A strong hybrid system is one of the best solutions to reduce the exhaust of carbon dioxide from vehicles. However it could be believed as being too big and complicated for a motorcycle. Yamaha has developed a unique power unit which consists of an engine, two motors, and a planetary gear drive train in a simple one-room construction with highly stiffened casing. The power unit has been installed onto a proto-type motorcycle called “HV-X”. The HV-X attains 40% or less exhaust of carbon dioxide with no degradation of drive performances, but better acceleration.