BOEING 737 MAINTENANCE MANUAL

航空发表于 2011-6-14 10:07:28

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航空发表于 2011-6-14 10:08:28

PNEUMATIC STARTING SYSTEM - DESCRIPTION AND OPERATION 1. General A. The pneumatic starting system provides means for rotating the engine to the rpm range where starting can be accomplished when fuel and ignition are supplied. B. The system consists of two pneumatic starters two starter valves, and the associated pneumatic plumbing. It is controlled by two start switches, located on the forward overhead panel. For component location, (see Fig. 1). C. With air pressure in the pneumatic manifold, actuating the engine start switch will supply electrical power to open the starter valve. Low pressure air acts on the turbine blades of the pneumatic starter causing it to rotate. Rotation of the starter is transmitted to the N2 compressor through the accessory drive gear system. When the engine has accelerated to starting speed, application of fuel and ignition, by advancing the start lever, result s in the engine starting. For engine start procedure, see Chapter 71, Power Plant. At starter cutout speed, electrical power is interrupted mechanically, the starter valve closes, and the starting cycle is ended. D. The pneumatic starting system can use low pressure air from three separate sources. Normally, the engines are started with bleed air from the auxiliary power unit (APU). Secondly, the low pressure air can be obtained from a ground source through the ground pneumatic connection (See Fig. 1). The third source of low pressure air is the cross-bleed air from an operating engine. The cross-bleed starting is not desirable because the operating engine must operate at approximately 80% power setting to develop adequate air pressure for starting the other engine.
2. Pneumatic Starter A. The pneumatic starter (Fig. 2) is a lightweight turbine-type air motor which converts the kinetic energy of compressed air into starting torque sufficient to accelerate the engine to starting speed. Low pressure air and electrical power are required for starter operation. The starter will continue to assist the engine until electrical power is removed. B. The starter consists of a scroll assembly, turbine wheel, reduction gear assembly, engaging mechanism and an output shaft. The starter is fitted with a start valve to control the inlet airflow. When the valve is open, it admits air to the inlet connection on the starter scroll assembly; the air then passes through the starter vanes of the scroll assembly and is directed radially inward through the turbine wheel imparting high-speed rotation. Exhaust air from the turbine wheel then passes through an air outlet screen. C. The reduction gear train translates the high speed, low torque of the turbine wheel into low speed, high torque. This output is transmitted through a pawl and ratchet engagement mechanism to the output shaft. From the starter output shaft the cranking torque is transmitted to the N2 compressor by way of the accessory drive gears. A clutch mechanism provides engagement of the reduction gear train with the output shaft for engine starting; when the speed of the output shaft exceeds the speed of the internal gear hub, the clutch mechanism overruns, thus providing automatic disengagement. The pneumatic starter is mounted on the accessory drive gear case beneath the engine.
3. Starter Valve (Fig. 3) A. The starter valve controls airflow to the pneumatic starter during engine starting. The valve is pneumatically operated and solenoid-controlled. It consists essentially of two assemblies; a valve body assembly with lever-operated butterfly valve, and a piston type pneumatic actuator assembly. The valve is located in the pneumatic duct to the starter. It is attached to the duct and starter inlet flange by clamps (Fig. 3). B. Valve P/N 898172-1-1 (AiResearch) (1) Valve inlet pressure is directed tttttttttthrough a pressure port, filter, and orifice, into a balltype solenoid-actuated switcher and through the pneumatic switcher to chamber B. With solenoid de-energized, chamber A is vented and inlet pressure in chamber B keeps the butterfly closed. With solenoid energized, the ball closes the vent, and inlet pressure is ported to chamber A and to the actuator of the pneumatic switcher. The pneumatic switcher closes off inlet pressure to chamber B and connects that chamber to sensing selector. (2) As chamber B vents through the bleed orifice, pressure differential across the actuator piston moves the butterfly toward open position. At inlet pressures lower than set valve (36 +5 psig), sensing selector remains closed and keeps the butterfly in fully open position. When inlet pressures equal or exceed set