A novel lambda-based EGR (exhaust gas recirculation) modulation method for a turbocharged diesel engine under transient operation

• A practical intake oxygen model contains EGR rate and air-fuel equivalence ratio (lambda) has been built and validated.
• Intake oxygen concentration is more correlated to NOx emission trace than EGR rate under transient.
• Lambda-based EGR modulation method is more effective in NOx overshot reduction under load transient operation.

Exhaust gas recirculation (EGR) is one of the major technical measures used on diesel engines to restrain NOx emission. Intake oxygen concentration has been proved to be the more substantial parameter to control NOx than EGR rate. However, the mechanism is hard to be utilized due to the difficulty of acquiring the real-time intake oxygen concentration. In the present study, a novel lambda-based intake oxygen model was built and validated. By using this model, two types of transient EGR modulation were performed, which illustrates the validity for NOx overshot elimination under the load transient. Based on the intake system schematic and the law of mass conservation, the model was deducted to find the relationship of intake oxygen concentration, EGR rate and lambda. Then the model was validated on a four-cylinder turbocharged diesel engine by following steps. Firstly, both steady and transient state test were conducted, the result shows that intake oxygen concentration is more related to NOx emission under transient working conditions. Then the model was validated under different speed and load. The result shows that intake oxygen concentration is linear to the ratio of EGR rate to lambda. Finally, transient EGR operation is performed. Under two types of transient operation, the emission performance is different. The lambda-based EGR modulation method is more effective in the constant-speed load transient. NOx overshot is eliminated without harm soot too much. In the constant-load speed transient, the modulation is still following the commonly accepted trade-off relationship. The study shows that the method could be effectively utilized in diesel load transient emissions control.