Vehicle emissions explained

As new vehicle efficiency and emissions targets tighten, we explain what comes out of the tailpipe when you turn the key?

Exhaust Gases Jpg

Along with all the usual new car figures and statistics such as power, fuel economy and performance, emissions data is playing an increasingly important role in new car information, particularly since the diesel emissions scandal that continues to trouble German car maker Volkswagen.

But what exactly are vehicle emissions and how do they affect you? Here’s the lowdown on what happens when you put your foot down.

Tailpipe emissions can be generally categorised into two types; greenhouse gasses and pollutants.


Pollutant emissions are the parts of car exhaust that can contribute to poor air quality and can even be found contaminating water sources as well. Thick smog in cities and choking fumes are caused by these pollutants and minimising them limits the problematic impact on human health and the environment.

Carbon Monoxide (CO)

This invisible gas is the product of incomplete combustion and is very toxic to humans. Most modern engines only produce minute amounts of CO thanks to efficient combustion processes, but tired engines with older technology are bigger offenders.

Nitrogen Oxides

Three different gasses – nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O) all fall under the same oxides of nitrogen categorisation, but have different effects when it comes to their pollution of the environment.

Nitric oxide has a minimal effect on the environment and health by itself but is highly reactive and can contribute to smog when combined with other airborne chemicals. Its relation NO2 is a brown irritant gas and can aggravate the respiratory tracts of humans, particularly those with medical conditions such as asthma. N2O is an anaesthetic and is used by drag racers in large quantities to allow the combustion of more fuel in highly turned engines. It is also produced in small quantities during normal combustion and has a minimal pollutant effect, however it is a greenhouse gas.

Nitrogen oxides are the emissions that got Volkswagen into hot water in 2015, when it was found to be using a so-called defeat device that allowed some of its diesel vehicles to pass strict lab testing, despite producing many times the maximum limit when in the real world.

The scandal prompted the introduction of a new testing system which assesses vehicles on the road in realistic conditions rather than simulated static rolling roads.


These tiny particles suspended in exhaust gasses are the visible plumes of black smoke sometimes produced by large trucks on acceleration. Generally speaking, diesel engines produce far more particulates than petrol engines and diesel exhaust is recognised as a group 1 carcinogen.

Clearly, all pollutant emissions are of concern but the good news is that modern technology has almost completely eliminated their release into the environment.

Diesel particulate filters, as fitted to the latest engines, trap the tiny pieces of carbon and other materials in a complex filter in which high exhaust temperatures actually burn the particles off and release them as less harmful gasses.

Catalytic converters use expensive noble metals and chemical reactions – also triggered by high exhaust temperatures, to convert the nasty gasses into less harmful and problematic components.

Exhaust gas recirculation or EGR is also another relatively new idea that recirculates a small amount of combustion gas back into the engine’s combustion chambers. This reduces the oxides of nitrogen content of the exhaust.

Generally speaking, the newer the vehicle, the more modern and effective the pollutant emissions control technology reducing the environmental impact of the vehicle.

Greenhouse gasses

Greenhouse gasses are largely harmless to humans in normal emissions quantities, but have a significant impact on a global level and are the major contributor to rising global temperatures. Unlike pollutants, which can be converted to less harmful products through clever chemistry, greenhouse gasses are an inevitable product of combustion.

Many gasses exist that contribute to the greenhouse effect including methane and nitrous oxide, but the main offender in transport is carbon dioxide (CO2).

Australia is a little behind the rest of the modern world for emissions standards and while Europe is already conforming to the toughest Euro 6 standard, Australia is lagging behind on Euro 4.

The amount of CO2 a vehicle produces is directly linked to the amount of fuel it burns and the type of fuel. The only way to reduce vehicle CO2 emissions is to increase fuel efficiency.

Significant progress has been made reducing fuel consumption and, in turn, reducing carbon emissions through the use of hybridised powertrains. Ultimately, fully electric vehicles are the solution to all vehicle emissions, but many argue the problem in many cases is merely shifted from the vehicle to the power station, so while much of Australia’s power generation comes from fossil fuel combustion, EVs offer little advantage in lowering overall CO2 emissions.


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