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After six years of development and 36 patents, Renault is getting ready to launch its own NOx trap, writes Richard Aucock. The system will be trialled from September in France and Germany in a private fleet of 2.0-litre diesel Espace MPVs.

Euro 5 emissions regulations make it harder for OEMs to meet NOx limits by standardising the NOx value for all weights of vehicles. Further tightening them for Euro 6 in 2014 from 0.18g to 0.8g/km puts OEMs under considerable pressure.

NOx aftertreatment is essential, but it can be expensive. Instead of buying in a urea additive system, Renault has chosen to develop its own zero-additive NOx trap. Studies show this increases consumption by 2 per cent, but improvement elsewhere offset it. Refinements to the injection system and EGR valve mean the Euro V Espace consumes no more fuel than the Euro IV model.

Programme engineer Sebastian d’Oria said: “The system's long development cycle is due to the complexity of the software modelling and the frequency of the purge. Premium manufacturers can fit two or three NOx sensors to solve the challenge, but it’s hard for us to economically justify even one. The customer won’t pay for it."

The NOx trap takes the place of a regular diesel oxidation catalyst. It still tackles HC and CO, but now controls NOx too, in a two-stage process. The first is a chemical trap. Platinum converts NO into NO2; then barium which oxidises into barium oxide before trapping the NO2 as NO3 in an aqueous barium nitrate solution.

When it becomes full – roughly every 10km/10 minutes – the trap purges. Changes in the injection timing and exhaust gas recirculation (EGR) rate run the engine rich to heat the catalyst. This breaks the chemical link, releasing NO3. The NO3 breaks down into nitrogen, releasing oxygen which reacts with the hydrocarbons to form CO2. 

The purge process modifies combustion, airflow, EGR rate and injector timing, while maintaining consistent torque and noise levels. "This had to be modelled for every point in the engine map. The customer should not detect it,” said d'Oria.

Because the process is modelled, the accuracy of the oxygen sensors was crucial. Traditional lambda sensors lose accuracy as engines run richer and temperatures increase: precisely the conditions that need to be closely monitored.

Renault has fitted two sensors that incorporate a current pump, accurately measuring oxygen concentration and outputting a current proportional to this, rather than a binary voltage. This lets it measure concentrations accurately across the entire operating range.

Because the particulate filter is now further down the exhaust, Renault sought to prevent particulate formation in the combustion chamber. It has reduced the size of fuel injection droplets, increased injection pressure and managed EGR rates more closely.