How the Valves Work.—The valves of both the two stroke and the four stroke cycle engines are worked by levers or cams which are operated by gears driven by the crankshaft.
The fuel valve is formed of a needle valve fitted in a nozzle which has a very small opening in it like those described for the larger makes of oil en gines in the preceding chapter.
Since the compression stroke compresses the air in the cylinder the fuel oil must be injected into the air chew by a blast of air which is under a much higher pressure, and to do this an air compressor pump, called an injector pump, is used.
In the original Diesel engines the needle valve sets upright in a little cylindrical fuel reservoir which has a small hole in the bottom of it; the fuel reservoir is fixed in the cylinder head and is con nected to the injector pump by a pipe.
When the pump makes its suction stroke it pulls a small amount of fuel oil into the reservoir. The lift of the needle valve is regulated by the governor, which determines the length of time and the distance it is opened, and, hence, the amount of oil that is pulled into the reservoir.
The instant the needle valve opens, the fuel oil in the reservoir is driven in a fine spray into the com pressed air charge in the cylinder by the blast of high pressure air, when it ignites.
The Air .Compressor, or Injector pump is generally a built-in part of the engine, and is driven by a crank on one end of the crankshaft, as shown in the side cross-section view Fig. 66 and end cross-section view Fig. 67.
The air compressor is of the two, or three stage type, that is the air is compressed twice and some times three times by a succession of pistons and cylinders, each succeeding one being smaller than the other. The one shown in Fig. 66 is a two stage air compressor.
The pressure of the air needed to spray the oil into the red-hot air charge in the cylinder of the , engine is between 800 and 1,200 pounds to the square inch, the pressure depending very largely on the nature of the fuel oil that is used. The air com pressor when in operation takes from 4 to 7 per cent of the total horsepower of the engine to run it.
The Scavenging a two cycle engine a blast of air, called scavenging air, must be used to drive the burnt gases out of the cylinder.
The scavenging air, which is under a low pres sure—usually from 4 to 8 pounds per square inch— is generally obtained from the low pressure cylinder of the air compressor, but in some Diesel engines a separate scavenging pump is belted to the engine.
Again, in some engines the valve which admits the scavenging air to the cylinder is a port opened and closed by the piston sliding across it, while others have a poppet valve in the head of the cylinder.
This latter kind of valve gives the best results for the scavenging air is then forced from the top of the cylinder, clear through it and then out of the exhaust port at the other end, and this thoroughly cleans out the burnt gases.
While it only takes about 4 per cent of the power developed by the engine to supply the scavenging air, where the cylinder is so scavenged there is a great saving effected in the use of the fuel oil, and at the same time there is a larger output in power. The reason for this higher efficiency is found in the fact that but very little of the burnt gases remains behind to.contaminate the fresh fuel mixture.
Governors for Diesel are three distinct kinds of governing devices used on Diesel engines, and these are (1) by controlling the time the suction valve is open, (2) by varying the length of the piston stroke of the fuel injector pump, and (3) by varying the clearance of the pump.
The purpose of the governor is, of course, to give close regulation, and any of the governors described will serve to regulate the speed of the engine close enough for running dynamos, provided it has two or more cylinders.
How a Diesel Engine Is a Diesel engine must be running before it will develop enough heat to ignite the fuel charge, some means for start ing it must be used.
The usual way to start a Diesel engine is to admit air which has been previously compressed in a tank to about 250 pounds to the square inch into the cylinder of the engine through the starting valves, of which there is one to each cylinder.