Tuesday, January 22, 2008

Tata Nano and Car Pollution

Karan Thapar interviewed Sunita Narain on CNN IBN regarding the Tata Nano and its impact on Indian roads in terms of traffic congestion and pollution. Nothing radically new was said. Ms. Narain was not against the Nano per se but just against encouraging any type of private vehicles on the road. Overall the interview went well with the usually impatient Mr. Thapar actually allowing Ms. Narain to complete a few sentences. A couple of salient points regarding car pollution:

KT: So how do you respond to the research done by the ‘Economic Times’ that says in terms of carbon dioxide equivalent emissions, the Nano is forty per cent better than the other cars? In other words, if cars are going to increase anyway, it is better if people buy the Nano than any other car.

To which Ms. Narain replied that compared to two wheelers the emissions from Nano are still large. No debate about that but I thought that both of them missed elaborating on this point a little more. For example will the implementation of Euro 4 and later Euro 5 emission standards lead to future reductions in CO2 emissions from Nano and other cars? The answer is very unlikely. This is important since transport will contribute an increasingly larger share of greenhouse gas emissions in India. The reason that Nano emits less CO2 than other cars is primarily because it has a smaller engine. Unlike other pollutants like SOx, NOx and PM10 where quality of fuel and engine technology make a big difference, the amount of CO2 emitted depends upon the type and amount of fuel burnt. Smaller engines use less fuel per km traveled, so Nano emits less. The diesel version of Nano has a bigger engine around 750-800 cc compared with the petrol version which is 624 cc. Diesel emits even more CO2 per litre burned (2.7 kg per litre) than petrol (2.3 kg per litre). As long as we are stuck with petrol and diesel as our fuels, we won't achieve any radical reductions in car CO2 emissions in the future.

Diesel was the main flashpoint in the interview with Ms. Narain coming out strongly against the use of diesel vehicles. Diesel emits much more of particulate matter and sulphur compounds than petrol. What she emphasized in words I have shown as a graph below. The calculations are for per passenger km travelled, which allows different vehicle types to be compared. The figures are for a generic car. I don't have emission factors for Nano, but although the absolute amounts will change, the overall trend shown by Nano will be similar to any other car, i.e. diesel more polluting for particulate matter and sulphur than petrol and CNG and the Nano more polluting that two wheelers and bus on a per passenger km basis.

Source: Emission factors for CO2, PM10 and SOx from World Bank: A Simple Model for Better Air Quality (2005); N.Harshadeep and S. Guttikunda. Assumed Occupancy: 2 wheels-1, Car-2, Bus- 40. A commute of 100 km is assumed.

Update: Figure changed. As a reader has so (un) kindly pointed out, the comparison of car diesel CO2 and car petrol CO2 emissions in the original figure was off by some amounts (it showed diesel CO2 emissions much more than petrol which is not correct) and I have removed that figure. The general trend between car diesel emissions and bus diesel emissions for CO2 still stand, as do trends for other pollutants. Using a more direct method of calculating CO2 emissions from diesel and petrol gives different results. For example a diesel car with mileage around 15km/litre will emit about 180 kg of CO2 for 1000 km use. A petrol car with mileage around 11.5 km/litre will emit about 200 kg of CO2 for 1000 km use. The above new figure represents these corrected results for diesel and petrol CO2 per pass km emissions. I don't know why the emission factors from the World Bank study are giving different results. I will investigate!

Diesel cars which are increasingly taking more of the market share in India are the most polluting per passenger km of travel for particulate matter and sulphur. Looking at the graph the way forward appears obvious. Massive increase in public transport running on clean fuel coupled with disincentives to use private vehicles especially diesel ones. Ms. Narain suggested that Ratan Tata will be a bigger hero if he brings to market an energy efficient bus that can transform public transport. Why is that not happening? An even more general question is why are car makers not rushing to bring a CNG car or a hybrid car to the market? Currently the incentives seem to be highly skewed. There is just no clear policy signal from the government in terms of favoring public transport over private vehicles and alternative fuels over petrol and diesel in transport.


  1. The chart shown is complete junk.

    Diesel engines are much more efficient than petrol engines especially in stop/go city driving (where pollution matters most). This means that diesels consume less CO2 per unit of energy produced than petrol engines period. A diesel engine producing double the CO2 of a petrol engine per unit of power produced is absolutely impossible.

    Apart from particulate soot, diesel engines produce far less pollutants than petrol engines - much less particulate lead, CO, NOx, hydrocarbons (which are responsible for petrochemical smog). The reason why people think that diesels are dirtier is because poorly maintained diesel engines (mainly from commercial vehicles) emit visible soot from burnt engine oil which leaks past the piston rings, while petrol pollutants are invisible.

    What India really needs to control pollution is annual emission checks to all road vehicles - two stroke, petrol and diesel, as in other countries so that the small proportion of vehicles that produce most of the pollution are removed from Indian roads.

  2. Ya, Anonymous is right.