Study On Effect Of Exhaust Gas Re-breathing Strategy On Gasoune CAI Combustion

Controlled auto ignition combustion mode can enhance heat efficiency and reduceNOx emission because of high compression rate and low combustion temperature. Owingto exhaust gas heating effect and diluting function, CAI combustion timing and processcan be controlled effectively with exhaust gas re-breathing strategyIn this article, a basic ZS1105 single-cylinder direct injection diesel engine wasrefitted with new inject system and new control system by installing an ignition systemand changing the combustion chamber. Equipped with a new electro-hydraulic variablevalve-train system, the CAI testing platform which can realize the SI combustion modeand CAI combustion mode was established. Mean while, CAI engine simulation modelwas set up with the software of GT-Power, CHEMKIN and Matlab.According to the timing exhaust gas being re-breathed into cylinder, the exhaust gasre-breathing strategy contains two modes such as earlier exhaust gas re-breathing and laterexhaust gas re-breathing. Because of different valve timing, the later exhaust gasre-breathing is composed of two methods. Using the engine testing platform, the exhaustgas re-breathing (EGR) strategy CAI combustion parameter and combustion characteristicwere well studied with different valve timings.Shutting exhaust valve later to intake stroke, the exhaust gas in the exhaust port wasre-breathed into cylinder, thus the earlier exhaust gas strategy was realized. The results ofthe earlier exhaust gas re-breathing strategy CAI combustion were showed as follow. Dueto the fact that the exhaust valve closing angle delayed, EGR rate in-cylinder increased,CAI combustion pressure peak, highest pressure rise rate, highest effective thermalefficiency were decreased so as to the IMEP, BMEP and NO as obvious when EGR rate was low anXemission. The exhaust gas eating function wd the exhaust gas dilutingfunction was evident on the contrary, so the influence of EGR on combustion timing wasdifferent between high EGR rate and low EGR rate. With the increase of EGR ratein-cylinder owing to the fact that the intake valve opening angle delay, CAI combustionpressure peak, highest increased rate of pressure, highest effective thermal efficiencydecreases so as to the IMEP, BMEP and NOXemission. In addition, CAI combustiontiming was delay because of the influence of fresh charge accumulated heating time byhigher exhaust gas. The combustion region of earlier exhaust gas re-breathing strategyCAI combustion was explored with the testing results as follows. When the intake valveopening time(IVO) was between 18.5°CAATDC and 61.5°CAATDC, at the same time,the exhaust valve closing time(EVC) was between 121°CAATDC and 236°CAATDC,stable CAI combustion can be realized. The EGR rate in the region showed a hyperbolicdistribution so as to the IMEP and BMEP and NOXemission and effective thermalefficiency, etc.The later exhaust gas re-breathing strategy CAI combustion was realized bysecondary exhaust valve opening during intake stroke. Later exhaust gas re-breathingstrategy was divided into two modes (SEVO 1 and SEVO 2) according to valve changing.The result of SEVO mode 1 shows when intake valve opening timing (IVO) wasbetween 19°CAATDC and 61.5°CAATDC, at the same time second exhaust valveopening timing(SEVO) was between 17.5°CAATDC and 42.5°CAATDC, CAIcombustion can be realized stably. The combustion performance parameters were asfollows. EGR rate changes from 0.383 to 0.577; CA10 changes were from 0°CAATDCto 7°CAATDC; combustion duration changes from 7°CA to 17°CA; IMEP changesfrom 0.302 MPa to 0.532 MPa; BMEP changes from 0.227 MPa to 0.416 MPa; effectivethermal efficiency changes from 0.248 to 0.314; the highest pressure rise rate changesfrom 0.066 MPa/°CA to 0.452 MPa/°CA; the highest heat release rate changes from 55 J/°CA to 173 J/°CA; The Specific NOXemission changes from 0.065 g/KW.h to 1.69g/KW.h. Delaying the intake valve opening angle has an obvious effect to combustionperformance parameters. For example, the EGR rate was increased, CAI combustionCA10 and CA50 were delayed, combustion duration was prolonged, the pressure rise rate,the effective thermal efficiency as well as the IMEP, BMEP and NO secondary exhaust valve opening angle hXemissions weredecreased, but delaying theas little effect to combustion performance parameters.The result of SEVO mode 2 shows that CAI combustion can be realized in the regionwhen intake vavle close timing (IVC) is between 87°CAATDC and 133.5°CAATDCand the second exhaust valve open timing (SEVO) is between 17°CAATDC and 127°CAATDC. The combustion performance parameters in the region are as follows. EGR ratein-cylinder changes from 0.349 to 0.59, CA10 changes from 5°CAATDC to 15°CAATDC, combustion duration changes from 6°CA to 21°CA, IMEP changes from 0.3MPa to 0.554Mpa, BMEP changes from 0.176 MPa to 0.430 MPa, effective thermalefficiency changes from 0.185 to 0.323, specific NOXemission changes from 0.1 g/KW.hto 1.6 g/KW.h. Delaying the intake valve closing angle had an obvious effect tocombustion performance parameters. For example, combustion duration was shortened,and the pressure rise rate, the effective thermal efficiency as well as the IMEP, BMEP andspecific NOXemissions were decreased. But delaying the secondary exhaust valveopening angle has little effect to combustion performance parameters.Comparing the load ranges and virtues of exhaust re-breathing strategy and exhaustgas residual strategy, the combustion performance was well analyzed, which will make acontribution to the further CAI combustion studying.In this paper, the CAI combustion regions and the combustion performance ofdifferent EGR strategies were predicted through the simulation platform. The simulationshowed the simulating CAI combustion region was similar to the testing combustionregion which means the simulating model can simulate combustion performancesuccessfully. The airflow interaction in gas exchanging process may cause a deviation inthe EGR rate in-cylinder, so the performance parameters may have some difference invalues. But this simulate model has a well potential for predicting the CAI combustionperformance.In summary, the exhaust gas re-breathing strategy CAI combustion was well studiedby means of experimental test and simulating technique. Simultaneously, the concept ofoxygen dilution was put forward which can be used to study different EGR strategy CAIcombustion performance successfully.