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. However, brake specific fuel consumption (BSFC) rises due to lower IMEP. In order to take advantage of the reduced ISFC, the intake section was optimised through virtual Design of Experiments (DoE). Subsequently, intake swirl was reduced, leading to further benefits in charging and engine efficiency. For both Miller and Atkinson cycle the efficiency was increased, while meeting the reference engine's baseline NOx emissions and IMEP.