Quantcast AIR FUEL RATIO CONTROL - TM-5-2815-232-140183

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limiter piston and the pressure of the oil starts to
increase.  When the oil pressure is high enough, the
limiter piston is pushed down against the limiter servo
spring force. The lower edge of the fuel limiter plunger
opens the port in the limiter piston when the piston has
moved down far enough. This lets supply oil go to drain.
The air fuel ratio control is now activated and can
operate as needed.
Engine Load Increases
As more load is put on the engine, the air
pressure in the inlet manifold is increased.
increased air pressure pushes the rolling diaphragm
assembly up and lifts the fuel limiter plunger more. Oil
pressure on the limiter piston decreases as the limiter
piston drain port is opened and the limiter piston moves
up until the port is closed again. At this time the limiter
piston is stopped in a new position that is proportional to
1. Air fuel ratio control.
the air pressure in the inlet manifold.
The air fuel ratio control assembly (1) is installed
Now, with the engine in operation at a steady
on the right rear corner of the top cover. The control is
speed, load added decreases engine speed.
factory calibrated and installed. This control is not for
governor moves to increase fuel as the power piston
field installation.
moves up to turn the output shafts in the "increase"
direction. As the output shafts turn, the right end of the
The air fuel ratio control is similar to hydraulic air
limit floating lever is lifted.  Because the limit floating
fuel ratio controls used on current Caterpillar engines.
lever is fastened to a pivot (pivot position is set by the air
This control automatically controls the governor output
fuel ratio control), the left end of the lever pushes the
shaft movement in the "fuel increase" direction, until air
limit/shutdown rod down. The limit/shutdown pilot valve
pressure in the engine inlet manifold is high enough to
plunger closes off governor control oil to the power
give complete fuel combustion.
piston and limits the power piston movement.
The air fuel ratio control limits the fuel to the
As the engine picks up load, air pressure to the
engine in proportion to the amount of turbocharger boost
air fuel ratio control increases. The rolling diaphragm
pressure (pressure above atmospheric) in the inlet
assembly moves up and lifts the fuel limiter plunger
manifold. The control is not activated during engine start
which opens the port in the limiter piston. Oil pressure
up and is cocked (activated) by a combination of oil
on the limiter piston is lowered, and lets the limit servo
pressure and the initial surge of boost when the engine is
spring push the piston and output rod up. This lets the
first loaded.
limit/shutdown pilot valve move up.  Fuel limit level is
then increased.
Engine Start Up
NOTE: The air fuel ratio control fuel limit range is set by
As the engine is started and the speed or load is
the position of the limit cam on the limit lever. When the
increased, air pressure from the inlet manifold increases
cam is moved away from the shutdown rod, the limit
and pushes up on the rolling diaphragm assembly to put
range is longer. When the cam is moved closer to the
the limiter spring under compression.  As the rolling
shutdown rod, the limit range is shorter.
diaphragm assembly moves up, the fuel limiter plunger
also moves up and closes off the port in the limiter
piston.  Now the supply oil can not drain through the


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