Study On Key Technologies Of Electronically-Controlled Injection Lean Burn CNG Engine

Recently, environmental improvement and energy issues have become moreand more important as worldwide concerns. Natural gas is a good alternative fuel toimprove these problems because of its plentiful resources and clean burningcharacteristics. As the increase of market demand for heavy-duty CNG(Compressed Natural Gas) engine in China, it is necessary to have an in-depthstudy on the key technologies of CNG engine. The key technologies of anelectronically controlled multipoint injection lean burn CNG engine wereinvestigated in this dissertation. A lean burn emission control strategy was putforward that Air/Fuel ratio was controlled differently in terms of areas and anoxidation catalyst was used to make CNG engine meet Euro Ⅲ emissionregulation.Based on CA6DE1-21 diesel engine, the main structures of CA6SE1-21NCNG engine were redesigned. Combustion chamber was designed as cylindrical orreentrant bowl with compression ratio of 12:1, which can be machined easily andno thermal stress concentration and can achieve the development targets for engineperformance and emission. The elasticity of piston rings was increased, the widthof flat part on the sliding surface of oil ring was reduced, and the second ring wasalso redesigned without distortion. The problem of excessive lubricating oilconsumption at low speed and low load conditions was solved. Water-cooledturbocharger with double passages was applied. The turbo housing was smaller andthe flow mass of the compressor was lower slightly than those of base engine. Theboost ratio is similar to that of base engine. Electronically controlled release valvewas designed to avoid transient compressor surge. The cam profile of base engineand reduced valve overlap could ensure engine performance and lower THCemission. Furthermore, NOx emission was under the limit of Euro Ⅲ emissionregulation.Lean burn combustion characteristics of CNG engine was tested andinvestigated. The studies showed that as air excess factor(λ)increased, in-cylindermixture concentration became leaner, the propagation of flame front becameslower, the combustion delay duration and the main duration became longer,maximum cylinder pressure decreased, heat release ratio became lower. Thecombustion cycle variability was higher. VARpi reached 3% whenλwas in therange from 1.0 to 1.5. VARpi increased considerably after λ was bigger than1.5. Combustion stability deteriorated seriously. NOx emission reached maximumwhenλ was in the range from 1.1 to 1.2. NOx emission reduced rapidly asλincreased afterλ was bigger than 1.2. NOx emission was about zero when λwas 1.56. CO emission reduced to minimum as λ increased from 1.0 to 1.2. COemission increased slowly as λ increased from 1.1 to 1.5. NMHC and CH4emissions increased gently when λ increased from 1.0 to 1.5. NMHC and CH4emissions increased sharply after λ was bigger than 1.5. The syntheticalemissions of NOx,CO and HC emissions were best when λ was in the rangefrom 1.4 to 1.5. Engine fuel consumption was lowest when λ was in the rangefrom 1.1 to 1.2. Fuel consumption increased slowly when λ changed from 1.1 to1.0 or from 1.2 to 1.5. But fuel consumption reached to an unacceptable extent afterλ was bigger than 1.5, and exhaust temperature reduced as λ increased.Based on the study results, a lean burn emission control strategy was putforward. The thresholds for determining main parameters such as λ at eachengine condition were determined:a).λ≤1.5b).VARpi ≤3%c). Special NOx emission≤5g/kW.hd). Exhaust temperature before turbo ≤680oC to ensure engine durabilityCharacteristics of the engine in-cylinder velocity field, turbulent kineticenergy field, mixture concentration field and temperature field were investigated byusing software STAR-CD. Calculated results indicated that in-cylinder flow wasdivided into intensive mixing phase and weak flow mixing phase. The gas velocityand turbulent kinetic energy were higher in intensive mixing phase and lower inweak flow mixing phase. The gas swirl velocity in horizontal direction was higherand tumble velocity in vertical direction was very lower. Turbulent kinetic energylocally at spark plug was lower which was advantageous to form stable flamekernel. At ignition timing, mixture concentration field displayed contrarystratification which mixture concentration in upper area of cylinder was leaner andricher in bottom area of cylinder. λ near the spark plug was about 1.7, λ in thebottom of combustion chamber was about 1.24. Because contrary stratification wasdisadvantageous to combustion stability, the viewpoint of retarding CNG injectiontiming to change in-cylinder mixture contrary stratification into weak stratificationwas put forward. The weak stratification was that mixture concentration in theupper area of cylinder was richer and leaner in the bottom area of cylinder. Thatrelatively richer mixture was distributed around the spark plug promotes ignitionability and early initial flame stability. Experiments validated that as CNG injectiontiming was retarded, cylinder pressure and heat release ratio increased, thecombustion delay duration and the main duration shortened, the combustion speedwas higher, the combustion cycle variability reduced, combustion stability wasimproved and thus the engine performance was improved.The effects of different combustion chambers on in-cylinder flow andcombustion were simulated. The calculated results showed a remarkably largeinfluence of the combustion chamber on in-cylinder flow and combustion. Localturbulence in No.2 combustion chamber was enhanced due to its reentrant shape.Turbulent kinetic energy in cylinder was enhanced and turbulent kinetic energywas lower near the spark plug and higher apart from the spark plug, whichquickened flame promulgation and accelerated combustion.Electronic control multipoint injection system for lean burn CNG engine wasdeveloped. For the control system, air mass, natural gas mass and ignition timingcould be controlled and adjusted accurately through close-looped feedback controlforλ. Through the calibration with this control system, CA6SE1-21N CNG enginereached the development targets for engine performance and fuel consumption andits emissions met Euro Ⅲ (ESC) regulation. Recently, a small number ofCA6SE1-21N engines have been mounted on the buses and trucks now inproduction.The mechanism was found that lubricating oil consumption of lean burn CNGengine at lower speed and lower load conditions was excessive. The reason of thisproblem was that the vacuum was formed in cylinder during intake stroke at lowerspeed and lower load conditions which pumps lubricating oil into cylinder. Afterthe piston rings were redesigned, the oil consumption of 8h idle speed ofCA6SE1-21N engine was reduced by 33.1%. Pistons, rings and cylinder linersweren't over worn and lubricating oil consumption was in normal range after 500hdurability test.