2 ms。The electronic-controlled ignition system was developed for automotive engines fueled with compressed natural gas (CNG). In order to evaluate the spark energy of the electronic-controlled system and optimize the electronic-controlled ignition system, referring to SAE J973—1999 standard, the spark energy measurement system was developed. Taking Zener string as simulation load, the voltage across the sampling resistance was acquired on time by digital oscilloscope, then the exact value of spark energy was obtained by integral calculations. The results show that the spark energy can be evaluated quantitatively and effectively. For the electronic-controlled automotive CNG engine JL465Q5, when the engine speed is lower than 2500r/min, the optimized charge time of the primary ignition coil is 6ms; when the engine speed is between 2500r/min and 5000r/min, the optimized charge time of the primary ignition coil is 3ms; when the engine speed is higher than 5000r/min, the optimized charge time of the primary ignition coil is 2ms. 