Abstract
KEYWORDS – Gas Engine, Large Bore, Combustion Development, Simulation, Testing
ABSTRACT
Due to ecological as well as economic reasons large size gas engines - especially within the medium speed range - are gaining increasing interest. In addition to the already established power generation application, these types of engines are of increasing interest for marine or rail propulsion as well. Besides the obligatory fulfillment of the stringent emission legislation, the achievable fuel efficiency level is a key factor for the end customer. Gas engines are a step forward when it comes to achieving good fuel efficiency combined with low emissions. The combustion system of a large bore gas engine is distinctly different compared to passenger car or commercial vehicle engines. The present publication addresses the requirements and challenges of the combustion system of large bore gas engines by application of a new methodology based on combination of simulation and experiment.
Compared to the combustion in an open – i.e. unseparated – combustion chamber, the pre-chamber allows a significant control over the process, both, by the design and by the operation mode of the pre-chamber. Therefore, it requires a specific combustion system layout by dimensioning the pre-chamber volume and the cross section of the nozzle orifices. A novel 3D CFD simulation methodology - Charge Motion Design (CMD) process, was successfully used in combination with experimental work on a single cylinder engine to optimize the pre-chamber layout. A prescribed burn rate combustion model was used to calculate heat transfer coefficients for the entire combustion chamber and further used for thermo-mechanical analysis of the cylinder head.
The excellent combustion stability of the optimized system allows BMEP levels in the range of 30 bar. In spite of a relatively low methane number of the fuel used and the NOx limit, high values for the efficiency could be realized. The investigations fulfilled the NOx limit of the TA-Luft legislation and showed the impressive capacity of the developed combustion system. By means of parameter variations, a very good correlation between simulation and experimental results, as well as the effect of different pre-chamber layouts was achieved. The temperature measurements on single cylinder engine were in agreement with results of thermo-mechanical analysis of cylinder head.