When the green flag drops at the 2013 Rolex 24 at Daytona on Jan. 26, the diesel engines powering the three Mazda6 sedans in the race will be making history.
The engines, modified versions of Mazda’s new 2.2-L four-cylinder Skyactiv-D, are the first diesels to compete in the famous 24-hour endurance race at the International Speedway. The stock-block units are dynamometer rated at more than 400 hp (298 kW) compared with 173 hp (129 kW) for the production “Sky-D” engine. They are equipped with a compounded turbocharging system and use a new synthetic diesel fuel that enables the cars to run without exhaust smoke.
They will not use exhaust aftertreatment systems in the Daytona race. The production Skyactiv-D, which launches on the U.S. 2014 Mazda6 later this year, is designed to comply with U.S. Tier 2 Bin 5, EU Stage 6, and Japan’s stringent Post New Long-Term Emission Standards without requiring selective catalytic reduction and lean-NOx traps.
The racecars are an official “factory” program. They were built by Florida-based SpeedSource Engineering, which has developed numerous Mazda racecars in close cooperation with Mazda North America and its Japan parent since 1995.
They are contesting the new GX class being launched this year by Grand-Am, the U.S. road-race sanctioning organization. The 11-race GX series aims to showcase diesel, hybrid, and other ”clean”-fueled racing machines as well as offer a competitive venue for cars that would be ill-matched in Grand-Am’s more powerful GT-class. The Mazda6s are the only diesels entered in the Daytona race. GX competitors will include gasoline-powered Porsche Caymans and the Lotus Evora.
Best known among enthusiasts for its piston-engined Miata and rotary-engined RX-7 and RX-8 sports cars, Mazda decided to race the production-bodied Mazda6 “Sky-D” in order to position the brand in the U.S. as a maker of clean-diesel technology. The company last marketed a diesel vehicle here (the B2200 pickup) in 1985.
“We think diesel is a great alternative to the expensive hybrid offerings in the marketplace,” said Jim O’Sullivan, President of Mazda North America. “The race program fits our Skyactiv strategy of optimizing everything we can do with ICEs and the rest of the vehicle—lower mass, better aero—to improve efficiency.” (See in-depth Skyactiv article: http://magazine.sae.org/11aeid0201.)
O’Sullivan observed that Mazda is “not just the only Japanese brand but also the only Asian brand bringing diesel into the U.S. market. Because we are an ‘enthusiast’ brand I think adding a diesel helps us get on the consideration sets of more people. That's because those who walk into the European brands’ showrooms looking for a diesel powertrain are primarily shopping for the diesel, rather than for a specific brand.”
In pre-race testing, the Mazda diesels consumed 25 to 30% less fuel than a comparable gasoline race car over the same distance, noted SpeedSource President Sylvain Tremblay. This allows them to be fitted with 14-gal (53 L) fuel tanks that are three to four gallons (11-15 L) smaller, and thus lighter, than the gasoline tanks of competitors.
The cars are also the first SpeedSource race vehicles developed entirely using computer tools, Tremblay told AEI. “Before this program we would reverse-engineer,” he noted. “But these cars were a clean-sheet, from-the-ground-up development using our full Ansys suite. Ansys is one of our technical partners and we use their tools extensively to do full engine and combustion simulation and chassis simulation, along with SolidWorks tools.
The diesel engine’s boosting setup, comprising a pair of Garrett turbochargers and a sophisticated air-to-air intercooler array, is based on the production Sky-D. It’s the first motorsports use of a compounded turbo system, Tremblay claimed. A large low-pressure turbocharger draws in and compresses ambient air, feeding it into a smaller high-pressure turbo which compresses the air again and forces it into the intake manifold.
Compared with a single turbocharger, the compounded-turbo arrangement provides a greater range of operating efficiency, Tremblay said. However, it is more complex and difficult to package in the car. The arrangement is not to be confused with turbo-compounding used on some reciprocating aircraft engines and the Detroit Diesel DD15 heavy-truck diesel, where one of the turbines is connected mechanically to the engine’s crankshaft.
“And it’s not sequential like on some gasoline turbocharged engines,” Tremblay explained. “One turbo charges the other and they’re not regulated mechanically in any other way other than by the actual computer control mapping of the turbos. As one compressor stalls, the other picks up that particular boost. It’s a very effective system for the Mazda race diesel.”
