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VW Diesel Owners Should Expect Lower Highway MPG After Compliance

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The expert leading the West Virginia University team that played a key role in exposing Volkswagen’s emissions cheating says the main change owners will see is reduced MPG. 

If anyone knows modern diesel Volkswagens and why customers buy them it is Jean Jennings.  Jean used to run a little magazine called Automobile and now has her own blog called Jean Knows Cars.  Jean owns a 2015 Golf SportWagen TDI diesel.  Her car before that was a 2013 Jetta SportWagen TDI diesel.  She and her husband loved the highway mileage their cars got.  What does she think of the mess VW has created?  In her words “Appalling, pathetic, cynical, and calculating.”  Loyal customers like the Jennings family are right to be angry, but let’s cut to the chase.  If you are a VW diesel owner how will the car differ after the inevitable “fix” is made by VW?  The expert that leads the team who uncovered VW’s trick has bad news.  Your highway mileage is going down.  Here’s why.

Golf TDI engine

In an interview with IEEE, Dan Carder, interim director of West Virginia University’s Center for Alternative Fuels, Engines and Emissions (CAFEE) lays out the chronology of how the EPA finally outed VW.  Back in 2011 researchers in the UK published data showing that NOx (oxides of nitrogen), the pollutant that tripped up VW, was not going down despite government changes to the allowable emissions of NOx.  A group funded in part by the Hewlett and Packard foundations called the International Council on Clean Transportation (ICCT) had been trying to get someone (anyone) to take notice of the dirty diesels for some time.  In an effort to get some more data, ICCT contracted with Carder’s team to rent some diesel VWs and measure the real-world emissions.  The difference was staggering.  After EPA staffers listened to a talk by Carder’s group they went to work on VW, eventually getting VW to admit to the scheme.

Golf rear 1200

Carder says that emissions control systems in a diesel car break down NOx into harmless compounds, but the systems only work when they are hot.  The way the system gets hot under low-load situations (highway driving) is by running a little rich, which means using more fuel than the car needs to propel itself.  Carder says, “There’s also a trap for NOX that acts like a sponge.  But if you saturate it with NOX, then you have to go to a rich-burning condition to use the catalyst to reduce the NOX.”  The pollution difference between the system working properly vs. the more fuel efficient over-ride VW programmed is huge.  EPA says up to 40 times more emissions than are allowable.

Carder told IEEE that although both performance and also fuel economy will suffer once the cars are modified to meet the current standards, the fuel economy is what customers will notice.  Volkswagen diesel models have plenty of torque, and they are geared to take advantage of that.  According to Carder, “There’d be a larger penalty on fuel than on performance.”  Volkswagens already had no meaningful advantage on other affordable cars in their segment when it comes to city and combined MPG.  A gasoline-powered Corolla LE Eco or Sentra FE has city and combined fuel economy within one MPG of a Jetta diesel.  If changes to the emissions controls are applied to low load highway situations, the single efficiency advantage the diesel cars had may disappear.

Photos by John Goreham are of a 2015 VW Golf TDI, just one of the VW and Audi vehicles EPA has called out.

For more information on the vehicles affected, please see: VW and Audi TDI Owners: Your Guide to Volkswagen’s TDI Crisis


John Goreham

John Goreham