Dfi Operation; Air Induction Through Crankcase; Air Compressor System; Fuel - Mercury Optimax 250 Manual

DIRECT FUEL INJECTION

DFI Operation

Air Induction Through Crankcase

Once inside the enclosure the air enters the plenum through the throttle shutter which is
located in the plenum assembly. The air then continues through the reed valves and into
the crankcase. The throttle shutter is actuated by the throttle shaft. Mounted on a separate
shaft is a throttle position sensor (TPS). This sensor tells the engine control unit (ECM)
the position of the throttle.
If the TPS should fail, the warning horn will sound. Engine speed will be reduced.

Air Compressor System

Air from inside the engine enclosure is drawn into the compressor through the air attenua-
tor. This attenuator 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 belt
from a pulley mounted on the flywheel 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 discharged through the adaptor plate.

Fuel

Fuel for the engine is stored in a typical fuel tank. A fuel lift pump draws fuel through the
fuel line and fuel filter, then pushes the fuel through a water separating fuel filter. This filter
removes any contaminates and water before the fuel reaches the vapor separator. Fuel
vapors are vented through a hose to the fuel tank. The electric fuel pump in the VST 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 the port fuel rail. A fuel
line connects the bottom of the first rail to the opposite fuel rail. Fuel is stored inside the
rail until an injector opens. A fuel pressure regulator controls pressure in the fuel rails, and
allows excess fuel to return into the vapor separator. The fuel regulator not only regulates
fuel pressure but also regulates it at approximately 10 p.s.i. higher than whatever the air
rail pressure is. The fuel regulator diaphragm is held closed with a spring that requires 10
p.s.i. to force the diaphragm off the diaphragm seat. The back side of the diaphragm is
exposed to air rail pressure. As the air rail pressure increases, the fuel pressure needed
to open the regulator will equally increase. Example: If there is 50 p.s.i. of air pressure on
the air rail side of the diaphragm, 60 p.s.i. of fuel pressure will be required to open the
regulator. The port fuel rail is water cooled.
Page 3B-18 90-888438 JUNE 2002