For many years now, the term “clean diesel” has been bandied about in the automotive industry. We even have “cleaner” diesel fuel now available in the U.S. (have you seen the high-performance fuel now available at the pump?), but what does that really mean? More importantly, have emissions equipment made diesel engines truly cleaner burning than they were years ago?

In Europe, where diesel-powered trucks and cars have long been favored over petrol (gasoline in Euro speak) burning vehicles, the thought that diesel is getting cleaner or is less polluting than gasoline is now being looked at differently, and the results don’t seem favorable.

A recent study published by the International Council of Clean Transportation (ICCT) found that modern diesel-burning vehicles might not be anywhere near as clean in the real world as their manufacturers claim. This seems especially true when it measures the emissions of nitrogen oxides (NOx). However, the study deviated from the standard methodology of lab testing.

The portable emissions measurement systems (PEMS) testing equipment provided a steady stream of data, including speed, acceleration, and various chemical outputs, and even looked at the grade of the roads the vehicle was climbing. During this research project, 15 modern diesel vehicles; 12 from Europe and three U.S. manufacturers, were tested during a cumulative 3,977 miles of driving.

The new data showed that these vehicles poured out NOx levels approximately seven times higher than the Euro 6 standard. This level of NOx emissions was occurring under normal driving conditions that included climbing hills, according to the ICCT. The sturdy concluded, “There is substantial evidence that the actual on-road emissions may not be sufficiently controlled under certain operating conditions that are not covered by the laboratory test.”

Although the emissions reduction efforts in new diesel engines have not been completely unsuccessful, according to the ICCT, with carbon monoxide and total hydrocarbons below the Euro 6 limit, the NOx output was far above mandated levels. Testing procedures are soon (2017) to move toward PEMS in Europe, and it may not be too long before the same happens here in the U.S. The researchers who performed the new study said that automotive manufacturers need to start developing better NOx equipment right now. How’s your rig looking?