Diesel cars

Touted as an eco answer to petrol cars, diesels are on the rise, but are they really the greener choice?


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It was a clear, spring morning in May 2000 when two policemen were monitoring the A9 motorway near Montpellier in southern France. They were bored; the road devoid of traffic. All of a sudden a BMW 3 series flashed past. They clocked it at 230 km/h, more than twice the 110 km/h speed limit.

Cranking up their sirens and giving chase down the deserted road, the coppers pulled over a red-faced 20-year-old Englishman and broke into smiles as they recognised Jenson Button, a driver for the BMW-Williams Formula One team.

The story hit the world's press the next day, but the headlines weren't screaming about an F1 driver having been caught speeding; they were focussed, instead, on Jenson's feat of pulling a roaring 230 km/h in a diesel car.

Diesel engines have traditionally been considered reliable workhorses: sturdy, tractor-pulling, truck-driving, long-distance beasts. No one imagined a diesel engine could be responsible for speeds more suited to a race car.

In Europe, diesel cars make up 53 per cent of all new car registrations. In Australia, that figure is around 6.5 per cent. However, with improvements in performance and a surge in popularity (thanks to Jenson), diesel engines are gaining traction in the car market Down Under. In the past few years, European manufacturers Fiat, VW, Peugeot and Renault have started promoting their small diesel passenger cars with some success in Australia. Taking into consideration the downturn in car sales precipitated by the global financial meltdown, sales of passenger diesel cars are up nearly 11 per cent on last year's numbers.

Korean manufacturer Hyundai is getting in on the act too, with its diesel i30 being awarded the CarsGuide Car of the Year in 2007.

Stephen Howard, spokesman for Hyundai says, "Hyundai see the diesel engine as a green option for the marketplace whilst offering performance and economy."

And the NRMA concurs with Howard. A recent study from the motoring organisation found that up to 28 per cent less fuel is used by the diesel version of a car.

The amount of climate-changing carbon dioxide that comes out the tailpipe of a diesel is much reduced too, thanks to its efficient use of fuel. According to the Federal Government's Green Vehicle Guide, the Holden Epica petrol model emits 221 g of carbon dioxide (CO2) per kilometre travelled, while the diesel version emits 199 g/km.

It seems an open and shut case. Diesels are more fuel-efficient and less damaging to the climate. But there's more to the story.


Rudolf Diesel invented the engine that bears his name. In a petrol-driven engine, air and petrol are mixed and then compressed in a chamber by a piston, where a spark from a spark plug ignites the fuel, pushing the piston back down and driving the engine. Diesel's engine cleverly did away with the spark plug by using heat from compressed air to ignite the fuel.

While diesel engines pack more punch than a petrol engine, petrol engines are nimbler and better able to dance along an autobahn at high speeds. It is true that diesels have better fuel economy than petrol cars. For example, over 100 km, a VW Golf with a 2-L diesel engine will use 5.3 L of fuel, while the petrol version will use 8 L - a 34 per cent saving.

This is for two reasons, the first is thanks to Rudolf Diesel's design: the diesel wastes less fuel, burning it more completely and turning more of the energy into motion. The second reason is that diesel contains more energy per litre of fuel than petrol, which means the engine needs less fuel (and a smaller tank) to go the same distance as a petrol engine. It also means that litre for litre, burning diesel will actually release more CO2 than petrol.

However, "because a diesel [engine] is much more efficient, you end up with a net benefit," says a spokesperson from the Department of Infrastructure, which is responsible for the Green Vehicle Guide. "Generally, speaking from a greenhouse gas point of view, diesels are positive."

Despite their good greenhouse rating, though, only two diesel-powered cars have made it into the Green Vehicle Guide's top 100: The BMW E87 1 Series 120d and the Volkswagen Golf. It's their air pollution rating that lets them down - while diesels are more fuel efficient, they also spew out pollutants such as soot, sulphates and hydrocarbons.

