Real-world tests have recorded the Mercedes C-Class C220 TDi BlueTec emitting 40 times more NOx than the legal limit in laboratory tests. However, Mercedes says the results are a consequence of real-world tests not reflecting official procedures, primarily because of the low ambient temperature during the test.
The tests were conducted by Holland’s Organisation for Applied Science Research (TNO) on behalf of the Dutch Minister of the Environment.
Mercedes says the discrepancy is partly down to the exhaust gas recirculation (EGR) process operating differently between five and seven degrees centigrade, a process it says is aimed at protecting the EGR cooler. Official laboratory tests that define whether a car is legal are conducted at 22 degrees centigrade.
Mercedes says this practice is fully legal, and that it is required to ensure the regulated requirement of delivering full functionality of the exhaust gas after-treatment system up to at least 160,000 kilometres. However, critics, while accepting the legality of the system, have suggested that such a device should be banned as it means the cars are hugely more polluting in low temperatures.