Shares of Navistar International Corp. fell 15 percent Friday after the company announced it will change its approach to pollution control in certain engines – a move some analysts and investors fear will be costly.

The truck manufacturer – which employs around 800 workers in Springfield – said it will mix its practices with those of its competitors to meet 2010 federal emission standards.

Troy Clarke, senior vice president of Navistar, called the new effort the “best of both worlds” in a live webcast Friday. Clarke said Navistar would be combining an in-cylinder technology with the solid state after-treatment model it has been developing.

But analyst Vicki Bryan from Gimme Credit say Navistar’s plan to abandon the technology that is 10 years and $700 million in the making could doom it financially.

“I was very wary about this,” Bryan said.

Selective catalytic reduction is the method that Navistar’s competitors use to meet the Environmental Protection Agency requirement to reduce nitrous oxide emissions by 95 percent by 2010. Navistar opted to use an EPA-patented procedure called EGR.

Auto industry analyst Dave Cole said he thinks Navistar will overcome this hurdle rather easily. He said the new system adds urea to the exhaust gas to reduce emissions.

“It’s a separate system that’s not cheap, but it’s not horribly expensive, either,” Cole said. “Everyone uses it. Had (Navistar) been able to achieve meeting the (emissions) requirement with a somewhat simpler system, that would’ve been a competitive advantage for them. But they just couldn’t make it work.”

Navistar said it will use a combination of banked emissions credits and noncompliance penalties to continue making trucks and engines while they work out the ICT-plus engine. Navistar currently pays about $2,000 per noncompliant engine sold, which in part has led to the turmoil surrounding the company.

After two large quarterly losses and a federal court decision against Navistar’s continued use of those the engines, Navistar’s stock price dropped. Activist investors swooped in and acquired more than 25 percent of the company’s shares in recent weeks. Navistar, fearing a possible hostile takeover, has adopted defensive measures.

During the webcast, Navistar management said the EPA was supportive and that the company is confident the new engine will be certified. Navistar officials said they will be discussing with the EPA on how to make this transition to the new engine without loss in production or sales.

The EPA would not comment specifically on Navistar’s announcement.

Bryan said she is also skeptical about how the company will be able to get the capital to pay for these changes.

Navistar’s CFO, A.J. Cederoth, said the company expects some “incremental” costs for additional hardware for the engine.

“There will be some opportunities to offset costs with pricing,” Cederoth said. “In the long term, we expect to reduce costs and expand margins.”

Jason Barlow, president of United Auto Workers Local 402 , was unavailable for comment. Local 402 represents local Navistar employees.

Previously he said that there was no concern for the local plant and that they would continue to ramp up hiring for increased production in the fall. The local plant does use some of the noncompliant engines, but mostly produces medium duty trucks that use a different engine.

“Their recent employment gains have been very positive in our community,” said Horton Hobbs, Greater Springfield Chamber of Commerce vice president of economic development. “Navistar hired for the first time in a decade this year. Hopefully this is a sign of things to come.”

Navistar shares (NYSE: NAV) closed Friday at $24.42, down $4.37.