Deep Dive: Inside the Mazda Skyactiv 2.5T Turbo Engine
Tuned for real-world fuel economy, not a window sticker.
With its newest Skyactiv engine, Mazda is trying to change the way people talk about engine output. As it launches a new 250-hp, 2.5-liter turbocharged four-cylinder in the redesigned 2016 CX-9, the brand that promises zoom-zoom is stressing two things: lots of torque, and real-world fuel economy.
"Instead of horsepower, we want people to start referring to torque," says North American product planning chief Tim Barnes.
Mazda is already thinking this way by designing its engines for responsiveness, and that comes from lots of torque early on in the rev range -- after all, the company's research showed most people don't often push their engines past 3,000 rpm. Keeping the revs low also helps fuel economy, especially on EPA test cycles. The trouble is that drivers don't always see the same efficiency in real-world driving that the EPA testers manage, so Mazda set all of its consumption targets according to driving habits of actual customers who don't drive on a dynamometer in a lab.
So, how does the new Mazda engine manage to squeeze out 310 lb-ft of torque at just 2,000 rpm, and deliver what the company claims is class-leading fuel economy? We talked nuts and bolts with Mazda to find out.
Smaller displacement, plus turbocharging
Mazda had planned from the beginning of its Skyactiv engine family development to build a four-cylinder turbo, because of some major advantages over a V-6. For one, a smaller engine weighs less, in this case about 150 pounds of savings over the CX-9's outgoing 3.7-liter six-cylinder. And although a turbo engine can't have the super-high compression ratio of Mazda's naturally aspirated Skyactiv engines, it boasts 1.6 times less mechanical loss as a result of less internal friction. Smaller engines also tend to be cheaper to build, which is crucial when you're working with expensive technologies like direct injection and variable valve timing.
Turbocharging allows the 2.5-liter four-cylinder to make a lot more power and torque, but it comes with its own set of disadvantages. One is turbo lag. Boosted engines also perform worse at high rpm and high load, as the engine is forced to run rich to keep temperatures down and prevent damage to the internal components. This is, of course, a waste of fuel. So, Mazda uses a few tricks to battle turbo lag and prevent fuel enrichment in the cylinders.
Variable nozzle turbocharging
Mazda calls its variable nozzle turbocharging Dynamic Pressure Turbo technology. By varying and strategically directing the amount of exhaust pulse used to run its turbocharger, the 2.5-liter Skyactiv-G engine can spool up more quickly at low rpm. The method works by sending exhaust air to the turbine through three smaller exhaust ports at low rpm, quickly creating boost pressure of up to 17.4 psi. Three larger ports open up via a flow control valve when the revs climb. Mazda describes the idea as putting your thumb on a running garden hose to restrict the water flow and increase the force of the stream, thus spooling up the turbo sooner.
Reflecting the way Mazda believes most people drive, the new engine is designed to spend most of its life turning less than 3,000 rpm. By offering so much instant torque at the low end of the rev range, the engine has enough grunt that the six-speed automatic doesn't have to downshift often. This not only saves fuel, but it also makes for more responsive power delivery for maneuvers like passing on the highway.
To make sure the turbo is not only responsive but also efficient, the exhaust ports are set up in an extremely clever way. It's a 4-3-1 exhaust, meaning the middle two cylinders share a single exhaust port, while the outer cylinders each get their own. That also means the ports are drawing on each other between pulses, helping suck residual exhaust from adjacent ports for maximum extraction. Mazda describes it as the same philosophy behind how a paint spray gun works, with a flow of air passing across a stream of paint and sucking it along for the ride.
Cooled EGR (exhaust gas recirculation)
Mazda also uses a special method uncommonly seen in gas-powered passenger cars to keep engine temperature down at high load to prevent fuel enrichment, a cooled EGR. By using a special (and rather pricey) module to cool exhaust gas that's recirculating through the turbo, the engine is able to reduce internal heat and thereby maintain an impressive 10:5:1 compression ratio. But the benefit will be seen more by consumers than the EPA testing lab, Mazda said.
"Without the cooled EGR, the engine wouldn't be able to offer the real-world fuel economy we want to make sure customers have," said research and development engineer Stan Hortinela. "And that component, which is not cheap, doesn't change our window sticker rating one iota. It's purely there to make sure people get the efficiency they expect from the rating." It also prevents internal temperatures from rising too high and damaging the engine.
The 3.7-liter V-6 in the old CX-9 was rated by the EPA for 17 mpg in the city and 24 mpg highway, and Mazda says it expects gains of about 20 percent for this new engine. When the ratings are finalized, it's very possible the 2016 Mazda CX-9 could get 30 mpg on the highway.
Other applications... Mazdaspeed3 anyone?
Mazda was dodging questions all week at the L.A. show about where else we'll see this new engine down the line. Over and over again the subject of the next Mazdaspeed3 came up, but Mazda refused to comment about future products despite the relentless barrage from journalists. The closest thing we got to a confirmation was from development engineer Dave Coleman, who said that while he wouldn't comment on future products, to "just remember that we are an enthusiast brand."
The outgoing Mazdaspeed3 also had a turbocharged four-cylinder engine, displacing 2.3 liters and making 263 hp and 280 lb-ft of torque. Our guess is it wouldn't take much to slap a bigger turbo on the 2.5-liter and tune it for fun and performance rather than fuel economy. We've long heard rumors, after all, of a resurrected turbocharged Speed3.
The 2016 Mazda CX-9 arrives in showrooms this spring.