Working Process Optimization And Emission Control Of 4SH-N Natural Gas Engine

As the oil shortage and environment pollution become more and more stringent, the clean alternative fuel vehicles play an important role in the automobile industry. Among them, the natural gas becomes the preferred choice due to its abundant resources, clean combustion and low CO2 emission level. However, it features higher ignition temperature, lower flame propagation speed and lower power density. With the wide application of natural gas vehicles in China, requirements of low fuel consumption and emission level, the research on CNG engine's burning mechanism, optimization of construction and working process, and control technology of pollutants emission has attracted the most attention. This dissertation emphasizes on the above problems by means of experiments and simulations.Engine power output and fuel economy are improved by the cylinder head design and optimized compression ratio(compression ratio increasing from 8.5 to 11), the rated power increases from 56.6kW to 69.8kW and the lowest gas consumption decreases by 15.4%. The intake system parameters and valve timing are optimized with respect to tuning effect for boost pressure through GT-Power. At the same time, the intake unevenness is evaluated with 3D simulation under steady and transient conditions. The results show that the maximum intake unevenness is controlled below 4%. The experiment results show that the engine's rated power further increases by 5.2kW and the minimum gas consumption decreases by 5.2% and the fuel economy is improved significantly in range of 1800~2200r/min, HC and CO emissions become lower, however NOx emission higher.A 3D geometry engine model is constructed to optimize the combustion system. The moving working space is meshed to visualize the in-cylinder flow field evolution and combustion process by means of 3D CFD software. The simulation results show that while approaching to compression TDC, the properly higher transverse gas velocity from spark plug to opposite wall can facilitate the flame propagation to the opposite area far away from spark plug. The suitable larger area of squish areas against the spark plug can increase in-cylinder turbulent kinetic energy at TDC. As the protrusion of piston near the spark plug limits the enlargement of flame front during the rapid combustion period resulting in worse combustion process, based on the simulation results analysis and overseas combustion chamber design experience, the combustion chamber shape is optimized. The simulation results show that the new designed two combustion chambers (2nd, 3rd) can both improve engine performance. The 2nd squish combustion chamber has better effect due to more stable initial flame kernel, higher turbulence kinetic energy by squish and tumble flow coupling by 6.08% than that of 1st (base) combustion chamber, larger flame front area at same crank angle which leads to more fuel burning. At 380oCA, 56.1% of fuel is burnt in 2nd combustion chamber, however only 45.9% and 48.8% of fuel are burnt in 1st and 3rd combustion chamber respectively. Although 3rd combustion chamber can shorten the distance of flame propagation, increase the turbulence kinetic energy by 8.31% compared to that of 1st combustion chamber, the higher gas velocity adjacent to the spark plug brings forward unstable initial flame kernel and longer flame development period. The experiments results show that the 2nd combustion chamber has better performance and fuel economy than that of 1st combustion chamber, its rated power increases from 75kW to 78.7kW, the lowest gas consumption decreases 4.4%, HC and CO emissions reduces slightly, however NOx emission becomes higher.In order to control its emission level effectively, this dissertation also pays attention to the air/fuel ratio effect from three after-treatments with different catalyst composition on engine emission under operating conditions (1940r/min and 2390r/min engine speed with 25-75% load). The experiment results show that the 1st catalyst converter has the largest HC conversion efficiency due to its higher Pd content, volume and cell density. Within the high efficient conversion window (λ=0.994~1.0022), HC is converted beyond 80%, CO and NOx beyond 93%. The 2nd and 3rd catalyst converters have lower precious metal content and volume, but Rh addition promotes the NOx dissociation during the CO and NOx oxidation and reduction reaction. Within the window (λ=0.994~1.002), CO and NOx are converted beyond 97% which is higher than that of 1st catalyst converter, but the HC conversion efficiency is below 80%. Therefore, this dissertation calibrates the engine with the 1st catalyst converter, determines the maps of gas injection pulse width and ignition advance angle within the catalyst converter high efficient air/fuel ratio window to get engine better fuel economy and emission. The transient cycle test results show that 4SH-N CNG engine can meet the National III emission legislation.In order to control its emission level effectively, this dissertation also pays attention to the effect of catalyst converter on engine emission. The experiment results show that the selected three three-way-catalyst (TWC) converters have the highest conversion efficiency windows (0.994≤λ≤1.002), which the conversion efficiency beyond 80% to NOx and CO. However, the HC conversion efficiency differs from each other apparently. The 1st catalyst has 16g precious metal content (among them Pd 12g), larger catalyst volume 5.79L, the hole porosity 400cpsi which increases the chemical reaction probability in catalyst and thus higher HC conversion efficiency. After the 1st catalyst match to 4SH-N natural gas engine and engine calibration, the engine test according to the emission transient cycle is done and the results show that it can meet the requirement by the National 3rd emission legislation.In order to study the potential for the improvement of fuel economy and lower emission level, the turbocharged and inter-cooled system, cooled EGR system and H2-SCR system are designed and corresponding turbo-charge, lean burn, EGR and H2-SCR technologies are researched. As the lean burn and EGR technologies can both reduce the NOx emission effectively, this dissertation compares the engine performance with them. The results show that the leaner mixture combined with EGR can reach higher thermal efficiency at the same NOx emission level compared to only lean burn technology. Therefore, the emission control strategy for the EGR combined withλis put forward and its control domain is determined which the NOx can reduce to below 1g/(kW?h) without after-treatment. The H2-SCR technology is used to reduce NOx with H2 injection into the exhaust pipe, while engine operates with lean mixture efficiently. The results show that more H2 injection amount can convert NOx thoroughly under different conditions. However, the lower oxygen content in exhaust gases, reasonability selecting hydrogen injection position and optimizing nozzle structure,the 100% NOx conversion efficiency require lower H2/NO ratio.The research on EGR with leaner mixture and H2-SCR can provide fundamental data and technology storage for future more stringent emission legislation.Innovations in this dissertation are as followed:1. Through the system integration technology, an advanced 4SH-N CNG engine is successfully developed which Emission Standards meets with National III and potentially upgrades to National IV.2. The combined control strategy of EGR and air fuel ratio has been put forward within the efficient economic engine operating range with trade-off between emission and fuel economy.3. The H2-SCR technical scheme is brought up to lower the NOx emission under rich oxygen condition for CNG engine and relative fundamental research are provided to the further application.