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

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.