Porsche Panamera

is two-pronging its approach to alt-fuel vehicles by expanding its stable of plug-in hybrid-electric vehicles (PHEVs) starting in 2014 while continuing to boost its push for more sales of its clean-diesel vehicles.

Europe's largest automaker is adding PHEV versions of the Porsche Panamera and 918 Spyder to the electric-drive vehicles set to debut in 2014, Reuters reports, citing an interview with VW electric-car head Rudolf Krebs.

VW, which will also start selling PHEV versions of the Golf, Passat and Audi Q7 SUV in 2014, will spend a "considerable" percentage of its $9.13 billion annual research and development budget on electric-drive advancements.

VW started selling its first hybrid, a gas-electric version of the Touareg SUV, in 2010, 13 years after Toyota introduced the Prius. But the company is looking to take advantage of what IHG Automotive says will be an eightfold annual increase in PHEV sales between 2013 and 2018.

Volkswagen was among the group of large German automakers that recently launched the "Clean Diesel. Clearly Better" campaign to push for clean-diesel sales in the US. Diesels account for more than half the new car sales in Europe, though less than three percent in the States.