Noted for their high efficiency compared to gasoline engines, diesels can be a great way to economize on fuel while providing plenty of pep. Their power is particularly manifest at low RPMs because a diesel engine’s high compression translates to superior torque for a given displacement. The newest diesels are also cleaner at the tailpipe than those of model years past, putting them well into the running on overall environmental friendliness.
Volkswagen offers TDI (“Turbocharged Direct Injection”) diesel options for many of its models, including the new-for-2011 Jetta. On the AutoEcoRating scale, the VW Jetta TDI with its 2.0 Liter, 140 horsepower diesel edges out its gasoline counterpart, scoring a 125 eRating compared to the 123 eRating of the Jetta SEL. The latter model is equipped with a 2.5 Liter, 5-cylinder gasoline engine that peaks at 170 horsepower. At the same time, the diesel cleans up on torque: the TDI twists to 236 pound-feet, a good bit more than the gasoline 2.5 Liter’s 177 pound-feet maximum.
Because a full environmental rating considers more than raw fuel economy, the TDI’s AutoEcoRating gain isn’t as large as its miles-per-gallon numbers might suggest. The diesel’s 30 city, 42 highway mpg values average out to 26 percent better than the 2.5 Liter gasoline model’s 24 city, 31 highway mpg. Two factors explain why the eRating benefit — 125 diesel versus 123 gasoline — nets out to only 2 percent.
Diesel engines do have an efficiency advantage over today’s gasoline engines, but the fuel economy numbers also simply reflect the fact that diesel fuel is denser than gasoline. A gallon of diesel weighs more; it contains more energy and at the same time more carbon. That difference in chemistry is partly counterbalanced because refining diesel fuel produces fewer emissions than refining gasoline. For the Jetta siblings, the 26 percent fuel economy benefit nets out to a 14 percent advantage in terms of lower greenhouse gas emissions (counting both combustion and refining).
The second factor is the tailpipe. Diesels can now match gasoline as far as meeting an average exhaust standard goes. But lower tailpipe pollution is a moving target and it’s difficult to make a diesel as clean as the newest, even-cleaner gasoline models. It just so happens that VW certifies the 2.5 Liter gasoline engine to an EPA standard that allows only half the tailpipe pollution of the average (EPA “Bin 5”) standard to which the TDI is certified. That takes another chunk out of the diesel’s advantage, resulting in the 2 point eRating lead in spite of the much larger fuel economy lead.
Both the Jetta 2.0 Liter TDI and the 2.5 Liter models rate better than the base gasoline version with its smaller, less powerful 2.0 Liter, 115 horsepower engine. The base VW Jetta, which lists at $17,095 with an automatic transmission (all of the ratings cited here are for automatics), has an eRating of 110, just under the average rating of 112 for compact cars overall.