Technical look at the VW scandal
Here’s how diesel cars are usually designed to produce lower emissions
It will probably be months, even years, before all the dust settles around the Volkswagen diesel emissions scandal.
There are still scores of questions to be answered, not the least being who authorized the ploy, who knew about it and when?
Easier to answer is why it happened, at least from a purely technical perspective.
The “what” is well known. VW programmed the emissions control system in vehicles equipped with many of its four-cylinder TDI diesel engines — about 600,000 in North America — so certain control functions were active only in conditions encountered during an emissions test.
In real-world driving, they were effectively bypassed.
The results were grossly excessive emissions in normal driving of one regulated group of pollutants, known as nitrogen oxides (NOx). But the cars satisfied the challenging U.S. emissions regulations in testing mode.
In addition to contributing to the formation of both smog and acid rain, Environment Canada says NOx can have adverse effects on human respiratory systems, aggravating conditions such as asthma, bronchitis and emphysema. It’s not good stuff.
Which is why it was one of the earliest automotive “criteria emissions” to be regulated.
Prior to vehicle emissions controls, vehicle exhaust typically contained about 2.24 g/km of NOx.
Because of the different fuels used and operating conditions between gasoline and diesel engines, diesels tend to generate greater concentrations of NOx
The U.S. Environmental Protection Agency established a limit of 0.25 g/km by 2004.
That limit was reduced to just 0.04 g/km in 2007, in both the U.S. and Canada: a 98-per-cent reduction from the uncontrolled state. And that’s where the diesel problems really began.
Because of the different fuels used and operating conditions between gasoline and diesel engines, diesels tend to generate greater concentrations of NOx.
For the same reasons, it can’t be as efficiently cleaned up by the catalytic converters used for gasoline engines.
Some other means of NOx control had to be developed for diesels to meet the new North American emissions standards. It was a challenge that kept VW diesels out of the market for a couple years.
For most diesel passenger car manufacturers, the solution was called Selective Catalytic Reduction (SCR).
These systems inject a ureabased Diesel Exhaust Fluid (DEF) — sometimes called AdBlue — into the exhaust system ahead of a separate SCR catalyst where the gases are reacted into nitrogen and water.
It’s a complex and costly system, but it works. Volkswagen used it in a few applications, but not on the subject engines, except in the Passat.
Instead, VW adopted an alternative system called a lean NOx trap. It costs less, takes up less space and doesn’t require a DEF tank and injection system. As the name implies, it traps NOx in a separate converter, where it is burned off every few minutes. And it also meets the test standards — when it’s fully operative.
Volkswagen’s ingenuity was widely acclaimed and awarded. It seems, however, that the system took a greater toll on performance and fuel economy than someone within VW felt was acceptable.
And the rest of the story is what the scandal was all about.