Hydraulic starting systems are used on various small diesel engines. In our discussion, we will use the General Motors hydrostarter system as an example.

Figure 10-7.-Hydraulic starting system.

has no detrimental effect upon the hydrostarter system. Engine starting torque for a given pressure will remain fairly constant regardless of the ambient temperature.

The hydrostarter system consists of a reservoir, an engine-driven charging pump, a manually operated pump, a piston-type accumulator (with a fluid side and a nitrogen side), a hydraulic, vane-type starting motor, and connecting lines and fittings. Hydraulic fluid oil flows by gravity (or by a slight vacuum) from the reservoir to the inlet of either the engine-driven pump or the hand pump. Fluid discharged by either pump is forced at high pressure into the accumulator and is stored at approximately 3250 psi under the pressure of compressed nitrogen gas. (Nitrogen is used instead of compressed air because nitrogen will not explode if seal leakage permits oil to enter the nitrogen side of the accumulator.) When the starter is engaged with the ring gear on the fly-wheel of the engine, and the control valve is opened, high pressure fluid is forced out of the accumulator by the action of the piston from the expanding nitrogen gas. The fluid flows into the starting motor, which rapidly accelerates the engine to a high cranking speed. When the starting lever is released, the spring action disengages the starting pinion and closes the control valve. This action stops the flow of hydraulic oil from the accumulator. The used fluid returns from the starter directly to the reservoir. (See directional arrows in fig. 10-7.)

During engine operation, the engine-driven charging pump runs continuously and auto-matically recharges the accumulator. When the required pressure is attained in the accumulator, a valve within the pump body opens and the fluid discharged by the pump is bypassed to the reservoir. When the system is shut down, the pressure in the accumulator will be maintained.

The hand pump of the hydrostarter system is a double-action piston pump. It serves to pump fluid into the accumulator for initial cranking when the accumulator has exhausted all the fluid stored in it. The starter is protected from high speeds of the engine by the action of an over-running clutch.

The hydraulic starting system may be used with most small engines now in service without modification other than the clutch and pinion assembly, which must be changed when a conversion from a left-hand to a right-hand rotation is made.