A new line of large vehicles opened up a new opportunity for Mazda. To build a new series of engines that would help elevate the models that used it, deliver on the company's desired power figures, and improve fuel economy and emissions. This is how and why the automaker ended up with an inline-six. A layout the company has never done before that has fallen out of favour in the industry.
Mazda engineering and development lead in Europe, Joachim Kunz, laid out the new platform and engines to be used in the CX-60, -70, -80., and -90. We spoke with him about the process and the ideas behind it.
In North America, we were told, Mazda is planning for 30 percent of its products be fully electric by 2030. That leaves plenty of life in the internal combustion engine, and the company recognized that there was still room to improve efficiency and economy as well as lower CO2 emissions. Mazda also left space for renewable fuels in the future. Where driving a combustion vehicle could be CO2 neutral, at least at the tailpipe, helping to clean up new models and the existing fleet.
It's Beautiful to Have the Six-Cylinder
For Mazda's new engine family, Kunz said that the decision was made for an inline six because "it's beautiful to have the six-cylinder. With perfect theoretical balance and beautiful engine sound."
A six-cylinder runs much more smoothly than other engine designs, which is why BMW and Mercedes-Benz have stuck with the layout for so long. With six cylinders all in a line, the explosions that happen inside a combustion engine push all of the pistons up and down on the same axis. Straight up and down. A V6 has the pistons pushing down from each side at an angle, working to tumble the engine around the crankshaft and increasing vibrations.
An inline-six also runs in perfect mechanical balance: If one piston is being forced downward by combustion a matching piston is moving upward in compression, further smoothing out the forces working to shake the engine apart.
Lastly, more combustion events in every revolution means shorter gaps between bangs. This effect is why 12-cylinder engines remain popular in luxury flagship models; they are inherently smoother and more refined than a V8.
There are practical and economic reasons as well. Practically, Mazda needed more power than a four-cylinder engine could deliver. "I cannot tell you which came first: we want to have a six-cylinder engine because it's beautiful or we need more power," Kunz said.
To get more power, Mazda needed more displacement. "We need a little bit more capacity to have higher torque. And we want to keep 500 cc cylinder volume because this is the sweet spot for the physics. For the thermodynamics," Kunz said. Mazda also didn't want to use turbocharging, the replacement for displacement, because a turbo doesn't work with the company's new engine combustion concept we'll cover further down the page.
What is so sweet about 500cc? A displacement of around 500cc per cylinder is a near-universal target for thermodynamic reasons. The Skyactiv I6, like most, has a narrow bore (89.5 mm) and long stroke (91.2 mm), instead of a square. It's a sort of the Goldilocks size that does an excellent job of turning combustion into usable power, and not just heating up the cylinder walls. It's why so many cars use 2.0L fours, 3.0L 6s, and 4.0L V8s.
Mazda showed us charts with other benefits that make having a large displacement engine with more cylinders favourable. Including the ability to make more power at lower RPM and that the design allows a better fuel consumption ratio when under load than a small-displacement turbo engine.
Economically, Kunz pointed out that manufacturing an I6 is cheaper than a V6. Fewer expensive components like camshafts and cylinder heads are required, and the engine can all be assembled from the same side instead of requiring machines on both sides. We'll note that it's also scalable to four-cylinder applications.
Back to Mazda's new combustion concept. Mazda's Spark Controlled Compression Ignition
(SPCCI) engines can run like a conventional spark-ignited gas engine in certain situations and with compression ignition like a diesel in others, with the technology allowing an ultra-high compression ratio of 16:1 compared with 12:1 in other engines considered "high compression" and single digits in turbocharged engines.
New egg-shaped combustion chambers effectively divide the air/fuel mix into two different areas, allowing the Skyactiv-X design to improve combustion. To waste less fuel and reduce the nitrogen oxide emissions associated with diesel engines (without using diesel exhaust fluid or similar exhaust additives). The result of this clever system from Mazda is 10 percent more power than conventional gas engines, up to 30 percent more torque, and better fuel efficiency than conventional diesel.
New Transmission Solves Cabin Space While Improving Everything Else
A new in-house designed eight-speed automatic goes with the new gas engine to help Mazda improve economy and aid electrification while also contributing to the company's desire to make its vehicles better to drive.
The new big trick: Instead of a traditional (and large) torque converter, a fluid connection like a turbine that sits between the engine and automatic transmission, Mazda here uses a wet multi-plate clutch. The clutch pack, which still sits in a fluid for cooling, can slip to make start-off smooth, but stay locked on the move. The design is lighter, narrower, and adds flexibility.
It's not like a double-clutch box, where gears are split between two input shafts, each controlled by its own clutch. This design is bulky, heavy, and expensive. Mazda's design is an otherwise conventional planetary-gear automatic that ditches the power-sucking torque converter.
Using a clutch makes the flow of power to the wheels feel more like a manual transmission, improving driving dynamics and meeting one of Mazda's key goals. The design reduces transmission losses by 22 percent, improving fuel economy and reducing heat transfer to the cabin. And, importantly, it is far narrower. So the centre tunnel doesn't cut into cabin space as much as a conventional unit.
Eight gears helps engine efficiency. The narrow spread between ratios makes sure that the engine is at the right RPM for peak efficiency more of the time. As Mazda puts it, the eight-speed "allows driving in the good fuel economy range" of engine operation.
The new gearbox design makes electrification easy. The six-cylinder gas and diesel engines place a 48V electric motor in between the engine and transmission, with one clutch pack on either side. The mild hybrid system lets Mazda shut off the engine when it would be idling and even under some low-speed conditions. Times where combustion engines are at their least efficient. Once moving, the internal combustion engines take over.
For plug-in hybrid systems, like the CX-60's, a larger electric motor is placed between engine and transmission. A smaller four-cylinder engine can be used because the electric motor is more powerful. The four-cylinder engine's shorter length makes room for the more powerful electric motor, keeping the entire package compact. An inline-six PHEV is not planned, Kunz said, unable to expand on the reasons why.
Sending power from the electric motor through the eight-speed transmission gives the motor the same torque multiplication enjoyed by gas engines. So a smaller motor can do more work. Mazda can also drive all four wheels with just one motor, instead of needing one to drive the fronts and one for the rear wheels as found in PHEVs like the Toyota RAV4 Prime. This ability, which allows Mazda to send almost 100 percent of power to the rear axle and a large percentage to the front tires as needed, is also a boost for vehicle dynamics.
Mazda needed a new engine with more displacement to make the power needed to move its large crossovers. A six-cylinder made the power Mazda needed, an inline-six offered the refinement and driving pleasure the company wanted. A new transmission boosted the efficiency of the engine and made adding electrification easier. All it needed was a platform to put the system in.