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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.

A large number of small size gas-fired cogeneration engines operate with homogenous lean air-fuel mixture. It allows for engine operation at high efficiency and low NOx emissions. As a result of the rising amount of installed cogeneration units, however, a tightening of the governmental emission limits regarding NOx is expected. While engine operation with further diluted mixture reduces NOx emissions, it also decreases engine efficiency. This leads to lower mean effective pressure, in particular for naturally aspirated engines. In order to improve the trade-off between engine efficiency, NOx emissions and mean effective pressure, numerical investigations of an alternative combustion process for a series small cogeneration engine were carried out. In a first step, Miller and Atkinson cycles were implemented by advanced or retarded inlet valve closing timings, respectively.