YPSILANTI, MI – Auto makers can increase fuel economy further and faster by switching to diesel or hybrid-electric powertrains, but the exorbitant cost of such moves likely means gasoline engines will remain dominant in the U.S. for the next 15 years, a study by the National Academy of Sciences indicates.

The report, compiled by the NAS’s National Research Council with the help of industry experts, spells out the type of technologies already available for improving light-vehicle fuel economy and their associated costs. The study was commissioned by the National Highway Traffic Safety Admin. as it set new corporate average fuel economy targets for 2016, and much of its findings were cited in the agency’s final ruling released in April.

It took two years to compile the report, say its authors, who presented the survey- and research-based study publicly for the first time here at an event hosted by the Ann Arbor, MI-based Center for Automotive Research.

“Spark-ignition engines will still be the primary technology over the next 15 years,” says Trevor Jones, CEO of ElectroSonics Medical Inc. and longtime automotive electronics expert who chaired the research committee. “Diesel engines offer a substantial fuel-consumption reduction but at a high cost.”

It costs $74 for every 1% gain in efficiency eked out of a gasoline engine in a midsize or large car, Jones says, quoting the report. That compares with $137 for a switch to an HEV powertrain and $157 to move to diesel.

Unless there’s a spike in gasoline prices or diesel fuel prices become more favorable, consumers won’t want to pay for the added cost of compression-ignition technology, says Christopher Baillie, project manager for engineering specialist FEV Inc., who headed part of the study.

“The OEs have to sell these vehicles,” he says. “With spark-ignited technology, you can have smaller, incremental (fuel economy) improvements, while spending a few hundred (dollars) here, a few hundred there. There’s not that big (investment) jump in adding a diesel. Consumers are more willing to pay for that.”

Baillie says adding direct injection hikes gasoline engine costs a more reasonable $230-$480 per vehicle, while incorporating a turbocharger adds up to $1,000 per unit. Combined, those two technologies can boost fuel economy 8%-10%, he says.

Variable valve event modulation is another way to improve gasoline-engine efficiency 1%-11% at a relatively low cost of $52-$760 per vehicle, Baillie says.

Roger Krieger, adjunct professor-Engine Research Center at the University of Wisconsin, says the recessionary economy also is a factor limiting diesel proliferation in the U.S.

“The economic problems of the industry are having a big impact,” he says. European auto makers, which already have diesels in production, can afford to offer them here. “But if you have to tool up for them, like U.S. manufacturers would, that may push (auto makers) toward gasoline technology.

“It will be very difficult for diesels to pick up their penetration rate in the next four to five years.”

Cost aside, there are huge fuel-economy gains to be had by switching from a gasoline engine to a smaller diesel, the report says. If auto makers replaced a 3.5L gasoline V-6 with a 2.8L diesel in a fullsize sedan, fuel consumption would decline some 33.2%, the NAS says.

Subbing a 1.9L diesel for a 2.4L gasoline 4-cyl. in a small multipurpose vehicle would improve fuel economy 31.9%. A switch from a 5.4L gasoline V-8 to a 4.8L diesel V-8 would boost fuel efficiency 34.1% in a fullsize pickup.

Those gains are possible with engines currently on the market. Adding more advanced technology already available could increase fuel economy another 13%, the study says, predicting such diesels will hit the market between 2011 and 2014.

However, the price of swapping gasoline engines for diesels is steep. Today’s 4-cyl. diesel costs about $2,393 more than a comparably sized gasoline engine, while a V-6 diesel runs about $3,174 higher than a V-6 gasoline powerplant.

If auto makers incorporated every fuel-economy trick available – from more efficient engines and transmissions to light weighting and friction reduction, they could squeeze out 29.0% greater fuel economy out of a gasoline-powered midsize or large car at a cost of about $2,159 per vehicle, the study concludes.

Taking an optimized diesel-powered path would cost $5,905 per unit and yield 37.5% greater efficiency, while a similar HEV vehicle would net a 43.9% gain in fuel economy at a cost of $6,027.

“We concluded (full) hybrids won’t be a significant fraction of the light-duty vehicle market in the next 15 years,” says Linos Jacovides, owner of Paphos LLC and former Delphi Corp. researcher.

Limited battery range will “limit near-term production” for plug-in hybrids, he adds. And the committee concluded there would be no breakthrough in the next 15 years that will allow full EVs to proliferate.

Simpler microhybrid (stop/start) systems are more economical, at $670-$1,100 per vehicle, but promise only about a 4% fuel economy gain, the study notes.