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Small natural gas cogeneration engines frequently operate with lean mixture and late ignition timing to comply with NOx emission standards. Late combustion phasing is the consequence, leading to significant losses in engine efficiency. When substituting a part of the excess air with exhaust gas, heat capacity increases, thus reducing NOx emissions. Combustion phasing can be advanced, resulting in a thermodynamically more favourable heat release without increasing NOx but improving engine efficiency. In this work, the effect of replacing a part of excess air with exhaust gas was investigated first in a constant volume combustion chamber. It enabled to analyse the influence of the exhaust gas under motionless initial conditions for several relative air-fuel ratios (λ = 1.3 to 1.7). Starting from the initial value of λ, the amount of CH4 was maintained constant as a part of the excess air was replaced by exhaust gas.

Operating an Otto-Engine with hydrogen as fuel the probability of combustion anomalies like knocking, pre-ignition and backfiring increases significantly. Knocking is strongly dependent on the operating point, while the cause for preignition is still investigated. Literature as well as preliminary investigations to this work suggest that especially the spark plug has a significant influence on the occurrence of preignition and backfiring. Hence, the scope of this paper is to identify and understand the influence of the spark plug on preignition in order to reduce their occurrence and receive more mechanical power from the engine. In the first step, the operating conditions leading to preignition and backfiring are determined experimentally using a naturally aspirated single cylinder gas engine and a state-of-the-art prechamber spark plug.