For a hydrogen engine to be successfully commercialized in a short period of time, it must have power the same level as gasoline engines, almost no NOx emissions, and high efficiency. This achievement would be possible with the use of low temperature combustion, which supercharges ultra-lean mixture, because NOx from a hydrogen engine is thermally produced. However, the energy input for higher output produces backfire, which is a significant problem for hydrogen-fueled engines with external mixture, and why the simultaneous achievement of high output and low-exhaust not easily accomplished. This research investigated the potential of simultaneously achieving output, similar to the output of a gasoline engine, without backfire, by using a complex valve variation and lean boosting. The study achieved almost zero NOx emission and high efficiency in a cylinder engine built for research purposes with a valve timing variation system. Experimental results revealed that retarding the intake valve opening timing could control the generated when increasing the output of the hydrogen engine, and the method above is also with lean boosting. Stable combustion in the lean region of Phi= 0.2, which can reduce temperature the NOx generation temperature, was possible due to the large ultra-lean limit of hydrogen. In the exhaust was almost NOx free due to the low temperature combustion, by supercharging ultra-lean mixture under high power operation similar to that of gasoline. The above results verified that almost pollution-free emission without backfire, and simultaneous power and efficiency could be achieved in a hydrogen engine through a method of intake valve retardation and lean overcharging. (C) 2014 EElsevier Ltd. All rights