On 25 July 2012, the Gujarat High Court ordered all public and private vehicles in the state to be converted to Compressed Natural Gas (CNG) within one year. The task is mammoth - in Ahmedabad alone, about 700,000 two-wheelers and 70,000 four-wheelers will have to be retrofitted with CNG kits. Discussions on the merits of this decision have focused almost entirely on the aspects of implementation - can so many vehicles be retrofitted within a year? Can CNG supply in the cities of the state be sufficiently augmented to meet the expected increase in demand? What will be the impact on the revenues of the state government? And so on.

One question has been conspicuous by its absence - given the goal of reducing local air pollution and greenhouse gas emissions, is CNG really that much superior to diesel and petrol as the preferred choice of fuel for urban transport?

In recent years, the use of CNG has been seen as a silver bullet, a cure-all, for the scourge of local air pollution and greenhouse gas emissions in Indian cities. In particular, the decision of the Supreme Court in 1998 mandating the conversion of all public transport vehicles in New Delhi to CNG has been considered a landmark, and a rallying call, by public agencies, civic groups and NGOs. Campaigns and PILs to promote conversion to CNG have gathered steam - and increasing success. However, several aspects surrounding the use and performance of CNG as a fuel suggest that its superiority in comparison to diesel and petrol is not quite so clear-cut.

Engine technology matters. Almost all CNG powered vehicles in India utilize retrofitted engines. The combustion of CNG in such vehicles is less than optimal, negating some of the 'cleaner' emissions claims made on its behalf.

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First, the perception of CNG as a clearly better fuel choice from the perspective of reduced emissions is not conclusively backed up by the evidence. A meta-analysis of emissions studies of various engine and fuel technologies for public transport vehicles, for instance, shows that there is no clear winner when it comes to fuel choice. The use of CNG results in lower Particulate Matter (PM) emissions, compared to conventionally used fuel and diesel. However it performs worse on other emissions components, particularly Nitrogen Oxides (NOx) and greenhouse gases.

Further, the study indicates that when vehicles employ post-treatment of emissions, the different fuel choices perform comparably. CNG vehicles fitted with Oxidation Catalysts, and diesel engines that utilise Ultra-Low Sulphur Diesel (ULSD) and diesel particulate filters have roughly the same level of emissions.

Second, a review of the several studies that have looked at air quality in New Delhi post-CNG conversion shows that the impacts are, largely, inconclusive. One study using daily ambient measurements for four years shows, for example, that levels of NOx have actually risen, and levels of PM show only marginal reductions. Another study that did find reductions in PM, Carbon Monoxide (CO) and Sulphur Dioxide post-CNG conversion attributed a significant part of the reduction to the lower sulphur content in petrol and diesel in recent years.

Third, engine technology matters. Almost all CNG powered vehicles in India utilize retrofitted engines. That is, regular petrol engines have been retrofitted to be able to use CNG. There are significant disadvantages with this method. Most importantly, the combustion of CNG in retrofitted vehicles is less than optimal, negating some of the 'cleaner' emissions claims made on its behalf.

For auto-rickshaws, in particular, one study found that as much as one-third of CNG is improperly burned in two-stroke engines, resulting in higher levels of greenhouse gas emissions and, more significantly, higher levels of PM emissions due to unburned lubrication oil (which appears as the blue smoke that has become a hallmark of CNG rickshaws). It concludes that New Delhi could have achieved greater emissions reductions, and at a lower cost, simply by upgrading rickshaws to higher efficiency four-stroke petrol engines.

Finally, anecdotal evidence from our work with rickshaw drivers suggests that engines retrofitted to use CNG are costlier to maintain, deteriorate sooner (in terms of mileage offered) and ultimately have shorter lives. This is also a story we hear repeated often in our experience working with public bus operators who use CNG. Any gains made at a 'per-unit of fuel used' level may well, therefore, disappear when the overall quantum of fuel use and the lifecycle costs of vehicles are taken into account.

CNG, then, is by no means a clear cut winner in the race to find a clean burning fuel for our vehicles.

This is not to say that there are no benefits from using CNG. Certainly there are non-environmental benefits from having a third fuel option in addition to petrol and diesel - such as the increase in energy security and the buffer from price shocks offered by a diversification of fuel choice. This is also not to say that CNG has no benefit as a potentially cleaner source of fuel for urban transport in India. However, until such a time as a 'clear winner' emerges, we are doing our urban citizens a disservice by proclaiming one particular fuel as the solution to all our air quality woes. We need, instead, to pursue improvements in fuels, technology and standards on all fronts.

In other words, truly improving the quality of air in our cities will require more than a blanket mandate to convert vehicles to CNG. There are several initial steps that we can take to reduce emissions from vehicles that use CNG as well as other fuels.

First, we must work directly with manufacturers to develop engines that are designed specifically for the use of CNG. In the long run, retrofitted engines are a sub-optimal, and potentially counterproductive, solution. Second, we must emphasise post-treatment of emissions as a relatively low cost strategy for reducing local air pollution - particularly the use of oxidation catalysts for CNG vehicles and diesel particulate filters for diesel vehicles.

Third, we must capitalize on the major investments being made internationally in improving diesel technology, especially with regard to the development of higher efficiency engines and Ultra Low Sulphur Diesel (ULSD) fuel variants. And fourth, we must insist on a strong regime of continuously improving mileage and emissions standards for vehicles themselves, regardless of the fuel and engine technology used.

As a final point it is worth emphasising that, given the phenomenal increase in both population and private vehicle ownership in Indian cities, any improvements in fuel and vehicle technology will at best result in marginal improvements in urban air quality. Any decrease in emissions from individual vehicles will be more than offset by the addition of an ever increasing number of private vehicles on Indian city roads.

In the long run, significant improvements in urban air quality can only come from shifting people to public transport and developing urban forms that avoid the need for excessive and overlong motorized travel. This, in turn, can only be achieved through significant and continual investments in high-quality public transport.


  • Mora, J.M., Martinez, H, & Unal, A. (2009) "Bus Technology Analysis". Centre for Sustainable Transport - EMBARQ Mexico

  • Kathuria, V. (2005) "Vehicular Pollution Control in Delhi: Impact of Compressed Natural Gas" Economic and Political Weekly, Vol 40, No. 18

  • Narain, U. & Krupnick, A. (2007) "The Impact of Delhi's CNG Program on Air Quality" Resources for the Future Discussion Paper 07-06

  • Conor, C.O., Reynolds, A.P., & Grieshop, M.K. (2011) "Climate and Health Relevant Emissions from in-use Indian Three Wheelers fueled by Natural Gas and Gasoline" Environmental Science & Technology 2011-45, 2406-2412