Switching to a diesel car

Last updated Wednesday 30 April 2008

Lean, mean, but not totally clean

Diesels are far more fuel-efficient than petrol alternatives. Out of town, some emit even less CO2 than hybrids.

Diesels are becoming a more and more popular choice: 40% of the cars sold in the UK in 2007 were diesels. They burn less fuel than petrol cars over a given distance, which means fewer CO2emissions and lower fuel costs. Diesels driven on motorways are actually more fuel efficient than hybrid cars. In addition, they can save on road tax.

Too good to be true? Maybe. In cities, hybrids are a better deal all round in terms of emissions and financial savings. Diesel is less refined than petrol and produces soot when burned, with mixed environmental impacts.

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Photo: Switching to a diesel car

Saves up to 600kg of CO2 a year

350 Bloomers are doing this

CO2 reduction 2 out of 5

Cheapness 2 out of 5

Popularity 4 out of 5

Cost 145 per year

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How will it make a difference?

The average UK driver could save 580kg of CO2 a year by swapping to the equivalent diesel model - 15% of the average Brit's direct carbon footprint.

Traditionally a smart move for the cash conscious, recent fuel price hikes mean diesels only overcome their extra purchase cost if driven very long distances. According to the AA a diesel car, costing about 1,400 more than its petrol equivalent, can take more than 45,000 miles before fuel efficiency savings recoup the extra cost of purchase.

To take the edge off the costs, choose a diesel with extremely low emissions and you could save 50 a year on car tax from the DVLA.

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What's the debate?

Pub Fact

  • Cars manufactured today produce 20 times fewer emissions than cars made in 1970
  • Diesel cars accounted for 40% of the 2.4 million new cars sold in Britain last year, compared with just 14% in 1999

Diesel cars' claims to environmental friendliness need to be qualified slightly. For one thing, estimates of how much diesels reduce emissions vary quite widely: from 15 to 25%. For another, whatever the CO2 savings, diesel gives us some other emissions to worry about.

Diesel also causes more smog-forming mono-oxides of nitrogen (NOx) emissions, which are implicated in the formation of ozone and acid rain. Ozone near the ground damages plants, reducing their ability to mop up carbon dioxide (CO2) from the atmosphere and has significant impacts on human health.

Diesels produce more soot (known as particulates) than petrol engines. While soot is categorically bad for human lungs, the jury's still out on its climate impact. Scientists claim they can both contribute to climate change (see BBC News: 'Soot 'influences arctic climate'') and slow it down (see BBC News: 'Why the Sun seems to be 'dimming'') - but research so far suggests the warming impact outweighs the cooling effect.

However, many diesel cars now come with a diesel particulate filter (DPF) either as standard or as an option. This filter removes nearly all of the particulates from the exhaust and goes a long way to improving air quality in our cities. In 2009, the new Euro V emissions regulations will require all new diesel cars to be fitted with a DPF.

Some environmentalists also argue there's no substitute for driving less, and that cheaper long distance motoring can act as an incentive to drive more. Climate consultant Chris Goodall unearthed a study in France suggesting that drivers who switched to diesel cars from petrol drove an average 17% further.

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How do I do it?

  • To pay nothing for road tax, get a car in Band A - one with emissions lower than 100g/km CO2
  • Choose manual transmission over automatic to save 10-15% more fuel
  • Fit a diesel particulate filter to reduce your impact on air quality
  • If most of your driving is in a town or city, consider a hybrid car instead
  • Drive more efficiently
  • Consider using recycled biodiesel to cut emissions much further

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Hungerford89, Flintshire 2009-01-21

We switched to a 1.4l Ford Fusion diesel from a 1.1l Toyota Yaris - just before the price of diesel went through the roof! But much lower emissions means lower tax, and we've managed to get about 58mpg average since we bought it - this could be better if I drove a bit easier (my wife can easily get 60 mpg!). Despite still having to pay more for the fuel, we're happy with the switch.

Baz, Shropshire 2009-01-07

Been running a diesel car for around 4 years now, its an old Peugot 405 estate, a good workhorse. tried running on veggie oil, runs fine with some cold starting issues but the main drawback is that the shaft seal on the mechanical injector pump leaks. Happily when I revert to diesel or a 10% mix, the leak fixes itself.

