Honda on Monday (25 September) said it had developed a new version of the 2.2-litre turbodiesel it already sells in Europe that will meet tough new US emission laws when launched, probably for the 2009 model year.

The so-called Next-Generation Clean Diesel Engine is capable of meeting the US' new, stringent 50-state 'Tier II Bin 5' emissions requirements (current diesels do not meet current stricter rules in California and four north-eastern states that have adopted the same emissions standards and hence are sold only in the other 45 states) and will be sold nationwide "within three years", the automaker said.

Honda claims its new diesel engine reduces exhaust gas emissions to a level equal to a petrol engine by employing a revolutionary new NOx catalytic converter that reduces NOx emissions sufficiently to meet the new emissions requirements.

This contrasts with the approach taken for Mercedes' recently-announced 50-state-compliant 'Bluetec' three-litre V6 diesel which uses urea injection, requiring regular top-ups, to reduce NOx emissions, similar to the particle-reduction systems fitted to earlier PSA Group-developed Euro 4-compliant diesels.

This catalytic converter has what Honda said is the world's first innovative system using the reductive reaction of ammonia generated within the catalytic converter to "detoxify" nitrogen oxide (NOx) by turning it into harmless nitrogen (N2).

The new 'cat' has a two-layer structure: one adsorbs NOx from the exhaust gas and converts a portion of it into ammonia, while the other layer adsorbs the resulting ammonia, and uses it later in a reaction that converts the remaining NOx in the exhaust into nitrogen (N2).

Ammonia is a highly effective reagent for reducing NOx into N2 in an oxygen-rich, lean-burn atmosphere. This ability to generate and store ammonia within the catalytic converter has enabled Honda to create a compact, lightweight NOx reduction system for diesel engines. The system also features enhanced NOx reduction performance at 200-300ºC, the main temperature range of diesel engines.

Honda designed the catalytic converter for use with its 2.2 i-CTDi diesel engine, already popular in Europe, where it has earned widespread praise from the enthusiast consumer press for quiet, clean operation and strong performance since its introduction in 2003 in the European-specification Accord [Euro] model.

By further advancing combustion control, the 2.2 i-CTDi delivers cleaner exhaust to the NOx catalytic converter. Honda achieved this by optimising the combustion chamber configuration, reducing fuel injection time with a 2,000-bar common rail injection system and boosting the efficiency of the EGR (exhaust gas recirculation) system.

Thanks to these improvements, Honda has reduced the amount of NOx and soot normally found in engine exhaust, while increasing power output.

Along with developing superior technology for cleaning exhaust gas, Honda plans to address other technical challenges in developing clean diesel engines, such as handling diesel fuels with different cetane numbers (the diesel equivalent of octane ratings for petrol) and meeting US on-board diagnostic system requirements.

Petrol engines presently employ three-way catalytic converters that offer NOx reduction rates as high as 99%, but this performance is possible only at the stoichiometric air-fuel ratio.

In the oxygen-rich environment of a lean-burn diesel engine, three-way catalytic converters only reduce NOx levels by approximately 10%. Honda's new catalytic converter efficiently reduces NOx in a lean-burn atmosphere, enabling diesel engines to rival petrol engines in cleanliness. The compact system is also easy to install in passenger vehicles, Honda noted.

The automaker said it plans to introduce the next-generation diesel engine in the US within three years; local media reports said Honda is aiming for the 2009 model year.