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A Diesely Dilemma

However sure you are of something it’s always nice to have confirmation. In the field of air quality recent research on emissions from vehicles in Southwark has confirmed what a study led by Kings College told us earlier this year – diesel cars are (relatively) dirty.

This won’t come as a surprise to many. Until recently diesel cars were thought to be dirty beasts suitable only for the thrifty and ultra-high mileage drivers. I know this only too well - when I was offered a new company car back in 2000 I made what I thought was a daring decision at the time and plumped for a diesel. My friends gasped in horror that I’d go for such an agricultural vehicle, whilst my uncle commented that when I pulled up he thought a taxi had arrived.

Oh how times have changed. Over the past decade UK sales of diesel cars have grown from a fraction of the market to parity with petrol car sales. In the market for larger cars this trend has gone even further, and diesel engined vehicles make up the vast majority of these cars sold. As a result the majority of the fuel sold from the UK’s filing stations is now diesel.

Technology has been one driver of this trend. Today’s diesel cars are vastly better than their ancestors – they’re quiet, fast, economical and in many cases better to drive than their petrol counterparts. But the real force behind diesel sales has been financial. Rising fuel costs have made diesel economy far more attractive, whilst Government policy linking Vehicle Excise Duty and Company Car Tax with the CO2 emissions of a vehicle means you’d be crazy (or very well off) not to go for diesel if you’re buying a larger car.

Whilst the rising use of diesel has had some beneficial impact on CO2 emissions it has come at a cost: poorer air quality. Diesel cars are inherently dirtier in terms of exhaust emissions than petrol cars, and cleaning up the exhaust of a diesel car can be a complex and expensive process.

When European vehicle emission regulations (known as the ‘Euro standards’) were first established back in the early 1990s legislators realised that setting the most demanding standards for all cars would wipe out the diesel car market, as the costs of producing diesels under these standards would be prohibitive. With diesel being a popular choice on the continent at the time diesel cars were given an easier ride than petrol cars, a situation that has continued from Euro 1 in 1992 up to the current Euro 5 standard that all new cars are now produced to meet.

However, these laxer standards aren’t the only problem: there also seem to be issues with the real world performance of diesel cars. If you’ve purchased a car of late you’ll be familiar with the official fuel economy ratings for new vehicles. You’ll also know that it’s almost impossible to match this fuel economy figure yourself. The figures are produced using lab based tests on a standardised test cycle, and this simply doesn’t match how most of us use our cars in the real world.

The two landmark pieces of research this year have used some clever technology that sniffs out emissions from individual vehicles as they drive past monitors on city streets. The results have shown that emissions of one pollutant (oxides of nitrogen or NOx) from diesel vehicles have not fallen as emission standards have tightened, which in part explains why levels of this pollutant in UK cities has remained stubbornly high. The Southwark study concluded that ‘Mean NO emissions from Euro 4 diesel cars was found to be 6-17 times higher than Euro 4 petrol cars’, much higher than expected.

So how can this situation be turned round? Unfortunately it’s probably too late for the vehicles currently on the road – there’s no easy way to fix the emissions from these vehicles. However we can make sure that the forthcoming Euro 6 standards (due in 2014) have the desired impact. A review of the vehicle test cycle is urgently needed, replacing the current cycle with one that’s actually representative of real world urban driving conditions.

But there’s also a case for a few other measures to check up on the performance of emission standards. If you’re putting a shelf up this weekend you’ll probably want to test it before filling it with books. And rather than use some standardised weights supplied by the scientific community for this test most of us would do something far simpler – give it a good shove.

Vehicle emissions regulations could do with some similar ‘idiot test’ style measures, perhaps just requiring manufactures to drive vehicles round city streets whilst monitoring emissions. If this simplistic testing indicated problems a more scientific approach could swing in to get to the bottom of it. In essence this is similar to what the research this year has done with its high tech road side sampling, but the fact that it’s taken so long to do this suggests that there’s a serious need for such testing to be incorporated into the European emissions regulation system.

Of course the other worry is that diesel car sales with continue to grow, and air pollution problems will grow with them. But the word from the vehicle industry is that, in the UK at least, we’ve probably reached a natural balance between petrol and diesel sales. One reason for this is that in every barrel of crude oil there’s only so much petrol and so much diesel. If we want refineries to produce more diesel then expensive and energy hungry processes to convert lighter fuels to diesel are needed, which would significantly push up th
e pump price of diesel.

Petrol cars are also starting to learn a thing or two from their diesel siblings. Radically downsized, turbo charged petrol engines are now becoming the norm offering much of the fuel economy benefits of diesel cars but at a lower cost. At the smaller, cheaper end of the car market these kind of technologies are likely to keep the diesel competition at bay, whilst even pleasing the
petrol heads at Top Gear.

Unfortunately the problems with NOx emissions from diesel cars also seem to apply to larger diesel vehicles such as buses and HGVs. I’ll cover this in a later post, but safe to say that sorting out emissions from diesel vehicles is probably the biggest issue we face if the UK is to meet its air quality obligations.


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