Research On Combustion Control And Optimization Strategy Of Ammonia/Diesel Dual Fuel Engine

To achieve the goal of"carbon peak,carbon neutral"and address the demands of energy structure adjustment,the development of green and low-carbon renewable energy,along with the decarbonization of the power system,has become a significant scientific concern both domestically and internationally.Among these,the promotion and application of ammonia as a zero-carbon fuel in internal combustion engines and other power systems have also garnered widespread attention.Currently,due to the high ignition energy and slow laminar flame speed characteristics in the combustion process of ammonia,it is typically employed in a dual fuel combustion mode with highly reactive fuel ignition in internal combustion engines.Based on the National Natural Science Foundation of China and the Natural Science Foundation of Jilin Province,this study addresses the demand for efficient and clean combustion of ammonia in future power systems.The research focuses on the combustion control and optimization strategies for ammonia/diesel dual fuel engines.Drawing on the concept of synergistic control of in-cylinder fuel stratification and combustion heat relase rate,the study aims to tackle challenges such as poor combustion stability,low thermal efficiency,and NOx and unburned ammonia emissions in ammonia-fueled engines.This is achieved by systematically organizing the in-cylinder fuel chemical reactivity and mixture stratification through the adjustment of boundary condition control parameters,including intake conditions,injection strategy,and pilot fuel reactivity.The goal is to optimize the combustion exothermic process,exploring methods to enhance the combustion efficiency of ammonia/diesel dual fuel engines and reduce NOx and unburned ammonia emissions.Additionally,an optical testing platform is employed to investigate the mixture formation,ignition kernel formation,and flame development in the cylinder of ammonia/diesel dual fuel combustion.In-depth research is conducted through numerical simulation platforms on the spatial distribution of the mixture,temperature field,and the generation and evolution processes of intermediate products and pollutants under the ammonia/diesel dual fuel combustion mode.The study aims to unveil the promoting mechanisms of in-cylinder activation/thermal ambiance adjustment on ammonia/diesel dual fuel combustion and the suppression mechanisms of pollutants,providing technical support and theoretical guidance for the optimization of ammonia/diesel dual fuel combustion and the expansion of operating conditions.The main research contents and conclusions are as follows:1.Thermodynamic engine experiment is conducted to investigate the impact of ammonia proportion on the combustion and emission characteristics of ammonia/diesel dual fuel combustion mode with intake manifold injection of ammonia into the cylinder.Additionally,the diesel injection timig is adjusted to optimize the combustion process.The results reveale that,compares to pure diesel combustion,at low ammonia ratios（20-50%）,the ammonia/diesel dual fuel premixed combustion ratio increases,the combustion duration shortens,and indicated thermal efficiency improves,resulting in reduced NOx emissions.However,at high ammonia ratios（>50%）,the decrease in the reactivity of the mixture leads to a further delay in the combustion phasing,increases cycle variations,decreases indicated thermal efficiency,and elevates NOx and unburned ammonia emissions.At high load,the impact of ammonia combustion inertness diminishes,and the CA50 approaches the top dead center,enhancing indicated thermal efficiency and combustion stability,thereby facilitating the increase in the ammonia ratio in the ammonia/diesel dual fuel combustion mode.Furthermore,the accumulation mode particles in ammonia/diesel dual fuel combustion are significantly reduced compared to the pure diesel combustion,with a reduction of over 88%at an ammonia proportion of 70%.Moreover,advancing the injection timing is beneficial for mitigating the combustion phasing delay caused by ammonia combustion inertness.After optimizing the injection timing,the indicated thermal efficiency increases by 2.7%and 5.5%at ammonia ratios of 50%and 70%,respectively.Optical testing and simulation reveale that the flames of ammonia/diesel combustion primarily exhibited an orange-yellow color due to the radiation emitted by combustion intermediates such as NO₂ and NH₂.Ammonia combustion flames are mainly concentrated around the diesel fuel jet flame,the center of the combustion chamber,and the boundary layer region,indicating insufficient premixed ammonia combustion,leading to increased ammonia emissions with higher ammonia ratio.Advancing the injection timing appropriately improves the distribution of the mixture in the cylinder,facilitating the propagation of ammonia combustion flames.However,the increase in combustion temperature leads to an elevation in NOx emissions.2.To address the issue of uneven mixing of gas in the cylinder leading to insufficient premixed ammonia combustion in localized lean regions,the study investigates the impact of a diesel split injection strategy on the combustion and emission characteristics of ammonia/diesel dual fuel combustion,based on the concept of in-cylinder mixture and fuel stratification control.The results indicate that the split injection strategy is beneficial for improving the reactivity of