Adjustable Injection Air Pressure.—The engineer can appre ciate the necessity of having a higher injection air pressure when the engine is carrying full load than when under a light load. When the fuel charge delivered to the fuel valve is large, as on full load, the resistance or "braking" action of the atomizer is high and requires a high pressure to force the entire charge of oil into the cylinder. light loads, the oil occupies only part of the a atomizing space, nd consequently a light air pressure is suffi cient. If the pressure is high on light loads, the oil is blown into the cylinder at an increased rate. The passage of the fuel would then require only part of the time during which the needle valve is opened. The remainder of the period of valve, opening would be devoted to the passage of pure air. The high velocity of the free air as it left the nozzle tip would chill the tip and lower the entire cylinder temperature, causing a decreased cylinder efficiency as well as a direct loss of air that has been compressed at a considerable expense of power. Furthermore, if the air pressure is high on low loads, a sharp knock is produced in the cylinder which results from the inrush of air at a pressure far above cylinder pressure. Conversely, if the air pressure is too low the engine will smoke since'the fuel has not been sufficiently atomized.
It is necessary for the successful operation of any Diesel that the injection air pressure be altered to conform to load change. This adjustment can be under manual control of the engineer, as is the general practice. The manual control can be obtained in several ways. The McIntosh & Seymour Marine Engine is pro vided with a clearance chamber on the low-pressure cylinder, whose volume can be altered, changing the air discharge pressure. Other builders arrange for the operator to adjust the low-pressure suction, obtaining the required air-pressure control. However, on fluctuating loads, this entails constant attention and is more suitably handled by some automatic arrangement. There are several designs of automatic injection control. The Busch Sulzer Diesel throttles the compressor suction through a linkage from the engine governor. The Standard Fuel Oil Engines, as has been outlined, use a governor-controlled air by-pass valve. These varied control arrangements will be taken up in the dis cussion on air compressors.
Adjustable Fuel Valve Timing.—The usual Diesel engine fuel valve is designed with a constant period of valve opening, regardless of load conditions. In the Otto-type explosive engine the efficiency of the engine depends on the maximum explosive pressure. With the Diesel engine the efficiency depends both on the combustion pressure, which should be identical with the maximum compression pressure, and on the duration of the fuel injection. It is very clear that with load changes the time during which the fuel is injected should also vary. Since the rate of combustion should be constant, the period of injection must vary if the greatest possible effi ciency is to be secured. Furthermore, a factor of operation also
enters into the problem. On low loads the amount of oil is small and will be entirely blown into the cylinder long before the valve closes. The balance of the valve opening period is taken up with the injection of high-pressure injection air. This air assists in no way toward the combustion. For these reasons several European builders, as well as the Busch-Sulzer Co., have adopted a form of injection timing control along the lines of the servomotor in Fig. 116.
Timing of Fuel Valves.—In timing a fuel valve the engine is pinched over until it is several degrees ahead of the desired point of fuel valve opening. The air line valve is "cracked," giving about 75 lbs. air pressure on the fuel valve. The indi cator plug is removed, and the engine is slowly barred over until the trammel cuts the opening mark on the flywheel. The injection valve should now start to open, as evidenced by the sound of injection air blowing into the cylinder. If the valve opens before the mark is reached, the rocker clearance can be increased, producing a later opening. If the valve opens late, the clearance can be reduced. The engine should be barred on to the closing mark, and the sound of the escaping air should cease as the mark is reached. Since the roller clear ance has been altered to make the opening earlier, the closing point will probably be late. It then becomes necessary to turn the engine back ahead of the valve opening mark and shift the cam nose. The nose should be shifted to produce the re quired opening with the roller clearance correct. Then, on checking the closing point, it should either be correct or early. If the latter, the nose must be shifted back a trifle and the roller clearance made less. This should produce the required opening and closure. If the nose is excessively worn, it is impossible to obtain a correct timing, and a new nose must be secured.
Back Lash.—The camshaft gears are not immune to wear, and the course of five to seven years of constant service the back lash between the gears becomes noticeable. The clear ance between the gear teeth has a very detrimental effect on the injection cam. As the cam nose contacts with the valve rocker roller, the pressure that the spring offers against the rocker movement is considerable. As the roller travels over the sur face of the cam nose and starts down along the back slope, this spring pressure forces the camshaft forward, causing the valve to close early. Since the wear is between the teeth, the camshaft is already behind its exact timing with the engine shaft, and consequently the opening of the fuel valve is late and the closure is early. A new cam nose of greater length will partially over come the defect, but new gears should be ordered to replace the worn ones.