Fuel System Diagnosis: Description

Alternative Fuel Engine Control Unit (AF ECU) controls fuel system while vehicle is operating on alternative fuel. AF ECU receives signals from various engine sensors and determines correct amount of fuel required by engine. Full tank can contain fuel at 3600 psi (24,820 kPa). A High Pressure Lock-Off (HPL) solenoid in the tank and Low Pressure Lock-Off (LPL) solenoid in fuel line prevent fuel flow. AF ECU commands the HPL ONLY open for one second at every ignition ON for a CNG prime pulse. CNG prime pulse is performed charge fuel line and allow Fuel Pressure Sensor (FPS) to monitor amount of fuel pressure in tank. AF ECU commands both HPL and LPL open when engine RPM indicates engine is cranking or running on CNG.

This fuel delivery system uses 3 different pressure stages to lower fuel pressure from tank pressure down to a delivery pressure which is slightly above atmospheric pressure.

• High Pressure (Tank) Stage  - When AF ECU commands HPL open, fuel begins to flow through High Pressure Regulator (HPR). HPR reduces fuel pressure to 115-220 psi (793-1517 kPa). Outlet of HPR is intermediate pressure stage.
• Intermediate Pressure Stage
  
  With pressure 115-220 psi (793-1517 kPa), fuel exiting HPR flows through intermediate fuel line to Low Pressure Lock-Off (LPL) solenoid. LPL is controlled by AF ECU. Fuel flows out of LPL through another portion of the intermediate fuel line and into Low Pressure Regulator (LPR). Inlet of LPR is the low pressure stage.
• Low Pressure Stage
  
  Low pressure regulator is a 2 stage regulator. Fuel enters LPR at intermediate stage pressure of 115-220 psi (793-1517 kPa). Primary stage of LPR operation reduces fuel pressure down to 4-6 psi (28-41 kPa). Fuel pressure is then again reduced in the secondary stage of LPR, down to 2-5 in. H² 0 (5.1-12.7 cm). Fuel exits LPR and flows through low pressure hose and into Gas Mass Sensor/Mixture Control Valve (GMS/MCV).