TOKYO – Major Japanese auto makers likely will sell diesel-powered passenger cars in the U.S. someday, but no one here is willing to say when that might be.

For now, the overall focus is on further improving gasoline engines and steadily expanding hybrid-electric vehicle (HEV) offerings.

Toyota Motor Corp., for example, plans to double HEV sales to 1 million units by 2012. That’s one-tenth of the auto maker’s global production, with more than half the volume earmarked for the U.S.

Nissan Motor Co. Ltd. will launch its first U.S. HEV early next year. The vehicle is based on the new ’07 Altima sedan and will be built at Nissan North America Inc.’s Smyrna, TN, plant.

Honda Motor Co. Ltd., already flush with Accord, Civic and Insight HEVs in North America, will expand its hybrid lineup by the end of the decade, as well.

But in a departure from its major competitors, Honda also is the first to formally announce plans to introduce a passenger-car diesel for the U.S. market.

The 2.4L, 4-cyl. turbodiesel, unlike European mills aimed at the U.S., reportedly will meet the U.S. Environmental Protection Agency’s new Tier 2 bin 5 emissions standards without the need for selective catalytic reduction, also known as SCR urea injection.

Applications for the new engine are unknown, but most likely it will be installed in Honda’s CR-V cross/utility vehicle or Civic small car by 2009.

Additionally, Honda reportedly is working on a 3L V-6 turbodiesel, also for introduction in the U.S.

Meanwhile, Fuji Heavy Industries Ltd., maker of Subaru cars, is thought to have developed its own 2L 4-cyl turbodiesel and is considering a 3L 6-cyl. mill.

Details of the powerplants are not available, but the 2L unit could appear by 2008 in the Legacy sedan and wagon, with the 3L version showing sometime later in the Tribeca B9 CUV, media reports say. There is no word on whether the engines will be offered in U.S. market.

Despite these developments, most Japanese auto makers believe economics remain unfavorable for diesels in the U.S.

Yo Usuba, Nissan senior vice president-powertrain engineering, says fuel prices ultimately hold the key, but he declines to speculate on the threshold that would make diesels more practical to develop.

“The initial cost of diesel technology is high and becoming higher with more stringent emission standards, thus it won’t be easy to overcome this disadvantage and make inroads in the market,” he says.

According to Usuba, the European market is most favorable for diesels, where the average yearly mileage is more than 12,500 miles (20,117 km). For drivers who exceed those levels in North America, he suggests there may be demand.

But that also is the demographic being targeted by HEVs. And Toyota is committed to cutting HEV-system costs (including batteries) by 50% over the next five years. For the Prius, that means about $1,250.

The main market for diesels will continue to be Europe, where all Japanese auto makers are in the process of expanding and improving their lineups.

Honda expects to sell 100,000 diesel vehicles this year. Toyota is projecting sales of 400,000 units, nearly 40% of total Toyota and Lexus sales in the market.

With the new Civic’s launch earlier this year, Honda formally ended its 3-year engine supply agreement with Isuzu Motors Ltd. The i-CTDi replaces Isuzu’s 1.7L diesel built at its Tychy, Poland, plant.

Honda’s new 2.2L i-CTDi 4-cyl., introduced in the European Accord (Acura TSX in the U.S.) in February 2004 and now available on the CR-V, FR-V and Civic, is expected to be the foundation for any future U.S. offering. The engine, which features a lightweight aluminum block, is produced at Honda of the U.K. Mfg. Ltd. in Swindon.

Toyota’s D-CAT diesel engines currently find use in the 2.0L Corolla and Avensis and 2.2L Avensis. The Corolla Verso features a 1.4L diesel produced at Toyota Motor Mfg. Turkey Inc. near Eskisehir, 15 miles (24 km) southwest of Istanbul.

Toyota’s Walbryzch, Poland, powertrain production complex supplies the 2.0L and 2.2L engines, along with 2.2L non-D-CAT units to Toyota Motor Mfg. France SA’s Valenciennes plant for the Yaris subcompact. The Aygo minicar employs a non-D-CAT 1.4L diesel supplied by PSA Peugeot Citroen.

Elsewhere, Nissan, which offers more than 10 European diesel models, including the 1.5L Micra and Note and 2.2L Primera and X-Trail, depends on alliance partner Renault SA to supply most of its small-displacement diesel powerplants.

Mazda, through its alliance with Ford Motor Co., fits its Mazda3 with a 1.3L turbodiesel produced by PSA. Mazda’s own 2.0L diesel, featuring a common-rail injection system supplied by Denso Corp., is installed on Mazda6 European export models.

To introduce a diesel in the U.S., Mazda research executive Seita Kanai warns his company “isn’t big enough” to promote diesels by itself. “We must work with Ford,” he says.

According to an internal study by Denso Corp., Toyota’s leading supplier and Japan’s leading manufacturer of common-rail systems, sales of diesels in the U.S. market will grow to 5% by 2015, up from an estimated 2% at present. In comparison, HEVs will increase to 10% of the light-vehicle market both in Japan and the U.S., Denso says.

In 2004, production of light-duty diesel vehicles in Japan totaled 1.3 million units, about 12% of total output during the year. Of these, two-thirds were trucks, with a significant number exported to Asia. However, most analysts do not expect Japanese consumers to switch to diesels.

Apart from the lingering image dating back to the 1960s and 1970s, when soot from truck tailpipes was identified as a national health concern, average mileage driven in Japan is just 6,000 miles (9,656 km) per year, which some here say is not enough to justify the higher prices that diesel engines require.

– with Mike Sutton