No smoke and mirrors here - just an engineer's lowdown on exhaust emissions
Tailpipe Tales PHOTOS: TORQUE

With all the excitement over CEVS (Carbon Emissions-based Vehicle Scheme), the majority of motorists might believe that the gas exiting an exhaust pipe is all carbon dioxide (CO2). After all, the CEVS uses automakers’ certifi ed CO2 output (in grams per kilometre) to determine the tax rebate granted ($5k to $20k for passenger cars), or as the case may be, the surcharge imposed ($5k to $20k, too).

In actual fact, CO2 is just one of the nasty gases that escape from vehicle tailpipes and affect us all – people, the air, flora and fauna. There’s a lot more where that came from, so to speak.


In theory, the complete combustion of pure fuel only produces water and carbon dioxide. "Real" fuel, however, has a certain level of impurity, while the conditions for internal combustion are far from ideal – which is why many of the by-products from engines' burning of petrol or diesel are classified as pollutants.

Both petrol and diesel are known as hydrocarbon fuels because their primary constituents are hydrogen and carbon. These are distillates of crude oil processed in an oil refinery.

In a petrol engine, the fuel-air mixture is ignited by an intense electric arc between the electrodes of a spark plug. The combustion that takes place invariably results in some unburnt hydrocarbons, oxides of nitrogen and oxides of sulphur, in addition to the water and carbon dioxide. Much of the odour from the exhaust tailpipes is due to the "loose" hydrocarbons, some of which are considered to be carcinogenic.

Carbon monoxide (CO) is another poisonous by-product of petrol combustion. Prolonged inhalation of CO causes displacement of oxygen in the human body and a gradual shutdown of the entire system.

Although nitrogen is an inert gas, during combustion of the air-fuel mixture (which contains naturally occurring nitrogen), oxidation of the nitrogen takes place, producing nitrogen monoxide (NO) and nitrogen dioxide (NO2). Collectively called NOx, these oxidised nitrogen compounds are toxic and one of the causes of acid rain (during rainfall, the nitrogen oxides dissolve in the atmosphere and become nitric acid).

Also causing acid rain are the oxides of sulphur (SOx). Like the nitrogen oxides mentioned earlier, the oxidised sulphur will dissolve in water to form, in this case, sulphuric acid. Although low in concentration, this contaminated rainwater is nonetheless acidic and poses a health risk, while its corrosive nature is obviously detrimental to metallic surfaces, too.

In a diesel engine, an air-fuel mixture isn't present in the combustion
chamber prior to ignition. Instead, only air is compressed by the
piston, with fl aming initiated by the injection of diesel fuel into the
hot, highly pressurised air near the end of every compression stroke.

This fundamental difference in the method of ignition (mainly the
higher combustion pressures compared to a petrol motor) intensifi es the oxidation of nitrogen. In addition to slightly higher quantities of NOx, diesel engines also generate small amounts of solid pollutants referred to as particulates.


So, it looks like there is plenty to be done before allowing the "problems" of combustion to exit via the tailpipe. The best of current emissions management is based, firstly, on fuels that have been de-sulphurised at the refinery. Such fuels are not yet available everywhere in the world, as most governments take a gradual approach to limiting the sulphur content of petrol and diesel dispensed at the pumps in their countries.

Low-sulphur automotive fuels are necessary for a signifi cant reduction in
sulphur oxides (which lead to acid rain). They are also needed for the reliable operation of the direct fuel injection tech that equips today’s state-of-the-art powerplants. In particular, regenerative particulate filters in the latest diesel exhaust systems can only perform effectively for prolonged periods if the fuel is of the low-sulphur variety.

Whether fuelled by diesel or petrol, the post-combustion hydrocarbons,carbon monoxide and nitrogen oxides are treated within the exhaust by the catalytic converter (see box story on the right). "Catalytic converting" produces water vapour, carbon dioxide and nitrogen, but the higher quantities of NOx and the presence of particulates in a diesel’s "downstream" require slightly more complex treatment than for a petrol setup. Either way, the end result after the "cat" has done its job is cleaner air.


What about the all-important CO2 that seems to be the only thing that legislators are "using" to reward/penalise the motorist? Well, carbon dioxide emission basically correlates to the amount of fuel consumed – more fuel burned means more CO2 emitted. A typical diesel engine actually spits out more CO2 than its petrol equivalent because of higher calorific value, but diesel propulsion is more efficient, so its overall CO2 figure is lower.


There hasn’t been a single worldwide standard for exhaust emissions so far, with Europe, Japan and the US having their own protocols and type-approval procedures. However, there is a general consensus to progressively tighten the guidelines on harmful fumes emitted by road vehicles.

Singapore's CEVS is based on the European "carbon" framework, and the
rebates only apply to vehicles that meet or exceed Euro 5, which took effect in 2010. With increasingly stringent emission standards, carmakers and engineers are busy developing (or improving) the technology to satisfy these rules as they get stricter. Some of the tricks in recent
years include forced induction, downsized engines, variable displacement, and full valve timing control. By the time Euro 6 comes into force (September 2014), there should be a few more interesting tricks and technical treats – and some new tailpipe tales to share.