Experimental study of CO2 and pollutant emission at various altitudes: Inconsistent results and reason analysis

Abstract How altitudes affect vehicle CO2/pollutant emission has long been studied, while different, sometimes conflicting conclusions are reported. To evaluate how altitudes impact vehicle CO2/pollutant emission, 21 vehicles were tested in six cities with altitudes ranging from 26.7 m above sea level (masl) to 2264.7 masl. Three of the vehicles were tested in the altitude chamber at five different heights (from 0 to 2400 masl) following the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). CO2/pollutant emissions, conformity factor, and extended factor were evaluated based on the results obtained. No uniform patterns were found between altitudes and CO2/pollutant emission in both laboratory and real driving tests. For laboratory tests, the different CO2 emission results could be explained by air-fuel ratio variations during the transient cycles and engine operation variations between tests. More variables were introduced to the real driving tests, which brings more uncertainty to the CO2 emission. The average CO2 emission of the turbocharged and naturally aspirated vehicles at various cities share similar patterns, indicating other parameters (e.g., driving patterns, road conditions, ambient conditions, et al.) might have a larger impact on the CO2 emission than the altitude growth. These reasons could account for the irregular pollutant results. Besides, the cylinder-to-cylinder variations in equivalence ratio about the mean value have more enormous impacts on the emission results. Discrete results mean that single engine/vehicle tests couldn’t reflect the real situations at various heights. The current conformity factors and the extended factors might be too loose.