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Spark ignited engines with direct injection (DISI) in fuel stratified mode promise an increase in efficiency mainly due to reduced pumping losses at part load. However, the need for expensive lean NOx catalysts may reduce this advantage. Therefore, a Bowl-Prechamber-Ignition (BPI) concept with flame jet ignition was developed to ignite premixed lean mixtures in DISI engines. It is characterised by a combination of a prechamber spark plug and a piston bowl. An important feature of the concept is its dual injection strategy. A pre injection in the inlet stroke produces a homogeneous lean mixture with an air fuel ratio of λ = 1.5 to λ = 1.7. A second injection with a small quantity of fuel is directed towards the piston bowl during the compression stroke. The enriched air fuel mixture of the piston bowl is transported by the pressure difference between main combustion chamber and prechamber into the prechamber.

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.

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.