In modern propulsion systems, an integrated system of pneumatic, hydraulic, and electric circuits provides control of the speed and direction of the propeller shaft. Each control system function may use only one of these three mediums, a combination of two, or a combina-tion of all three. The choice of control medium in each instance is based on the performance of a given control function in the most reliable and efficient manner. In general, the different control mediums are used in the following functions:

1. The basic control medium is pneumatic, and the majority of propulsion control functions are performed by pneumatic circuits.

2. Hydraulic circuits are used whenever a large amount of control element actuating power is required, such as when pitch is applied to controllable pitch propellers.

3. Electric circuits are used extensively for the sensing and indicating of control system conditions and for alarm systems.

The basic requirements for a propulsion control system are threefold. First, it must control the main engines load, keeping the engines equally loaded. Next, it must maintain propeller shaft speed and direction. Finally, it must maintain the desired pitch since gas turbine ships and most diesel-driven ships have controllable pitch propellers.

The engine speed and load are controlled by the governor of each main engine, through a pneumatic signal sent to the governor, which increases or decreases the tension of the speeder spring.

Control of the propeller shaft speed is done by control of the main engine speed as stated previously. Propeller shaft direction of rotation, ahead or astern, on noncontrollable pitch propeller ships is controlled by clutches and reduction gears. (These mechanisms will be discussed in chapter 12 of this manual.) In ships with controllable pitch propellers, the pitch is controlled by a signal, either pneumatic or hydraulic, that is sent to an oil distribution (OD) box. There, the signal is converted to a high-pressure hydraulic force which actuates the propeller blades through a piston and cylinder assembly in the propeller hub.

In most ships with propulsion control systems, the machinery can be operated from three different locations. Local control is usually from a panel mounted on or near the machinery to be operated. The local control station is used for the operation of a single unit, such as one main engine, or for setting the pitch on one propeller. The enclosed operating station (EOS) has a console for the operation of one complete propeller shaft, including main engines, propeller pitch control unit, clutches, and all other machinery required for propulsion. On large ships, there may be one EOS for each propeller shaft. On smaller ships, one EOS is used to control and monitor all propulsion machinery for the ship. The third operating station is the pilot-house console, which controls propeller shaft speed and pitch or direction. Generally, this station cannot control the starting or stopping of main engines, operate clutches, or control other individual pieces of propulsion machinery. Both the EOS console and pilot-house console will have instruments that indicate shaft rpm, propeller pitch, and other indicators required for the monitoring of the propulsion plant. Generally, propulsion control systems should operate trouble-free for many years with a minimum of care.

Pneumatic systems need a constant source of clean dry air to operate correctly. If the supply of air is dirty or contains oil or water, the various control valves throughout the system will stick and cause malfunctions. All connections should be checked periodically. If leaks should start, a drop in line pressure will create a faulty signal. You can locate any leaks in a pneumatic system by brushing a solution of soapy water on the connectors of the system. An air leak will be indicated by bubbles that will form at the site of the leak.

The worst enemy of any hydraulic system is dirt. Dirt that is allowed to enter the system, either when oil is being added or when other work is being performed, will create problems. Dirt will cause the extremely close clearances of parts in hydraulic components to become damaged. Also, dirt will cause the valves in the system to malfunction.

Electrical systems are all but trouble-free with little routine maintenance required. Occasionally, problems occur because of loose connections or from component failure.

Most propulsion control systems will use a combination of all three mediums: pneumatic, hydraulic, and electric. In troubleshooting a malfunction, you should check only one medium at a time until you can isolate the trouble and make the necessary repairs.

SUMMARY

You should be familiar with factors related to combustion and how these factors affect diesel engines. Know the meaning of turbulence and precombustion and the significance of each to the combustion process of a diesel engine.

In our discussion of fuel systems, we can never overstress the importance of clean fuel. Devices that help maintain the quality of fuel and oil are known as purifiers. Purifiers may be of the disc or tubular type. These purifiers differ in construction and method; however, both operate to remove water and dirt from fuel by centrifugal force. As you can see from the information you have read in this chapter, a great deal of importance is placed on keeping the fuel system in a high state of purity.

From reading our discussion of fuel injection systems, you should be aware of three types of mechanical diesel fuel injection systems: (1) jerk-type, (2) distributor-type, and (3) unit fuel injector type, and how these three systems differ in construction and the methods by which fuel injection is achieved.

Our discussion of fuel injection would not have been complete without an explanation on how the amount of fuel delivered to the cylinders is controlled. The control of the engine speed is dependent on speed-sensitive devices known as governors. There are two basic types of governors, mechanical and hydraulic. The mechanical governor is used when extremely sensitive operation is not required. The hydraulic governors are more sensitive to speed variations. They are quicker acting and more accurate due to the reduction in friction of the mass of moving parts.

A complete understanding of fuel injection and engine control is a must if you are to operate a diesel in a safe and effective manner. If you are uncertain about any of the information in this chapter, we recommend you reread the sections that are giving you trouble before you continue to chapter 10.