A Review on Exhaust Emissions from Vehicles

Vehicular emissions are one of the most critical sources of urban air pollution, especially in densely populated cities like Delhi, where rapid urbanization and increasing vehicle numbers exacerbate air quality concerns. These emissions can be categorized into exhaust and non-exhaust sources. Exhaust emissions result from the combustion of fuel and comprise carbon dioxide (CO₂), nitrogen oxides (NOₓ), hydrocarbons (HC), and Mobile Source Air Toxics (MSATs) that lead to adverse health effects and environmental degradation. Contributing significantly to the total particulate pollution with non-exhaust emissions are road dust resuspension, tyre wear, and brake wear among others. Efforts to mitigate vehicular emissions have led to the growth of interest in alternative fuels, particularly Compressed Natural Gas (CNG) and Liquefied Petroleum Gas (LPG), as possible alternatives to traditional petrol and diesel. A series of studies undertaken in European countries showed that vehicles running on LPG and CNG emit lower CO, HC, and CO₂, thus making them environmentally viable alternatives. However, several challenges are identified, including limited fuel infrastructure, vehicle calibration problems, and a need for supportive policies to induce large-scale use. This study evaluates the impact of fuel types on emissions using emission inventory models and chassis dynamometer testing. The results indicate that while CNG and LPG significantly reduce CO₂ and HC emissions. These measures, when combined, can help create a more effective long-term strategy for reducing vehicular pollution and improving urban air quality.