ENGINE SYSTEMS
29
Fuel injector operation
The injection operation has three stages:
•
Fill stage
•
Main injection
•
End of main injection
Fill stage
During the fill stage both coils are de-energized and
the spool valve is closed. High pressure oil from the
high pressure oil rail is dead headed at the spool valve.
Low pressure fuel fills the port below the plunger. The
needle control spring holds the needle on its seat to
prevent fuel from entering the combustion chamber.
Main injection (Step 1)
A pulse width current energizes the OPEN coil.
Magnetic force moves the spool open. High-pressure
oil flows past the spool valve into the intensifier piston
chamber. Oil pressure overcomes the force of the
intensifier piston spring and the intensifier starts to
move. An increase in fuel pressure under the plunger
seats the fuel inlet check ball. Fuel pressure starts to
build once the plunger passes the fuel spill port of the
barrel. Force on the needle begins to build.
Main injection (Step 2)
The pulse width controlled current to the OPEN coil
is shut off, but the spool valve remains open. High
pressure oil from high-pressure oil rail continues to
flow past the spool valve. The intensifier piston and
plunger continue to move and fuel pressure increases
in the barrel. When fuel pressure rises above the VOP
- about 21 MPa (3100 psi) - the needle lifts of its seat
and injection begins.
End of main injection (Step 1)
When the Injector Drive Module (IDM) determines
that the correct injector On Time has been reached
(meaning the correct amount of fuel has been
delivered), the IDM sends pulse width controlled
current to the CLOSE coil of the injector. The current
energizes the CLOSE coil and magnetic force closes
the spool valve. High pressure oil is dead headed
against the spool valve.
End of main injection (Step 2)
The pulse width controlled current to the CLOSE
coil is shut off, but the spool valve remains closed.
The intensifier piston and plunger return to their initial
position. Oil above the intensifier piston flows past the
spool valve through the exhaust ports. Fuel pressure
decreases until the needle control spring forces the
needle back onto its seat.