The NSW Department of Environment and Climate Change (DECC) says 24 per cent of soot particles in Sydney's air are produced by motor vehicles. "Of this, up to 80 per cent comes from diesel emissions, even though diesel vehicles account for only 15 per cent of vehicle kilometres travelled," states the DECC's Action for Air Plan.

The soot particles, also referred to as particulates, are a health hazard because they can lodge in the lungs when inhaled and cause problems. A paper from the Victorian Environmental Protection Agency, released in 2002, found that "numerous studies have linked elevated particle levels in the air to increased hospital admissions, emergency room visits, asthma attacks and premature deaths among those suffering from heart and respiratory problems."

The same paper also lists arsenic, benzene, formaldehyde and nickel as other chemicals contained in diesel exhaust that are of concern to human health.

The US Environmental Protection Agency has also investigated, and their study goes into a litany of concerns
about diesel exhaust, coming to the conclusion that "diesel exhaust is likely to be carcinogenic to humans by inhalation".

That being said, no one recommends you go around inhaling the exhaust of a petrol-driven car either! Both kinds of fuels create a toxic mix of gases as they are burned.

Diesel, however, has one other downside when compared with petrol: it's a little group of compounds called nitrogen oxides (NOx's). These are found at higher levels in diesel exhaust than petrol and, when the sun hits them, they create smog.

"It's really a gimmick to suggest that diesels are helping with the climate problem," says Mark Jacobson, a professor of civil and environmental engineering at Stanford University in the US, who is also an expert in soot pollution.

Jacobson's biggest concern is that the soot released from diesel engines - as well as being a recognised health issue - is also contributing to global warming.

"Black carbon [soot] causes about 300,000 times more warming than the same mass of carbon dioxide," he says.

The effect of soot particles on the climate "far overwhelms" the reduction in carbon dioxide emissions, he adds.


Because of the health implications, the government regulates the amount of soot emitted from cars.

To meet these emissions standards and to counteract the concerns about soot particles belching out of the exhaust pipes of cars, many modern diesel engines contain filters that are designed to strain out the worst of the pollutants.

And as of this month, all new European-imported diesels will be subject to the more stringent EU5 standard, which requires a reduction of diesel particulate emissions by between 80 and 90 per cent of the current EU4 standard, a standard is met by most Australian cars. And the coming EU6 standard will also address the emission of nitrogen oxides. "The NOx emissions required under the EU6 standard are almost on par with petrol engines," says BMW spokesperson Tim James.

Adding a filter reduces health impacts, but it also causes a reduction in mileage because of the additional fuel required, Jacobson says. "It's generally around three to eight per cent additional fuel use to run [them]."
But those in the industry argue a filter doesn't significantly impact the overall efficiency of a diesel.
"I think there would be a general acceptance that diesel vehicles are no longer the noisy, dirty things they have the reputation for. They're a lot cleaner than they used to be," the Department of Infrastructure spokesperson told G Magazine.

Using high-efficiency filters that meet the EU5 standard means "you could say with some confidence that [particulate] emissions are probably on par for diesel and petrol engines," he says.

The future

These days many auto manufacturers are turning towards more 21st century options, including hybrid petrol-electric vehicles and fully electric cars.

Toyota, a well-known proponent of hybrid cars, believes that "within the next five years...small passenger car diesels will begin to disappear as their ability to meet [emissions standards] will become prohibitively expensive and will no longer make it economical to build small diesels," says company spokesman Mike Breen.

"We believe hybrid vehicles will continue to develop and will form the basis of the ultimate eco-car."

Stanford University's Jacobson drives a car. He recently upgraded from a hybrid to a totally electric vehicle.
"Battery electric vehicles are so much more efficient than internal combustion engines," he says. "Of the electricity you put into an electric vehicle, 80 to 86 per cent of it is converted to work to move the car. If you compare that to a diesel or gasoline vehicle, only about 17 to 20 per cent of the fuel that you put into the tank is converted to work."

Modern diesels may be able to perform like race cars while being more efficient than their petrol counterparts, but at the end of the day they're not necessarily eco-saviours.