With grumbles about the marketability of electrified vehicles growing louder, auto makers haven’t been shy about their reluctance to let go of the internal-combustion engine.

The question becomes, what’s the best fuel to power one?

A handful of new or revised diesel powertrains come to market in 2013, and executives have spoken openly about getting Americans to warm to a fuel-saving engine commonly available in Europe. The path to achieving corporate average fuel economy standards isn’t solely guided by diesel, however.

Smaller numbers of vehicles powered by compressed natural gas and hydrogen fuel cells will be found alongside diesel mills on dealers’ lots next year.

“In states like California, I think the investment will be made in hydrogen,” Toyota’ s U.S. President Jim Lentz tells reporters on the sidelines of the Los Angeles auto show last week.

“Our desire is to have a portfolio approach as we continue to improve internal-combustion/gas engines. Gasoline (prices are) going to go up, not down. People are going to have to find more efficient ways to squeeze more out of their vehicle,” he says.

Asked about slower-than-anticipated hybrid sales, Lentz says, “I don’t think that consumers are necessarily shying away from hybrids,” but notes more awareness and consumer education is needed for greater acceptance.

Toyota plans to sell a hydrogen fuel-cell vehicle starting in 2015.

Honda also is taking a cover-all-bases approach. “You can’t accomplish what we’re trying to accomplish, which is essentially doubling the fuel economy of the industry of today by 2025 without a portfolio of products,” American Honda President John Mendel says.

“You can’t just do it with hybrids. You have to have some different alternative fuel in there. I think that we certainly have a large investment in hydrogen as well with our (fuel-cell-powered) FCX Clarity, (but) the infrastructure prevents that from being as aggressive as we could be.

“Everybody wants to pick what’s the one golden bean that’s going to get me there, and I think there’s going to be a lot of different golden beans,” he adds.

Honda also offers a Civic powered by CNG, something Mendel believes in.

“CNG is a very popular option for the next 20 to 25 years,” he says. “It’s abundant, it’s inexpensive, it’s clean. There’s mileage issues, there’s range issues, there’s infrastructure issues, but it could be part of it. It could also be part of a hybrid strategy. It could also be part of a dual-fuel strategy.”

Lentz is skeptical of CNG powertrains because of the lack of infrastructure and because he believes hydrogen is the future. But, he says, “I think there will be a point in time where natural gas will be one of the solutions in the next 25 to 30 years as we develop more and more hydrogen solutions.”

Regardless of future technological questions and market acceptance, auto makers remain concerned about U.S. policymakers’ increased influence on powertrain development.

“From a governmental standpoint, we ought not to be mandating a technology whether it’s battery or anything else,” Mendel says. “Set the target, let us cheat – and everybody will cheat a little bit differently.”