Reverting to the action of the pump,, if the regulator be ad justed to stop the plunger on its return stroke at P', then the eccentric will strike the plunger when the crank is along the line OH, a' degrees ahead of dead-center. The total pump stroke would then be P'R. This is not enough to keep the engine up to speed. The eccentric increases its throw with its center at A'. The eccentric then strikes the plunger at G, fi degrees from dead center, continuing to inject oil until dead-center at R' is reached. The total pump stroke S' equals 8, the original plunger travel. It is evident, then, from the diagram that the point of beginning of injection occurs liter in the compression stroke.
It follows that the regulator may be used to advance or re tard the point of admission. If the fuel be light and displays a proneness to preignite, the regulator can reduce the pump stroke. The eccentricity increases as a 'result of the slight decrease in speed, and, while the pump stroke is now the same as before, the injection is later. This will assist in reducing the preignition. The reversal of this action may be used if the oil is very heavy, re quiring a longer time for vaporization. This manual adjustment cannot well be used to prevent preignitions which occur with varying load conditions, since it would require constant attention from the operator.
Little Giant Engine Governor.—Figure 308 outlines schematic ally the governor used on the Little Giant engine. The gov ernor sleeve A is connected by a system of links to a cam B that bears against a collar C on the pump plunger D. The eccentric is keyed to the engine shaft and moves the pump plunger through a reach-rod, not shown. Any movement of the cam will allow the plunger to take a longer or shorter suction stroke, as the case may be, and thus will regulate the amount of oil injected into the cylinder.
The spring-loaded governor is belted to the crankshaft, and any movement of the balls is communicated to the cam. It is apparent that each variation in the return stroke of the pump plunger through a movement of the cam causes the injection angle to change. Since the eccentric is keyed to the shaft, the crank turns until the eccentric push-rod strikes the pump plunger. If the plunger has had a short return stroke, then the crank must turn through a greater angle before the eccentric rod comes into contact. Unlike the pumps previously discussed,-,there is no way to change the injection angle other than by a permanent shifting of the fixed eccentric. All preignitions must be con trolled by means of the water injection.
Little Giant Fuel Pump.—Figure 309 shows the pump used in
conjunction with the above governor. It is a very simple casting and is provided with ball valves, both for the suction and for the discharge line. It will be noticed that each line is fitted with two ball valves. This is unnecessary since leakage is evident only when both balls fail; consequently one may leak without detection, bringing conditions back to the use of a single ball. If the operator runs out of spare valves, the pump will operate quite successfully with one set. A ball-bearing ball, which can be procured at any garage, makes a very satisfactory valve. The governor is set to inject the oil at approximately 90 degrees ahead of dead-center.
Governors with Constant Injection Angle.—A governor of a design that will maintain a constant fuel injection point, regard less of the load carried, is without question the most advantageous. Such a governor will relieve the engine of preignition shocks, when these are due to too early injection and not to poor gas stratification. In an engine where the oil is vaporized directly in the cylinder, preignition may occur practically regardless of the injection point. The vaporized fuel and the air are fairly well mixed as soon as the oil is sprayed into the cylinder. If the temperature existent in the cylinder, resulting from the heat absorbed from the previous charge as well as from the compres sion, is high enough, the charge will burn considerably before the crank reaches dead-center; in fact, preignition has occurred when the piston is at mid-stroke. On the other hand, if the fuel is introduced at a point but little ahead of dead-center, there can be no preignition.
With governors that vary the angle of advance with the load, there is no danger of preignition at low loads. The angle of advance, or the injection angle, at low loads is small, and the fuel is injected but little ahead of dead-center. On heavy loads, due to the early injection, the danger of preignition is consider able. The contrary is true of the governor with a fixed injection point. If the injection point be such as to preclude preignition at light loads, there is but little danger of full-load prematures; however, if the eccentric is so set as to cause the engine to pre mature on heavy loads, preignition will probably occur even at light loads. The statement that the governor with a fixed in jection point is the better in operation applies only where a com bustion chamber is used. In this construction the injection point selected must be a favorable one for full-load conditions.