Regulated emission characteristics of in-use LNG and diesel semi-trailer towing vehicles under real driving conditions using PEMS

On-road driving emissions of six liquefied natural gas (LNG) and diesel semi-trailer towing vehicles (STTVs) which met China Emission Standard IV and V were tested using Portable Emission Measurement System (PEMS) in northern China. Emission characteristics of these vehicles under real driving conditions were analyzed and proved that on-road emissions of heavy-duty vehicles (HDVs) were underestimated in the past. There were large differences among LNG and diesel vehicles, which also existed between China V vehicles and China IV vehicles. Emission factors showed the highest level under real driving conditions, which probably be caused by frequent acceleration, deceleration, and start-stop. NOx emission factors ranged from 2.855 to 20.939 g/km based on distance-traveled and 6.719–90.557 g/kg based on fuel consumption during whole tests, which were much higher than previous researches on chassis dynamometer. It was inferred from tests that the fuel consumption rate of the test vehicles had a strong correlation with NOx emission, and the exhaust temperature also affected the efficiency of Selected Catalytic Reduction (SCR) after-treatment system, thus changing the NOx emission greatly. THC emission factors of LNG vehicles were 2.012–10.636 g/km, which were much higher than that of diesel vehicles (0.029–0.185 g/km). Unburned CH₄ may be an important reason for this phenomenon. Further on-road emission tests, especially CH₄ emission test should be carried out in subsequent research. In addition, the Particulate Number (PN) emission factors of diesel vehicles were at a very high level during whole tests, and Diesel Particulate Filter (DPF) should be installed to reduce PN emission.