CNG Car - problems in performance

In place of the carbuetor, there is a mixing valve similar to that used in LPG fueled fork lift engines. Generally there is a pressure regulator to control the CNG at the mixiing valve and some type of cam actuated valve for allowing it into the intake manifold airstream as the throttle is opened. Sometimes there is also a solenoid valve in the gas line operated by sensing of ignition pulses from the distributor, or in older systems, some means of insuring that the engine is running or cranking, to prevent uncontrolled release of fuel gas. In other systems, the fuel pressure regulator is set to deliever fuel when a slight vacuum exists. At load, the vacuum is developed by a venturi in the mixing valve, similar to the venturi in a carburetor. At idle and low speeds the vacuum is provided by intake manifold vacuum, much higher in magnitude than venturi vacuum, so orifices or needle valve adjustments are used to set primarily the idle and idle to venturi transition port fuel flow (just the same as in a gasoline carburetor). Porting within the mixing valve in addition to orifices and air bleeds (again as in a carburetor) sets the fuel/air ratio at various throttle setting.

You have either insufficient air or insufficient fuel at idle and low speeds.

Insure first that you have enought fuel at low speeds. Run the engine at the no-load low speed where it will just sustain operation, and feed gas into the air inlet of the mixing valve from an unlit propane torch, gradudually increasing the amount. If the engine runs better, you have insufficient fuel at idle and low speed. If the opposite occurs, the engine is running too rich at idle, and you need to adjust the idle fuel quantity down.

You may also have a possibility that the throttle closes too far at idle. However, that is not likely if low speed operation in general is affected. Some systems provide idle air by means of an air bypass passage, again with an adjsuting screw, with the throttle essentially closed.

The fuel diagnositic procedure involving addition of small amounts of propane will probably start you off in the right direction. Good luck.

BernieK

hi bernie,

thanks for the elaborate explanation. perhaps i may not be able to carry out the suggestion you gave me..but sure i will share it with my service centre guy and check the outcome. thnx again!

When driving a CNG fitted Car you must follow the instruction what most car mechanics gives.

when you stop and park the car at night transfer it on petrol and again start it on petrol in the morning .When driving on CNG, deposits of remains of fuel accumulate on various components of engine and creat trouble.Take a good 10 km drive on petrol after every 8-10 days gap.

On CNG, when starting the car ,it demands more current to crank as compare to the car on Petrol . Make sure that your car ignition system particularly coil is perfect and produce good current. Replace them if you doubt their faithfulness.

On CNG ,you get good fuel economy though you have to pay price in trems of more check up and tunning. Epecially the Air filter needs to be replaced after every 1-2 month period , to produce good air fuel mixture.

Go to a fork lift dealer and have their mechanic adjust the fuel system.

The car dealers do not have enough experience with these systems.

Ther have been a lot of good answers. It does sound like a fuel/air mixture problem. But also check the accelarator postion sensor. If it has a dead spot, it could be giving a false reading to the computer and therefore making the fuel air mixture too rich or too lean.

I'd suggest having a technician with an exhaust gas analyzer check the air/fuel ratio and adjust it precisely, at least within the limits of the conversion system.

Perhaps you have fitted a CNG kit on the car which is not adequate.

All the taxis operating in Mumbai by law have converted to CNG to cut down on pollution and there are 50,000 of them. I don't see this problem with any of them.

Which Ford dealer's service station have you tried in Mumbai? Suggest you find an alternative one.

dear all,

thnk u for the inputs..will now be able to attack the problem better. thanx again.

Satish

Dear All,

Finally, the problem was traced to an electronic signal transmission failure when the accelerator was at the min/idle position. The control console was repalced and since then i have had no problems. Thanks for all the inputs!!

Satish

how much was the price of the replacement

no idea...it was replaced under warranty and hence i do not know the cost.