Generator Problem

I'm not an engine expert, but here's my 2¢...

If it runs, it means the basic demands are met, at least initially. My guess is that the cng supply pressure is fine to start (when you're not on it) but slowly lowers as it is shifted to run on cng, until after 10 minutes it no longer has the amount of fuel it needs and shuts down.

The problem could be too small of a regulator/compressor from the source. If there is a supply pressure gauge for the cng, start it normally, and watch what happens to the pressure as it is shifted. Check that the regulator/compressor is sized to meet the engine's demands.

It makes sense to me, it runs on one fuel, but not the other. So something seems to be going on with the cng fuel supply (regulator, compressor, and/or pipe sizing).

I would check the regulator. CNG is stored at high pressure if the demand of the engine is to much for the regulator it could cause the diaphragms to freeze. Which the indication of black smoke it's running rich until it floods out with too much gas..

If this is the case just looking at or feeling the regulator will tell. Is it frosted up or very cold to the touch when the engine dies?

Agreed,

It certainly sounds like fuel starvation at about the 10 minute mark.

Can you monitor fuel flow, exhaust temperatures, etc which may give you a clue ....

or one other thought ... does the load vary significantly at this point, effectively stalling the engine, particularly if it cannot respond quickly enough to a sudden load increase ....

"I have found some carbon like black material from exhaust gas." Do you observe any unusual noise? Some times inner perforated wall of exhaust silenser get damaged and acaustic filler material displace and hanging, fine pieces of it come out blackened due to heat of exhaust gas. Ristriction posed by it in exhaust pipe may cause stop in few minutes.

I have found some carbon like black material from exhaust gas.

I would guess that the needle in the petrol carby is vibrating off it's seat while the engine is running. This will overfill the bowl and and cause the engine to 'flood'. This is probably the easiest to test of all the possible faults.

Simply turn the petrol tap to 'off' when you change over to cng. If the engine runs without belching black smoke and stopping this is the cause. It is harder to cure however. Just easier to turn off the petrol tap each time.

This sounds correct. The soot is caused by running too rich, that's classic. I am guessing that your liquid fuel is somehow leaking into the system as you are running on CNG creating an over-rich mix.

If the CNG system is similar to the LPG system, you will have a converter, where the liquefied gas is converted to a gas or vapour. This conversion is carried out by the hot coolant.

The coolant or CNG lines could be incorrectly connected or your converter is probably faulty. They can and do corrode particularly if incorrect coolant is used.

I would look for an electrical problem also. It might take 10 mins or so for a connection or broken wire to heat up and expand and break the contact. If you don't have the proper tools to check for spark you can disconnect the spark-plug wire and secure it about a 1/8 inch anway from the plug,that will be close enought to let the generator run but will also let you see the spark jump. Start the generator cold and watch the spark around the 10 min mark and see if it is lost when the engine quits. If you still see the spark as the engine is dying it is likey a fuel problem but if the loss of spark stops the engine it is a electrical problem. Primitive but it works. I would change the spark-plug also as I have seen heat problems with those also. Just a suggestion............Good luck

Also check for an air lock if it is fuel starvation.