the premixed ammonia/air mixture before ignition,reducing the ignition delay,and decreasing the peak pressure rise rate and cycle variations.However,the split injection strategy also leads to an increase in combustion temperature,resulting in elevated NOx emissions.Combining optical testing with simulation results,it is revealed that the pre-injection of diesel in the split injection strategy effectively improves the distribution of the mixture in the central region and the boundary layer of the combustion chamber,promoting the gradual propagation of the ammonia flame from the end of the fuel jet towards the center of the combustion chamber,facilitating the complete combustion of ammonia.Furthermore,the optimization of the split injection strategy indicates that the ignition in-cylinder combustion is jointly controlled by the main pre-injection timing,with a pre-injection timing between-50 and-40°CA ATDC being favorable for enhancing the reactivity of the ammonia premixed gas and forming a well-defined fuel stratification.The influence of the main-injection timing on combustion is similar to that of a single injection strategy.Advancing the main-injection timing appropriately enhances the heat release rate and improves combustion stability.Simultaneously,the pre-injection diesel ratio plays a dominant role in influencing the in-cylinder mixture and fuel reactivity stratification,with a pre-injection ratio between 33.3%and50%being favorable for forming a stable ignition source and reducing the quenching phenomenon of the ammonia combustion flame.Overall,the split injection strategy,after optimization,achieves a higher indicated thermal efficiency compared to the single injection strategy,with an improvement ranging from 1.3%to 2.6%.3.To address the issues of combustion phasing delay,decreased thermal efficiency,and high unburned ammonia emissions resulting from the inert nature of ammonia,the study further investigates the impact of varying intake conditions on the combustion and emission characteristics of ammonia/diesel dual fuel combustion.By adjusting the intake flow rate and temperature to regulate the mixture concentration and in-cylinder activation/thermal environment,the study explores the influence of different intake conditions on ammonia/diesel dual fuel combustion.The results indicate that increasing the intake flow rate significantly improves combustion conditions at high ammonia ratios,resulting in increased indicated thermal efficiency and reduced CO and particulate emissions.However,the increase in intake flow rate results in an increase in the excessively lean region in the cylinder,leading to lower combustion temperatures and increased unburned ammonia emissions.Increasing the intake temperature is beneficial for improving the temperature distribution in the cylinder,enhancing combustion stability,and indicated thermal efficiency at high ammonia ratios,and reducing CO and unburned ammonia emissions.However,under low ammonia ratio conditions,the shortened fuel-air mixing time results in more ammonia/diesel mixture concentrated at the bottom of the combustion chamber,leading to increased heat transfer losses to the piston walls and a decrease in indicated thermal efficiency.Joint control of intake flow rate and temperature can raise the indicated thermal efficiency of the ammonia/diesel dual fuel engine to 44.4%at a high ammonia ratio（70%）,with minimal change in NOx emissions.4.Employing the concept of synergistic control of fuel characteristics and combustion boundary conditions,the study actively adjusts the pilot fuel reactivity to investigate the impact of different physicochemical properties of pilot fuels on ammonia-fueled engine combustion and emissions.Furthermore,an Exhaust Gas Recirculation（EGR）co-injection strategy is applied for joint optimization.The results demonstrate that using high cetane number pilot fuels enhances the fuel reactivity stratification gradient between the ignition fuel and premixed ammonia/air mixture,leading to an advancement of the combustion phasing,improved combustion stability,and increased indicated thermal efficiency.Consequently,this approach contributes to expanding the range of ammonia substitution.At an ammonia ratio of 70%,the indicated thermal efficiency of ammonia/Coal-to-liquid（CTL）is increased by 4.8%and 3.1%,compared to ammonia/diesel and ammonia/biodiesel.Additionally,the combination of ammonia with high cetane number fuels reduces the premixed combustion ratio,lowers combustion temperatures,and decreases NOx emissions.Optical testing results reveal that high cetane number pilot fuels aid in improving the reactivity of ammonia/air msxture,causing more ignition cores to appear in the early stages of combustion,facilitating the propagation of flames in the central region of the combustion chamber.Furthermore,the introduction of EGR slows down the heat release rate,mitigating the issues associated with excessive pressure rise rates resulting from advanced injection timing and reduces NOx emissions.EGR’s impact varies with different pilot fuels,with high cetane number pilot fuel CTL showing greater sensitivity to changes in EGR rates when the injection timing is advanced.Oxygen-containing biodiesel/ammonia combustion exhibit higher EGR tolerance and help alleviate the increase in particulate matter caused by EGR.When combined with optimized injection timing at a 30%EGR rate,this strategy can enhance thermal efficiency while reducing NOx emissions.