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Hybrid electric vehicle with internal combustion engine fueled with hydrogen can be a competitor to the fuel cell electric vehicle that is thought to be the ultimately clean and efficient vehicle. The objective in this research is to pursue higher thermal efficiency and lower exhaust emissions in a hydrogen-fueled engine for the series type hybrid vehicle system. Influences of compression ratio, surface / volume ratio of combustion chamber, and boost pressure on thermal efficiency and exhaust emissions were analyzed. Results showed that reduction of the surface / volume ratio by increased cylinder bore was effective to improve indicated thermal efficiency, and it was possible to achieve 44% of indicated thermal efficiency. However, brake thermal efficiency resulted in 35.5%. It is considered that an improved mechanical efficiency by an optimized engine design could increase the brake thermal efficiency largely.