Knowing the right thing to do in an attempt to be environmentally-friendly isn’t always a simple matter. Often I’m left with the feeling that the problem is not really being struck firmly and squarely. Take as an example New Delhi’s laudable attempt to clean up its vehicle emissions. With a population of over 12 million, and becoming more motorised, it was decided to shift  all 90,000 public vehicles (taxis, buses, rubbish trucks etc.) over to compressed natural gas rather than running on petrol or diesel.

So what is compressed natural gas? Well, it’s natural gas that has been compressed, doh! It is essentially methane gas (CH₄) that is stored in pressurised tanks

As the smallest of the alkanes, methane offers some really big advantages as a clean fuel. A lot of this comes down to the cleanliness of the burn. To completely burn a fuel you need to provide enough oxygen for every carbon and hydrogen atom in the fuel’s molecules to combine to form carbon dioxide and water. The bigger the molecule is, the more oxygen you have to deliver.

Now remember that the volume taken up by a fixed number of particles of a gas is constant, for a given temperature and pressure, irrespective of the gas and the size of its particles. This means benzene, for example, will require four and a half times the amount of oxygen to completely combust than the same volume of methane. And it would also need to be very intimately mixed.

In effect, bigger molecules are more difficult to mix to ensure complete combustion in a combustion engine and so rather than getting carbon dioxide, you get the oxygen-lite versions i.e. carbon particulates (soot) and carbon monoxide. You’ll also get the remnants of any unburnt hydrocarbon molecules as well.

But methane is a small molecule, and so burns completely and therefore cleanly. And as a bonus, because you end up burning all the molecule you release all the available energy. This is very good news because after all, this is what you bought the stuff for in the first place.

So here we have a fuel that burns cleaner than petrol/diesel, delivers more energy per mole of carbon dioxde produced and then there’s the cost……….At New Delhi prices CNG is a third the price of petrol and half the price of diesel. Is there no end to the good news here?

Environmental stories never seem to be straightforward however, and this is where this one begins to get ‘nuanced’. It’s all fine talking about different fuels, but we also really need to factor in another important variable; different types of engine.

Not all engines are built the same and the villain of this piece is the two-stroke engine. This is much simpler and cheaper than the four-stroke engines that most car owners will be familiar with. They are used a lot in the west for powering model airplanes and pizza delivery scooters. In the east they have a much more prolific role in that they commonly power auto-rickshaws, also known by the onomatopoeiac: tuk-tuks. New Dehli alone has 5000 tuk-tuks.

A study by the University of British Columbia have found that two-stroke engines have the ability to produce some very unwholesome emissions when they run CNG. Admittedly based on a small number of vehicles (30 in total, 17 of which were two-strokes), the study found that some two-stroke engines left as much as 30% of the CNG unburnt. This gives two-stroke tuk-tuks burning CNG a particulates problem, per kilo of fuel burnt, at the same level as a bus running diesel! Two-stroke engines give off 30 times particulates when running petrol so CNG isn’t so green in this context.

You also need to consider what else is in the exhaust if the fuel isn’t being burnt; methane. Now you’d be producing carbon doxide anyway, so how bad can methane be? Well, pretty bad actually. Methane is also a global warming gas like carbon dioxide, but is many times more potent.

And so back to saving the planet. At first glance swapping to CNG looked like a good option, but after a closer look it would do the environment a big favour if two-stroke vehicles stayed with petrol. This story highlights the problems inherent in analysing complex systems, such as the impact of technology on the environment, and why policy makers need to ensure that they have an understanding of the issues that are based on comprehensive, sound and on-going scientific investigations.

See the original article Climate and Health Relevant Emissions from in-Use Indian Three-Wheelers Fueled by Natural Gas and Gasoline