The new diesel purification system currently under field tests with selected Toyata Avensis customers in a number of European countries is aimed at overcoming what the company refers to as the 'final obstacle' to clean diesels.

The Diesel Particulate-NOx Reduction system (DPNR) reduces particulates and NOx simultaneously, brings emissions to well below Euro Step IV levels, and is servicing-free.

The launch of DPNR is yet another chapter in Toyota's long history of developing technology for cleaner diesel engines by using catalytic converters and improved combustion technologies, such as direct fuel injection, electronically controlled exhaust gas recirculation (EGR) and, most recently, high pressure common rail fuel injection.

The widespread commercial introduction of DPNR, following successful field trials, is likely from next year, and would eliminate one of the few remaining possible objections to diesel as an environmentally clean fuel.

A fleet of 60 Toyota Avensis 2-litre D-4D models were offered to selected customers in Germany, UK, Austria, Italy, Norway, Finland and Belgium. These customers are involved in a wide range of commercial activity.

The world's first common rail diesel system, developed by Toyota subsidiary company Denso, was introduced for truck application in 1995. In 1999, Toyota and Denso introduced the system to passenger cars and, such is the pressure for improved environmental control, the majority of diesel engines are forecast to use common rail by 2004.

Now a second generation common rail system, capable of rail pressures of 180 MPa, is complementing the DPNR technology. A high response, large amount flow, high resolution EGR system has been added to create low temperature combustion, itself a factor in greatly reducing the base emission levels from the engine.

The DPNR catalytic converter is mounted close to the exhaust manifold and a simple oxidation catalytic converter is further downstream in the exhaust system.

The DPNR converter features a newly developed, highly porous ceramic filter coated with a catalyst exclusively developed by Toyota for its NOx storage reduction catalytic converter, initially designed for use with Toyota's lean-burn (high-oxygen) gasoline engines.

In the DPNR purification process, during conventional lean-burn combustion, particulate matter is first oxidised using active oxygen which has been created when NOx is temporarily stored inside the catalytic converter.

Then, when the engine momentarily switches to low-oxygen stoichiometric (rich) combustion – through a 'rich spike' exhaust port injection - the stored NOx is reduced producing more active oxygen. This additional oxygen is used to further oxidise particulate matter inside the catalytic converter.

Unlike other particule filters, it is servicing-free, meaning that during the entire vehicle's life is not scheduled a periodic replacement of any of DPNR's components and it doesn't require the use of any fuel additive.

For maximum effect and to avoid catalyst deterioration, DPNR requires the use of diesel fuel with less than 10ppm of sulphur, which is currently being introduced across many countries in Europe.