Effects of differential pressure measurement characteristics on high pressure-EGR estimation error in SI-engines

Abstract

With proper control, exhaust gas recirculation (EGR) can be used for knock mitigation in SI engines, enabling fuel economy improvements through more optimal combustion phasing and lower fuel-enrichment at high loads. Due to significant pressure pulsations across the EGR valve, estimating the mass flow rates at transient and reversing flows across the valve can be challenging. Many systems utilize the pressure drop across a restriction in a flow path as an indication of mass flow. In automotive applications, the measurement of the pressure drop is accomplished by the use of a delta pressure sensor and the pressure drop mechanism could be a sharp edge orifice or other restrictions such as a valve opening. This technique works well for steady-state flow; however, if pressure fluctuations exist, especially at higher frequencies, this method of measuring mass flow can have large errors. The accuracy of EGR mass flow estimation based on a pressure differential (Δ P) measurement and the steady compressible flow orifice equation is investigated for various Δ P sensor frequencies and sampling rates on Ford 3.5L V6 GTDI and 2.3L I4 GTDI engines. This paper identifies the two major contributors of the error are the long length of the gauge lines and the close location of the downstream tap. The long length of the gauge line results in resonances at low frequency which affects the sensor performance. When the downstream tap is placed too close to the orifice, the accuracy of sensor reading is reduced. The proposed solution suggests keeping the gauge line as short as possible and the downstream tap at least two diameters away from the orifice and using a high-speed sensor. The result from engine testing shows a great improvement in the measurement accuracy by 43%. It is demonstrated that using a slow responding sensor with a low sample rate leads to erroneous mass flow calculation when significant pressure pulsations exist. A minimum of 1 kHz sample rate is needed to increase the accuracy of the EGR estimate within ±1%.