CNG: The Cleaner Fuel? – Part Two

Hundreds of thousands of vehicles have been court ordered to use convert to CNG in the Indian State of Gujarat. Photo by vipez.

This post is the second of a two-part series on CNG fuel in Indian cities. The first post on TheCityFix can be found here.

A version of this post was originally published online on India Together.

On July 25, 2012, the Gujarat High Court ordered all 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 aspects of implementation—whether it’s possible to retrofit so many vehicles within a year; whether the CNG supply in the city of Ahmedabad can be sufficiently augmented to meet the expected increase in demand; and what the impact on the revenues of the state government would be.

However, one question that has been conspicuous by its absence is this: 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 ruling and a rallying call by public agencies, civic groups and NGOs. Campaigns and public interest lawsuits to promote conversion to CNG have gathered steam, with increasing success. However, the presumption that CNG is superior to diesel and petrol fails to take into consideration several aspects surrounding the use and performance of CNG as a fuel.

Firstly, the perception of reduced emissions from using CNG has not been conclusively backed by 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 utilize Ultra-Low Sulphur Diesel (ULSD) and diesel particulate filters have roughly the same level of emissions.

Secondly, a review of the several studies that have looked at air quality in New Delhi post-CNG conversion shows that the impacts are largely insignificant. 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.

Thirdly, engine technology matters. Almost all CNG-powered vehicles in India use retrofitted engines. That is, regular petrol engines have been modified for CNG use. 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 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 EMBARQ’s work with rickshaw drivers suggests that engines retrofitted to use CNG are costlier to maintain, deteriorate faster 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 disappear, therefore, 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 finding a clean burning fuel for 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 can be taken 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 emphasize 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, regardless of the fuel and engine technology used.

As a final point, it is worth emphasizing 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 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 can only be achieved through significant and continual investments in high-quality public transport.

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