Deviations between steady-state and transient NOx emissions, due to limitations in current air-path control strategies were identified during transient operation. Three different engine and vehicle installations were operated under transient conditions i.e., the European Extra-Urban Drive Cycle (EUDC), in an emission rolls and powertrain testing facility. Air-path control factors and emissions were then identified by first, running a powertrain on the EUDC; second, selecting various operating points on this cycle (according either to speed-fuel or speed-torque); and third, running the same powertrain at these operating points in steady state.
EUDC test results show that Mass Air Flow (MAF) and Manifold Ambient Pressure (MAP), as determined from steady-state optimisation, are achievable, during transient operation, by employing alternative air-path control parameters, relative to steady-state e.g. Exhaust Gas Recirculation, EGR (valve lift) and Variable Nozzle Turbocharger, VNT (vane position) - accommodating, that is, transient air-path lags. However, the strategy thus employed, during transient operation, also varies levels of EGR; and, therefore, may cause transient NOx emissions to deviate from those of optimized steady-states. Comparison between European EUDC test results and corresponding steady-state operation (i.e. operation at the same engine speed with either same torque or same fuel consumption), demonstrated parity in NOx emissions. Hence, provided no transient delays in MAP and MAF are realized, transient emissions of NOx, at any instant during a EUDC, are approximately those emitted under steady-state operation. Detailed steady-state results show that MAF demand may not be the most appropriate set-point with which to control EGR levels during transient operation. It is proposed that a control system based on accurate prediction of the percentage of EGR as set point may provide superior robustness.