Implementation aspects of an exhaust back pressure (P3) estimator for a series sequential 2-stage boosting system

Abstract

Exhaust back pressure (EBP) is an important parameter for controlling the flow of exhaust gases, so-called exhaust gas recirculation (EGR), for the reduction of harmful nitrogen oxides, NOx and for turbine durability, limiting the pre-turbine pressure. Since the exhaust conditions are severe, including soot, thermal effects and harmful gases, normally a sensor is not considered for mass production. Hence, a cost benefit can be realised which also reduces warranty claims. However, the EBP needs therefore to be estimated, not measured. In single-stage turbocharged engines, a model-based algorithm controls the EGR flow using the available airpath actuators, namely throttle and EGR valve, scheduling the flows using the standard orifice flow equation and estimating EBP using normal turbine design parameters like corrected flow, pressure ratio and variable nozzle turbine position (VNT where applicable). However, for two-stage systems, with two turbines in series, the estimator concept needs to be revised, as well as the sensor set, and this is the topic of this technical paper, complete with both simulation, engine and vehicle results.