Search Results

An operation with a lean air-fuel mixture enables smaller cogeneration gas engines to operate at both high efficiency and low NOx emissions. Conventionally, the combustion process is induced through spark ignition. However, its small reactive mixture volume sets limits on increasing the air-fuel ratio, as a higher dilution reduces mixture inflammability as well as flame propagation speed. In addition, the spark plug durability is limited due to electrode wear, particularly through spark erosion, causing high maintenance costs. The ignition by means of a hot surface has great potential to extend the frequency of servicing intervals as well as to improve the trade-off between engine efficiency and NOx emissions. Compared to conventional spark ignition, ignition by means of a hot surface is achieved by accelerated combustion. The latter is produced by an increased initial reactive mixture volume.

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.

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.