MADRID - Formula One teams will finally take their 2018 cars to the track when preseason testing begins on Monday in Spain, with Mercedes and Ferrari promising significant improvements to try to keep running up front.

This week's test session in Barcelona will also allow McLaren to debut its Renault engine after three dismal years with Honda, and give Red Bull a chance to show it has a car fast enough to finally contest for titles again.

Force India and Williams will look to move closer to the top three, while Renault and Hass will try to keep improving. Toro Rosso will be the team trying to succeed with the Honda engine, while Sauber will bring partner Alfa Romeo back to F1, hoping to take a step forward after a last-place finish a year ago.

What all teams will have is the halo, the protective cockpit device that is mandatory this season. The odd-looking cover that goes around the driver's head is the most visible change to cars from a year ago.

The "shark fins" and their adjacent tiny wings on top of the cars are gone this season, and each driver will only be allowed to use three power units - instead of four - during the 21-race calendar.

The season opens on March 25 in Australia. Drivers will participate in another four-day test session in Barcelona from March 6-9.

Most teams unveiled their cars last week, when drivers were allowed to get a first taste of action in limited outings for video filming.

Mercedes dominated the last four seasons, but reigning champ Lewis Hamilton faced a tough challenge from Ferrari's Sebastian Vettel last year.

"The car was fantastic but there were so many things we could still improve on," Hamilton said. "What we've done is taken a lot of the DNA from last year's car... this is the sister car, an evolution of that. This is better than last year's car in every aspect. It looks quite similar but inside, underneath the shell and even the bodywork, it's all refined to perform even better."

Ferrari won five races with Vettel last season, but the Italian team hasn't won a drivers' title since 2007 with Kimi Raikkonen. Its last constructors' championship came in 2008.

"We can't wait to get out on to the track to see how the car performs, how it behaves," Vettel said. "This car is a big step from last year."

Red Bull, which won four straight titles with Vettel from 2010-13, was the third force last season and at times it got extremely close to Mercedes and Ferrari. Max Verstappen won two races and Daniel Ricciardo one.

Red Bull will debut a new suspension - in addition to new livery - that it hopes will allow the team to become a title contender.

McLaren is most anxious to get to the track after swapping from Honda to Renault in a bid to address its reliability and power issues.

"I think good times are coming," two-time world champion Fernando Alonso told Sky Sports. "We struggled, we kept motivation very high. I think the team is stronger now than three years ago."

There will be two rookie drivers on the grid: Russian Sergey Sirotkin with Williams and Monaco-born Charles Leclerc with Sauber, which is actually calling itself Alfa Romeo Sauber F1 Team after inking a technical and commercial partnership with the Italian brand that has a long F1 history.

Polish driver Robert Kubica, who hasn't raced in F1 since 2010 after being badly injured in a rally crash, signed as a reserve driver with Williams and is expected to participate in three practice sessions this season.

"We won the championship last year but it was difficult at times, some really strong opposition," Mercedes chief Toto Wolff said.

"We needed to find the right balance between developing our car without losing its raw speed. Last year's car was the fastest on track - it had the most wins and poles. We needed to preserve that."

Associated Press