NOW HEIR-R WORKS
Page 17
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• Caterpillar's HEUI system is not like a typical "common rail" system, in which fuel is pressurised by a common pump and fed at injection pressure to electronically controlled injectors. Cat's system combines a low-pressure fuel rail common to all cylinders (this is shown light green in diagram, right) with a high-pressure oil rail (shown red) which feeds each unit injector. The oil rail is fed from the conventional engine oil system via a pump that raises the oil pressure to 280 bar— the rest of the engine's oil system remains at conventional pressure, When the electronic injection control opens a valve, the oil actuates an "intensifier piston that pressurises one shot of fuel. The piston is seven times as large on the oil side as it is on the fuel side, so the fuel can be pressurised to seven times the oil pressure, ie almost 2,000 bar The high-pressure fuel (shown dark green) travels to the nozzle tip, where it is released as the check valve lifts. Fuel pressure at the tip is up to 1,750 bar—a con ventional fuel rail would have to hold fuel at about 2,100 bar to achieve this pressure at the tip, due to pressure losses in the system.
In an HEUI-A injector, the check valve is a straightforward poppettype design. It lifts when the fuel pressure is sufficient to act against a spring, and closes when the fuel pressure falls.
In the HEUI-B injector, the solenoid valve has two functions: it allows the oil to pressurise the fuel as before, but it also directs some oil to the check valve—the check valve is opened and closed hydraulically, controlled by the engine control unit (ECU). This allows the ECU to control the rate at which fuel is injected, and to allow pilot injection or split injection.