The study deals the effect of different compression ratios on maximum hydrogen energy share, thermal efficiency, and emissions in a 7.4 kW direct injection CI (compression ignition) engine under dual-fuel mode. Experimental tests were conducted on the engine with three different compression ratios (19.5:1 base (CR1), 16.5:1 (CR2), and 15.4:1 (CR3)) using hydrogen as main fuel and diesel as pilot fuel at 100% load and constant speed of 1500 rpm. Knock limited maximum hydrogen energy share enhanced significantly from 19% with CR1 to 59% and 63% with CR2 and CR3. The percentage reductions of NOX emission in the engine with CR2 and CR3 are about 43% and 48% respectively. HC (Hydrocarbon) and CO (carbon monoxide) emissions reached to zero level with the hydrogen addition at all compression ratios. The optimum compression ratio is 16.5:1 in view of higher thermal efficiency and lower emissions (HC, CO, smoke, and NOR). A notable conclusion emerged from the study is that the reduction in compression ratio of the engine is a promising option for the improvement in hydrogen energy share and thermal efficiency along with benefits of lower emissions. (C) 2015 Elsevier Ltd. All rights reserved.