Principal Engine Emissions

CO = Carbon monoxide, HC = Unburned hydrocarbons, NO_x = Nitrogen oxides mainly  mixture of NO and NO₂  ,
PM = Particulate matter
Other engine emissions include aldehydes such as formaldehyde and acetaldehyde primarily from the alcohol fuelled engines, benzene and polyaromatic hydrocarbons (PAH).

Sources of Engine/Vehicle Emissions

Figure 1.3 shows the sources of emissions from a gasoline fuelled SI engine viz., exhaust, crankcase blow by and fuel evaporation from fuel tank and fuel system

From a diesel engine powered vehicle the emission sources are shown in Fig. 1.4.