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I recently ran tests to fix a stumble & tip in throttle miss on a Chevrolet 350 engine that occurred only on cng. The following are a few test results to compare running conditions between gasoline and CNG.
Hot Idle -- Gasoline average KV 10 - 11 ---- CNG average KV 12 - 15
Gas Ms burn time average 1.5ms ---- CNG 1.3 ms average
I had anticipated the burn time to be longer because of the difference in octane, but the different mixture of cng required more KV to start the burn.
Another note - a spark plug miss, bad wire, or vacuum leak will be more noticible running on cng than on gasoline.
Spark plug wires 1.5KV resistance during burn, did not change between fuels
Gas 114 ppm HC ---- CNG 78 ppm HC
Gas .1 CO ---- CNG 0 CO
2000 rpm
Gas 88 ppm HC ---- CNG 79 ppm HC
Gas .2 CO ---- CNG 0 CO
Gas 16.3 CO2 ---- CNG 7.4 CO2
Gas 6.5 O2 ---- CNG 7.7 O2
I did not have time for much else as I had located the problem and needed to complete the repair. A spark plug problem caused the customer to feel a miss at idle on & tip inacceleration on cng, but not on gasoline. The problem was located with above tests.
As I have said before, I am not a cng tech & do not claim to be one. I am frusterated at not being able to find information. I still think the cng repair business has the feel of a secret society where nobody shares technical data or helps anyone else.
Curtis -- I am still working on my secret decoder ring and will continue to send what I consider to be helpful tidbits learned the hard way & hopefully help someone else.
Stan M.
Hot Idle -- Gasoline average KV 10 - 11 ---- CNG average KV 12 - 15
Gas Ms burn time average 1.5ms ---- CNG 1.3 ms average
I had anticipated the burn time to be longer because of the difference in octane, but the different mixture of cng required more KV to start the burn.
Another note - a spark plug miss, bad wire, or vacuum leak will be more noticible running on cng than on gasoline.
Spark plug wires 1.5KV resistance during burn, did not change between fuels
Gas 114 ppm HC ---- CNG 78 ppm HC
Gas .1 CO ---- CNG 0 CO
2000 rpm
Gas 88 ppm HC ---- CNG 79 ppm HC
Gas .2 CO ---- CNG 0 CO
Gas 16.3 CO2 ---- CNG 7.4 CO2
Gas 6.5 O2 ---- CNG 7.7 O2
I did not have time for much else as I had located the problem and needed to complete the repair. A spark plug problem caused the customer to feel a miss at idle on & tip inacceleration on cng, but not on gasoline. The problem was located with above tests.
As I have said before, I am not a cng tech & do not claim to be one. I am frusterated at not being able to find information. I still think the cng repair business has the feel of a secret society where nobody shares technical data or helps anyone else.
Curtis -- I am still working on my secret decoder ring and will continue to send what I consider to be helpful tidbits learned the hard way & hopefully help someone else.
Stan M.
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