Toyota will swap hybrid technology used in models like the 2012 Prius v for BMW's diesel technology.

BMW and Toyota have reportedly agreed to swap diesel and hybrid powertrains in an effort to improve their fleet fuel economy.

Such partnerships are becoming increasingly popular as automakers face the rapid increase in government-mandated mileage standards in key markets such as the U.S., Europe, Japan and China.  The challenge for even the best manufacturers is to ensure they can not only meet new regulations but do it in a way that lives up to customer expectations.

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As part of their new alliance BMW would supply some of its advanced diesel technology to Toyota in return for some of the Japanese maker’s Hybrid Synergy Drive powertrains.  Toyota is the world’s biggest seller of hybrid-electric vehicles, its Prius model accounting for roughly half of global hybrid demand.

According to Automotive News a formal announcement of the partnership would likely be made this week at the Tokyo Motor Show.

BMW was a latecomer to hybrid power and has been struggling to play catch-up, with a gas-electric version of its 5-Series line to be announced at the biennial Tokyo event.  While Toyota has dominated the hybrid field it, in turn, has been slow to embrace diesel technology.

So have its key Japanese rivals – but that appears to be changing.  Mazda plans to offer a diesel version of its new high-mileage Skyactiv system and a senior Honda official told TheDetroitBureau.com that the maker is also working to expand its diesel line-up.  Significantly, as the Tokyo debut of the 5-Series “oil-burner” suggests, there are signs that diesel demand is expected to grow out of its traditional European stronghold, with sales on the rise in both the U.S. and Japan.

It’s not the first time the two makers have turned to partnerships to expand their powertrain and vehicle options.

BMW previously joined a hybrid consortium that also included General Motors, Chrysler and Daimler AG.  That helped it develop the Bavarian maker’s first hybrid offering though BMW has since walked away from that alliance.

Toyota, meanwhile, will unveil its new GT86 sports car at the Tokyo Motor Show, a vehicle it has developed in partnership with Japan’s smaller automaker Subaru – which will market its own version of the two-seater as the Subaru BRZ.  Toyota is also allied with Silicon Valley-based Tesla Motors.  The start-up battery car maker will, among other things, handle the development of the Toyota RAV-4 EV, due out in less than two years.

Toyota also announced a partnership with Ford, in August, to jointly develop new hybrid SUVs and trucks.  The U.S. maker, for its part, pays a licensing fee to Toyota for patents covering the gas-electric technologies it uses in models such as the Ford Fusion Hybrid.