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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.

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.

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.

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.

A climate and altitude conditioning test bench was developed at the Institute of Energy Efficient Mobility (IEEM) of Karlsruhe University of Applied Sciences to evaluate the overall sustainability of using innovative biofuels in handheld power tools such as chainsaws, trimmers and blowers under any typical operating condition worldwide. The 6 m3 hermetically sealed and thermally insulated test chamber is large enough to fit the entire power tool. A two-stage refrigeration system with intake air drying and electric heating allows for realistic temperature conditions to be set in the test chamber, ranging from arctic cold to tropical heat (-28 to 45 °C). Altitudes of up to 3500 m above sea level can be simulated using a throttle valve at the inlet of the chamber and a pressure-controlled rotary screw compressor positioned downstream the test chamber outlet.

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.