However, Mazda's latest engine, the SKYACTIV-G 2.5T, is a realization of excellent real- world and catalog fuel-economy as well as spry acceleration, featuring a host of technological advancements to achieve this goal.
One piece of technology is Mazda's Dynamic Pressure Turbo, the world's first turbocharger with the ability to vary the degree of exhaust pulsation depending on engine speed. The system routes engine exhaust to the turbocharger's turbine through smaller ports at low rpm. It works similarly to when one might place his or her thumb on a garden hose, creating a strong amount of pressure through a smaller outlet. This allows the turbocharger to spool up quickly, creating instant boost—up to 1.2 bar (17.4 psi) of pressure. When the engine is in the heart of its rev range, it opens up secondary valves, allowing for greater amounts of exhaust gas to pass through the turbocharger. The system is complemented by the 2.5-liter engine that already benefits from more torque at atmospheric pressure than a comparable 2.0-liter by virtue of its size.
Further assisting CX-9 to maximize turbocharger efficiency is a 4-3-1 exhaust. With this setup, the exhaust from the middle two cylinders (2 and 3) is joined into a single port, while the exhausts from the outer cylinders (1 and 4) each have their own ports. These three ports come together at the entrance to the turbocharger's exhaust side, where there is always one exhaust pulse arriving every 180 degrees of crankshaft rotation. Not only does this very compact manifold keeps the exhaust pulses separate for maximum energy extraction, it also harnesses each exhaust pulse to suck the residual exhaust from the adjacent ports.
That only tells so much of the story. In order to increase fuel-efficiency, SKYACTIV-G 2.5T employs the efficient combustion of Mazda's SKYACTIV-G 2.5-liter engine and marries it to a cooled exhaust gas recirculation (EGR) system, which helps prevent burning excessive amounts of fuel when the engine is running at higher temperatures. In many turbocharged cars, heat is controlled through adding more fuel to the combustion chamber; Mazda's cooled EGR reduces the need for that.
While Mazda sees no discernible benefits on regulated test cycles from this technology, it benefits the consumer through real-world gains.
The cooled EGR helps bring engine temperatures down from approximately 500 degrees C (932 F) to just over 100 degrees C (212 F), allowing SKYACTIV-G 2.5T to operate with a compression ratio of 10.5:1—one of the highest numbers of any gasoline-powered, turbocharged engine.
The net result: 310 lb-ft (420 N-m) of torque at a low 2,000 rpm and 250 horsepower at 5,000 rpm on 93-octane gasoline (227 horsepower on 87-octane gasoline). At 55 mph (88 km/h), a driver needs just 18 horsepower to maintain speed on flat roads with a front- wheel-drive model—a four-horsepower reduction versus the outgoing model, illustrating reduced friction in all parts of the driveline and enhanced aerodynamics.