DIESEL : to be or not to be CLEAN ?
The VW tsunami stimulated my curiosity to update the NOx emission of diesel.
This is the summary of my search.
1/ It is clear that the homologation test cycle NEDC does not reflect the road emission and consumption. This is a weak point but not a legal case.
2/ Specific adjustments only for an homologation test cycle are clearly illegal.
3/ Because the new test cycle WLTP under final elaboration is supposed to reflect better than NEDC or FTP tests the true on road emissions there is a lot of studies around this question.
For me it is quite clear that the NEDC numbers, such as 80mgNOx/km diesel Euro6 cannot be identical with the WLTP. But what is acceptable such as x2=160mg or more or less ??? Difficult debate with the auto ayatollahs.
4/ The EU auto industry is focussing on the future 90gCO2/km rule, quite a big challenge for the "Premium" (German) leaders. The combustion specialists know that the trade off CO2 vs. NO is hard to de-correlate.
5/ To comply with Euro 6, 3 de-NOx technologies are presently implemented: Exhaust gas recirculation (EGR), NOx trap, Selective Catalytic Reduction (SCR) with urea injection and catalyst. EGR is the simplest for limited de-NOx but its extension to higher rates of de-NOx has detrimental effects on the efficiency of the combustion. NOx trap efficiency over a wide range of operating conditions is difficult and a regeneration phase might cause very high NOx emissions. SCR is the most efficient but technology intensive and expensive. SCR is widely proven on trucks.
6/ To me, the origin of the VW affair came from studies measuring on-road CO2 and emission of diesel cars implementing various de-NOx technologies exacerbing their differences in efficiencies at road conditions.
The ICCT study with ADAC collaboration (guaranty of quality) shows that most of Euro6 diesel emit ON-ROAD much more than 80mgNOx/km up to a factor of 25 more. Only one with SCR maintains 80mg on road. The study insists also on the other pollutants CO, THC that remain on road close to their test values. I suppose that it is also the case with particulate which is not mentioned.
The study shows that in their present state the 3 NOx after treatments are far from identical efficiencies acceptable for on road conditions in spite of their ability to satisfy the Euro6 test.
I still believe that exhaust de-NOx technologies can considerably progress and produce acceptable road efficiencies. The SCR is already proven. What about the 2 others? They have to because similar technologies will become mandatory for DI gasoline engines also under constrain of low CO2 emission and efficiency target close to diesel. The Homogeneous Combustion process, potentially very low NO producer and demonstrated at prototype level could come back at the front of the scene.