Research On Air-fuel Ratio Control Of SI Engines Burning Ethanol-gasoline Blends

As global automobile production and population increase every year, energy shortage and environmental pollution are growing in tensity. Alcohol fuels, especially alcohol-gasoline blends, were considered as the most promising alternative fuels, and have aroused extensive attentions for their low emission level and wide resources. Like ordinary gasoline engines, air-fuel ratio control is still one of the most important parts in engine management systems under different alcohol-gasoline blends, and it directly affects the whole engine power, economy and emission levels. Unfortunately, as existence of intake air-pump effect, fuel wall-wetting effect and air-fuel ratio control loop delay, traditional air-fuel ratio control is still very poor. In addition, special physicochemical properties of alcohol-gasoline blends, especially the evaporation characteristics, make the air-fuel ratio control design of alcohol-gasoline blend engines even more difficult.This thesis focused on the subject of air-fuel ratio control with ordinary gasoline engines burning ethanol-gasoline blends, and carried out a series of theoretical and experimental researches, including:Research on fuel film evaporation characteristics of ethanol-gasoline blends:Basic physical properties of ethanol-gasoline blends were analyzed and accurately determined at first. Then the Gilliland experimental platform was set up, fuel film evaporation and diffusion characteristics of different ethanol-gasoline blends in air streams were studied, and critical influencing factors were discussed as well. Results showed that the evaporation and diffusion process of ethanol-gasoline blends into intake air streams can be also described by the traditional correlation of forced convective mass transfer inside pipes.Modeling of intake manifold air dynamic characteristics:An intake air dynamic mean value model was built. In this model, a speed correction term was introduced to compensate the saturated nonlinear characteristic of throttle air mass flow in large throttle opening areas. Both model parameters identification and model validation were carried out by means of experimental and simulation methods.Modeling and analyzing of intake manifold fuel dynamic characteristics:An intake fuel dynamic mean value model was built. Unlike traditional methods of fuel film dynamic parameters identification, Extended Kalman Filter algorithm was introduced to construct an offline intake port fuel dynamic observer which could estimate fuel film dynamic parameters efficiently. According to the identification results, critical influencing factors of fuel film dynamic parameters, especially mixing ratio of ethanol-gasoline blends, were discussed. A fuel film evaporation time constant model based on Gilliland test results was then built to reduce calibration parameters and improve model accuracy simultaneously.Strategy research on model-based air-fuel ratio control:A complete model-based air-fuel ratio control solution under different ethanol-gasoline blends was presented. Unlike traditional detecting method of intake air mass flow, Extended Kalman Filter algorithm was introduced to construct a model-based intake air state observer which could estimate intake port air mass flow precisely. To eliminate intake fuel wall-wetting effect, a discrete model-based fuel dynamic compensator was also designed and verified, and its control parameters were always from prediction of fuel film evaporation time constant model. In addition, fuel injector correction strategy under different ethanol-gasoline blends and battery voltages was also designed, according to the results of fuel injector flow characteristic test.Designing and verification of air-fuel ration controller:According to the features of MR479Q gasoline engine, an electrical control unit and its corresponding calibration system were developed. ECU software followed AUTOSAR specifications, while calibration system communications based on CCP. Verification tests of ECU embedded model-based air-fuel ratio control strategy were carried out under several different ethanol-gasoline blends. Test results show that under varies of ethanol-gasoline blends good control effectiveness could be always achieved by the model-based air-fuel ratio control strategy and algorithm, the control error is less than 2% under steady conditions, while not exceed 4% under transient conditions.The thesis built an accurate air-fuel ratio model of ethanol-gasoline blend engines, designed model-based air-fuel ratio control strategy and algorithm, and achieved perfect air-fuel ratio control effect under varies of ethanol-gasoline blends, made good foundations for the further research on development and performance prediction of ethanol-gasoline blend engines.