Compared to direct diesel drives, diesel electric propulsion systems are technically and operationally superior in virtually all applications. This superiority has been a major reason for the steadily growing demand for diesel-electric main drives in marine engineering applications.
Electrical propulsion system offers numerous advantages for ships that are subject to specific requirements. They are rated as particularly economical, environmentally friendly and reliable, offer considerable comfort in terms of operation and control, have optimal manoeuvring and positioning properties, low vibration and noise levels, and additionally enable the best possible utilization of space owing to their reduced noise levels.
The electrical side of all systems will be based on a direct current or an alternating current motor, coupled to the ship’s propeller shaft, with the speed and direction of propeller rotation being governed by electric control of the motor itself or by the alternation of the power supply.
Layout of Diesel Electric Propulsion
The electrical propulsion arrangement for a ship is often described as a diesel-electric or turbo-electric system. It is characterized only by the type of prime mover with no reference to the type of electrical propulsion motor. When the prime mover is a diesel engine, then it is called Diesel-Electrical Propulsion. The most commonly used diesel electrical propulsion systems are not a new concept. In the past these systems were usually diesel engine driven D.C generators that supplied power to D.C motors. Their applications were generally limited to vessels that required a degree of low speed manoeuvring.
Vessels such as ferries, harbour tugs, and various other applications used diesel electrical systems for features that were not available in mechanical systems at that time like speed control and manoeuvrability. To date, electrical propulsion systems have been used mainly for specialized vessels rather than for cargo ships in general. These include dredgers, tugs, trawlers, lighthouse tenders, cable ships, ice breakers, research ships, floating cranes, and vessels for the offshore industries. Electrical-drive systems have made substantial progress in recent years.
Types of Diesel Electric Propulsion
The two systems dominating the market today are Frequency controlled A.C Motors and SCR controlled D.C Motors.
Frequency controlled A.C Motor drive system were generally more cost effective below 500 H.P and SCR controlled D.C motors systems at the higher end. The offshore drilling industries favour SCR controlled DC drives.
Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue. The deep draft cruise ship industry, due to the high hotel-like power requirements, is adopting high-power diesel electrical propulsion systems in most of its new builds.
Both technologies have a proven record of efficiencies and reliability. For a direct current propulsion motor, the electrical power may be from one or more DC generators or may be form an alternator and then delivered through a rectifier as a DC supply. The power for direct current motors is limited to about 8 MW, and so AC machines are used for high outputs unless an effort is made to install DC motors in tandem. The rectification scheme can incorporate speed control and a means of reversing.
Power for AC propulsion motor is supplied obviously by an alternator; the prime movers may be a diesel engine, a gas turbine, or a boiler and steam turbine installation.
The choice of diesel electrical system as the power source for a propulsion system of a vessel has nothing to do with hydrodynamic efficiency. The propulsion system of a vessel provides thrust to move the vessel and is still chosen by the designer based on merits for the vessel’s application. Conventional propellers, controllable pitch propellers, azimuthing Z drives, transverse tunnel thrusters, and low speed water jet systems can be driven with equal effectiveness by a diesel-electrical system.
Diesel-electrical propulsion becomes viable when the installed KW for propulsion approaches or is exceeded by the KW installed for other purposes. The convenience of electric power distribution makes it possible to optimally locate the primary power source, i.e. diesel generators, exclusive of consideration as to whether it is for propulsion, thrusters, or cargo handling purposes. A large variation in propulsion power requirements, such as long periods of low speed operation or the necessity to shift power from main propulsion to thrusters for dynamic positioning purposes, can also justify diesel electric systems.
Modern turbo-charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long period of low speed, low load, low RPM, high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto-start, and load shedding features make it possible to efficiently utilize the installed horsepower of a diesel electrical system.
Lay Out of Diesel Electric Propulsion in Ships
Advantages of Diesel Electric Propulsion
Diesel electrical propulsion can overcome the following design problems:
When propulsive or station-keeping power requirements are a small or relatively small percentage of total power requirements, research vessels with special manoeuvring requirements, and gaming vessels where speed is inconsequential (such as a gaming vessel operating in a river).
When space and propulsion machinery limitations either exclude the use of direct diesel or adversely affect the construction costs resulting from using direct diesels:
1. Vessels with hull and struts too small to accommodate diesel engines, access, ventilation, etc.
2. Vessels with potential trim problems, such as stern wheelers, where machinery needs to be located forward to avoid trim problems.
3. Vessels that require, due to space limitations, more than one machinery space are subject to increased construction cost due to duplication of increases in system such as: engine cooling, space ventilation, control facilities, exhaust, etc.
4. Vessels that have a large variation in power consumption.
The fact that the propulsion power may be supplied by an electric motor instead of a direct driven diesel engine does not makes equipment aboard the vessel any less familiar to the operator.
The utilization of the diesel engines is transferred from direct propulsion power to generate power. This provides greater flexibility in the use of installed KW, and in some instances, reduces the number of diesel engines installed. The ability to generate only the power required to meet the needs of the duty cycle of vessels utilizing multiple generator sets reduces fuel consumption and maintenance cost. It also provides redundancy in power capacity.
Salient Features of Diesel- Electric Propulsion
1. Economic Reasons
Diesel electric propulsion is especially economical for a number of reasons:
- Optimal utilization of fuel for diesel engines to generate electrical power, even in partial load ranges.
- High efficiency across the entire speed range.
- Reduced maintenance costs through longer service intervals based on the optimized operating times of diesel engines with constant speed.
- Minimal standstill time for maintenance and service.
- Flexible and need-oriented use of diesel generator sets in combination power plant for drives and on-board power systems.
Diesel electric propulsion systems demonstrate high availability for reasons that include:
- Modular design with small probability of total loss of propulsion power.
- Sharply reduced number of moving mechanical parts.
- Proven technologies based on decades of operating experiences.
- Redundant drives with one propeller are also possible.
- Designs are also possible for maximum redundancy requirements.
3. Environmental Compatibility
Diesel electrical propulsion systems protect the environment because the pollution emissions of diesel engines are reduced by operating the engine at the optimal speed and load ranges.
4. Operating Convenience
Diesel electrical propulsion is very convenient for the users, because of the following:
- Excellent dynamic response from zero to maximum propelling speed.
- Short reversing time.
- Availability of maximum torque across the entire speed range at the propeller.
- Quite operation.
- Minimum mechanical vibrations.
- Flexible arrangement of components in the ship.
- Simplified mechanical requirements for the propeller shaft.
- Reduced space requirements in the shaft system.
- Design and engineering of propeller is independent of the drive.
- Flexibility in the choice of diesel engine speed.