CNG Bi-Fuel Ignition Timing
A bi-fuel bus (CNG and gasoline) exhibits poor performance and reduced fuel economy on CNG compared to gasoline. The bus has a 5.9L engine with a distributorless ignition system. The technician notices that the engine runs fine on gasoline but has a slight surge and lacks power on CNG. The spark plugs are gapped at 0.045 inch, which is correct for gasoline but the CNG specification is 0.035 inch. The ignition coils are original with 150,000 miles. The technician checks the ignition timing and finds it is set to the gasoline specification (10° BTDC) but the CNG specification is 15° BTDC. The engine has a knock sensor and the ECM adjusts timing based on fuel type. The bus has a dedicated CNG mode and gasoline mode, selected by a switch. The technician suspects the ignition system is not optimized for CNG.
1. What are the likely causes of the poor CNG performance, and what adjustments should be made?
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CNG has a higher octane rating than gasoline, so it can tolerate more advanced timing. The timing is set to 10° BTDC, but CNG spec is 15° BTDC. This retarded timing reduces power and efficiency on CNG. Also, the spark plug gap is too large for CNG; a smaller gap (0.035 inch) is needed because CNG is harder to ignite. The larger gap may cause misfire under load, leading to surge and poor performance. Additionally, the ignition coils have high mileage and may be weak, which is more critical with CNG's higher voltage requirement. The technician should adjust the base timing to 15° BTDC for CNG mode (if adjustable), replace spark plugs with correct gap, and consider replacing ignition coils. Also, verify that the ECM is properly switching between fuel and timing maps; the switch may be faulty. Check for DTCs related to knock sensor or fuel type.
2. How should the technician verify that the ignition system is functioning correctly on CNG?
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Use an oscilloscope to check the secondary ignition pattern for each cylinder on CNG. Look for consistent burn time and voltage. A longer burn time or higher voltage may indicate a worn plug or weak coil. Compare patterns between CNG and gasoline modes. Also, check the knock sensor operation: with CNG's higher octane, knock should be less, but if the sensor is faulty, the ECM may retard timing unnecessarily. Use a scan tool to monitor ignition timing advance while driving on CNG; it should be higher than on gasoline. If timing is not advancing, check the fuel type signal to the ECM. Perform a road test with a load to simulate the surge condition and monitor misfire counters. Replace coils if patterns are abnormal.