Mazda’s SkyActiv-D Engine Performing Well Despite On-track Woes

Mazda’s ambition to race its 2.2-liter SkyActiv-D engine in the prototype class has certainly been challenging on the 2014 Tudor United Sports Car circuit, especially considering the success the spirited 4-cylinder diesel engine enjoyed last year in the GX class. Both Mazda Prototype LMP2 cars campaigned by SpeedSource Race Engineering had DNFs at the Rolex 24 Hours of Daytona. The #70 car did finish at Sebring, 13 laps back of the winner, while the #07 car dropped out after 104 laps. Then at Long Beach, the 07 car made it to the checkered flag, trailing six laps behind the winner while the 70 car completed only three laps.

Left-side view of the SkyActive-D.

Both cars have suffered a variety of teething problems, including overheating due to track debris clogging the radiator, a wheel coming off after a pit stop, thrown oil-pump belt and a punctured fuel cell. Despite the early drama of running the only diesel in the Tudor series, the team continues to meet many objectives that emphasize green initiatives and production relevance for the racing venture. And there’s a measure of reassurance knowing that none of the early problems were directly related to the engine.

“Part of the story for us is to find the ultimate limits of the components,” John Doonan, Mazda’s director of motorsports tells EngineLabs.

Right-side view of the compound turbo arrangement.

In stepping up to the prototype class, Mazda added about 75 horsepower to the GX SkyActiv-D engine while maintaining more than 50 percent factory production parts. Last year the GX class had a target of about 400 horsepower, and the Mazda 6 won its class in nine of 12 races (a factory SkyActiv engine is rated at 173 horsepower). Now the team is asking for about 475 horsepower and revs up to 5,200 rpm while pumping out nearly 600 lb-ft of torque. The block and heads are straight production pieces, but the Pankl billet crank, Carillo steel rods and Mahle pistons are aftermarket race components.

“We are now using steel pistons,” confirms Doonan, noting aluminum alloy was in the engine last year and the new Mahle slugs are a 2-piece, 3-ring design. “The steel pistons have been a huge gain as we turn the torque up.”

The intake and exhaust manifolds as well as other race-related plumbing and accessories were fabricated by SpeedSource

Lowering the compression ratio

The production SkyActive engine runs a relatively low 14:1 compression ratio, and the race team dropped that number even lower to help improve durability.

“We still have cylinder pressures in the 250 bar (3,625 psi) range,” says Doonan. “And for EGT, we’re trying to maintain 1,500 to 1,600 degrees.”

Action from the garage at the season third race on the Long Beach circuit.

The production cylinder head is modified only slightly to handle the upgraded Bosch fuel injection system that operates with 30,000 psi. Twin compound turbos from Garrett provide up to 70 pounds of boost, and the charge is cooled through a pair of air-to-air intercoolers. A closer look at the compound turbo setup on last year’s engine can be seen in this previous EngineLabs story. Overall the engine weighs about 340 pounds, compared to 385 for a stock engine.

Closeup of the Bosch fuel system.

Unique for this diesel application is a renewable synthetic fuel from Dynamic Fuels, which is a joint venture established by Tyson Foods and Syntroleum. The fuel is refined using non-foodchain byproducts as feedstock, combined with plant-based waste such as corn and algae oils. The fuel is clear and odorless, yet offers a higher cetane level in the 70s, compared to conventional diesel with a cetane level in the high 40s or low 50s.

“It doesn’t have the typical diesel aromatics. Right away on the dyno we saw huge gains, it’s such a quality fuel,” says Doonan. “Also, it doesn’t smoke, and that’s a critical message for us to be clean.”

The Mazda Prototype cars — built by SpeedSource and Multimatic Engineering — are based on previous Lola designs.

About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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