Carbon dioxide, the greenhouse gas produced by the combustion of all fuels, is considered by many observers to represent the greatest long-term threat to our planet’s environment. The automotive industry has accepted its responsibility in this context and is working intensively not only on advanced engines and more efficient power trains, but also on alternatives to fossil fuels. Meanwhile, more and more people believe that the diesel is a better alternative than the petrol engine. For various reasons, however, the diesel is still regarded with caution and scepticism in some countries, including Greece, Sweden and the USA, where it is consistently overtaxed and restricted by law.
In Sweden, for example, leading green politicians describe carbon dioxide emissions from fossil fuels used in road vehicles (which account for 12% of the total according to the World Business Council for Sustainable Development) as a major contribution the world’s greatest environmental threat – the enhanced greenhouse effect – and also describe the diesel as destructive to the environment. As a result, the tax on diesel fuel is four times that on petrol – although the latter contributes about 20% more carbon dioxide to the atmosphere.
The ambiguity of the message is clear – if the diesel is 20% better in the context of the Earth’s biggest environmental threat, why so much opposition to it? However, the Swedish government has indicated that new cars equipped with particulate filters may be taxed more favourably in the future.
While the proportion of diesels in the vehicle population in certain European countries, and in certain vehicle segments, is as high as 90%, it is less than 10% in Sweden. Many Swedes remain cautious and slightly opposed to the diesel, and knowledge of what modern diesel technology has to offer is relatively poor. One argument used in support of higher taxation on the diesel is that emissions from these engines are somewhat higher than from petrol engines. However, this ignores the fact that the diesels used for comparison purposes are not equipped with nitrogen oxide traps or particulate filters.
Higher efficiency and fuel economy
The driver of a modern diesel car has the satisfaction of knowing that every unit of fuel burned emits 20% less carbon dioxide than his neighbour’s petrol-engined model. And the knowledge that the fuel consumption of a diesel-driven Volvo V70 is about 5 litres per 100 kilometres in extra-urban driving, compared with about 8 litres for a petrol-driven car of comparable performance, is another cause for satisfaction.
When the British authorities altered their company car taxation rules and introduced a CO2-based scale, the proportion of diesel registrations increased dramatically. In Sweden, surveys also show that car buyers have become generally more positively disposed to the diesel, with at least 40% stating that they would consider diesels if vehicle taxation levels were comparable. However, resistance in the USA is higher, although for different reasons.
“From the perspective of the US customer, the diesel is less attractive since low fuel prices there make the potential benefits small. In addition, there is widespread scepticism about this type in the USA,” notes Anders Eugensson of Volvo Cars who, as head of Governmental Affairs, liases with official agencies all around the world in consultation with representative bodies like ACEA in Europe.
“Fuel specifications in the USA are also lower and emission control standards are tougher, making it almost technically impossible to use diesel fuel in cars. Much of the development in the States is based both on federal regulations and on California’s special legislation – which means that the focus there is on hybrids using electricity and petrol/gas,” continues Anders Eugensson.
Particulate filter a prime issue
He looks instead at Europe, where the diesel is a much more obvious and accepted option, both for environmental and economic reasons:
“In recent times, we have detected a change in attitudes. Swedish politicians and environmental officials are also beginning to see the potential of the diesel’s efficiency and exhaust purification technology.”
However, Anders Eugensson points out that the harmful effects of particulates have prompted French and British politicians to demand incentives of various kinds to equipping diesels with particulate filters. Germany already provides a grant of 518 Euro for retrofitting older cars with the feature.
“The particulate filter issue was debated extensively in 2004,” points out Anders Eugensson. “But we are now seeing development taking a major step forward with their actual introduction. This is switching the focus of the political debate to other environmental areas.”
Tough commitments for industry
So what is the Volvo Cars view?
