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The presented concept in this study consists of a state of the art passenger car natural-gas engine fired by different hydrogen (H2) and compressed-natural-gas (CNG) fuel blends. The hydrogen content in the fuel was varied among 5 and 15vol% corresponding to 0.6-2.1 mass%, while comparisons include also engine operation on pure CNG. Increasing hydrogen content of the fuel accelerated combustion leading to modest efficiency improvements. Combustion analysis showed that the increasing burning rates mainly affected the initial combustion phase (duration for 5% mass fraction burned). With optimal combinations of spark timing and EGR rate the achievements are additional efficiency increase with substantially lower engine-out NOx while total unburned hydrocarbons or CO-engine-out emissions are not affected. Investigations using Design of Experiments (DoE) algorithms provided a comprehensive picture of the entire parameter space.