TM  1-1520-240-10            2-3-1 SECTION  III.  ENGINES AND RELATED SYSTEMS 2-3-1.  Engines. The CH-47D is powered by either two T55-L-712 or two T55-GA-714A engines. The engines are housed in sepa- rate nacelles mounted externally on each side of the aft pylon. The engines have the capability to produce emer- gency power on pilot demand. See Performance Charts in Chapter 7  712   or Chapter 7A  714A   . 2-3-2.  General Each engine has a gas producer section and a power turbine section. The gas producer supplies hot gases to drive the power turbine. It also mechanically drives the engine accessory gearbox. The power turbine shaft ex- tends coaxially through the gas producer rotor and ro- tates independently of it. The gas producer section and the power turbine section are connected by only the hot gases which pass from one section to the other. During engine starting, air enters the engine inlet and is compressed as it passes through seven axial stages and one centrifugal stage of the compressor rotor. The com- pressed air passes through a diffuser. Some of the air enters the combustion chamber where it is mixed with start fuel. The mixture ignited by four igniter plugs. Some of the air is directed to the fuel nozzles. After the engine is started, it continues to operate on metered fuel supplied to the fuel nozzles. Hot expanding gases leave the combustion chamber and drive a two-stage gas producer turbine. Energy from the combustion gases also drives the two-stage power tur- bine, which drives the power turbine shaft to the engine transmission. The engine lubrication system has an inte- gral oil tank which is inside the air inlet housing and is serviced with approximately 12 quarts. (Refer to table 2-15-1.) 2-3-3.  Engine Inlet Screens. An engine inlet screen which minimizes foreign object damage (FOD) is installed on each engine. The reduc- tion in engine power available with screens installed is negligible. The engine inlet screens have bypass panels. These  two  panels  are  on  the  aft  end  of  each  screen. Refer to Chapter 5 for information on use of bypass pan- els. Helicopters with engine air particle separator (EAPS) installed,  refer  to  TM  1-1520-240-10  EAPS  SUPPLE- MENT. 2-3-4.  Engine Anti–Icing. The engine air inlet fairing and engine drive shaft fairing receive anti-icing protection from the thermal radiation produced by the oil tank in the engine inlet housing. The hot oil in the oil cavity of the inlet housing warms the air as it passes into the engine inlet. 2-3-5.  Engine Power Control System. 712 Each engine is controlled by a separate power control system which includes cockpit controls and an engine fuel control unit. Each system provides automatic control of engine gas producer rotor speed and power turbine speed in response to any setting of the engine controls selected by the pilot. Engine gas producer rotor speed (N1) and power turbine speed (N2) are controlled by the fuel control unit, which varies the amount of fuel delivered to the engine fuel nozzles. During normal operation, the fuel control unit automatically controls fuel flow metering during power changes, thus protecting the engine from overspeed  and  overtemp.  Fuel  flow  is  automatically monitored to compensate for changes in outside air tem- perature and compressor discharge pressure. 2-3-6.  Engine Fuel Control Units.  712 Each engine fuel control unit contains a single element fuel pump, a gas producer speed governor, a power tur- bine speed governor, an acceleration-deceleration con- trol, a fuel flow limiter, a fuel control shutoff valve, and a main metering valve. A gas producer (N1) lever and a power turbine (N2) lever are mounted on the fuel control unit. Output  power  of  the  power  turbine  (a  function  of  the speed and torque) is restricted by limiting the maximum fuel flow to the gas producer. Maximum gas producer rotor speed is set by the ENG COND (engine condition) levers in the cockpit. The ENG COND levers electrome- chanically positions the gas producer lever, which con- trols the fuel control fuel shutoff valve and operating level of the gas producer. During flight, the ENG COND levers are left at FLT and the output shaft speed is regulated by the power turbine speed (N2) governor. The  power  turbine  lever  is  electromechanically  posi- tioned by the ENGINE BEEP TRIM switches, thrust con- trol, and EMERG ENG TRIM (emergency engine trim) 712   switches. Output shaft torques are limited by the fuel flow limiter, which limits the maximum fuel flow. The position of the main metering valve is determined by the gas producer speed governor, power turbine speed gov- ernor, the acceleration-deceleration control, or the fuel flow limiter, depending on engine requirements at that time.  The  governor  or  the  control  unit  demanding  the least fuel flow overrides the other in regulating the meter- ing valve. 2-3-7.  Speed Governing. The power turbine speed governor senses the speed of the power turbine and regulates the amount of fuel which is  supplied  to  the  gas  producer.  This  slows  down  or speeds up the gas producer rotor so that power turbine and rotor system speed remains nearly constant as loads vary. At minimum rotor blade pitch, the amount of power re- quired is at minimum. As pitch is increased, power tur- bine speed (N2) starts to decrease since more power is required  from  the  engine  to  maintain  a  constant  rotor speed. The power turbine speed governor senses the decrease of N2 RPM and increases the flow of fuel to the gas producer. Decreasing pitch causes N2 to increase.