Development of CO2 and NOx emission factors for heavy-duty vehicles based on engine dynamometer and real-world emission tests

Abstract

This study aimed to develop emission factors (EFs) for older heavy-duty vehicles (HDVs). Although their market share has declined, these vehicles still represent a substantial portion of the fleet and disproportionately contribute to road emissions. For pre-Euro 6 vehicles, deriving EFs as functions of speed and tractive power is challenging because certification tests are limited to engine dynamometers, where emissions are measured only with respect to engine speed and torque. To overcome this limitation, we combined engine emission maps from dynamometer tests with a vehicle longitudinal dynamics model to estimate second-by-second emissions. The method was validated using two Euro 6d/e vehicles tested with both engine dynamometers and Portable Emission Measurement Systems (PEMS), showing strong agreement for CO2 (R-2 > 0.97). Based on this framework, CO2 and NOx EFs were developed for Euro 4 and 5 vehicles using dynamometer data and simulation, while EFs for Euro 6d Temp and 6d/e vehicles were derived directly from PEMS on-road tests. Results indicated that Euro 6d/e vehicles emitted about 1.7 times less CO2 and 82 times less NOx than Euro 4 vehicles, confirming the effectiveness of regulatory tightening. Applying the developed factors to heavy heavy-duty trucks in Seoul showed that Euro 4 and 5 vehicles accounted for 52 % of total CO2 emissions and 93 % of total NOx emissions in 2022. These findings demonstrate that the proposed approach provides a reliable tool for estimating emissions from older HDVs lacking on-road data, and offer valuable insights for emission inventory improvement and policy design.