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Operating an Otto-Engine with hydrogen as fuel the probability of combustion anomalies like knocking, pre-ignition and backfiring increases significantly. Knocking is strongly dependent on the operating point, while the cause for preignition is still investigated. Literature as well as preliminary investigations to this work suggest that especially the spark plug has a significant influence on the occurrence of preignition and backfiring. Hence, the scope of this paper is to identify and understand the influence of the spark plug on preignition in order to reduce their occurrence and receive more mechanical power from the engine. In the first step, the operating conditions leading to preignition and backfiring are determined experimentally using a naturally aspirated single cylinder gas engine and a state-of-the-art prechamber spark plug.

The concept of hot surface assisted compression ignition (HSACI) was previously shown to allow for control of combustion timing and to enable combustion beyond the limits of pure homogeneous charge compression ignition (HCCI) combustion. This work investigates the potential of HSACI to extend the operating limits of a naturally aspirated single-cylinder natural gas fueled HCCI engine. A zero-dimensional (0D) thermo-kinetic modeling framework was set up and coupled with the chemical reaction mechanism AramcoMech 1.3. The results of the 0D study show that reasonable ignition timings in the range 0-12°CA after top dead center (TDC) in HCCI can be expressed by constant volume ignition delays at TDC conditions of 9-15°CA. Simulations featuring the two-stage combustion in HSACI point out the capability of the initial heat release as a means to shorten bulk-gas ignition delay.

Small-scale cogeneration units (Pel < 50 kW) frequently use lean mixture and late ignition timing to comply with current NOx emission limits. Future tightened NOx limits might still be met by means of increased dilution, though both indicated and brake efficiency drop due to further retarded combustion phasing and reduced brake power. As an alternative, when changing the combustion process from lean burn to stoichiometric, a three-way-catalyst allows for a significant reduction of NOx emissions. Combustion timing can be advanced, resulting in enhanced heat release and thus increased engine efficiency. Based on this approach, this work presents the development of a stoichiometric combustion process for a small naturally aspirated single cylinder gas engine (Pel = 5.5 kW) originally operated with lean mixture. To ensure low NOx emissions, a three-way-catalyst is used.

Small natural gas cogeneration engines usually operate with lean mixture and late combustion phasing to comply with NOx emission standards, leading to significant losses in engine efficiency. Owing to water evaporation heat and high specific heat capacity of the water vapor, leads the water injection to cooling the combustion chamber charge, which enables earlier combustion phasing, higher compression ratio and thus higher engine efficiency. Therefore, water injection enables mitigating the tradeoff between NOx emissions and engine performance, without loss in engine efficiency. The intake port injection represents, because of the low required injection pressure and the simple injector integration, a cost-effective way to introduce water into the engine. Hence, the purpose of this work is to adapt the intake port water injection timing to the charge mixture flow conditions in the intake port.

Homogeneous charge compression ignition (HCCI) in natural gas fueled engines is thought to achieve high efficiency and low NOx emissions. While automotive applications require various load and speed regions, the operation range of stationary cogeneration engines is narrower. Hence, HCCI operation is easier to reach and more applicable to comply with future emission standards. This study presents computationally investigations of the auto-ignition ranges of a stationary natural gas HCCI engine. Starting from a detailed 1D engine cycle simulation model, a reduced engine model was developed and coupled to chemical kinetics using AVL Boost. Compression ratio, air-fuel ratio, internal EGR rate (iEGR) and intake temperature were varied for three different speeds, namely 1200, 1700 and 2200 rpm. Each examination includes a full factorial design study of 375 configurations. In the first step, the combustion was calculated using the GRI-mechanism 3.0 and a single zone combustion model.

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.

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.

Lean burn operation allows small cogeneration engines to achieve both high efficiency and low NOx emissions. While further mixture dilution enables future emission standards to be met, it leads to retarded combustion phasing and losses in indicated engine efficiency. In the case of naturally aspirated engines, IMEP drops due to lower fuel fraction, increasing brake specific fuel consumption. In this work, an alternative engine configuration was investigated that improves the trade-off between engine efficiency, NOx emissions and IMEP. It combines well-established means such as Miller/Atkinson valve timing and optimised intake system for a single-cylinder cogeneration engine, operating with homogenous lean air-natural gas mixture. First, the engine configuration was analysed using a detailed 1D CFD model, implying a significant potential in reaching the project target.

In an effort to reduce both maintenance costs and NOx emissions of small cogeneration engines operated with natural gas, an alternative ignition system that allows stable operation at very lean homogeneous air-fuel mixtures has been developed. Combustion is induced by an electrically heated ceramic glow plug, whose temperature is controlled by an ECU. Adjusting hot surface temperature allows shifting the inflammation timing of the mixture and, therefore, the phasing of combustion in the engine cycle. The main aim of this work was to determine the effect of intake pressure and air-fuel ratio on the parameters of hot surface ignition (HSI) and understand which are the factors limiting stable HSI operation in terms of cycle-by-cycle variations.

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.

A large number of small size gas-fired cogeneration engines operate with homogenous lean air-fuel mixture. It allows for engine operation at high efficiency and low NOx emissions. As a result of the rising amount of installed cogeneration units, however, a tightening of the governmental emission limits regarding NOx is expected. While engine operation with further diluted mixture reduces NOx emissions, it also decreases engine efficiency. This leads to lower mean effective pressure, in particular for naturally aspirated engines. In order to improve the trade-off between engine efficiency, NOx emissions and mean effective pressure, numerical investigations of an alternative combustion process for a series small cogeneration engine were carried out. In a first step, Miller and Atkinson cycles were implemented by advanced or retarded inlet valve closing timings, respectively.

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.