Study On Combustion Characteristics Of Methanol/Methanol-Hydrogen Fueled Engine

As the energy crisis and global warming become increasingly stringent,optimizing combustion modes and finding clean alternative fuels have become the research focus of energy-saving and clean internal combustion engines at this stage.Methanol is an ideal alternative fuel for engines.It has the advantages of multiple synthesis routes,high oxygen content,good anti-knock properties and zero sulfur content.In addition,methanol can be dissociated into dissociated methanol gas（DMG）mainly composed of hydrogen and carbon monoxide through the high temperature of the engine exhaust.And part of the exhaust heat can be recovered,thereby improving the thermal efficiency of the engine.Methanol is mixed with DMG to form"methanol-hydrogen"fuel.Combined with the pre-chamber to achieve stratified lean combustion,it will further save energy and reduce emissions.A large-bore pre-chamber marine engine was taken as the research object in this paper.The coupled numerical simulation of computational fluid dynamics（CFD）and chemical reaction kinetics was employed to investigate the combustion process of engine.The effects of excess air ratio and ignition timing on the performance of a large-bore pre-chamber engine using pure methanol were studied.And the influence of DMG blending ratio and excess air ratio on the performance of large-bore pre-chamber methanol-hydrogen engine.The results showed:（1）In a large-bore pre-chamber engine using pure methanol,as the excess air ratio of the main chamber increased from 2.0 to 2.8,the combustion duration was extended from 10.0°CA to22.3°CA,and the in-cylinder maximum pressure was decreased from 23.0 MPa to 18.0 MPa,ringing intensity（RI）decreased from 7.2 MW·m^-2 to 1.3 MW·m^-2,and NO_x emissions decrease from 0.7 g·（k W·h）^-1 to 0.1 g·（k W·h）^-1.When the excess air ratio was 2.4,the highest indicated thermal efficiency of 49.2%could be obtained.After that,the combustion efficiency dropped rapidly,while the thermodynamic efficiency continued to increase,resulting in a decrease in the indicated thermal efficiency,and a sharp increase in CO and HC emissions.（2）Under the condition that the excess air ratio of the main chamber was 2.1,as the ignition timing was delayed from-8°CA ATDC to the top dead center,the maximum pressure rise rate,RI,and indicated thermal efficiency were decreased.The burning center of gravity moved back,and the burning duration was prolonged.When the ignition timing was-4°CA ATDC,the RI was 3.5MW·m^-2,which was lower than the limit of 5.0 MW·m^-2,and the IMEP was 2.3 MPa.And at this time,lower emissions and higher thermal efficiency could be obtained.The comprehensive performance could reach the level of advanced alternative fuel engines on the market.（3）Methanol was blended with DMG as the fuel of the engine.Under the condition that the excess air ratio of the main chamber was 2.1,the blending ratio of DMG was increased from 0%to 20%,and the hydrogen content was continuously increased.The flame propagation velocity was effectively increased,which shortened the combustion duration from 14°CA to 11.5°CA.The constant volume ratio of the engine was improved,and the peak in-cylinder pressure,the RI and the combustion efficiency were increased.When the DMG blending ratio was 20%,the indicated thermal efficiency reached 47.3%,the total indicated thermal efficiency reached 48.7%,and the RI was 1.9 MW·m^-2,which was far below the limit of 5.0 MW·m^-2.With the increase of the DMG blending ratio,the engine’s IMEP was slightly reduced and could be maintained above 2.2 MPa.Compared with the pure methanol scheme,the maximum NO_x emission of the scheme with DMG was 0.7 g·（k W·h）^-1.HC emissions could be reduced by a maximum of 82.1%,and CO emissions could be reduced by a maximum of 98.4%.They were all lower than the corresponding limit value of marine engines emission regulations.（4）Blending DMG could increase the concentration of radicals and promoted the combustion of the mixture gas,among which the H radicals had the leading role.However,H radicals were 5orders of magnitude lower than the rate of change of OH radicals,and OH radicals played a major role in the oxidation reaction of methanol and DMG.The OH radicals were also an indicator of the high temperature reaction zone.CH₂O were mainly distributed on the flame front with lower temperature.With the increase of methanol blending ratio,the peak of the net increase of CH₂O appeared earlier.（5）Under the condition of methanol blending with 15%DMG,when excess air ratio was 2.4,the indicated thermal efficiency reached the maximum value of 48.7%,and the total indicated thermal efficiency was 49.8%.Under the condition that the excess air ratio was very large（higher than 2.6）,compared with the pure methanol fuel scheme,the methanol blending 15%DNG scheme had a more obvious effect on promoting the combustion of the mixture gas,and the combustion efficiency was improved even more.With the increase of excess air ratio,NO_x emissions continued to decrease,the minimum was 0.5 g·（k W·h）^-1;HC and CO emissions continued to increase,the maximum was 0.5 g·（k W·h）^-1 and respectively 20.7 g·（k W·h）^-1.The combustion duration increased and the RI decreased.