Isuzu 4HK-1, Manual, Page: 16 / 50

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Isuzu 4HK-1 and 6HK-1 Engine Fuel Schematic Overview
The common rail system stores high pressure fuel between the supply pump and the
injectors. The common rail also serves as an accumulator to dampen the fuel
pulsations from the pump. Flow dampers are located at the outlet of the common rail to
the lines to the injectors. They minimize the pulsation of fuel pressure inside the
common rail. The flow dampers also cut off the fuel flow in case an injection line or
injector leaks. An orifice in the flow damper piston supplies the fuel to the injectors. An
orifice in a spring loaded damper piston acts as a pulsation damper. In the event that
there is excessive flow through the orifice, the leading end of the piston closes off the
fuel supply to the injection pipe or injectors. The piston again allows fuel flow when the
fuel pressure inside the common rail becomes about 0.6 MPa (87 psi).
High pressure fuel from the common rail is available to the inlet of all of the injectors. A
high pressure line connects the common rail to each injector. The high pressure supply
tube is to be replaced each time it is removed. Each injector has a solenoid valve
which, when triggered, causes the injection of a high pressure fuel mist into the
combustion chamber. This injection can happen more that once during each firing cycle
to control emissions and also make the engine run quieter.
When each injector fires a small amount of drain flow is generated. The injector drain
flow collects into a common drain tube which returns to the fuel tank.
A safety relief valve, located at the rear of the common rail, limits the maximum
pressure to a maximum of approximately 33,350 PSI (230 Mpa). This can happen if the
flow control solenoid valve (SCV) becomes defective. The common rail pressure
sensor is located at the bottom of the common rail.