“We hope to reach the EU average for diesels in the various countries and car populations. We hope for a situation in which the customer’s choice is governed by himself or herself, not by the politicians. Although in the case of the USA, we are not counting on the diesel at this time,” is Anders Eugenssson’s summing-up.
He lists the options and comments on each:
Electric hybrids: “These are promising. We can expect the technology to advance on a wide front.”
Electric power: “This is not a runner at present, and will be only if battery development takes off.”
Hydrogen: “A definite option, provided it is produced from renewable resources, although it will be at least 20 years before the technology matures.”
Methane: “The combination of natural gas and biogas is a very promising alternative while hydrogen is being developed – especially from the energy supply viewpoint.”
Ethanol (‘FlexiFuel’): “Although this is renewable, the challenge of health-related emissions from combustion is greater.”
Methanol: “Difficult to manage and not an option, except possibly as a fuel in fuel cells.”
DiMethylEther (DME): “Useful mainly in heavy vehicles, although it may be an option in cars in the longer term.”
ACEA, the representative body of the automotive industry in Europe, has agreed with the European Commission to reduce CO2 emissions from new cars significantly in the years ahead. Future requirements are also likely to include a provision that all diesels are equipped with particulate filters, while NOx traps are also likely to become a requirement further on.
More diesels needed
The European automotive industry has undertaken to reduce average CO2 emissions from new cars sold in 2008 by 25% compared with the 1995 level. This means that the average figure will be 140 g/km. The target applies to the European fleets of the region’s carmakers, not to individual manufacturers.
“This illustrates the crucial importance of a working representative body for the industry,” comments Niklas Gustavsson, who is responsible for future environmental legislation provisions at Governmental Affairs. “The agreement will require us to market a higher proportion of diesels and in this context, the introduction of particulate filters will be very important to political decisions, such as those designed to improve conditions for diesel buyers in Sweden.”
He notes that noise emission controls are also being tightened. In this respect, major advances have also been made in the diesel area, at both the absolute and subjective level.
“Fuel injection technology has been refined to the point that the characteristic ‘diesel knock’ has all but been eliminated. The modern diesel also has excellent driving characteristics – and needs refuelling less often,” says Niklas Gustavsson.
He emphasises that Volvo Cars’ superb five-cylinder engine is a Swedish product and supports many jobs in the company’s own engine plant in Skövde, where a considerable proportion of the French PSA four-cylinder diesels for the Volvo S40 and V50 engine will be manufactured and assembled in about a year’s time.
Jan-Erling Rydquist was technical project manager in the early stages of development of Volvo’s own D5 diesel. He has a comprehensive knowledge of the history of diesel engine development.
Thirty-year lead now three
“Rudolf Diesel invented the working principle of the diesel engine about 30 years after Nikolaus August Otto invented the petrol engine. In practice, however, this lead in terms of time and development has shrunk to perhaps three to four years,” believes Jan-Erling Rydquist. “In those European countries, such as Greece, Finland and Sweden, which have the oldest national car fleets, this means that large numbers of petrol-engined cars five or more years old are inferior to new diesels in terms of emission levels.”
He maintains that Volvo’s D5 engine was basically adaptable to Euro 4 requirements even before it was equipped with a particulate filter in 2005. In terms of combustion and operation, the basic design has been refined continuously by means of a lower compression ratio, optimised fuel injection and swirl-assisted scavenging (controlled swirl of the air in the combustion chamber), as well as a more efficient turbo and exhaust gas recirculation (EGR).
At the same time, he concedes that particulates have been a problem, despite the low levels achieved before the advent of the particulate filter. Emissions of particulate matter (PM) consist of minor quantities of soot from unburned fuel – small particles that can coalesce into harmful clusters. Health risks have been associated with these minute particles of unburned carbon, which contain small quantities of polycyclic aromatic hydrocarbons (PAH). These occur in varying sizes; however, particles smaller than 2.5 µm are the ones that cause most concern.
