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Here is an illustration of Caterpillar 6D16 diesel engine fuel system structure guide.

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Fuel from the fuel tank 4 is drawn up by the feed pump 5 and strained by the fuel filter 2. The feed pump is driven by a cam in the injection pump 6.
After filtration, fuel is fed to the injection pump 6. From there, it is fed under high pressure to the injection nozzles

1. The injection nozzles spray the fuel into the combustion chambers.
If the fuel pressure in the injection pump 6 exceeds a preset level, the overflow valve 7 opens to allow excess fuel to return to the fuel tank 4

The fuel filter separates any water content out of fuel fed from the injection pump's feed pump, and its element removes any impurities.

The injection pump feeds fuel to the injection nozzles under high pressure and incorporates a mechanism for increasing and decreasing the fuel flow.

As the plunger 6 rises and its lead E meets the fuel inlet/outlet hole D,fuel flows through the center of the plunger and is expelled from the inlet/outlet hole. Regardless of how much further the plunger rises, no fuel feed takes place thereafter.
The stroke length F of the plunger 6 during which pressure feed takes place is known as the effective stroke.

Injection volume adjustment mechanism
To match changes in engine loading, an adjustment mechanism controls the amount of fuel injected. This mechanism turns the plunger 6 by a given angle, thereby altering the point at which the fuel inlet/outlet hole D meets the lead E. Simply stated, the effective stroke is made longer or shorter. A single control rack 7 is used to rotate every plunger in the engine, so the plungers rotate simultaneously and by the same angle.

Fuel highly pressurized by plunger pushes up delivery valve 3 for injection, and when delivery of pressurized fuel ends, delivery valve returns by the force of delivery valve spring to close fuel passage, thus avoiding reverse flow of fuel. Delivery valve lowers further to rest at its seat, and for this stroke L, residual pressure between delivery valve
and injection nozzle is for an instant lowered. This return suction makes fuel-cutting at nozzles effective and avoids post-injection dripping.

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Overflow valve
When the fuel pressure in the injection pump exceeds a preset level, the steel ball R is pushed up, allowing fuel to flow out from the injection pump and return to the fuel tank. This stabilizes the fuel temperature and temperature distribution in the injection pump and keeps the injection rate constant in each cylinder.

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