[Zurück]

T. Lauer, W. Holly, F. Winter, R. Pachler, S. Murakami:
"Impact of the Fuel Gas Quality on the Efficiency of a Large Gas Engine";
Vortrag: 28th CIMAC World Congress, Helsinki, Finland; 06.06.2016 - 10.06.2016; in: "Meeting the Future of Combustion Engines", (2016), Paper-Nr. 043, 12 S.

More stringent emission legislations will increase the pressure on conventional propulsion systems like diesel engines operated with HFO. On the other hand, large gas engines with lean air/fuel-mixture are becoming an interesting alternative, particularly if the combustion process is flexible with respect to the methane number of the fuel gas.However, the combustion efficiency of large gas engines is strongly limited by the self-ignition of the endgas (knock)
depending on the gas quality. As a consequence, it is an important goal for future gas engine development to optimise the combustion process for different gas qualities and compositions. Due to the complexity of the kinetically controlled processes of the irregular combustion (knock) and emission formation these investigations were carried out experimentally in the past, what is time and cost consuming and does not allow a full understanding of the processes
involved. Therefore, a numerical method was developed to model the knocking combustion and the formation of nitric oxides for a large gas engine in dependency of the fuel gas chemistry. With this approach optimal operating parameters were investigated for a given fuel composition and methane number.

A comprehensive simulation approach based on a 0D-2-Zone combustion model was enhanced for These investigations. The burn rate was predicted with a phenomenological combustion model. The knocking combustion and nitric oxide emissions for different fuel compositions were modelled with detailed reaction chemistry. Therefore, a suitable CH4/C3H8-mechanism was chosen, validated and reduced by means of a sensitivity analyses. Since the knocking combustion is related to the fast burning cycles, an empirical cycle-to-cycle model was implemented.
All models were calibrated with measurements of a large gas engine with pre-chamber spark plug. A good correlation of the simulation results with the measured burn rates, NOx concentrations and knock intensities could be observed.

With this thermodynamic approach detailed studies were carried out for fuel gas with methane numbers between 65 and 100 in order to identify the impact of engine parameters like the compression ratio, inlet valve closing, EGR-rate and the boost pressure on fuel efficiency. The boundaries for these investigations were the knock limit of the combustion process and the legislation for the nitric oxide emissions. Detailed investigations were carried out for IMO
TIER III, TA Luft and TA Luft ½ and the impact of the mentioned engine parameters is discussed.

In an additional step, the thermodynamic model was combined with a metaheuristic optimisation method in order to achieve optimal results for the process efficiency and a given limit of the NOx-emissions by searching the best combination of spark timing, compression ratio, boost pressure, equivalence ratio and miller timing in the multiparameter space. The optimization results are presented in dependency of the fuel gas quality, too. The impact of different fuel parameters like the methane number and the fraction of inert gases (N2,CO2) on the engine process are described with respect to the burn rate,the knock limit and the NO-formation.
The impact of the fuel quality on the engine design is documented.

The numerical method based on 1D-simulation and detailed chemistry is regarded as new and innovative. The discussion of different fuel gas qualities and emission limits provides a detailed understanding of the thermodynamic process of large gas engines.