Same raw material

Both petrol and diesel are derived from crude oil, a mixture of hundreds of hydrocarbons — compounds containing mostly carbon and hydrogen, and some other elements like nitrogen and sulphur. Petrol and diesel are hydrocarbons too, petrol being lighter. Petrol has only carbon and hydrogen, and very few nitrogen atoms. It typically has chains of nine carbon atoms; diesel mostly comprises chains of 14 carbon atoms.

Naturally occurring crude can be burnt directly to produce energy, but because different hydrocarbons have different boiling points, the combustion will be very uneven. As such, crude oil is ‘refined’ into compounds like kerosene, LPG, paraffin wax, naphtha, etc., apart from petrol and diesel. Each of these derived products has a specific use.

‘Refinement’ happens by heating the crude. As different hydrocarbons vapourise at different temperatures, the vapourised gases are taken out and turned back into liquids. Petrol is more ‘refined’ than diesel.

Diesel more ‘powerful’

Because of longer carbon chains, diesel packs more energy per unit volume compared to petrol. Higher energy and better combustion efficiency means diesel delivers greater power and ensures better mileage. Longer the carbon chain, higher is the probability of finding other elements like sulphur and nitrogen attached to it. Petrol is thus a more uniform carbon compound than diesel. Crucially, diesel has higher sulphur content.

Exhaust products Petrol

produces carbon monoxide (CO) and some nitrous oxides (NOx) during combustion. The CO combines with oxygen again to produce carbon dioxide (CO2), which is what comes out of the exhaust pipes of petrol vehicles. The relatively high nitrogen and sulphur content in diesel results in the formation of higher amounts of NOx, and particulate matter containing mainly sulphurous compounds. CO2 is a greenhouse gas, and contributes to global warming; NOx and particulate matter carry major health risks.

Climate friendliness

CO2 is an invisible, odourless gas, and does not cause direct or immediate problems for human beings. NOx and particulate matter, on the other hand, affect human health, causing a range of respiratory diseases. Emissions from diesel vehicles, especially particulate matter and black carbon, are the major reasons for air pollution as well. Sulphates, like sulphur dioxide, released in the atmosphere may lead to acid rain. Amid rising concern about global warming in the 1990s, many countries had started shifting to diesels because they did not produce CO2.

‘Dirtiness’ of diesel

Older diesel vehicles were significantly more polluting than petrols, and an immediate health risk. But because some countries, mainly in Europe, had been moving to diesel following concerns over global warming, a lot of R&D investment was made in diesel engines. Modern diesels are a huge improvement over engines produced 15-20 years ago.

Dr Avinash Kumar Agarwal of the Department of Mechanical Engineering at IIT Kanpur says that a Bharat Stage-I (BS-I) rated diesel engine produced about 15 years ago in India would probably be 50 times more polluting than the BS-IV engines of today. Tech upgrades in petrol engines have been much less and slower than the advances made in diesel engine technology.

Agarwal is of the opinion that diesel vehicles older than 15 years, especially trucks, and all BS-I and BS-II vehicles need to be taken off the roads across India. But the latest diesel engines run the petrols close, he says, each being roughly as polluting or as environment friendly as the other.

Others, however, argue that despite the tech advances, diesels retain a higher potential to pollute than petrols. Another professor at IIT Kanpur, who did not wish to be named, said the quality of diesel in India was an issue. The sulphur in emissions from diesels is about 10 parts per million in Europe, while it is 50 ppm in India, he pointed out.