TM 5-3810-307-24-1-2
Impact Hammer - Fuel Injection System Operation (Refer To Figure K-8.)
The fuel injection system consistently delivers a precise amount of highly atomized fuel to the
combustion chamber of the lower cylinder. The fuel injection system is both driven and timed by the
pre-combustion pressure in the lower cylinder.
As the intake/exhaust port on the lower cylinder closes during the downward stroke, air in the lower
cylinder combustion chamber begins to compress. As the air pressure increases, the increased force
on the air piston in the fuel pump pushes the fuel plunger up into the fuel pump head. The fuel plunger,
whose diameter is smaller than the air piston, produces a greater pressure in the fuel above the fuel
plunger. As the fuel pressure increases, the inlet check valve closes and the outlet check valve opens,
transferring high pressure fuel through the fuel injector lines to the two fuel injectors.
The fuel injectors contain an injector nozzle, needle, adjustable spring, and a small ring-shaped piston
around the injector needle. Fuel is injected into the combustion chamber via two holes in the injector
nozzle. The needle, held against its seat by the adjustable spring, blocks the flow of fuel when fuel is
not required. As fuel enters the chamber behind the needle tip, it acts upon the needle in the opposite
direction of the spring. When the fuel pressure is high enough, the pressure overcomes the spring and
the needle is lifted off its seat, allowing fuel to pass through the nozzle.
NOTE
The fuel is injected into the ring-shaped area around the piston nose that
forms the combustion chamber with the lower cylinder. It is not injected
beneath the piston nose.
Fuel injection occurs one millisecond before the piston strikes the impact block. The instant one of the
fuel injectors opens, the combustion process begins. The combustion of diesel fuel and air produces a
rapid increase in the cylinder pressure, which is transmitted to the fuel pump where the fuel pressure
will increase to approximately twice the opening pressure of the fuel injectors. The rapid increase in
fuel pressure ensures that both fuel injectors deactivate within the same initial millisecond.
Fuel injection lasts for some five milliseconds and produces peak combustion pressure for ten
milliseconds. Fuel injection continues until the air piston in the fuel pump reaches the end of its stroke.
During the upward impact hammer piston stroke, the lower cylinder intake/exhaust ports open, relieving
the cylinder pressure acting upon the fuel pump. The fuel plunger spring returns the fuel plunger and
air piston to their original positions. As the fuel plunger retracts, the outlet check valve closes and the
inlet check valve opens, refilling the fuel pump for the next cycle.
The remote throttle controls how much fuel pump output is transferred to the fuel injectors by controlling
how much fuel is returned to the fuel tank via the return line. This controls the stroke rate (blows per
minute) of the impact hammer operation. During fuel injection the throttle valve (needle block) allows a
very small amount of fuel to return to the fuel tank. The throttle valve (needle block) is hydraulically-
controlled by the remote throttle to vary the amount of fuel returned to the fuel tank. When the remote
throttle is placed in the off position (no pressure), the throttle valve (needle block) is completely open
and all of the fuel can return to the fuel tank before high pressures develop.
K-22 Change-1
