In the frame of the decentralised electrical power and heat cogeneration with stationary I. C. engines operating on natural gas, the Swiss limit for exhaust gas emissions, with 80 mg/mN3 (5% O2), is the most severe in force on an international level. On the other hand, the only technology able to meet this requirement, with the use of a stoechiometric mixture and a three way catalyst, allows a relatively low global engine efficiency and is characterised by a limited reliability of the exhaust gas treatment system. In this context, the Laboratory of Industrial Energy Systems of the EPFL has undertaken, on the basis of a converted diesel engine for natural gas operation, to develop new technologies (mixture, ignition, ...) and to evaluate their potential to reduce exhaust gas emission, particilarly NOx, and reliability. The objective of the first part of this project is to study the behaviour of a naturally aspirated version of the engine “Liebherr G926”, fuelled with a stoechiometric mixture. The main engine characteristics and perfomances have been used to reduce noxious emissions. The main results obtained are: A variation of the spark timing at wide open throttle has allowed to determine the best operating condition with a mixture of l=1.001. The maximal mechanical power output of 106.3 ± 0.5 kW at 1502 ± 5 rpm and the maximal global efficiency of 35.9 + 1,7/-0.6% have been achieved at a spark timing of 20 °CA BTDC. The lowest cycle to cycle variability, with a coefficitent of variability of the mean effective pressure smaller than 1.5%, is reached at the same spark timing (Figure 5.11). No occurrence of knock has been noticed on the spark timing range between 5 and 30 °CA BTDC. An increase of the spark gap from 0.2 up to 1.2 mm reduced the combustion cycle by cycle variability (Figure 5.21) without modifying the engine performances and emissions (Figures 5.15 and 5.17), at full as well as at half load. A increase, respectively a decrease of two units of the spark thermal index from a basis value of 5 has no significant influence on engine performances and emission (Figures 5.23 to 5.26). Similarly, an increase of the central electrode length from 1 up to 3 mm has no detectable effect on the engine behaviour (Figures 5.23 5o 5.26). The use of a three way catalyst for the exhaust gas treatment allows to reduce the NOx and CO emissions well under the Swiss limits (Figure 5.35).