“The particulate filter is the solution to this problem,” says Jan-Erling Rydquist. “Otherwise, the diesel has the advantage of being consistently more efficient, regardless of the size of car.
“But the cost of achieving minor results in relation to total PM emissions to atmosphere is, of course, very high. Diesel cars account for an extremely small proportion of all particulate matter in the air that we breathe.” (See Factfile on PM emissions.)
Jan-Erling explains that, in practice, all hydrocarbon-related particles can be burned since development is focused on the achievement of perfect combustion. In other words, with modern engines and further refined combustion systems, particulate emissions from the modern diesel are extremely small.
The particulate filter system is obviously fully integrated in the engine’s electronic management system. Among other things, it is equipped with oxygen (Lambdasond) and temperature sensors, as well as a pressure sensor, to measure the increase in back pressure and determine when the accumulated soot in the filter requires to be removed in a short, controlled burnoff operation. This is performed with the aid of the engine management system about every 500 kilometres, or as required, unnoticed by the driver. The estimated life of a particulate filter is approximately 250,000 km, or over ten years for the average diesel driver.
Volvo Cars employs two different systems in its cars. One of these is used in the smaller four-cylinder S40 and V50, which are equipped with an additive particulate filter system (ADPF). This features a small tank containing a special liquid additive which, when added to the fuel, stimulates and facilitates the initiation of combustion of the soot in the filter. Since the exhaust temperature in a conventional diesel is normally too low to guarantee the initiation of this efficient catalytic cleaning process, the additive permits a lower activation temperature. The additive is topped up as part of normal service and, as a result, need not concern the driver. The additive fluid contains a proportion of metals, including Cerium, which cannot be burned and contributes a small quantity of ash to that inevitably generated by incombustible fuel and engine oil additives. When this quantity and the pressure drop across the filter reach a certain level, the filter must be removed for removal of the ash by washing and blow-drying. This procedure is carried out about every 120,000 km.
Used in Volvo’s bigger five-cylinder cars, the second system employs a CDPF-type filter (‘C’ standing for ‘Catalysed’) in which a specific noble metal coating on the substrate, combined with an even further refined engine management system, enables the soot to be burned off in this case also, without the use of an additive. This eliminates the need for a major service intervention to clean the filter as described above for ADPF. In this case, the life of the filter is governed by the quantity of ash normally produced by the engine in the form of wear particles, and fuel and engine oil additives. The life of a filter in the CDPF system is over 240,000 kilometres.
Living on air
Jan-Erling Rydquist notes that particulate filter systems call for very comprehensive engine and software development:
“Both types of filter require sophisticated engines offering low oil consumption and low PM emissions, as well as highly advanced engine management in which the fuel injection, EGR and other systems are continually adjusted with the aid of sensors. Unnoticed by the driver, the exhaust temperature is increased as required while driving by various control inputs, activating the soot combustion and filter cleaning functions. Overall, the development of a filter is costly.”
He adds that the combustion process may be further improved with the introduction of advanced systems in the future. For example, if more homogeneous combustion can be developed and controlled at low temperatures (below 1,700°C), the potential will exist for extremely low nitrogen oxide formation accompanied by almost negligible PM production.
“Diesel technology is fundamentally efficient,” says Jan-Erling Rydquist. “A great deal of research is under way around the world to develop methods of controlling the process, and when further progress has been made, we may no longer need a particulate filter...
“Since a diesel engine ‘lives on air’, efficient combustion is inextricably linked to the most efficient possible scavenging. In future, once more advanced turbo technology has been developed, it will be possible to apply it not only to reduce emissions, but also to improve performance. In another words, there is nothing contradictory in developing the two aspects together in the diesel!”
1. Volvo’s new five-cylinder diesel is equipped with a particulate filter and boasts considerably lower emission levels. (P2005_1795)
2. Volvo’s own upgraded D5 engine delivers high power together with low emissions and low fuel consumption. (P2005_1897)
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