Emission Characteristics of In-use Cng Vehicles in Delhi

Road transportation has most adversely affected the air quality in Delhi. The existing fleet of 3.5 million motor vehicles in Delhi contributes nearly 70% of the total air pollution load. To improve the air quality status in Delhi various measures viz. improvement in fuel quality and engine technology, use of exhaust treatment devices, use of cleaner alternative fuels (CNG) etc. have been adopted in the recent past. The Supreme Court of India has promulgated the mandatory use of CNG as automotive fuel in Delhi for public transport vehicles, which include diesel-driven buses, taxis (diesel and petrol) and petrol-driven three wheelers (autorickshaw: intermediate public transport). These vehicles present a combination of new, converted and retrofitted vehicles operating on CNG. In addition, a small number of private vehicles also operate on CNG (duel-fuel: petrol and CNG). In Delhi, more than 8-year old buses and pre-1990 taxis and autrickshaws have already been scrapped since March 2000. Hence, nearly 25,000 autos, 9,000 buses and 8000 taxis are required to run on CNG. The existing 87 dispensing stations are unable to meet the demand due to various factors. This has resulted in long queues (at dispensing stations) along the roads, thus seriously hampering the flow of traffic and has induced congestion and unwarranted pollution loading along the corridors. The CNG vehicles have been assumed as non-polluting vehicles, hence no certification (PUC: pollution under control certificate) was required until recently for tail-pipe emissions, which otherwise is mandatory for all other vehicles. To understand how these in-use CNG-powered vehicles compared against their petrol-driven counterparts, the present study was undertaken in Delhi. The tailpipe CO and HC emission characteristics of CNG vehicles were analysed and compared against the emission characteristics of similar vehicle categories earlier using petrol as fuel. The emission levels were compared against the legislated tailpipe CO emission standards of 4.5% and 3% (by volume) for three and four wheelers respectively. All CNG-powered three wheelers are four-stroke engine vehicles, whereas petrol-driven conventional three wheelers are powered by two-stroke engines. This difference appears to have influenced the emission characteristics from CNG-powered three wheelers. The comparison has revealed notable reduction in CO and HC emissions from CNG-driven vehicles, especially from three wheelers. We have also tried to ascertain whether the lower emissions from three wheelers were due to the combination of CNG and four-stroke technology or because of the use of CNG alone. In addition, the present scenario has also been analysed, in limited scope, with respect to CNG-induced chaos, congestion and pollution. Nevertheless, there is a pressing need to initiate the process of certification and generation of database for CNG-powered vehicles, when the true picture would emerge as to how effective the CNG conversion is?