The cycle variation characteristics of a port fuel injection hydrogen internal combustion engine (PFI-HICE) have been extensively investigated. The covariance of indicated mean effective pressure (COVimep) is the best parameter for evaluating the cycle variations in the PFI-HICE. COVimep decreases as fuel-air ratio increases from 1000 to 5500 rpm, and engine speed minimally affects COVimep. The effect of ignition advance angle on COVimep is determined by fuel-air ratio. The ignition advance angles that correspond to the minimum COVimep of the PFI-HICE decrease as fuel-air ratio increases. The effect of ignition advance angle on COVimep diminishes as fuel-air ratio increases. The COVimep of the PFI-HICE rapidly decreases as throttle increases when the throttle is less than 20%. Injection timing only slightly affects COVimep under high-speed conditions, and COVimep increases when hydrogen is injected in intake periods under low-speed conditions. These results indicate that studying COVimep improves the stability of PFI-HICEs. Crown Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.