Fig 1

The flexibility of choosing a different fuel altogether may be the best choice in the ongoing battle with unpredictable fuel prices. In the new 2013 Chevrolet Silverado and GMC Sierra 2500 HD bi-fuel pickup trucks, gas is gas — whether it’s gasoline or Compressed Natural Gas (CNG) — and either fuel can be used with the push of a button. The 6.0L V8 engine can run on gasoline or CNG and seamlessly transitions between the two fuels without any loss of performance or fuel economy. The CNG and gasoline tanks have a combined range of 650 miles.

The bi-fuel system is designed for primary operation on CNG as long as there is sufficient fuel in the CNG fuel tank. The trucks feature a 17.5 GGE, gasoline gallon equivalent, (66.2 L) CNG tank and a 36 gallon (136.2 L) gasoline tank.

Bi-Fuel Engine

The 6.0L V8 engine uses an Engine Control Module (ECM) with both gasoline and CNG calibrations to ensure all engine operating parameters are within acceptable performance operating limits. The engine is designed to meet strict Environmental Protection Agency (EPA) and California Air Resources Board (CARB) certification. It features a gasoline fuel injection system and a CNG fuel injection system.

The hardened bi-fuel engine provides the same durability as a gasoline only engine. The hardened engine components for CNG fuel use include hardened intake and exhaust valve seats and hardened exhaust valves.

The engine always starts on gasoline. When the engine reaches a pre-determined operating temperature it seamlessly switches over to the CNG fuel system. If the CNG fuel is depleted, the system automatically switches over to the gasoline system.

A minimum gasoline level of at least one-quarter tank should always be maintained. To protect engine components, a switch to gasoline may occur at any time. The vehicle will not start if the engine stalls due to a lack of gasoline.

Switching Between Fuels

The CNG fuel mode switch and LED fuel gauge is located on the center of the instrument panel. (Fig. 2) The CNG fuel gauge indicates how much CNG is left in the tank. CNG quantity is affected by changes in fuel temperature and fuel pressure.

Fig 2

The single light at the top of the switch indicates which fuel system (mode) is currently in operation. Red indicates gasoline mode and green indicates CNG mode. When the system is changing from gasoline to CNG, the indicator will turn orange for a few seconds.

Four lights are at the bottom of the button. The number of illuminated lights indicates the CNG fuel level. Four lights indicate a full tank (approximately 3,600–3,100 psi; 24,800–21,400 kPa) while only one light indicate the fuel level is less than a quarter tank full (approximately 1,000–500 psi; 6,900–3,450 kPa).

The driver can press the center of the button to switch between fuels at almost any time. The fuel mode can be changed almost any time the ignition is ON, or the engine is running.

CNG Fuel Supply

The CNG fuel supply and metering system consists of the following components:

• CNG 1/4 turn isolation valve (manual shut-off valve)
• Alternative fuel rail pressure/temperature sensor
• CNG coalescing fuel filter
• CNG cut-off solenoid valves (2 solenoid valves)
• Engine Control Module (ECM)
• Excess flow valve
• Fill line check valve
• Fill filter
• Fill valve (NGV1 fill receptacle)
• Fuel injectors/injector hoses
• Fuel injector control module (FICM)
• Fuel injector rails
• Fuel mode switch
• CNG fuel pressure regulator
• Fuel pump disable module
• CNG fuel tank
• CNG Fuel Tank Pressure (FTP) sensor
• CNG fuel tank temperature sensor
• CNG tank Pressure Relief Device (PRD) (2 devices)
• High pressure line and hose (stainless steel tubing and stainless steel jacketed PTFE hose)
• LED fuel gauge
• Low pressure fuel line and hose (stainless steel tubing and stainless steel jacketed PTFE hose)

The high pressure fuel system is equipped with a manually operated 1/4 turn isolation valve (manual shut-off valve), located in the high pressure supply line near the CNG fuel tank outlet on the driver’s side of the vehicle. (Fig. 3) The purpose of the valve is to isolate the high pressure side of the fuel system for some service procedures. If this valve is inadvertently left in the OFF position, the vehicle will not operate in CNG mode. Turn the manual shut-off valve 90 degrees to turn OFF the CNG.

Fig 3

The coalescing fuel filter is located between the front CNG cut-off solenoid valve 1 and the 1/4 turn isolation valve (manual shut-off valve). It contains a 6 micron filter element and a drain fitting. The filter is designed to trap contaminates and liquids that may damage the fuel injectors. It must be periodically inspected, drained and replaced.

CNG Operation

The FICM controls the three operating modes of the bi-fuel system. In gasoline mode, the fuel injector and fuel pump control signals are bypassed through the FICM to the proper components, allowing the vehicle to operate the same as a conventional vehicle.

In change-over mode, the FICM supplies voltage to the CNG cut-off solenoid valves, opening them at the CNG fuel tank and the pressure regulator, which allows the CNG system pressure to adjust to the normal operating pressure from the tank to the injectors. The FICM will then switch off the gasoline injectors sequentially beginning with cylinder #1. The appropriate CNG injectors are switched on sequentially as each gasoline injector is switched off.

Once the engine is operating in CNG mode, the gasoline fuel pump is switched off by the FICM. The ECM performs all injector pulse and ignition timing calculations to ensure proper engine performance.

The Tech 2/MDI scan tools are used for vehicle diagnosis. Diagnostic Trouble Codes (DTCs) are the same as with gasoline only engines.

A 40 micron filter is integrated with the fill valve to help prevent contaminates from entering the CNG fuel system during refueling. When the filter becomes restricted, the amount of time required to refill the CNG tank will increase.

The CNG fill valve (receptacle) is a NGV1 profile and mates to any NGV1 fill dispenser valve. The fill dispenser seals to the receptacle with an internal O-ring. The fill valve is mounted in the side of the CNG fuel tank cover. Remove and reinstall the fill valve dust cover when refueling.

The CNG fuel tank, located at the front of the truck bed, is constructed of aluminum liner wrapped in carbon fiber and conforms to NGV2-2000 (Type 3) specifications. The CNG cut-off solenoid valve 2 is threaded into the outlet end (driver side) of the fuel tank and is used to prevent fuel flow during non-operational running conditions. The service life of a Type 3 CNG fuel tank is 15 years from the date it was manufactured. Metal shields are used to protect the tank from road debris or other contact conditions that may occur.

All service parts, including the gaseous fuel components, are available through the GM parts distribution system.

– Thanks to Sherman Dixon and Chris Graham