Can a diesel engine be too fuel-efficient?

Maybe, at least if you are Audi of America and you’re looking for a way to market diesel and hybrid powertrains side by side in the same vehicle.

That’s why the importer says it will put more of the accent on performance, rather than fuel economy, with its upcoming diesel-powered Q5, leaving room to emphasize miles per gallon with the cross/utility vehicle’s upcoming hybrid-electric powertrain.

The unfavorable comparison also is to blame for the decision to hold back from the U.S. the A8 Hybrid, bowing in Europe this year. The gasoline-electric model actually is less fuel-efficient on the European Union driving cycle than the A8 diesel, according to the auto maker’s estimates.

Audi offers diesels in just two vehicles in the U.S., the Q7 CUV and A3, but will roll out oil-burners in several more lines later this year, beginning with the A8 flagship. That will be followed by the A6 and Q5 in 2013. The next-generation A4 also will boast a diesel option once that model makes it to market.

Diesels will play an important part in the auto maker’s strategy to meet toughening corporate average fuel economy regulations in the U.S., which escalate to 35.5 mpg (6.6 L/100 km) for 2016, Audi of America President Johan de Nysschen says in an interview.

Diesel powertrains now account for about 41% of Q7 sales and 55% of A3 deliveries in the U.S., but those penetrations likely have been restricted by tight supplies.

“Our current A8 with the 4.2L gasoline V-8 gets the same fuel economy as hybrids from BMW, Lexus and Mercedes” due to its emphasis on weight-saving aluminum construction, de Nysschen points out. “Imagine now what happens when you put a diesel in the car. You have all the drivability and driving enjoyment with even better fuel economy.”

The A8 will get the 3.0L V-6 diesel as an option this year. In Europe the diesel car is rated at 39.2 mpg (6.0 L/100/km), compared with 36.8 mpg (6.4 L/100 km) for the A8 Hybrid, which combines a 211-hp 2.0L turbocharged gasoline engine with a 40-kW (54-hp) electric motor and a lithium-ion battery pack.

“We therefore have decided not to do the hybrid in the A8 for the U.S.,” de Nysschen tells WardsAuto.

That fuel-efficiency equation also has Audi opting for the bigger of two turbodiesels for its Q5. Rather than go with a 2.0L, the importer will market in the Q5 the same 3.0L V-6 found in the larger Q7.

That will give it better performance but concede the fuel-efficiency advantage to the Q5 Hybrid, which uses the same gasoline-electric powertrain as the A8. In addition to leaving some room in the market for the hybrid, the 3.0L is a better fit for Audi in America.

The V-6 diesel “already has been thoroughly developed for the U.S. for applications in the Q7, and it will be in the A6 and A8 as well, so we get economies of scale with that,” de Nysschen says. “And the hybrid’s specific consumption will be better. I think the case would be somewhat muddied if we had a 2.0L TDI (in the Q5), which tends to give the hybrids a run for the money.”

In the Q5, the 2.0L TDI engine is about 3% more efficient than the hybrid powertrain in the European drive cycle, while the 240-hp 3.0L diesel, at about 30.6 mpg (7.7 L/100 km), is some 10% less efficient.

Fuel economy still is not a big selling point for Audi’s American customers.

“Customers are interested in efficiency, of course, but it is not the most important driver,” Audi’s top U.S. executive says. “They make that decision base on rational criteria – the cost of the technology vs. the benefit it brings. If it is not a positive answer, then it doesn’t make sense.

“And this is actually part of our concern (in marketing hybrids), because we believe our customers are behaving quite rationally.”

Still, de Nysschen expects Audi to “get sufficient critical mass” with hybrid vehicles in the U.S. “Q5 is our first one, and we’ll see where it goes from there,” he says. “But the mainstay is going to be diesel technology.”

The auto maker also will tiptoe into electric vehicles, with the R8 the first to carry the e-tron EV branding in the U.S.

Shown in concept form in 2009 and powered by twin electric motors and a 45-kWh Li-ion battery pack, the R8 e-tron goes into production later this year. It probably will come to the U.S. about a year behind its European introduction.

Audi also will fleet-test electric A3s in the U.S. this year, with the program likely to get under way in the first quarter.

dzoia@wardsauto.com