2013 Volkswagen Passat TDI: Our Long-Term MPG Results May Shed Light on Future Emissions Recall Expectations
by Dan Edmunds, Director of Vehicle Testing on September 23, 2015
A cloud hangs over Volkswagen and its TDI "clean diesel" engine offerings. The target is the 2.0-liter TDI engine, the US market's best-selling passenger car diesel engine. Volkswagen has been caught using illegal software that recognizes when the car is undergoing an emissions test and engages a special ECU calibration that meets all emissions standards.
All other times, including every mile anyone actually drives one of these, a second calibration takes over, one that apparently misses the Oxides of Nitrogen (NOx) emission standard by a huge margin. The test group that initially uncovered this scheme reported NOx levels as much as 40 times above the legal limit.
That is not an insignificant amount.
All of the affected cars use the 2.0-liter TDI engine: the 2009-2014 Jetta, 2010-2014 Golf, 2012-2014 Beetle, the 2012-2014 Passat and the 2010-2014 Audi A3. Of these, the Passat is the lone example with Selective Catalytic Reduction (SCR), a system that injects diesel exhaust fluid (DEF) into a special catalyst to control NOx.
The EPA hit-list also includes the 2015 TDI versions of all of these cars, and has refused to certify any 2016 versions for sale. But these use a totally new 2.0-liter TDI engine, one that belongs to a whole new engine family that goes by the name EA288. All of them have SCR systems that use DEF, not just the Passat.
VW's response includes a statement that says the bad software was limited to the older EA189 engine family, which only goes as far as the 2014 models. If true, the 2015 and later vehicles may fall off the EPA's list with no need for updated software or other recall measures to be in compliance. Time will tell.
A recall is inevitable, but current owners are worried about how the coming changes may affect the future performance of their vehicles.
What does that have to do with our long-term test fleet?
A 2013 Volkswagen Passat TDI diesel spent an entire year with us. If that wasn't enough, we also hosted a gasoline-powered 2014 Volkswagen Passat TSI at about the same time. They actually overlapped for a few months. We put about 20,000 miles on each and kept scrupulous fuel economy records.
No one could have engineered a more perfect matchup. Both were fitted with the 6-speed DSG transmission, they rolled on the same wheels and tires and were both SEL Premium models with no options. They were even indistinguishable in photographs due to their matching Reflex Silver paint and Titan Black interiors. The TDI was a 2013 and the TSI was a 2014, but there were no important changes between the two years.
The only meaningful difference was their engines. One had the 2.0-liter TDI turbo-diesel engine (first photo) the other had the 1.8-liter TSI turbocharged gasoline engine (second photo).
We've long been impressed with Volkswagen's official EPA MPG ratings. It has always come across as a company that didn't over-promise and under-deliver when it came to MPG on the window sticker. It didn't always claim the highest ratings in their class, but the numbers were achievable — and believable.
These two Passats did nothing to quash that impression. Both met or exceeded their EPA window sticker fuel economy ratings during their year with us.
And that's where this gets interesting in the context of the current troubles.
Fuel economy ratings are derived from the same series of indoor dynamometer (dyno) tests that are used to measure emissions compliance. The two are so interlinked that MPG is wholly based on one of the emission components that are measured: CO2. The amount of fuel burned is directly related to the amount of CO2 produced, so MPG is actually back-calculated from the measured quantity of CO2 emitted during the tests.
What this means is the illegal software trick that was triggered by the dyno test also had a direct bearing on the Passat TDI's fuel economy test results. Likewise, the on-road calibration that does not comply with emission standards would determine the TDI's observed fuel economy in real-world circumstances.
How does the gasoline-powered 2014 Passat fit in? That car is not under suspicion. As far as we know, the illegal dual-calibration dyno test shenanigans are not working behind the scenes here. In this case the rated fuel economy should closely match our observed fuel economy, and that's exactly what we see.
Here are the gasoline-powered Passat TSI's test result highlights after 21,050 miles:
Average Test MPG: 28.0
EPA Combined rating: 28
Best tank MPG: 36.5
EPA Highway rating: 34
Our staff matched the EPA rating over the span of the test, and a couple of drivers managed to beat the highway rating a few times. This example shows ratings that are in line with real-world fuel economy.
In contrast, here are the Passat TDI's test result highlights after 20,724 miles:
Average Test MPG: 37.2
EPA Combined rating: 34
Best tank MPG: 46.5
EPA Highway rating: 40
Our end-of-test results almost never exceed EPA combined by more a tenth or two, and coming in low by as much as a single mpg is considered a reasonable result. Moreover, we never exceed highway mpg anything close to that much, but our TDI exceeded its 40 mpg highway rating on 18 different tanks of fuel.
The TDI results were frankly amazing, but we were at a loss to explain them.
The recent revelations offer a possible explanation. Based on how MPG is derived, the window sticker ratings were surely derived from the emissions-complaint dyno calibration. Meanwhile, our observed fuel economy can only be the result of the illegal non-compliant engine calibration that takes over when the car is not being tested.
What does this mean for the expected performance of these cars after their software is brought into full compliance with the law? In theory, any new calibration would move real-world performance closer to the test-only calibration that currently passes the dyno tests. Since that calibration drives today's window sticker fuel economy, a customer's observed mpg should fall, but only to the level of the current window sticker. VW's mpg-based marketing claims may still hold up after the dust settles.
That's based on what we know now. Things could turn out different if the recalibration process is not that cut-and-dried.
Dan Edmunds, Director of Vehicle Testing