Vehicle emissions explained – buying a diesel or petrol car?October 5, 2015
The recent news about Volkswagen cheating emissions tests has thrown the whole issue of vehicle emissions into the spotlight, but there has been much inaccurate and confusing reporting of the whole emissions issue in the media, so here’s a user-friendly guide to vehicle emissions, and what car you should choose – diesel, petrol, or something else.
Volkswagen has been in the news due to the Environmental Protection Agency (EPA) in America discovering that diesel engines in certain Volkswagen Group vehicles have been fitted with a ‘defeat device’, which senses when the vehicle is being officially tested, and lowers the performance of the vehicle and therefore also lowers the emissions. The emissions in question are nitrogen oxides (NOx). It seems that Volkswagen couldn’t successfully engineer some of its diesel engines to pass the stringent US NOx tests, so the company resorted to using software to cheat the system instead.
Although Vokswagen’s ‘defeat device’ is likely to have an impact on all emissions, it is NOx emissions that are the issue. NOx emissions have an impact on local air quality – they help to form smog and have serious health effects, including contributing to the development of respiratory and lung diseases and cancer.
Some national media in the UK have been confusing NOx emissions with CO2 emissions. Whereas NOx emissions impact on local air quality, CO2 emissions impact upon climate change, and road tax is based on CO2 emissions, it is not influenced by NOx emissions, despite the impression that some national media have recently given.
So how have we reached the point where there is so much focus – and confusion – around vehicle emissions?
Petrol engines were popular in the UK for many years, then sales of diesel engines started to increase. When there was much political focus on climate change and CO2 levels, diesel engines were encouraged in the UK, because diesel engines emitted lower levels of CO2, which also had a direct relationship to better fuel economy. Vehicle tax systems – Vehicle Excise Duty (Road Tax) and Company Car Tax – were developed to encourage motorists to buy cars with lower CO2 emissions – which in turn encouraged people to buy diesel cars.
However as well as CO2 emissions, cars also have emissions that impact on local air quality, such as NOx, and particulates. Euro standards ensured that cars became cleaner in terms of local air quality emissions, but such emissions didn’t influence any tax incentives or penalties.
So motorists were encouraged by government incentives to buy diesel cars, primarily due to lower CO2 emissions, and these cars were also more economical than their petrol equivalents.
Petrol v diesel emissions and economy
To demonstrate the difference in emissions and economy between a petrol and a diesel car, over the last week Green Car Guide has had two cars on test. Both are very similar cars, but one was a petrol (automatic), and one was a diesel (manual), as follows:
Volvo V40 T3 Geartronic SE Lux Nav
V40 T3 SE Lux Nav available from: £25,470
CO2 emissions: 124g/km (manual)
Official combined NEDC fuel consumption: 53.3mpg (manual)
Real-life average fuel consumption: 28.8mpg (automatic)
NOx Emissions: 15 mg/km (manual)
Emissions Particles: 0 mg/km
Ford Focus ECOnetic 5 Door 1.5 TDCi with Stop/Start 6 Speed Manual
Focus ECOnetic 5 Door 1.5 TDCi available from: £19,145
CO2 emissions: 88g/km
Official combined NEDC fuel consumption: 83.1mpg
Real-life average fuel consumption: 67.3mpg
NOx Emissions: 66 mg/km
Emissions Particles: 0.70 mg/km
Both cars are small family hatchbacks, although Volvo would argue that its V40 is more ‘premium’ (it’s certainly more expensive). The Volvo is a good car, and very refined car to drive, but the Focus – which is also perfectly acceptable to drive – is almost two and a half times more economical in real-life driving (Focus 67.3mpg v V40 28.8mpg). (Note that our V40 test car was automatic, but NOx data was only available for the manual transmission model, and combined mpg is also quoted for a manual). The official CO2 emissions are 124g/km for the Volvo (again based on the manual), and 88g/km for the Focus.
However the NOx emissions of the (manual) petrol V40 are 15 mg/km; whereas the NOx emissions of the diesel Focus are 66 mg/km – ie. because it’s a diesel, the Focus has 4.4 times higher NOx emissions.
Official v real-life emissions and economy
Apart from the issue of NOx emissions, there’s also the issue of official CO2 emissions and economy compared to real-life CO2 emissions and economy. There has been increasing awareness over recent years that it is impossible to come close to the official NEDC test mpg (and CO2 emissions) figures in real-life driving.
Having achieved 103mpg in a diesel BMW 1 Series in the RAC Future Car Challenge in 2012 (and having achieved a 40% range improvement in an electric BMW 1 Series in the same event in 2011), we’re aware that eco-driving can improve on the official figures, but it’s not realistic for most motorists to drive in such a way in everyday driving. We measure real-life miles per gallon for all the cars that we test, and on average our real-life mpg is up to 25% worse than the official figures. This differential seems to be growing wider as cars gain greener tech – which is engineered to ensure that cars perform well on the short and low-load NEDC test, but the green tech doesn’t work as well when the cars are driven outside of this cycle.
A new test – the Worldwide harmonized Light vehicles Test Procedure (WLTP) – is promised to replace the NEDC test, but it isn’t likely to be here for a good few years yet. There should now be political pressure to introduce the new test sooner, and to make it more representative of real-life driving. And manufacturers should also be made to publish NOx and particulate emissions as well as mpg and CO2.
However there’s also the issue of emissions targets. By 2021, phased in from 2020, the EU fleet average CO2 emissions to be achieved by all new cars will be 95 grams per kilometre. If the test for CO2 emissions is made more realistic, and therefore cars will perform worse, then the trajectory that manufacturers are currently on to lower emissions to an average of 95g/km CO2 by 2020 will be seriously damaged.
So which car should you choose – petrol or diesel?
There is currently a media backlash against diesels, which have higher levels of emissions that impact on local air quality, as can be seen from the above data, but there is no financial disincentive for buying a diesel car, apart from the extra purchase cost of a diesel engine versus a petrol engine. So should diesel cars be demonised? If they are, then motorists will lose out on diesel cars potentially being more than twice as economical as petrol cars in real-life driving – as well as having lower CO2 emissions. With petrol and diesel prices currently being on par with each other, this means that a petrol car could be more than twice as expensive in terms of fuel costs. So which engine technology will motorists be tempted to choose?: diesel.
However the answer is to choose the fuel or engine technology that is most efficient for your individual driving patterns. For lots of long distance journeys, diesel is likely to be best. For low mileage drivers undertaking a range of different journeys, a petrol car might offer the lowest whole life running costs. For people mainly driving short distances in urban areas, with occasional longer distances, a plug-in petrol-electric hybrid may be best. For drivers who only ever drive short distances, an electric car would be ideal, as it has zero tailpipe emissions – both in terms of CO2 and local air quality.
One thing to come out of the Volkswagen emissions scandal, along with the increased focus on local air quality and the intense scrutiny of diesel emissions, is that the increasing uptake of electric cars is likely to gain even more pace.
View the economy and CO2 emissions of all cars, and compare car running costs, at: www.greencarguide.co.uk/features/car-running-costs-comparison-tool
Car manufacturers generally don’t publish the local air quality emissions of their vehicles, however you can find all these emissions figures at: