TM5-3810-307-10
Impact Hammer - Fuel Injection System Operation (Refer To Figure H-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.
H-24 Change 1