Anonymous 2008-10-18

In an ironic twist, one of the cleanest running cars on the road, the Toyota Prius, is having trouble passing the Georgia emissions test. This problem was first reported in April 2007. Prius owners have become dumbfounded and frustrated over the ordeal. The same goes for Georgia emissions testing officials who discovered the glitch that causes the vehicle to fail the test every time.


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Glossary terms used on this page
Acid rain
Acid rain is simply rain with higher than normal acidity. Its main cause is emissions of nitrogen and sulphur compounds from burning fossil fuels to power transport, power stations, forest fires, fertiliser and industry.
Biodiesel is fuel generated from vegetable oil that can be used pure or blended with regular diesel (diesel produced by refining crude oil) in conventional, unmodified diesel engines. It is not the same as waste vegetable oil, otherwise known as 'unwashed biodiesel', which requires engine modification.
CO2, or carbon dioxide, is made up of the elements carbon and oxygen. It exists quite naturally in our atmosphere, as part of the carbon cycle. Everyday processes in the plant and animal world both add CO2 to the atmosphere and take it out. However, because it is a greenhouse gas - meaning it affects the temperature of the earth - the exact level of CO2 is important. Burning fossil fuels releases CO2 into the atmosphere, hence the anxiety that extensive use of these fuels is causing climate change.
Carbon footprint
A person's carbon footprint (or that of a particular household, business or entire community) refers to the CO2 for which they are responsible - whether directly, via their home energy use, their transport use, or indirectly via the embodied energy in the products and services they buy and use. You can work out your carbon footprint using calculators such as the Government's Act On CO2 Calculator.
Climate change
Climate change is the variation in the average global or regional climate as measured by yardsticks such as average temperature and rainfall. This variation is caused by both natural processes and human activity. Weather is what happens over days or even hours, whereas climate is the average weather measured over a longer period. Increasingly when people refer to climate change, however, they specifically mean the phenomenon of global warming.
Ordinary diesel, like regular petrol, is refined from oil but it is a thicker, heavier liquid with a higher 'energy density' - meaning it offers better fuel economy. On the down side, unless you buy an air filter, diesel exhaust is a significant source of particulates and other sources of air pollution. A type of diesel not derived from petroleum is increasingly widely available, commonly referred to as biodiesel.
Emissions are the CO2 (and other greenhouse gases) produced by energy use, usually calculated and stated as an annual tally: also referred to as your carbon footprint. Your personal emissions can be direct - such as the gas you personally use to heat your home or the petrol you burn to power your car - or indirect - meaning the energy use that has gone into the products or services you buy. The latter, such as the emissions caused by the manufacture of your new TV, or the packaging your food comes in, are also referred to as embodied emissions.
A hybrid car is one that has both a petrol engine and an electric motor and switches between them according to the driving conditions. It is different from a dual-fuel vehicle, which uses two types of liquid fuel.
Nitrogen oxides
Nitrogen oxides are compounds of nitrogen and oxygen, two elements that do not normally react with each other but will do so during high temperature combustion – such as in a car engine. Examples include nitric oxide (NO) and nitrogen dioxide (NO2), which contribute to air pollution, and nitrous oxide (N2O), which is a major greenhouse gas. Although its warming effect is far less than CO2, it persists in the atmosphere for far longer, so measured over 100 years its impact is 298 times greater.
Ozone is made up of three oxygen atoms. It has benefits and dangers depending on where in our atmosphere it occurs: near ground level, it is a pollutant that affects respiration; 10-50km up, in the stratosphere, the ozone layer filters out potentially harmful ultraviolet rays (which cause skin cancer) from reaching the earth. Ozone also functions as a greenhouse gas, though it is considered a less potent one than CO2.
Particulates - sometimes referred to as particulate matter or just particles - are tiny pieces of dust, soot and other materials suspended in the atmosphere. These can be produced naturally, by volcanoes or forest fires for example, but are also caused by incomplete combustion of fossil fuels, particularly from diesel engines. Because diesel is a denser fuel than petrol, it needs more oxygen for all its hydrocarbons to react completely, and the unburnt carbon - soot or black smoke - is emitted through the exhaust pipe.

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