A Method for Comparing Transient NOx Emissions With Weighted Steady State Test Results

This paper describes a method used to compare the emissions from transient operation of an engine with the emissions from steady state operating modes of the engine. Weightings were assigned to each mode based on the transient cycle under evaluation. The method of assigning the weightings for each mode took into account several factors, including the distance between each second of the transient cycle's speed-and-torque point requests (in a speed vs. torque coordinate system) and the given mode. Two transient cycles were chosen. The transient cycles were taken from actual in-use data collected on nonroad engines during in-field operation. The steady state modes selected were based on both International Standard Organization (ISO) test modes, as well as, augmentation based on contour plots of the emissions from nonroad diesel engines. Twenty-four (24) steady-state modes were used.

The transient cycle's speed-and-torque points are used to weight each steady state mode in the method. Based on this assumption, a larger number of speed-and-torque points in the transient cycle will be located closer to those modes¹ with higher weightings. The actual method used to distribute and weight the modes is termed the proportional modal weighting method. It is discussed below.

The method broached in this paper is one of many methods that may be used in the evaluation of emissions from transient and steady state engine operation. Weights are assigned based on actual in-use data. This will allow for a more representative comparison of steady-state and transient test cycles, where the steady-state modes are weighted specifically for the transient cycle with which it is being compared. This will demonstrate the adequacy of using this weighting method for a steady state characterization of transient nitrogen oxides (NOx), hydrocarbons (HCs), and particulate matter (PM) emissions from actual engine operation.