LONDON, England - MotoGP champion Casey Stoner will have a bigger challenge in more ways than one during the 2012 season, even if he is the overwhelming favourite to win the opening floodlit race in Qatar this weekend.The Australian is chasing his third title in the top category but this time, thanks to the decision to increase engine capacity, on a 1000cc machine more powerful than the 800cc Honda with which he triumphed in 2011.His Honda team won the last four races of 2011 and Stoner has set the pace in pre-season testing, lapping fastest on four of the five days in Malaysia and Spain.'FEELING POSITIVE'The 26-year-old, who won 10 races in 2011, has also triumphed in four of the previous five Qatar GP's, four time from pole. Despite that formidable record, Stoner still expects 2012 to be tougher."We head to Qatar feeling positive, but still have some work to do," he said in a team preview. "This will be the first time with the new bike on this circuit so we need to spend some time on the set-up to put ourselves in the best position."We've always enjoyed good results at Losail but this is a new season and there are more competitors contesting for victory this year."The bigger engine could also make the racing more spectacular, with greater power surging through the rear wheel. 2012 will also see the introduction of the Claiming Rule Teams concept which allows new entrants to run under lower-cost regulations with a prototype chassis and an affordable production engine.Riders on such machines include American veteran Colin Edwards with a Suter-BMW for Forward Racing while Britain's James Ellison returns on an Aprilia for the Paul Bird Motorsport former World Superbike team after a five-year absence.Nine of the 21-strong MotoGP field will be on CRT bikes.NEW FACESSix factory riders will stay with their 2011 teams but newcomers include Moto2 champion Stefan Bradl of Germany, Italians Michele Pirro, Mattia Pasini and Danilo Petrucci, and Colombian Yonny Hernandez. Italian Andrea Dovizioso has joined the Tech3 Yamaha team from Honda while Spaniard Alvaro Bautista has switched to Honda Gresini after the death last October of Italian Marco Simoncelli.Stoner's main rivals will be familiar faces: Spanish team mate Dani Pedrosa is injury-free at last while Yamaha's 2010 champion Jorge Lorenzo and American Ben Spies will also be strong contenders. Lorenzo, 2011 overall runner-up who needed plastic surgery on a finger in October that year, said: "I think this season we have a slightly more competitive bike. I am hoping for good things to happen."Because of my hand injury it has been a long time since I was in a race so I am really excited for Sunday. I have been training a lot, I'm feeling very good and very happy with the bike."'BETTER SPECTACLE'Pedrosa missed four races in 2011 to injury, including a broken collarbone, but the 26-year-old told Reuters last month he was feeling great. "Stoner and Lorenzo are riding to a good level but so am I," declared the Spaniard. "Casey is very fast and so is Jorge, they are great opponents."I think the change in the bikes is positive... they are more powerful, harder on the tyres, but it should be a better spectacle because there will be more overtaking."Among all the optimism, Italian great Valentino Rossi has struck a discordant note with his Ducati still lagging behind the leaders after a 2011 that saw the 33-year-old fail to chalk up a victory for the first time in his GP career."I'm a realist and I know well that our times are still a long way off and that there are still some things to fix," the nine-times World champion across all categories said in a team statement."We won't be able to address some of them right away but it's also true that both we at the track and the guys back home now have a direction to work in and we must try to do as well as we can."