| With the strict restrictions on fuel consumption and emission regulations,it is of great significance to improve engine economy and reduce emissions.In order to suppress knock,the methods of delayed ignition and thicken mixture are often adopted in boosted downsized GDI engines under large load conditions,which is not conducive to engine power performance or economy.Water injection technology is an effective method to suppress knock and has the advantages of improving engine economy and reducing emissions.In this study,the port water injection system and the water direct injection system were respectively constructed with the in-line 4-cylinder GDI engine and the thermodynamic single-cylinder GDI engine as the objects.And the effects of different water injection parameters and optimization strategies on engine power performance,economy and emission characteristics were analyzed by experimental studies.In addition,the possibility of expanding the engine load was explored by combining the water direct injection technology with the lean combustion technology,and the effects on the thermal efficiency and emission characteristics were studied.First of all,the influence of different water injection time and mass on the combustion,power,economy and emission characteristics of the engine was studied in the port water injection test.Studies have shown that port water injection can effectively reduce NO_x,CO and carbon smoke emissions,but HC emissions increase.The effects of reducing NO_x and carbon smoke emissions are enhanced with the delay of water injection time and the increase of water injection mass due to the cooling effect of water which reduces the in-cylinder combustion temperature.Water injection reduces heat transfer loss and has the advantage of energy saving under small water injection mass.However,more water reduces the in-cylinder peak pressure and CA 50 lags behind,which is not conducive to the power performance and economy of the engine.Then,the trade-off among power output,fuel consumption and emissions was studied by optimizing the port water injection without engine knock.Studies have shown that at 2000 r/min,100%load,port injection of 25%volume-fraction aqueous ethanol increases BMEP,reduces BSFC,and reduces NO_x,CO,and carbon smoke.Finally,the effects of different water injection time,water injection mass,and excess air coefficient(λ)on the engine indicative thermal efficiency(ITE)and emission characteristics were investigated in the water direct injection test,and the engine power loss caused by water injection was compensated by the advance ignition time.Studies have shown that the ITE,NO_x,CO and HC can be significantly reduced by water direct injection,and the reduction is intensified with the delay of water injection.With the increase of the water direct injection mass,the rising trend of the in-cylinder average temperature becomes flat,ITE first increases and then decreases,NO_x significantly decreases,CO first decreases and then increases,HC increases.ITE and NO_x decreases by 1.3%and 48%respectively compared with the test basic condition When the water-oil mass ratio(μ)is 50%.Advance ignition time and optimize combustion phase can further improve ITE and compensate power loss caused by injecting water,but it will weaken the ability to reduce emissions.At 2000 r/min,the maximum IMEP of the engine can be extended from 8.8 bar to 10～13 bar by adopting different water injection mass.With the increase ofλ,the in-cylinder peak temperature decreases,ITE increases,NO_x rises first and then decreases,CO and HC decrease first and then rises,and exhaust temperature decreases.At IMEP of 10 bar,λ=1.8,ignition time can be advanced by26.6°CA and ITE increases 7.9%over the test basic condition without engine knock.μ,λand ignition time co-control improves ITE and IMEP with better emissions,but is limited by COV or CA50. |
