Fuel Pressure Regulator: Description and Operation

HIGH PRESSURE LOCK-OFF (HPL) SOLENOID

The HPL is a normally closed, solenoid valve. The HPL solenoid and a low pressure lock-off (LPL) solenoid in the fuel line prevent fuel flow. The powertrain control module (PCM) commands ONLY the HPL open for 1 second at EVERY ignition ON in order to prime the CNG fuel system. The CNG prime pulse charges the fuel line which allows the fuel tank pressure (FTP) sensor to monitor the amount of fuel pressure in the system. The PCM commands BOTH the HPL and the LPL open when engine RPM indicates that the engine is cranking or running on CNG.

HIGH PRESSURE REGULATOR (HPR)

The HPR is supplied with fuel from the CNG fuel tank at up to 24 821 kPa (3,600 psi) through high pressure stainless steel lines. The HPR reduces fuel pressure to between 758-1 448 kPa (110-210 psi). The outlet of the HPR is the Intermediate pressure stage. Fuel flows out of the HPR and into the low pressure lock-off (LPL) solenoid. The pressure drop within the regulator causes fuel temperature to drop. In order to prevent HPR freeze-up, the HPR is connected to the vehicle engine cooling system. The HPR contains an over-pressure relief device (PRD) which will not allow pressure above 1 896 kPa (275 psi) on the output stage of the HPR. The PRD is removable in order to allow the installation of a pressure gage for diagnostic purposes. The HPR has an internal, serviceable 40-micron filter. This filter must be serviced periodically. Refer to High Pressure Regulator (HPR) Filter Replacement for replacement procedures.

LOW PRESSURE LOCK-OFF (LPL) SOLENOID

The LPL is a normally closed, solenoid valve. The LPL is mounted in the Intermediate pressure line. The PCM controls LPL solenoid operation. The PCM energizes the LPL solenoid only when engine cranking or running is detected. Anytime the engine is stopped the PCM de-energizes the LPL solenoid in order to stop fuel flow. The PCM operates the LPL and the high pressure lock-off (HPL) solenoid at the same time except at ignition ON. During ignition ON only the HPL is opened, while the LPL remains closed, in order to charge the fuel system with CNG. The LPL is supplied fuel from the HPR with fuel pressure between 758-1 448 kPa (110-210 psi). The outlet of the LPL supplies the intermediate pressure regulator (IPR) with fuel.

INTERMEDIATE PRESSURE REGULATOR (IPR)

Fuel enters the IPR at the intermediate stage pressure of 758-1 448 kPa (110-210 psi). The IPR reduces the fuel pressure down to about 276 kPa (40 psi). The fuel exits the IPR and flows to the CNG fuel rails. The IPR is equipped with a schrader valve for testing the inlet pressure. Refer to Fuel System Diagnosis for pressure checking procedures.