Abstract

Intra-city buses are crucial for urban transportation, providing essential mobility services. However, in India, the public sector's total bus fleet expanded from 1.89 million units in 2014 to its current size in 2023, reflecting a compound annual growth rate of 2.19%. This growth has led to significant emissions and impacts on air quality in urban areas. This study evaluates the real-world fuel efficiency and emission characteristics of gaseous pollutants from urban transit diesel buses under heterogeneous traffic conditions using a portable emissions measurement system. Fuel consumption rates were determined in real-time using a carbon balance method based on the emission rates of major carbonaceous pollutants. Under typical real-world driving conditions, the average distance-specific fuel consumption for diesel buses is 29.3 L/100 km with the air conditioning off and 42.7 L/100 km with the air conditioning on. The average emission rates (g/s) for CO, HC, CO2, and NO gases from transit buses were measured at 0.092 ± 0.057, 0.054 ± 0.032, 0.436 ± 0.186, and 0.089 ± 0.058, respectively. The maximum emission factors observed were 17.98 g/km for CO and 12.93 g/km for HC+NO, which exceed the BS emission standards by 10.3 to 15.2 times (BS III: CO = 0.95 g/km, HC+NO = 0.86 g/km; BS IV: CO = 0.74 g/km, HC+NO = 0.46 g/km). Fuel efficiency in buses is influenced by operating conditions such as land use, ridership, speed, and accessibility. These insights underscore the need for enhanced transit management strategies to optimize fuel use and limit emissions.