All About Engines

25o All About Engines oil engines, and less expensive to run because they used cheaper fuel. The semi-Diesel engine has a compression which may vary in different makes from 120 lb. to 350 lb. on the square inch. In construction they are more like the ordinary oil engine, having a combustion chamber at the end of the cylinder. Perhaps the most interesting feature is the way in which the fuel supply is controlled by the governor. The pump plunger is operated by a cam acting through a bell- crank lever, and the oil is sucked up at every alter- nate stroke from the tank through the left-hand tube. From the pump barrel it has two possible paths—one through the wide tube to the combustion chamber, and the other through the narrow tube on the right, which passes down behind the pump and into the tank from which the oil was originally drawn. But the second path is not always open. It is nor- mally closed by a valve, the spindle of which is pushed in by an arm on the plunger, and at other times remains closed by the pressure of the oil in the pump. Between the arm and the valve spindle is a wedge which hangs from a lever operated by the governor. As the speed increases, this wedge is raised, the thicker portion is interposed, the upper plunger is pushed in, and the overflow valve, as it is called, is opened. Some oil then flows back into the tank instead of passing to the engine. Generally speaking, for small powers the ordinary oil engine is used, for moderate powers the semi-Diesel, and for large powers the Diesel engine.