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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.

To meet future emission standards with gasoline direct injection engines it is important to have a reliable process robustness during stratified charge operation. Especially engines with a wide spacing arrangement of fuel injector and spark plug which operate with an air-guided concept are very sensitive concerning misfire operation caused by cyclic variations of the mixture formation and transport. Primarily the turbulent in-cylinder gas motion and the interaction with the fuel injection indicate these fluctuations. To reduce these cycle-to-cycle variations and to generate a steady flow behavior an adjustable air-guiding system was developed and attached to the inlet port of a single-cylinder DI engine. The following examinations show that the air-guiding system can lead to a significant reduction of the cycle-to-cycle-variation of the in-cylinder air flow. As a result of these improvements, the deviation of imep in the fired engine decreases obviously.

Engines with gasoline direct injection promise an increase in efficiency mainly due to the overall lean mixture and reduced pumping losses at part load. But the near stoichiometric combustion of the stratified mixture with high combustion temperature leads to high NOx emissions. The need for expensive lean NOx catalysts in combination with complex operation strategies may reduce the advantages in efficiency significantly. The Bowl-Prechamber-Ignition (BPI) concept with flame jet ignition was developed to ignite premixed lean mixtures in DISI engines. The mainly homogeneous lean mixture leads to low combustion temperatures and subsequently to low NOx emissions. By additional EGR a further reduction of the combustion temperature is achievable. The BPI concept is realized by a prechamber spark plug and a piston bowl. The main feature of the concept is its dual injection strategy.

Electrical power and efficiency are decisive factors to minimise payoff time of cogeneration units and thus increase their profitability. In the case of (small-scale) cogeneration engines, low-NOx operation and high engine efficiency are frequently achieved through lean burn operation. Whereas higher diluted mixture enables future emission standards to be met, it reduces engine power. It further leads to poor combustion phasing, reducing engine efficiency. In this work, an engine concept that improves the trade-off between engine efficiency, NOx emissions and engine power, was investigated numerically. It combines individual measures such as lean burn operation, overexpanded cycle as well as a power- and efficiency-optimised intake system. Miller and Atkinson valve timings were examined using a detailed 1D model (AVL BOOST). Indicated specific fuel consumption (ISFC) was improved while maintaining effective compression ratio constant.

One promising alternative for meeting stringent NOx limits while attaining high engine efficiency in lean-burn operation are NOx storage catalysts (NSC), an established technology in passenger car aftertreatment systems. For this reason, a NSC system for a stationary single-cylinder CHP gas engine with a rated electric power of 5.5 kW comprising series automotive parts was developed. Main aim of the work presented in this paper was maximising NOx conversion performance and determining the overall potential of NSC aftertreatment with regard to min-NOx operation. The experiments showed that both NOx storage and reduction are highly sensitive to exhaust gas temperature and purge time. While NOx adsorption rate peaks at a NSC inlet temperature of around 290 °C, higher temperatures are beneficial for a fast desorption during the regeneration phase. Combining a relatively large catalyst (1.9 l) with a small exhaust gas mass flow leads to a low space velocity inside the NSC.