Electric Starting Systems

Figure 7-11.—Electric starting system. charging the battery with an engine-driven generator. The main components of the electric starting system, as shown in figure 7-11, are the battery, cranking motor, and associated control and protective devices. Electric Starting Systems The starting motor for diesel and gasoline engines operates on the same principle as a direct current electric motor. The motor is designed to carry extremely heavy loads but, because it draws a high current (300 to 665 amperes), it tends to overheat quickly. To avoid overheating, NEVER allow the motor to run more than the specified amount of time, usually 30 seconds at a time. Then allow it to cool for 2 or 3 minutes before using it again. To start a diesel engine, you must turn it over rapidly to obtain sufficient heat to ignite the fuel. The starting motor is located near the flywheel, and the drive gear on the starter is arranged so that it can mesh with the teeth on the flywheel when the starting switch is closed. The drive mechanism must function to (1) transmit the turning power to the engine when the starting motor runs, (2) disconnect the starting motor from the engine immediately after the engine has started, and (3) provide a gear reduction ratio between the starting motor and the engine. The drive mechanism must disengage the pinion from the flywheel immediately after the engine starts. After the engine starts, its speed may increase rapidly to approximately 1,500 rpm. If the drive pinion remained meshed with the flywheel and also locked with the shaft of the starting motor at a normal engine speed (1,500 rpm), the shaft would be spun at a rapid rate (22,500 to 30,000 rpm). At such speeds, the starting motor would be badly damaged. Hydraulic Starting Systems There are several types of hydraulic starting systems in use. In most installations, the system consists of a hydraulic starting motor, a piston- type accumulator, a manually operated hydraulic pump, an engine-driven hydraulic pump, and a reservoir for the hydraulic fluid. Hydraulic pressure is provided in the accumu- lator by the manually operated hand pump or from the engine-driven pump when the engine is operating. When the starting lever is operated, the control valve allows hydraulic oil (under pressure of nitrogen gas) from the accumulator to pass through the hydraulic starting motor, thereby cranking the engine. When the starting lever is released, spring action disengages the starting pinion and closes the control valve. This stops the flow of hydraulic oil from the accumulator. The starter is protected from the high speeds of the engine by the action of an overrunning clutch. The hydraulic starting system is used on some smaller diesel engines. This system can be applied to most engines now in service without modification. Air Starting Systems Starting air comes directly from the ship’s medium-pressure (MP) or high-pressure (HP) air service line or from the starting air flasks which are included in some systems for the purpose of storing starting air. From either source, the air, 7-12