fuel oil is blown in the form of a very fine and uniformly diffuse mist into this compressed and heated air. Spontaneous ignition or " explosion " of the mixture instantly takes place, but the fuel supply is so regulated that the pressure is but little, if at all, increased by the explosion, the end aimed at being to cause combustion to take place at as nearly as possible constant pressure; a typical dia- gram is reproduced in fig. 8.

On the cessation of the fuel injection the exploded charge expand at rapidly falling pressure during the working stroke; towards th end of this stroke the exhaust valve opens, and continues open until the completion of the succeeding up-stroke, the burnt gases thence escaping into the atmosphere; this completes the cycle. The engine is thus of the " four-stroke " type, receiving one working impulse in each two revolutions of the crankshaft, the order of op- erations being (l) suction of air; (2) compression of air; (3) working stroke; (4) exhaust.

The valves are operated by rocking levers actuated from a half- speed overhead cam-shaft H, driven from the crankshaft.

Compressed air reservoirs primarily charged and thenceforward maintained by the engine itself-^-containing air stored at a pressure of from 750 to 1,000 Ib. per sq. in., are used for starting the engine, and providing the air blast for the fuel oil injection into the cylinder.

Many Diesel engines are also built operating upon the 2-stroke cycle, and fig. 9 shows a typical design in section.

Here there is no exhaust valve, but towards the end of the down- stroke the piston over-runs exhaust ports AA formed in the cylinder walls whence the burnt gases escape into the atmosphere. Simulta- neously an air pump, operated by the engine, delivers a charge of fresh air at a pressure of 3-5 Ib. per sq. in. into the cylinder through an air valve, or valves, in its head. This air hastens the discharge of the burnt gases, and is compressed on the return up-stroke of the piston at or near the end of which the fuel oil is injected, as be- fore, and the working stroke then follows. This type has been largely developed by Carets Bros, of Ghent ; it will be noted that every down- stroke is a working stroke, but due to the less perfect scavenging of the exhaust, and to the power absorbed by the air pumps, the power output is found to be only from ij to ij times that of an equal