Mercedes leads fire ups of 2019 cars

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Three of Formula One's four engines were heard roaring into life over the weekend.

The 2019 season begins officially next Monday with the start of pre-season testing at Barcelona's Circuit de Catalunya. World champions Mercedes posted a clip of its new engine being fired up on Friday, followed by Renault and Toro Rosso doing the same with its Honda engine -- the unit it will now share with parent team Red Bull.

Ferrari is the only engine yet to be heard ahead of the new campaign -- the Italian team also supplies Haas and the Alfa Romeo team (formerly Sauber). McLaren, a Renault customer, was the first team to release a fire-up clip, doing so earlier this month.

Mercedes has also continued to tease what looks set to be a camouflaged livery ahead of its 2019 car launch on Wednesday. Such a colour scheme would likely be for pre-season testing only -- Red Bull ran a similar concept ahead of the 2015 campaign.