Power Plant Schematic
5TH STAGE BLEED AIR
9TH STAGE BLEED AIR BOOSTER
COMBUSTOR
FAN

TURBINES
FROM FROM ENGINE ELECTRICAL INDICATING SYSTEM

FROM FROM SYSTEM
PNEUMATIC FUEL
SYSTEM SYSTEM

FAN AIR
Copyright . The Boeing Company. See title page for details.
7.20.4  December 1, 2000

Engines, APU -Engine System Description 737 Operations Manual
Engine Fuel System
Fuel is delivered under pressure from fuel pumps located in the fuel tanks. The fuel enters the engine through the fuel shutoff valve. The fuel passes through the first stage engine fuel pump where pressure is increased. It then passes through two fuel/oil heat exchangers where engine oil heats the fuel. A fuel filter then removes contaminants. Fuel automatically bypasses the filter if the filter becomes saturated. Before the fuel bypass occurs, the fuel FILTER BYPASS alert illuminates on the fuel control panel. The second stage engine fuel pump provides high pressure fuel to the main engine control (MEC). As the fuel leaves the second stage pump, a portion of the fuel is diverted to run the hydromechanical portion of the MEC. This fuel is filtered again and then routed through the fuel heater a second time. The MEC meters the correct amount of fuel to the combustor.
The engine fuel shutoff valve and MEC fuel shutoff valve allow fuel flow to the engine when both valves are open. The valves are open when the engine fire warning switch is in and the start lever is in IDLE. The engine fuel shutoff valve closes when either the start lever is in CUTOFF or the engine fire warning switch is out. The MEC fuel shutoff valve closes only when the start lever is in CUTOFF. The FUEL VALVE CLOSED light on the fuel control panel indicates engine fuel shutoff valve position.
Fuel flow is measured after the MEC fuel shutoff valve and is displayed on the center instrument panel. Fuel flow information is also provided to the FMS.
Engine Oil System
Oil from the individual engine tank is circulated under pressure through the engine to lubricate the engine bearings and accessory gearbox. Oil quantity is displayed on the oil quantity indicator, located on the center instrument panel.
The oil system is pressurized by the engine driven oil pump. Oil from the pump is filtered and then passes to the engine bearings and gearbox. Sensors for the oil pressure indicator and LOW OIL PRESSURE light are located downstream of the oil filter prior to engine lubrication.
Oil is returned to the oil tank by engine driven scavenge pumps. From the scavenge pumps the oil passes through a scavenge filter. If the filter becomes saturated with contaminants, oil automatically bypasses the filter. Prior to the oil bypassing the scavenge filter, the OIL FILTER BYPASS illuminates on the center instrument panel.
Scavenge oil temperature is sensed as the oil returns to the oil tank and is displayed on the oil temperature indicator, located on the center instrument panel. The oil then passes through the fuel/oil heat exchanger, where it is cooled by engine fuel prior to returning to the oil tank.
Engines, APU -Engine System Description
 
737 Operations Manual
Engine Fuel and Oil System Schematic
TO BEARINGS AND GEARBOX
LOW OIL PRESSURE
OIL RETURN
COMBUSTOR OIL PRESS
FUEL FLOW TRANSMITTERSCAVENGE FUELPUMPS
FLOW MEC FUEL SHUTOFF VALVE OIL
IDLE
CONTROL
FILTER
MEC
PMC SCAVENGE
FUEL
OIL FILTER
B
FILTERC
B
FILTER
B
BYPASS
SECOND
OIL
STAGE
OIL
QUANT
FUEL PUMP PUMP
OIL
FILTER
C
FUEL
TEMP
B
BYPASS
FILTER 
FUEL HEATER B  FUEL/OIL HEAT EXCHANGER  OIL TANK 
FIRST STAGE FUEL PUMP 
FUEL OIL  FUEL VALVE CLOSED SHUTOFF VALVE FROM FUEL TANK ENGINE FUEL  OPERATION CONDITION: NORMAL ENGINE 

Copyright . The Boeing Company. See title page for details.
7.20.6  December 5, 2003

Engines, APU -Engine System Description 737 Operations Manual
Engine Start System
Starter operation requires pressurized air and electrical power. Air from the bleed air system powers the starter motor. The APU, an external ground cart, or the other operating engine provides the bleed air source.
In the GRD position, the engine start switch uses DC power from the battery bus to close the engine bleed air valve (on all 737–300 airplanes delivered after November, 1988) and open the start valve to allow pressure to rotate the starter. When the start valve opens, an amber START VALVE OPEN light on the center instrument panel illuminates. The starter rotates the N2 compressor through the accessory drive gear system. When the engine accelerates to the recommended value (25% or max motoring), moving the engine start lever to the IDLE position opens the fuel valves and causes the MEC to supply fuel to the combustor where the fuel ignites. At starter cutout speed (approximately 46% N2), power is removed from the engine start switch holding solenoid. The engine start switch returns to OFF, the engine bleed air valve returns to the selected position, and the start valve closes.
　
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