Certified Natural Gas

Using natural gas for fuel

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Do you ever wonder how the Compressed Natural Gas (CNG) Civic GX works?

Let me explain it to you…

The components of the Natural Gas system consist of:

  • Numerous sensors
  • Injectors
  • ECU (electronic control unit)
  • Engine check light
  • Fuel Gauge
  • Fuel
  • Fuel Cutoff Valve
  • CNG Fuel tank
  • Two Part Fuel Regulator
  • Filler Nozzle

The Compressed Natural Gas (CNG) system, like the fuel injection system, consists of several different SENSORS located in various locations all around the car. There are sensors to detect fuel tank pressure, fuel tank temperature,  air pressure, air temperature, throttle angle, air density, fuel temperature, oil pressure, coolant temperature, exhaust temperature, crank angle, timing, engine rpm, and speed, to name a few. Every time the car is started, the ECU (electronic control unit) scans all of the sensors to determine their functionality. The ENGINE CHECK LIGHT comes on during this function and goes off if all sensors are OK.

All the sensors work together to determine the amount of time the fuel INJECTORS are open. The fuel injectors for a CNG vehicle are different from regular injectors. They are much heavier to handle the gaseous pressure of the CNG instead of liquid fuel. The Two Part fuel pressure regulator on the intake manifold lowers the 3,600 PSI pressure in the CNG tank, in a two step process to 26-38 PSI. The fuel is then injected into the cylinders in a gaseous form by the injectors.

Normally the fuel injectors are only open for a few milliseconds at a time. The sensors report to the ECU about one thousand times a second. All the various sensors operate within a certain electrical range: Anytime any one of these sensors goes out of this preset range more than three times in one second, the ECU detects this out of range condition and goes into what is called “backup mode”. The ECU then turns on the engine check light on the dash. At the same time the ECU ignores the signal from that sensor and assumes a preprogrammed value at the appropriate time for that sensor. At the same time the ECU records the malfunction into its memory. In backup mode, the car is still drivable at a reduced pace.

Filling Station

Filling Station

The fuel filler is a high pressure fitting with a quick disconnect. The fitting is designed to provide a secure, leak proof connection to the supply station. The supply station may be a local fuel station with CNG capability or may be a pump assembly installed in your own garage. This particular unit taps into the CNG supply for your house. Fuel is transferred to the vehicle in a liquid form. The slower the fuel is introduced to the fuel tank, the more liquid fuel is transferred. While filling the tank, when the tank registers full, turn off the supply for about a minute, then resume filling. This way, the full amount of fuel (26.4 gallons; 100 liters) is inserted. The amount of fuel remaining in the tank is determined by the ECU by calculating the fuel pressure and the fuel temperature in the tank, and then showing the result of that calculation on the gauge as the fuel remaining.

When maintenance is preformed on the vehicle, or in case of an emergency, there is a manual fuel cutoff valve located under the car near the left rear wheel.

The fuel tank is a fiberglass or aluminum cylinder reinforced with carbon fiber. Extensive crash testing has proven that this tank is very robust.

The ECU has a built in function that does not allow the engine to rev beyond the redline on the tachometer. When the car is not moving, the rev limit is restrained to 4,000 RPM.

The ECU also controls the idle rpm at all times. If the engine is cold, the fuel injectors stay open a little longer to allow the engine to idle faster. Additional air is allowed to enter the throttle as well. Fuel injected cars do not have a choke. When the air conditioner or lights are on, or the power steering is being used, the ECU detects the extra load and adjusts the idle accordingly.

Owning a CNG car usually qualifies the owner for a Federal $4000 credit, but you should check with your tax accountant before purchasing one to see if you qualify. The California Air Resourses Board has discontinued the $3000 rebate for buying a natural gas car. The Civic GX model is only sold at selected dealers in California and New York, due to the availability of refueling stations in these areas.

…and that’s the way the CNG fuel system works!!

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