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data on the controls which connect to the engine fuel system. Section 77-40 tells you more about the
engine integrated display system.
2. Description/Operation
Figure 1 shows the schematic diagram for DA 40. The engine fuel system has these components:
A. Injector
The injector has a hollow body attached to the front of the engine sump. Air flows through the
injector to the inlet manifold. A throttle valve in the body of the fuel injector controls the air flow.
The body has a venturi. When air flows through the venturi, the air pressure decreases. The injector
uses the decrease in pressure as a measure of air flow to the inlet manifold. Venturi pressure goes
to one side of an air diaphragm and air inlet pressure goes to the other side of the diaphragm.
Fuel goes into the injector under pressure. The fuel goes through a fuel strainer to a manual
mixture control valve. The fuel flows from the manual mixture control valve through a metering jet
to an idle valve. The fuel flows from the idle valve to a fuel diaphragm. This is metered fuel.
The fuel diaphragm is connected to the air diaphragm and to a ball valve. The ball valve controls
the fuel flow to the engine. Fuel pressure at the fuel strainer goes to the other side of the fuel
diaphragm.
The pressure difference across the air diaphragm causes a force which opens the ball valve. The
pressure difference across the fuel diaphragm causes a force which closes the ball valve. The fuel
flow out of the ball valve causes the metered fuel pressure to decrease. When the fuel pressure
differential balances the air pressure differential, then the system is stable and the correct amount
of fuel goes to the fuel-flow-divider.
AIRCRAFT
Engine Fuel and Control DA 40 Series
AMM
Page 2
20 Sep 2007 73-00-00 Doc # 6.02.01
Rev. 5
A
A
SECTION A-A
Fuel Pump Diaphragm
Diaphragm
Inlet Valve
Outlet Valve
Spring
Spring
Drain
Manual Mixture
Control Valve
Nozzles
Hollow
Needle
Valve
Idle Valve Metering Jet
Ball
Valve
Idle
Spring
Air
Diaphragm
Fuel
Diaphragm
Venturi
Throttle
Valve
Idle
Stop-Screw
Idle Mixture
Thumb-Screw
Mixture Linkage
Vent To Atmosphere
Fuel Inlet Strainer
Fuel Pressure Sensor Fuel
Pressure
Sensor
Fuel
Flow
Sensor
To Integrated
Engine Display
Fuel Flow Divider
CWP
Figure 1: Engine Fuel System Schematic
AIRCRAFT
DA 40 Series
AMM
Engine Fuel and Control
Doc # 6.02.01
Rev. 5 73-00-00 Page 3
20 Sep 2007
Air density decreases as the airplane climbs. The air diaphragm does not measure the air density
and the system gives a continuously rich mixture. The pilot can adjust the mixture strength with the
manual mixture control.
If you move the mixture control lever aft, then the mixture becomes leaner. If you move the mixture
control fully aft, then all fuel flow is cut off and the engine stops.
Different mixture strengths are necessary for different power settings. A linkage connects the
throttle lever to the idle valve. It controls the fuel flow at idle to give a rich mixture. When the throttle
is opened, the idle valve changes the outlet from the metering jet to give the correct mixture
strength.
The pressures acting on the air diaphragm at idle are very small. A small spring in the air
diaphragm makes sure that the ball valve stays open and the system supplies sufficient fuel for idle.
The injector has two adjustments:
) Idle speed. You can adjust the idle speed with the stop screw on the throttle lever. Turn the
stop-screw clockwise to increase the idle speed.
) Idle mixture. You can adjust the idle mixture with the thumb-screw. The thumb-screw is in the
link between the throttler lever and the idle mixture lever. The thumb-screw has right-hand
threads on both ends. One thread is coarse and the other thread is fine.
If you turn the thumb-screw so that the coarse thread is moves into its end-block, then the link
becomes shorter and the idle mixture becomes leaner. If you turn the thumb-screw so that the
coarse thread moves out of its end-block, then the link becomes longer and the idle mixture
becomes richer. Section 71-00 tells you how to do an engine run-up to set the mixture strength.
B. Fuel Flow Divider
The fuel outlet of the injector connects to the fuel-flow-divider. The fuel-flow-divider is on top of the
engine crankcase and has an outlet for each cylinder.
The fuel-flow-divider has a diaphragm. Fuel injector outlet pressure connects to the bottom of the
diaphragm and atmospheric pressure connects to the top of the diaphragm. A hollow needle-valve
　
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