Dfi Operation; Air Induction Through Crankcase; Air Compressor System; Fuel - Mercury Optimax 115 Service Manual

DIRECT FUEL INJECTION

DFI Operation

Air Induction Through Crankcase

Combustion air enters the cowl through holes located in the top aft end of the cowl. The
cowl liner directs this air to the bottom of the powerhead. This limits the exposure of salt
air to the components inside the engine cowl.
Once inside the cowl the air enters the plenum through the throttle shutter which are lo-
cated in the plenum assembly. The air then continues through the reed valves and into
the crankcase. The throttle shutter are actuated by the throttle shaft. Mounted onto a sep-
arate shaft are two throttle position sensors (TPS). These sensors tell the engine control
unit (ECM) the position of the throttle.
2 TPS's are installed on the DFI engine for safety redundancy. If one TPS should fail, the
dash mounted CHECK ENGINE light will flash and the warning horn will sound. engine
speed will be reduced to 3000 RPM. If both TPS's should fail, engine speed will be re-
duced to idle. DFI engines require large amounts of air into the cylinders at idle speed.
To accomplish this, the throttle shutters are partially open at low engine speeds. The dual
TPS system reads the shaft movement in both directions, one reads movement up (in-
creasing resistance), while the other reads the same movement as down (decreasing re-
sistance). The ECM reads both and calculates the throttle shutter position.

Air Compressor System

Air from inside the engine cowl is drawn into the compressor through the flywheel cover.
This cover acts like a muffler to quiet compressor noise and contains a filter to prevent
the ingestion of debris into the compressor. The compressor is driven by a serpentine belt
from a pulley mounted on the crankshaft, and is automatically self adjusted using a single
idler pulley. This air compressor is a single cylinder unit containing a connecting rod, pis-
ton, rings, bearings, reed valves, and a crankshaft. The compressor is water cooled to
lower the temperature of the air charge and is lubricated by oil from the engine oil pump
assembly. As the compressor piston moves downward inside the cylinder, air is pulled
through the filter, reed valves and into the cylinder. After the compressor piston changes
direction, the intake reeds close and the exhaust reeds open allowing compressed air into
the hose leading to the air/fuel rails.
The air/fuel rails contain two passages; one for fuel, the second is the air passage. The
air passage is common between all the cylinders included in the rail. A hose connects the
starboard rail air passage to the air compressor. Another hose connects the starboard air
rail passage to the port air rail passage. An air pressure regulator will limit the amount of
pressure developed inside the air passages to approximately 10 psi below the pressure
of the fuel inside the fuel passages (i.e. 80 psi air vs 90 psi fuel). Air exiting the pressure
regulator is returned into the exhaust adaptor and exits thru the propeller.

Fuel

Fuel for the engine is stored in a typical fuel tank. A primer bulb is installed into the fuel
line to allow priming of the fuel system. A crankcase mounted pulse driven diaphragm fuel
pump draws fuel through the fuel line, primer bulb, fuel pump assembly and then pushes
the fuel thru a water separating fuel filter. This filter removes any contaminates and water
before the fuel reaches the vapor separator. Fuel vapors are bled through a vent cannister
into the air compressor inlet in the front of the flywheel cover. The electric fuel pump is
different than the fuel pump that is utilized on the standard EFI engine (non DFI), and is
capable of developing fuel pressures in excess of 90 psi. Fuel inside the rail must remain
pressurized at exactly 10 psi over the air rail pressure or the ECM (map) calibrations will
be incorrect. Fuel from the vapor separator is supplied to the top of one fuel rail. A fuel
line connects the bottom of the first rail to the opposite fuel rail. Fuel is stored inside the
Page 3B-18 90-855347R1 JANUARY 1999