Principles of naval engineering - Historic Naval Ships Association

Chapter 23. -GAS TURBINEThe starting and ignition circuits receivepower from storage batteries. The warning andsafety circuits receive power from the ship'spower supply panel. Power for the indicatingcircuits is self generated by thermocouplesand other units in the circuits.Ehgine StartersOf the various methods used for starting gasturbine engines, the three most common devicesare the air turbine, the hydraulic starting device,and the electric starter-generator.The air turbinestarter is a turbine-air motorwith a radial inward-flow turbine wheel assemblyand an engaging and disengaging mechanism.Compressed air is supplied to the air turbinefrom an external source.The hydraulic motor starter consists of amotor-driven hydraulic pump mounted separately.It supplies high pressure hydraulic oilto the hydraulic motor starter, which is mountedon the accessory pad along with its engagingand disengaging mechanism. The hydraulicmotor starter is quite similar to the air turbinestarter; however, the hydraulic motor starteris usually used for larger and higher horsepowergas turbine engines.The electric starter- generator is a shuntwoundd-c generator with compensating windingsand a series winding, using a 24-volt batterypower source. The generator is usually mountedon the accessory drive pad. The generator is sodesigned and controlled that it can be used as anengine starter. When the designed engine speedis reached, the starter- generator is automaticallyswitched from a starter to a generator.TRANSMISSION OF ENGINE POWERThe two main types of gas turbine engineinstallations used for ship propulsion are (1)the geared drive, and (2) the turboelectricdrive.The fundamental characteristics of the gasturbine engine make it necessary for the drivemechanism to change both the speed and thedirection of shaft rotation in the driven mechanism.The process of transmitting engine powerto a point where it can be used in performinguseful work involves a number of factors, twoof which are torque and speed. The gas turbineengine does not produce high torque, but it doesproduce high speed. Therefore, a gear train isused with most gas turbine engines to lowerspeed and increase torque. This is true inboth types of installations. In the case of thegeared drive installation, the gears are usedbetween the gas turbine engine and the propellershaft. In the case of the turboelectric drive,the gears are usually used between the gasturbine engine and the generator shaft, to reducethe rpm of the generator to a practicable operatingvalue.The propelling equipment of a boat or shipmust be capable of providing reversing poweras well as forward power. In a few ships andboats, reversing is accomplished by the use ofcontrollable pitch propellers.^ In mostvessels,however, reversing is accomplished by the useof reversing gears.Reducing the speed of rotation and reversingthe direction of shaft rotation are not the onlyrequirements of the drive mechanism of a shipor boat. It is also necessary to make some provisionfor the fact that the engine must be ableto operate at times without transmitting powerto the propeller shaft. In the electric drive,this is no problem because the transmissionof power is controlled electrically. With thegear type of drive, however, it is necessary toinclude a means of disconnecting the enginefrom the propeller shaft. Devices used for thispurpose are called clutches .The arrangement of components in a geartypedrive varies, depending upon the type andsize of the installation. In some of the smallinstallations, the clutch, the reversing gear,and the reduction gear may be combined in asingle unit. This type of arrangement is shownin figure 23-20. In other installations, the clutchand the reversing gear may be in one housing andthe reduction gear in a separate housing attachedto the reversing gear housing. Drive mechanismsarranged in either manner are calledtransmissions .GAS TURBINE ENGINES ANDJET PROPULSIONThus far, we have considered the gas turbineengine as a prime mover which delivers powerin the form of torque on an output shaft. In concludingthis chapter, it should be noted thatthe gas turbine engine also serves as the primeControllable pitch propellers are discussed in chapter5 of this text.611