Heuwetter, D., Glewen, W., Foster, D., Krieger, R. et al., "Experimental Investigation of Transient Response and Turbocharger Coupling for High and Low Pressure EGR Systems," SAE Int. J. Engines 7(2):977-985, 2014, doi:10.4271/2014-01-1367.
The transient response of an engine with both High Pressure (HP) and Low Pressure (LP) EGR loops was compared by conducting step changes in EGR fraction at a constant engine speed and load. The HP EGR loop performance was shown to be closely linked to turbocharger performance, whereas the LP EGR loop was relatively independent of turbocharger performance and vice versa. The same experiment was repeated with the variable geometry turbine vanes completely open to reduce turbocharger action and achieve similar EGR rate changes with the HP and LP EGR loops. Under these conditions, the increased loop volume of the LP EGR loop prolonged the response of intake O2 concentration following the change in air-fuel ratio. The prolonged change of intake O2 concentration caused emissions to require more time to reach steady state as well. Strong coupling between the HP EGR loop and turbochargers was again observed using a hybrid EGR strategy. The potential benefit of the HP EGR loop's smaller volume and shorter residence time was largely negated by the simultaneous use of the larger LP EGR loop. Under these conditions, the intake charge oxygen concentration responded more quickly to actuation of the LP EGR valve than the HP EGR valve.