Circuit Description

The Alternative Fuel Engine Control Unit (AF ECU) controls fuel system when vehicle is operating on CNG. CNG is stored in a tank at pressures up to 3600 psi. A High Pressure Lock-Off (HPL) in the tank and a Low Pressure Lock-Off (LPL) in fuel line prevent fuel flow. When ignition is turned on, AF ECU commands HPL open for one second to charge fuel system, and allows Fuel Pressure Sensor (FPS) to monitor tank pressure. When engine is cranking or running, AF ECU opens both HPL and LPL. System pressure is reduced at three stages. Fuel tank and system before high pressure regulator is at up to 3600 psi. Intermediate section pressure after high pressure regulator is 130-200 psi. Fuel pressure is reduced to 4-6 psi inside low pressure regulator. Secondary stage of low pressure regulator reduces pressure to 4.5-7 in. Hg. Fuel exits low pressure regulator and enters Gas Mass Sensor/Mixture Control Valve (GMS/MCV).