Ricardo's annual diesel report, although concentrating on Europe, shows that a small but significant diesel passenger car market is starting to emerge in North America. This is in addition to the established market for diesel sports utility and pick-ups, which are commonly used as passenger vehicles. It is the first time the report has referred to the US market.

"Only a few years ago it would have been unthinkable to be discussing the potential development of diesel car markets in the USA," says Ricardo's director of diesel engineering, Ian Penny. "However, the technology has moved so fast in Europe that there is rapidly growing interest in new diesel applications in America."

Diesel penetration of the USA passenger car market (excluding pick-ups) rose to a peak of 6% in 1981, when the figure for Europe at 10% was not much higher. However, diesel noise, exhaust smoke and poor performance of engines 20 years ago were unacceptable to American motorists. And they had become less interested in the fuel economy benefits of diesel as fuel prices were reduced following the second oil crisis. However, dramatic advances in diesel engine technology mean that new compression ignition powerplants now offer an interesting alternative for US SUVs (sport utility vehicles) and light trucks, where high torque is a key requirement.

Today, the driving force for the diesel market in the US will be the driveability benefits of high torque diesel engines coupled with 30 to 50% better fuel economy and, perhaps more importantly to American consumers, a third fewer tank refills. For vehicle manufacturers the benefits will be in meeting the CAFE (corporate average fuel economy) requirements and gaining market share.

Governments will also benefit from reduced CO2 emissions and lower fuel imports, both key political issues which are likely to grow in importance.

Meanwhile, in the Western European car market, earlier predictions from previous Ricardo reports that diesel penetration would eventually exceed 50% seem almost certain to be correct according to the company's ongoing studies. In 2002, two of the major European manufacturers, VW Audi Group and DaimlerChrysler, for the first time sold more diesel than petrol powered cars. Another company, Peugeot SA, was very close to 50% diesel sales.

Overall, diesel penetration has reached 41%, up from 28% four years ago, and remains on a continuing upward trend gaining an average 3% of penetration each year. Over the past decade the diesel car market has tripled in size.

Analysis of European sales by country shows that the French diesel car market increased to a record 1.35 million cars in 2002 and that diesel penetration, which jumped to 63%, now accounts for almost two out of every three cars sold. At the same time sales of petrol powered cars in France fell by 19%. In Germany, diesel sales reached 1.24 million and penetration increased to 38%, while petrol car sales fell by 8%. In Belgium diesels represented 64% of car sales and in Spain 59%.

The UK is the fifth largest but fastest growing of the major diesel markets where impetus has been provided by changes in tax rules to benefit vehicles with lower CO2 emissions. Diesel penetration continued to increase throughout 2002 and the year-end average was 23.5%, accounting for almost one in four new cars sold. Ricardo's latest study concludes that the reasons behind the explosive expansion of the European diesel markets are generally to do with rapid advances in technology rather than simple fuel economy benefits. The savings in running cost enjoyed by diesel motorists have always existed but have never before attracted buyers in such numbers. Diesel cars are now available with power outputs up to 180kW (240bhp) and levels of low speed torque that were previously unimaginable. Performance matches that of equivalent petrol powered vehicles with similar levels of refinement.

In the past 10 years changes in legislated emissions levels have required HC plus NOx limits to fall by 42% and particulates to fall by 64%. Ricardo says that the technical challenges of meeting Euro 4 and possibly Euro 5 emissions legislation in Europe will require increasing sophistication in the engine air and exhaust gas recirculation (EGR) handling systems with the primary aim of reducing NOx emissions. The additional air supply will also offer the possibility of very high specific power outputs (power per unit displacement), which will be required to stretch the performance and fuel economy boundaries even further. For the American market the more stringent NOx limits will require additional aftertreatment technology, and particulate filters will become the norm for diesel engine applications both in European and American passenger cars.

One of the additional challenges facing European vehicle manufacturers is the requirement to limit average CO2 emissions to the EC/ACEA 140g/km target for 2008. This means very high fuel economy vehicles requiring small engines with a high specific power output. Ricardo has already demonstrated a 1.2 litre diesel, for example, which produces an output of 74 kW (62 kW per litre) using production-ready technology. The company says there will be increasing numbers of engines of this type either used as direct powerplants or as part of a mild hybrid system.