valve, bleedoff reference regulator opens to maintain a constant reference pressure in chamber A, and the sensing selector opens to direct downstream pressure to chamber B. This modulates the butterfly position to maintain the desired downstream pressure. (3) When solenoid is de-energized, pneumatic switcher and chamber A are quickly vented and upstream pressure is ported to chamber B or closing side of the actuator, causing rapid closing of the butterfly. As downstream pressure drops, sensing selector closes, chamber B pressure rises, assisting the torsion spring in closing the valve completely. (4) An additional pneumatic bleed located on the upstream side of the butterfly vane directs hot air onto the solenoid switcher and pneumatic switcher housings to prevent icing and to reduce moisture during cold weather operation.
3. Starter Valve (Fig. 3) A. The starter valve controls airflow to the pneumatic starter during engine starting. The valve is pneumatically operated and solenoid-controlled. It consists essentially of two assemblies; a valve body assembly with lever-operated butterfly valve, and a piston type pneumatic actuator assembly. The valve is located in the pneumatic duct to the starter. It is attached to the duct and starter inlet flange by clamps (Fig. 3). B. Valve P/N 2730426 (Parker Hannifin) (1) Valve opening is initiated by energizing the solenoid. The energized solenoid moves the 3- way poppet to close the vent to ambient and allow control air to pass through an orifice and into the opening side chamber of the actuator diaphragm. The control air pressure exerts a force on the actuator diaphragm and overcomes the spring closure force thereby opening the butterfly. A slow opening rate is achieved by restricting the flow of high pressure air by the upstream orifice. (2) When upstream pressure is not available at the actuator diaphragm, a compression spring exerts a closing force on the actuator diaphragm to move or hold the butterfly in the closed position. The open side of the diaphragm is vented to ambient through the open passage to the 3-way solenoid positioned valve to prevent any force during the shutoff operation. (3) On airplanes with start valve open light on center instrument panel, the shutoff valve includes an additional port downstream of the butterfly valve for connecting a START VALVE OPEN light pressure switch (Fig. 1 and 3). C. The solenoid switcher can be operated by the manual override button when the solenoid malfunctions (Fig. 3, Detail A). The valve also incorporates a second override feature that requires opening the engine left cowl panel CSD servicing access door and turning the valve butterfly position indicator with an allen wrench (Fig. 3, Detail B). A hole forward of the CSD servicing access door in the cowl panel provides access to the valve manual override button.
4. Operation A. Electrical power for the starting system is obtained from the 28-volt dc battery bus (Fig. 5). The system is controlled by two start switches, one for each engine. The start switches are four position rotary switches, located on the forward overhead panel (Fig. 1). The four positions are: GRD start, OFF, LOW IGN, and FLT start. The switches are momentary in ground start position. Once a switch is positioned to ground start, a holding solenoid in the switch is energized and holds the momentary contacts closed. When the starter cutout switch opens at cutout speed, the holding solenoid is de-energized, and the start switch returns to off position. In all other positions the switch will remain as positioned until another selection is made (Fig. 5). B. Other controls used in conjunction with the starting system are the air conditioning control switches, wing thermal anti-ice control switches and the start levers. Refer to Chapter 21, Air Conditioning; Chapter 28, Fuel; Chapter 30, Ice and Rain Protection; Chapter 36, Pneumatics; Chapter 49, Airborne Auxiliary Power Unit; and Chapter 74, Ignition for integration of these systems with Chapter 80, Starting. C. Engine starting can be performed in the following ways: (1) Using auxiliary power unit (APU). (2) Using ground air source. (3) Using bleed-air from the other engine. D. Starter valve inlet pressure limits: NOTE: The maximum pressure limits shown in the table are just guides to normally expected values of these pneumatic starting sources.

baokan 发表于 2011-6-14 17:11:04

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chengrichie 发表于 2011-7-12 08:38:33

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altairrr 发表于 2014-6-17 09:44:25

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fxhuang_07 发表于 2014-8-23 20:32:33

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BOEING 737 MAINTENANCE MANUAL