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Experimental study focusing on the combustion and cyclic variation characteristics of a pure hydrogen-fueled engine at idle and lean conditions was introduced. The experiment was carried out on a modified four-cylinder spark-ignition engine equipped with an electronic controlled hydrogen injection system. The test results showed that when excess air ratio was varied within 2.08 to 3.20, the engine original electronic control unit could maintain idle speed of the pure hydrogen-fueled engine at its target value (790r/min) by automatic adjusting the spark timing and idle bypass valve opening. The engine cooling loss was decreased with the increase of excess air ratio. However, the cyclic variation in indicated mean effective pressure and combustion duration were slightly raised when the engine was gradually leaned out. When excess air ratio rose from 2.08 to 3.20, the engine fuel energy flow rate was reduced by about 15.4%. 