Improving the Efficiency of Gas Engines using Pre-chamber Ignition
Sammanfattning: The aim of this project is to explore and understand the combustion phenomenon in engines operating on gaseous fuels, and develop technologies as an alternative to the present diesel engine technologies for heavy duty applications; which are facing severe challenges like stringent emissions norms, high technology costs and unsustainable fuel supply. The studies presented in this thesis focus on the application of pre-chamber ignition system in heavy duty natural gas engines, as a means to improve fuel efficiency and reduce NOx emissions. Initial experiments using pre-chamber spark plugs without auxiliary fueling showed improvements in combustion stability, but with only a marginal extension in the dilution limit with excess air and EGR; and hence no significant fuel efficiency improvements. Following these observations, a literature survey was conducted and it was soon realized that additional fueling to the pre-chamber will help scavenge the pre-chamber at the beginning of every cycle and also lead to the formation of an easily combustible mixture inside the pre-chamber, even while the main chamber is extremely fuel-lean. Further experiments conducted on a single cylinder engine with a custom made pre-chamber assembly with auxiliary fueling showed considerable extension of the dilution limit of main chamber combustion, from an excess air ratio of about 1.7 (with an un-fueled pre-chamber) to over 2.6. The maximum indicated efficiency observed at an operating load of 10 bar IMEPg was over 47% with engine-out NOx emission levels below the 'EURO 6' limits (for heavy duty natural gas engines). Following these finding, experiments to study the effect of pre-chamber volume and nozzle diameter on resulting main chamber ignition were conducted, where the pre-chamber volume fraction of 2.4% and the nozzle diameter ratio in the range of 0.025-0.035 (1/cm) were found to be optimum. CFD simulations were then conducted to understand the fluid dynamic aspects of interactions between the pre-chamber jets and the main chamber charge, which revealed the importance of jet momentum and jets-wall interaction on main chamber ignition. Further experiments on a large bore marine engine to understand scaling requirements for pre-chamber design showed that the optimal pre-chamber volume (and with it the nozzle diameter) scales with the displacement volume of the engine. Gross indicated efficiency of 50% was also recorded.
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