A Super Clean Diesel Vehicle for US LEV-III SULEV Category: Second Report; Advanced A/F Control for NOx Reduction and for SCR Heat Up

摘要

The objective for this research is to develop LEVIII SULEV technology for midsize passenger cars with SCR equipped diesel engines. The key for SULEV is to lower NOx over the whole emission cycle. From the engine control standpoint, this paper describes a unique NOx reduction technology with three-way conversion reaction until the SCR is heated up. Operating the engine at stoichiometric A/F allows a high conversion efficiency of both NOx and HC using a DOC. Furthermore, it allows rapid heat up of the SCR due to exothermal effect and reduced mass flow. The control system regulates the airflow in order to achieve stoichiometric A/F by controlling EGR flow as first priority. The amount of fuel is determined by the advanced torque control modelled by combustion efficiency so that the A/F can be controlled to stoichiometry maintaining driver requested torque. In combination with the unique SCR system, the FTP75 and US06 emissions test results for NOx + NMHC are below the proposed LEVIII SULEV regulation by a reasonable margin [1]. Stoichiometric mode is activated to control the NOx before the SCR is activated and when SCR efficiency is dropping at high exhaust flow; Stoichiometric mode supplement for lack of efficiency of SCR to achieve the LEV III targets. Fuel consumption penalty is minimized by a unique stoichiometric A/F control concept where AF target is achieved mostly with EGR control. The base structured engine control has advanced model-based functions such as air management with simultaneous control of VGT/EGR/throttle, torque structure, temperature and pressure estimator and precise A/F control, which are the main enablers to realize stoichiometric control.