Before the engine is started a clutch, is thrown in by a lever and this throws the cams that operate the fuel valves over so that they are cut out and the starting valves are cut in and the engine is made to run by compressed air. After the pistons have made a few strokes and the flywheel has gathered enough momentum the clutch is thrown out and the cams are moved over so that they again work the fuel valves. As soon as the fuel is injected into the cylinders the engine begins to develop power.
Cooling Schemes for Diesel to the high temperatures developed by the Diesel en gine not only the cylinder but the cylinder head and, in large sizes, the piston as well must be water cooled. The water is circulated by a force pump and from 3 to 9 gallons are used per horsepower per hour, the amount being determined by the heat developed.
The Lubrication of Diesel very high temperatures are produced in the cylinders these and the working parts of the engine, such as wrist pins, crankshaft bearings and other bearings in the crankcase, must be oiled with a good quality of lubricating oil having a medium body.
For lubricating the cylinders of the air compres sor only the best light body lubricating oil should be used. Where only a little lubricating oil is fed into the cylinders of the compressor and the com pressed air is well cooled between the successive stages a poor lubricating oil will carbonize and this is the beginning of grief.
But if the intercooling of the air between the com pression stages is not good the high temperature and high pressure are very liable to crack the lubricating oil, that is break it up and liberate the gases in it when it will burn just like a fuel oil does in the cyl inder of an engine. The moral is use good lubricat
ing oils and save trouble.
Advantages of the Two and Four Stroke Cycle Diesel Engines.—The Four Stroke Cycle Engine.— This type of engine is more economical of fuel than a two stroke cycle engine because the expansive force of the burning fuel charge does not stop until the end of the power stroke is reached. Again, no scav tinging pump is needed to force out the burnt gases, and this also saves power.
The cylinders are not heated to so high a tempera ture because the fuel oil is ignited only once in every four strokes, as against once in every two strokes in a two stroke cycle engine. All of the burnt gases are forced out of the cylinder of a four stroke cycle engine on the exhaust stroke, while in a two stroke cycle engine it is quite impossible to get all of the waste gases out.
The Two Stroke Cycle Engine.—Since every other stroke is a power stroke in a two stroke cycle engine the power developed is nearly twice as great as in a four stroke cycle engine hairing the same size cylinder and length of stroke.
This makes the weight of the engine lighter for the same horsepower, and as a two stroke cycle en gine has fewer parts it is easier to move about, set up and keep in repair than one of the four stroke cycle type. Engines under 700 horsepower are often of the four stroke cycle type, but where they are over the above-named horsepower they are generally of the two stroke cycle type. Fig. 68 shows an upright, four cylinder, four stroke cycle, 400 horsepower Diesel engine.