Effect Of Water Direct Injection Technology On Combustion And Emissions Of Diesel Engine

In-cylinder water injection can reduce the average temperature of the combustion process.On the one hand,it is beneficial to improve the indicated thermal efficiency,and on the other hand,it also reduces the thermal load and pollutant emissions of the diesel engine.Therefore,studying the influence of in-cylinder water injection on diesel engine combustion and emission characteristics has important theoretical guiding significance and practical application value for improving the performance of diesel engines.Under low-speed,high-load and high-speed and high-load conditions,the paper uses a three-cylinder diesel engine as a prototype to establish a three-dimensional full-airway model,and uses numerical simulation to study the effects of water-fuel ratio,water injection pressure,water injection temperature and water injection strategies on diesel engine combustion and Influence of emission characteristics.Research indicates:Increasing the water-to-oil ratio under high-speed operating conditions can significantly reduce the thermal load of the engine,but the effect is not significant under low-speed operating conditions.This is because the water mist entering the cylinder will entrain the diesel oil and jointly drive the piston to do work to increase the temperature in the cylinder.At low speeds,the atomization and diffusion of water droplets are poor,so the thermal load has not been improved;the diffusion of water molecules in the air at high speeds The speed is accelerated,and the cooling effect of water is obvious,so the heat load is significantly reduced.By comparison,it is found that the best performance can be obtained with a water-to-oil mass ratio of 0.3 at both speeds.The change of spray water pressure will affect the atomization degree and diffusion speed of the water droplets injected into the cylinder.Increasing the spray water pressure within a certain range can obtain more thermal efficiency increments.This is because as the injection pressure increases,the rate of establishment of the high-temperature and high-pressure steam environment in the cylinder increases,and the combustion becomes more complete,so the indicated power increases;and when the injection pressure is too large,the droplet size is too small,resulting in an excessive evaporation rate.Faster,the combustion level drops instead.By comparison,it is found that the water injection pressure of 50 MPa under low-speed operating conditions brings an increase of 2.55% to the thermal efficiency of the engine,and NOx and Soot are reduced by 17.1% and 3.18% respectively;while under high-speed operating conditions,when the water injection pressure of 60 MPa is used,Engine combustion and emission levels are the best,indicating a 2.44% increase in thermal efficiency.Increasing the spray water temperature will speed up the work done by the additional working fluid,and the isobaric expansion process will take longer,so the thermal efficiency has been further improved.When the spray water temperature is 100°C,the diesel engine thermal efficiency increase will increase to 2.83%,NOx,Soot The reduction of HC and CO reached18.3%,4.9% and 18.4%,respectively,and the amount of HC produced was also reduced.The difference in the time of spraying water will affect the turbulent kinetic energy,which in turn affects the airflow movement in the cylinder.The water injection at the early stage of the compression stroke has a significant effect on reducing the thermal load of the engine,but has little effect on the improvement of thermal efficiency;when the water injection is at the late stage of the compression stroke,the duration of the water droplet and diesel injection overlaps,which reduces it to a certain extent.Compressing negative work and increasing positive expansion work,both of which will improve IMEP,further affecting the engine’s indicated power and indicated fuel consumption.By comparison,it is found that when the water injection timing is set to 10°CA BTDC,the increase in thermal efficiency of the engine is the highest,and the effect of improving harmful pollutants is the best.