Effect of spark ignition location on the turbulent jet ignition characteristics in a lean burning natural gas engine

The Turbulent Jet Ignition is an effective concept to achieve stable lean burning for natural gas engines due to the multiple ignition sources, high ignition energy, and fast combustion rate. A variation of the ignition location has a non-negligible effect on the ignition performance of the TJI system. The present work aims to provide more details on this effect by numerical simulations. Both factors of the additional fuel supply to the pre-chamber and the in-cylinder flow field are taken into consideration in this study. A numerical model is built based on a lean burning natural gas engine and validated by experimental results. Five different spark ignition sources are equally arranged on the vertical axis of the pre-chamber, with different distances from the connecting orifices. Simulations are carried out under the same initial and boundary conditions except for the location of the ignition source. Combustion pressure, in-cylinder flow field, fuel mass fraction distribution, and heat release rate are analyzed to study the in-cylinder ignition and combustion process. The results show that a rotational flow and a non-uniform fuel distribution are formed in the pre-chamber during the compression stroke. The turbulent jet characteristics are significantly influenced by the coupling of two factors: the combustion rate inside the pre-chamber as well as the flame propagation distance from the ignition source to the connecting orifices. Rapid combustion rate and shorter flame propagation distance both lead to the earlier ejection of cold jets and hot jets. Among five ignition sources, the one located closest to the connecting orifices generates earlier hot jets with the highest mean velocity. The jets are more effective to ignite the lean mixture and could decrease the combustion duration of the main chamber.

The Effect Mechanisms of Intake Manifold Water Injection on the Emissions of a Natural Gas Engine

NOx is the main emission of lean burn natural gas engine (NGE). Water injection (WI) is an effective method to reduce NOx, which has been widely studied in conventional fuel engine. Currently, there are few researches on the application of WI in NGE. The influences of WI on NGE are not clear. In the paper, the effect mechanisms of WI on the emissions of NGE are studied. Based on the thermodynamic properties of water and the combustion mechanism of natural gas, the emissions generation mechanism of NGE with WI was analyzed. According to the experimental system, the effects of intake manifold water injection (IMWI) on the emissions of a lean burn NGE was carried out. The results show that, with WI, the in-cylinder temperature decreased greatly, which effectively inhibited the formation of thermal NO. Water generated a lot of OH groups, which effectively inhibited the formation of rapid NO. At 1800 rpm and 0.92g/s WI rate, NOx is reduced by 70.4%. OH group could effectively promote CO oxidize to CO2. At 1000 rpm and 0.92g/s WI rate, CO is decreased by 22.2%. However, since the decrease of the total activation energy of combustion reaction, the chain breaking reaction increased, resulting in a significant increase in HC. At 800rpm and 0.92g/s WI rate, HC was increased by 11.6%.