Maximum boost pressure used during the race will be in the 50-55-psi (345-379 kPa) range.
Tremblay, who will share driving duties on the No. 70 car, noted that the team’s intercooling strategy continues to evolve. “It’s a huge development window for us. In all of our dynamometer testing we’ve used liquid-based coolers, but we’re using air-to-air on the racecars. We’ve played with intercooler sizing quite a bit,” he noted.
“It’s really an art to find the proper size to handle the pressure drop across the cores, how much aerodynamic drag you’re willing to suffer—the larger the openings, the more air is close to the car so you lose downforce, and how much intake air temperature you can handle. But at Daytona, we're running an ‘equal’ system where the high pressure and low pressure intercoolers are the same volume and the same density.”
Stock engine a strong platform
When the Mazda and SpeedSource engineers first examined the new Skyactiv-D engine, they quickly realized the engine had potential for excellent power density, Tremblay recalled. An added benefit was the production engine’s ample tuning latitude.
“We didn’t have to resort to a lot of special aftermarket parts, or develop a bespoke racing diesel like Audi and Peugeot did with their LeMans prototype engines,” he explained. The team discovered that more than 50% of the stock engine’s parts, by weight—more than 275 part numbers—could be used with little or no modification in the racing version. Included in the long list are the aluminum, open-deck cylinder block and (unmodified) cylinder head, valves, and coolant pump. Tremblay noted that the stock Mazda pump outperformed every trick racing pump the team tested.
Many other parts are modified stock items (such as the mildly race-ground camshafts) or racing parts that use production geometries (including the pistons’ bowl-shaped domes). An aftermarket race crankshaft is substituted for the stock crank only to extend the rebuild interval to at least 30 hours during the Grand-Am season, Tremblay said.
The racing diesel’s peak speed on Daytona’s infield road circuit will be 5250 rpm. Its broad, flat torque curve, produced at much lower engine speed than most gasoline competitors, requires a special transmission supplied by SpeedSource’s gearbox partner, Emco Gears. The combination made the team reduce the cars’ effective drive ratio by half (to 2.16:1), in order to compensate for the engine torque.
Flowing through the Mazda diesel’s Bosch Motorsports injectors is a synthetic diesel fuel supplied by Dynamic Fuels, a 50-50 joint venture between Tyson Foods and Syntroleum. The fuel is refined from non-foodstream feedstock based on chicken, pork, and beef fats left over from Tyson’s processing, as well as from various post-processing plant-based waste including inedible soybean and corn oils, jatropha, and algae oils.
“Any triglyceride [the main constituent of animal fats and vegetable oils) works in our diesel-fuel process,” said Jeff Drake, Senior Director of Operations at Tyson Renewable Energy. The company’s manufacturing facility near Baton Rouge, LA, launched in December 2010, is capacitized for 5000 barrels of the clear, almost odorless fuel per day, and up to 75 million gallons annually.
Dynamic Fuels has had significant success with its synthetic diesel fuel. It was used in the U.S. Navy’s recent “green fleet” exercise to validate cleaner-burning, non-petroleum-based fuels in naval vessels and military aircraft. It also has been used successfully in over-the-road trucks, off-highway equipment, and tested in Alamo and National rental-car fleets.
In Mazda’s racing diesel, the synthetic fuel provides clean, smokeless combustion, per SpeedSource and Mazda specification. The fuel features a cetane rating “in the 70s,” Drake noted, compared with conventional diesel fuel’s cetane average in the high 40s to low-50s. And importantly for quick fueling needed in racing, the synthetic fuel is much more resistant to foaming than typical pump diesel fuel, he said.
Prior to the team’s pre-Daytona testing, Tremblay reflected on developing the Mazda6 diesel racer. “All the testing we’ve done in the shop, including all of the engine and chassis dynamometer work, the simulation work, should make me feel warm and fuzzy. But this is motorsports and anything can happen. Everyday what scares me is the stuff that we don’t know.
“The only ones who have played with racing diesels before us are Audi and Peugeot, and those were money’s-no-object programs,” he continued. “There’s so little technology transfer from what they’ve done to what we’re doing, that we’re basically alone. We’re forging new ground with this Mazda6 diesel racecar which is interesting and scary all at once.”