Effects of Differential Pressure Measurement Characteristics on Low Pressure-EGR Estimation Error in Si-Engines*

Abstract With proper control, cooled LP-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. In addition, it can allow more aggressive downsizing and boosting. Due to the inherent pressure pulsations and low differential pressures across the EGR valve, however, estimating the LP-EGR within the inducted charge can be problematic. The accuracy of this estimation, based on a pressure differential (Δ P ) measurement and the steady compressible flow orifice equation is investigated for various Δ P sensor response speeds and sampling rates using a GT-Power model of a modified Ford 1.6 L EcoBoost engine. In addition, an unsteady compressible flow orifice equation that accounts for flow inertia is derived and used to estimate LP-EGR for the case of a fast response Δ P sensor. Errors in the estimated EGR percentage using the steady compressible orifice equation with averaged Δ P measurement can be as high as 30%, and errors within ±1% require a Δ P of at least 10 kPa. These two measures can be improved up to a maximum EGR estimation error of 10% and a minimum Δ P of 4 kPa respectively through the use of crank-angle resolved Δ P measurement. Further improvements are possible with the new unsteady orifice equation, where all errors are reduced roughly to within ±1%. The effect of inertia, however, can be mimicked in the steady orifice equation with a realistic sampling rate and a slower sensor with an appropriately selected response speed, resulting in a maximum error of 5% and errors within ±1% for Δ P exceeding 1 kPa.