For the last 2 1/2 years, Chrysler and Fiat worked together harder, faster and better than Sergio Marchionne imagined possible, and the work is just beginning.

"2012 is the most difficult year," Chrysler CEO Marchionne said. "We will have the least product introductions."

The fewest new vehicles but arguably the most important.

The Dodge Dart compact sedan is the first car Chrysler and Fiat developed together. It will use the first engines that combine the companies’ technology, and it’s in line for advanced new transmissions. The Dart goes on sale in the second quarter.

"The execution of the launch of the Dart is indicative of how we will do" with new midsize sedans and Jeep SUVs coming in 2013, Marchionne said.

"It’s the first rollout of Fiat’s endowment of architectures" to underpin new Chrysler, Dodge and Jeep vehicles, he said. "The Dart will also become the donor platform for the car we will build in China."

That’s the symbiosis Marchionne expects from Fiat and Chrysler. Fiat provided the architecture, or basic structure, that the Dart shares with the sporty Alfa Romeo Giulietta. Chrysler engineers and designers created a bigger, roomier car using the architecture. The Dart’s platform then becomes the spearhead of Fiat-Chrysler’s push into China.

"We’re 20 percent done" with Chrysler’s turnaround plan, Marchionne told me in December. "The architecture choices, powertrain choices … all that stuff is done.

"The plan is laid out. Execution is the key."

The new vehicles coming in 2013 include new midsize sedans to replace the Chrysler 200 and Dodge Avenger and new versions of the Compass and Liberty. They’ll be based on the platform that underpins the Dart and built in Toledo, Ohio.

Modifying the platform to meet Jeep owners’ demands will be a crucial test of the Fiat-Chrysler strategy.

"It’s gonna be a trail-rated, full-blooded Jeep that has its origins in the architecture of a sports car," Marchionne said.

Chrysler is also considering a compact hatchback — Marchionne calls it the "Chrysler 100." If that car gets the green light, Chrysler will also export it as a Lancia, Fiat’s European luxury brand.

He also plans to export sporty Alfa Romeos, probably a midsize sport sedan and wagon. Alfa’s return to the U.S. has been frustratingly delayed until 2013, but Marchionne remains adamant it will happen.

"We had to make sure they were lightweight, fast cars with Alfa powertrains," he said, citing an upcoming 1.8-liter engine that produces a mind-boggling 300 horsepower.

"There will be plants making Alfas in the United States and Europe," he said. "I need to export to Europe from the U.S. The volumes I get here are how I reintroduce Alfa to Europe," where it has become a bit player compared to brands like Audi and BMW.

Marchionne must also solve the riddle of Chrysler and Dodge’s minivans in 2013. He hates building nearly identical vehicles for two different brands, but he can’t afford to throw away tens of thousands of sales by rashly killing a model.

"Retention and growth of volume is key," he said. "I’ve got to protect (the golden) goose."