CO2 Emissions

Carbon dioxide emissions describe how much of this gas a vehicle releases for every kilometre travelled, expressed in grams per kilometre. Unlike the pollutants targeted by a catalytic converter, carbon dioxide is not toxic in the conventional sense; it is the principal greenhouse gas and the main contributor to a vehicle's impact on the climate. The figure has become one of the most consequential numbers on any car's specification sheet, shaping taxation, manufacturer strategy and buyer choice across much of the world.

The defining feature of carbon dioxide output is that it is directly proportional to the amount of fuel burned. Combustion of any hydrocarbon fuel produces carbon dioxide as an unavoidable end product, so the more petrol or diesel an engine consumes, the more it emits, in a fixed chemical relationship. Burning a litre of petrol releases roughly 2.3 kilograms of carbon dioxide, and a litre of diesel around 2.6 kilograms, because diesel is denser and more carbon-rich. This means the emissions figure is effectively another way of stating fuel consumption, and the two move in lockstep.

Because the link is chemical rather than mechanical, carbon dioxide cannot be removed by any after-treatment device. There is no filter or catalyst that can strip it from the exhaust, as the carbon in the fuel has to go somewhere once oxidised. The only ways to cut carbon dioxide are to burn less fuel — through lighter vehicles, more efficient engines, hybridisation or smaller capacities — or to switch to a fuel that contains less fossil carbon. This is the fundamental distinction between carbon dioxide and pollutants such as nitrogen oxides, which can be neutralised chemically after combustion.

The figures quoted for new cars are measured under standardised laboratory procedures, currently the Worldwide Harmonised Light Vehicles Test Procedure, or WLTP, which replaced the older and more optimistic NEDC cycle. WLTP subjects the car to a defined sequence of speeds and loads designed to approximate real driving more closely, producing a repeatable number that allows different models to be compared on equal terms. Real-world emissions still vary with driving style, load and conditions, but the test value provides the official benchmark used for regulation and labelling.

That official figure carries real financial and regulatory weight. Many countries scale vehicle taxation directly to carbon dioxide output, so a cleaner car attracts lower duty, and manufacturers face fleet-average targets that penalise them heavily for exceeding a set grams-per-kilometre limit across everything they sell. These pressures have driven the rapid adoption of hybrid and electric powertrains, since a battery-electric car emits no carbon dioxide at the tailpipe at all, transforming the emissions calculation entirely.