Ignition and Timing Systems
Alternative fuel systems are gaseous fuels systems that require a higher spark plug firing voltage to ignite the fuel. This increases the load on the vehicle ignition system. A marginally operating secondary ignition system may exhibit problems when operating on compressed natural gas (CNG) such as backfires or stalling, even though these problems do not exist while operating on gasoline.
CNG burns slower than gasoline under normal operating conditions. When required, the CNG system advances the ignition timing over the base gasoline timing by 0 to 15 degrees to optimize driveability and emissions of the vehicle. The system may or may not advance spark timing at idle depending on calibration. This timing adjustment is accomplished by the powertrain control module (PCM).