The operation of a cogeneration internal combustion engine with unscavenged prechamber ignition was investigated. The objective was to evaluate the potential to reduce the exhaust gas emissions, particularly the CO emissions, without exhaust gas after treatment. The investigation was carried out on a small size gas engine (150kW) and required the development of cooled prechambers and the modification of the engine cylinder heads. The limit of the conventional lean burn operating mode with direct ignition is discussed and the prechamber geometrical configuration is presented. Through the generation of gas jets in the main chamber, the use of a prechamber strongly intensifies and accelerates the combustion process. The prechamber operation reduces significantly the emissions of CO and THC for same NOx emissions. The use of a piston generating significantly more turbulence leads to a somewhat higher fuel conversion efficiency without affecting significantly the CO and THC emissions at low NOx emissions. Further improvement associated with the adjustment of the engine operating parameters and the turbocharger characteristics, as well as a comparison between direct and prechamber ignition operation are presented in the second part (II) of this publication.