Engine Fuel & Control
TTP
Cessna 172
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Cat.B1
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Level -3
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ATA73
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Engine Fuel & Control
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Cat.B2
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Level -1
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Description - The fuel injection system is a low pressure, multi nozzle, continuous flow system which injects raw fuel into the engine cylinder heads. The injection system is based on the principle of measuring engine air consumption to control fuel flow. More air flow through the venturi will result in more fuel being delivered to the engine, and less air flow through the venturi results in a decreased flow of fuel to engine.
System components consist of the fuel/air control unit, the fuel distribution valve (flow divider), injection nozzles (4 total) and lines used to connect the components. A description of the components is as follows:
Fuel/Air Control Unit - The fuel/air control unit, also known as the 'servo regulator,’ is located on the underside of the engine and integrates the functions of measuring airflow and controlling fuel flow.
Control unit consists of an airflow sensing system, a regulator section and a fuel metering section.
Fuel Distribution Valve - The fuel distribution valve, also known as a 'spider’ or a flow divider, is located on top of the engine and serves to distribute fuel evenly to the four cylinders once it has been regulated by the fuel/air control unit. Also attached to the fuel distribution valve is a rigid line which feeds into a pressure transducer. This transducer measures fuel pressure and translates that reading into fuel flow at the cockpit indicator.
(3) Injection Nozzles - Each cylinder contains an injection nozzle, also known as an air bleed nozzle or a fuel injector. This nozzle incorporates a calibrated jet that determines, in conjunction with fuel pressure, the fuel flow entering each cylinder. Fuel entering the nozzle is discharged through the jet into an ambient air pressure chamber within the nozzle assembly. This nozzle assembly also contains a calibrated opening which is vented to the atmosphere, and allows fuel to be dispersed into the intake portion of the cylinder in an atomized, cone-shaped pattern.
Operation - Fuel is stored in the wing tanks and is delivered to the fuel injection system via a series of lines, valves and pumps. From the engine-driven fuel pump, fuel enters the fuel/air control unit, passes through the fuel distribution valve, and is routed to individual injection nozzles at each cylinder.
The heart of the injection system is the fuel/air control unit, which occupies the position ordinarily used by the carburetor at the engine intake manifold inlet. The fuel/air control unit is comprised of an integrated airflow sensing system, a regulator section and a fuel metering section. Operation of the fuel injection system is based on the principle of measuring airflow and using the airflow signal to operate a servo valve. The accurately regulated fuel pressure established by the servo valve, when applied across the fuel control system, makes fuel flow proportional to airflow.
AIRFLOW SENSING SYSTEM consists of a throttle body which houses the air throttle valve, the venturi, servo valve and fuel control unit. The differential pressure between impact air and the venturi throat pressure is a measurement of the velocity of the air entering the engine. These pressures are vented through drilled channels in the throttle body to both sides of an air diaphragm and create a force across the diaphragm. A change in air throttle position or a change in engine speed will change the air velocity, which in turn changes the force across the air diaphragm.
REGULATOR SECTION contains the air diaphragm mentioned in the preceding paragraph and a fuel diaphragm. Fuel inlet pressure is applied to one side of the fuel diaphragm. The other side of the fuel diaphragm is exposed to fuel that has passed through the metering jet (metered fuel pressure). The differential pressure across the fuel diaphragm is referred to as the fuel metering force.
(a) The air metering force applied to the air diaphragm is transmitted through the regulator stem and tends to move the ball valve in the opening direction. The fuel metering force across the fuel diaphragm acts to oppose the air metering force and tends to close the ball valve. Because the air forces are very low in the idle range, a constant head idle spring is provided to maintain an adequate fuel metering force at low rpm.
(b) As the air metering force increases, the spring compresses until the spring retainer touches the air diaphragm and acts as a solid member. The constant effort spring produces a force which provides a smooth transfer from idle to low power cruise operation. Whenever the air metering, fuel metering and spring forces are balanced, the ball valve maintains a fixed position.
FUEL METERING SECTION is contained within the throttle body casting and consists of an inlet fuel screen, a rotary idle valve and a rotary mixture valve. Both idle speed (closed throttle position) and idle mixture (relationship between throttle position and idle valve position) may be adjusted externally to meet individual engine requirements.
(a) The idle valve is connected to the throttle valve by means of an external adjustable link. The idle valve controls fuel flow through the low speed range of operation and is adjustable to obtain good idling characteristics without affecting fuel metering in the high power range.
(b) The mixture control valve gives full rich mixture on one stop and a progressively leaner mixture as it is moved toward idle cutoff. The full rich stop defines sea level requirements and the mixture control provides for altitude leaning.
730001 Engine-Driven Fuel Pump - Check for evidence of leakage, security of attachment,
and general condition. B 05-12-02 120
730002 Fuel Injection System - Check system for security and condition. Clean fuel inlet
screen, check and clean injection nozzles and screens (if evidence of contamination
is found), and lubricate air throttle shaft. B 05-12-02 120
730003 Idle and Mixture - Run the airplane engine to determine satisfactory performance. If
required, adjust the idle rpm and fuel mixture.
Fuel Strainer Cleaning Procedures
(1) Remove fuel inlet hose to access fuel strainer.
(2) Remove and clean fuel strainer in Stoddard solvent.
(3) Using new O-rings, install fuel strainer to control unit. Torque 65 to 70 inch-pound.
(4) Install the fuel inlet hose. Use a wrench to hold the fuel strainer adapter and torque to 270 to 300 inch-pound.
(5) Perform leak check.
Nozzle Cleaning Procedures.
(1) Remove nozzles from engine. Individual two-piece nozzles should be kept as matched assemblies.
(2) Inspect carefully for evidence of varnish build up and/or contaminated screens.
(3) Soak nozzles in Methyl Ethyl Ketone, Acetone or other suitable solvent to remove all contamination and varnish from nozzle. Stubborn deposits may benefit from ultrasonic cleaning methods.
(4) Dry nozzles using compressed shop air not to exceed 30 PSI. Blow through nozzle in direction opposite of fuel flow.
(5) Install nozzles to intake cylinders. Torque from 55 to 60 inch-pound.
(6) Install rigid fuel lines to nozzles. Torque 25 to 50 inch-pound.
(7) Perform leak check.
9. Fuel
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