Development Of Electronic Controlled Multipoint Injection CNG Engine

Recently, environmental improvement and energy issues have become more and more important as worldwide concerns. Natural gas is a good alternative fuel to improve these problems because of its plentiful resources and clean burning characteristics. As the increase of market demand for heavy-duty CNG (Compressed Natural Gas) engine in China, the higher performance CNG engine which is more powerful and lower exhaust emission is in dire need of home market. Thus the electronic multi-injection lean burned CA6SE3-21E3N CNG engine is developed to meet the requirements of the bus and truck market. And it exhaust performance can reach the EUROⅢESC level. The key technologies of an electronically controlled multipoint injection lean burn CNG engine were investigated in this dissertation. A lean burn emission control strategy was put forward that air excess factor(λ) was controlled differently in terms of areas and an oxidation catalyst was used to make CNG engine meet EuroⅢemission regulation.Based on CA6DE1 diesel engine, the main structures of CA6SE1-21E3N CNG engine were redesigned. Combustion chamber was designed as cylindrical or reentrant bowl with compression ratio of 12:1, which can be machined easily and no thermal stress concentration and can achieve the development targets for engine performance and emission. The elasticity of piston rings was increased, the width of flat part on the sliding surface of oil ring was reduced, and the second ring was also redesigned without distortion. The problem of exorbitant lubricating oil consumption at low speed and low load conditions was solved. Water-cooled turbocharger with double passages was applied. The turbo housing was smaller and the flow mass of the compressor was lower slightly than those of base engine. The boost ratio is similar to that of base engine. Electronically controlled release valve was designed to avoid transient compressor surge. The cam profile of base engine and reduced valve overlap can ensure engine performance and lower THC emission. Furthermore, NOx emission is under the limit of EuroⅢemission regulation.The 1D simulation of turbocharger matching and camshaft phase optimization by GT-POWER software and also the CFD simulation of the coolant flow in the water jacket are conducted during the engine development. All these works assure the engine performance and reliability.The influence of the CNG engine combustion chamber structure and compression ratio on the performance, the influence of advanced angle of ignition on combustion and emission and the rules of lean burned CNG engine combustion are investigated during the experiments. The conclusion are as follows: the maximum combustion pressure and heat release rate are increased by higher compression ratio, which improve the engine power characteristics, decrease the THC emission but increase the NOx emission. The decreasing of THC emission is helpful to catalyst loads under the condition of NOx emission less than the development target; the increasing of advanced angle of ignition, higher maximum combustion pressure and heat release rate, faster combustion rate and higher combustion temperature and heat efficiency, more completely combustion process, higher combustion stability and less late combustion increase the NOx emission and decrease the HC emission, and it also decrease the break specific gas consumption and exhaust gas temperature;The studies showed that asλincreased, in-cylinder mixture concentration became leaner, the propagation of flame front became slower, the combustion delay duration and the main duration became longer, maximum cylinder pressure decreased, heat release ratio became lower. The combustion cycle variability was higher, combustion stability deteriorated. NOx emission reduced considerably, CO, NMHC and CH4 emissions increased slowly. Engine fuel consumption was higher, and exhaust temperature reduced. But the synthetical emissions of NOx,CO and HC emissions were best when a certainλvalue was reached.Based on the study results, a lean burn emission control strategy was put forward. The thresholds for determining main parameters such asλat each engine condition were determined:a).λ≤1.5b).VARpi≤3%c). Special NOx emission≤5g/kW.hd). Exhaust temperature before turbo≤680oC to ensure engine durabilityElectronic control multipoint injection system for lean burn CNG engine was developed. For the control system, air mass, natural gas mass and ignition timing could be controlled and adjusted accurately through close-looped feedback control forλ. Through the calibration with this control system, CA6SE1-21E3N CNG engine reached the development targets for engine performance and fuel consumption and its emissions met EuroⅢ(ESC) regulation. Recently, a small number of CA6SE1-21E3N engines have been mounted on the buses and trucks now in production.