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, [SpeedSource President Sylvain] 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.

Mazda races compounded-turbodiesel, new synthetic fuel - SAE International