Figure 226 shows an indicator card from this engine. The card reveals combustion conditions that are not along the true Hvid plan. It would appear that the primary charge did not explode until dead-center and that the piston retreated some distance before the pressure difference caused the injection of the main charge and its consequent combustion. Under such injection conditions a dull thump would be heard, produced by the burning gases striking the receding piston. If the injection had been early, as is usual, a sharp sound would have been emitted as the primary charge impinged on the advancing piston.
Fuel Nordberg semi-Diesel engine is 'equipped with a fuel pump shown in Fig. 227. The pump plunger is driven by an eccentric keyed to the engine shaft, while the fuel charge is regulated by the closure of the by-pass valve V which is under governor control. The fixed eccentric moves the pump plunger P on the outward stroke, drawing in a charge of oil. As the plunger reverses and moves inward on the discharge stroke, the oil, displaced by the plunger, escapes through the by-pass valve, which is held open by the cam F. - This cam is rocked through an angle by the governor eccentric rod G. At the proper time the cam is moved to its mid-position, as shown in Fig. 228. The by-pass valve closes, and the oil displaced by the further movement of the pump plunger passes through the discharge valve H and enters the fuel atomizer.
228 outlines the relative positions of the governor and pump at the beginning of the fuel-injection period. The governor is of the inertia type and carries a pin B to which is fastened the by-pass valve rod G. The governor is fulcrumed at ,T on the flywheel; the position of the governor pin,B is such that the by-pass valve is just closed when the engine crank is A on full load. As the engine revolves, the cam moves away from the by-pass valve stem and, returning, opens it when the point B has moved 180 degrees. Figure 228 shows the crank position A when the cam has just closed the by-pass valve. I is the position of the pump eccentric at this time; this gives a total maximum discharge angle of 30 degrees, as shown. D represents the position of the crank at no load; this gives a discharge angle of 5 degrees. From the figure it is evident that the oil is injected before mid-stroke of the engine piston.
Valve Timing.—The timing diagram of the Nordberg semi Diesel appears in Fig. 229. Since this engine is two-cycle, the
piston acts as the exhaust and air-admission valves; the exhaust and admission periods are dependent on the location of the ports in the cylinder and are fixed.
Nordberg Engine 230 illustrates the engine section and plan. The frame is of heavy construction and has two main bearings. The piston is provided with a piston rod and crosshead. The front end of the cylinder A is closed and acts as the air compressor to furnish the scavenging air charge to the power cylinder. The air enters the compressor through the piston valve V.
This engine employs water injection to control the cylinder temperature, thus preventing preignition. The operator- should examine the cylinder at frequent intervals as the water often scores the piston and cylinder. The question of water injection is fully discussed in the chapter on Water Injection.
Muller Ignition.—While no American firm has adopted this design, it is, nevertheless, of interest. This is especially true since some claim that the American engines using the Hvid principle are actually operating along the lines covered by the Muller patent.
This ignition or injection method has simplicity to recommend it since it does not require either an air compressor, as does the Diesel, or a fuel injection pump, as found on the majority of semi-Diesels. The plan which is illustrated in Fig. 231 is as follows: The cylinder head is provided with two compartments. Into one of these runs the fuel line from the measuring device controlled by the governor. This cavity is connected to the engine cylinder by means of a needle valve. The valve is better shown in the drawing to the right. It has a drilled passage and is actuated from the engine camshaft, the timing of which allows the cylinder to be in communication with the fuel chamber during the engine suction and compression stroke. During the early part of the compression stroke the fuel is deposited in the space around the hollow valve stem. At the end of the com pression stroke this valve is raised and, in so doing, cuts off the communication of the cylindei with the portion of the fuel chamber above the surface of the fuel charge. The drawing illustrates the manner by which the valve stem, in raising, brings the cross passages up into the casting.