"Just as taking the lead out of gasoline in the 1970s enabled a new generation of emissions control technologies that made gasoline vehicles over 95 per cent cleaner, so will removing the sulphur from diesel help usher in a new generation of clean diesel technology across all applications."
That's according to Allen Schaeffer in an early 2006 report by the Energy Saving Trust. He's talking about the future of diesel engine power. And he knows what he's talking about, because he was the executive director of the Diesel Technology Forum when he said it.
What does it mean? It means that diesels are getting cleaner by the day, cleaner even than petrol engines, with the larger car manufacturers injecting vast resources and money into the research and development of cleaner diesel engines. That the future will produce the cleanest diesel engines, capable of astonishing fuel economy while limiting emissions to a negligible level, is a universally accepted concept. But let's focus on the present before we study the future.
Common Terms in Diesel Emission Research
Before we get into the meat, we should clarify a few of the terms which are inevitably employed when boffins talk about diesel emissions.
Euro 1, 2, 3, 4 & 5
Euro 1-5 refer to a group of European emission standards. In essence, they're a set of requirements which define tolerable levels of exhaust emissions across the European Union, with each new stage becoming more rigorous than the last. Euro 1, the first stage, was introduced in 1993 and simply covered passenger cars. It was amended soon afterwards to include light trucks as well. Euro 2 was introduced in 1996 for passenger cars, Euro 3 was begun in 2000 for any vehicle, Euro 4 in 2005 also covered any vehicle as will Euro 5, due to be introduced in 2008/9. Put briefly, the Euro regulations stipulate a reduction in carbon monoxide and dioxide from the 1992 levels of 2.72g/km to 0.5g/km in 2014, a reduction of NOx from around 0.97g/km in 1992 to 0.08g/km in 2014, and a reduction of particulate matter from 0.14g/km in 1992 to 0.005g/km in 2014.
NOx
Nitrogen Oxide, or NOx, is a general term that refers to a group of gases created in high-temperature combustion. In air, and at normal temperatures, nitrogen and oxygen don't react with one another. But in an engine, the ignition of fuel and air creates a chemical reaction between oxygen and atmospheric nitrogen, which generates various oxides of nitrogen.
NOx is nasty stuff. It can contribute up to 50% of the overall exhaust emissions when oil is burned and, more worryingly, when it reacts with volatile organic compounds in the presence of sunlight, NOx creates smog. Asthmatics and people who exercise in the smog - commuting cyclists, for example - are at risk of health issues; in particular lung tissue damage.
PM
Particulate matter, or PM, is synonymous with the term 'aerosol'. Particulates are tiny particles of solid or liquid matter suspended in the air. While there are many natural causes of particulate matter, we'll focus on the burning of fossil fuels here, in particular PM10.
PM10
PM10 is a particular size of particulate matter - 10 microns or less - which are of most concern to scientists and government watchdogs, since these particles remain suspended in air and can easily be inhaled into the lungs.
Particulate Matter & Modern Diesels
While modern diesel engines are cleaner generally than petrol engines, with significantly lower overall emissions per kilometer, it's the particulate matter that it is concerning researchers. PM is often confused with the black smoke that is emitted from the exhausts of older or dirtier cars. However, PM is different from the smoke; PM is microscopic - invisible to the naked eye. Petrol doesn't emit any particulates from combustion in modern engines, so manufacturers are working hard to eliminate it from diesels too, along with the sulphur and sulphates which contribute to acid rain.
One way car manufacturers are reducing the emissions from their diesel engines is by demanding cleaner diesel from the oil producers. Under pressure from the manufacturers of diesel engines, and with a desire to appear proactive on the thorny subject of climate change, the fuel producers have indeed begun over the last few years to filter those noxious chemicals - sulphur, nitrogen oxide et cetera - out of diesel.
The diesel engine's superior fuel economy over the petrol engine's (superior by an average of around 40% in terms of mileage according recent studies) means that much less carbon dioxide is emitted from the diesel compared with a petrol engine of similar performance over a similar distance. Current diesel engines produce very little carbon monoxide or carbon dioxide since the fuel (or the large majority of it) is burned in excess air even at full load.
PM10 & Health Issues
The particulate matter which is emitted by unfiltered diesel combustion is created by unburned fuel, and the rate at which this unburned fuel is emitted increases the faster the engine revolves. The particulates are usually caused by worn injectors, which tend to inject more fuel into the cylinder than can be burned in time. PM10 has been linked to health problems, even being labeled a carcinogen in towns and cities, where large collections of older diesel engines can cause the PM10 to coalesce.
Particulate Filters to Combat Emissions
Manufacturers of diesel engines have been taking all this research extremely seriously for some time now, and judging by the purity of the diesel engines already on the road and those planned for the near future, they're committed to public health and the environment at large. To fight the health issues and ameliorate the air condition, manufacturers are fitting diesel particulate filters to their cars which reduce or prevent the emission of diesel particulates. The filters were being built into diesel cars by 1996 and, while they're still not mandatory, many regulations have become impossible to meet without the use of one. This, and the fact that there have been widespread European protests applying pressure to car manufacturers to fit the filters regardless of legislation, has ensured that the particulate filters are now used in most modern diesels.
Particulate filters are highly effective at preventing the issue of diesel particulates; under the right circumstances a wall-flow particulate filter can have a particulate removal efficiency of nearly 100%, and even a partial filter will still remove up to 85% of your diesel engine's particulate matter emissions.
This means that a diesel engine constructed after around 2000, fitted with a high-quality wall-flow filter, filled with clean diesel and driven according to the manufacturer's guidelines will be far healthier than an equivalent petrol engine. Combined with the far superior fuel economy and significantly lower tax group, a diesel engined car seems like the obvious choice.