The most notable difference between linear hydrogen engine (LHE) and conventional crank hydrogen engine (CHE) is that the former supports variable motion operation for combustion optimizing. This article presents a study to reveal the effect of variable motion on hydrogen fuel diffusion and mixing of a direct injection LHE. A multi-dimensional model of coupled dynamics is proposed to describe the motion effect on mixture formation. The fuel mixing characteristic of the LHE is evaluated by comparing a corresponding CHE. Result indicates that compared with the CHE, the LHE behaves a slight lower uniformity of the hydrogen-air mixture during injection stage due to its slower motion, but the slower motion also provides longer time for hydrogen diffusion to form more homogeneous mixture before sparking ignition. Moreover, the effect of variable motion stroke on the hydrogen-air mixing of the LHE is also analyzed. Study reveals that the variable stroke has little effect on the hydrogen-air mixing during the injection stage, and the mixture uniformity varies in positive correlation with the stroke length at the occurrence of spark ignition. Suggesting that in order to achieve homogeneous combustion mode, the long motion stroke operation is a good choice for the LHE. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.