What’s different about a Miller-Cycle engine and why did Mazda use it? – Jalopnik

The Mazda CX-90 feels like a bona fide luxury SUV, but the company’s current push isn’t its first. One day, Mazda would launch its own luxury sub-brand, with innovations such as a supercharged Miller cycle engine. With that engine, customers could look forward to approximately 50% more power than they would find in a comparable engine running on the traditional Otto cycle.

Now the planned premium brand – Amati – never fully took off, but Mazda did not want to lose its engine investment. As a result, a supercharged 2.3-liter Miller-cycle V6 made its premiere in the 1995 Mazda Millenia sedan, which was originally supposed to be an Amati model. The engine produced 210 horsepower and 210 pound-feet of torque DepartmentsAuto raved about the power station at the time, calling it “the biggest piece of surprise and joy in the business.”

The secret sauce was in the Miller cycle itself. The system, invented by Ralph Miller in 1957, can increase efficiency by about 15% compared to regular combustion engines, thanks to the way the valves work during the compression stroke. Let’s see how it works.

The four basic steps in the combustion cycle of a typical gas engine are probably not new to Jalopnik readers: the pistons move along the cylinders to draw in air/fuel, move up to compress the mixture, move down to generate power as the air/fuel burns, and then move up to push out the remaining exhaust gas. That hasn’t changed since the first four-stroke engine was developed in 1876 by Nikolas Otto – who also named the engine style after him.

However, the Otto cycle – despite having become the go-to system for gas-powered engines – is not the only way to run an engine. Most hybrid powertrains today, including the hybrid perfection of the Toyota Prius, have gas engines that rely on the Atkinson cycle, which produces less power more efficiently by shortening the compression stroke. Hybrids then benefit from their electric motors to compensate for the lower power.

The Miller cycle takes the Atkinson cycle to the next level by adjusting when the intake valves close. With the Miller setup, the intake valves are open for about 20 to 30% of the compression stroke. The result is that less motor power is required for compression. While hybrid cars with Atkinson cycle engines use electric motors to make up for lost power, Miller and Mazda both turned to forced induction in the form of superchargers.

Mazda’s original Miller cycle engine added cost and complexity to the production process, and the Millenia never gained much traction with customers. As a result, use of the engine ceased when Mazda discontinued the Millenia. But that wasn’t the end of the story. For starters, Mazda’s current Skyactiv engine technologies are derived from the Miller cycle experiment. Dave Coleman, former vehicle evaluation manager and current technical manager of Mazda North America, told WardsAuto that: “We call it the Mazda Miller cycle, because the Miller cycle is our method of obtaining an Atkinson cycle.”

Another Miller cycle machine is the Volvo XC90 that has been modified to Miller level for 2025. Already noticeably efficient, the XC90 welcomed a 4% improvement in fuel efficiency with the new setup.

Furthermore, Volkswagen’s Budack cycle engine, the 2.0-liter turbo mill for vehicles like the VW Tiguan among others, actually runs on a variant of the Miller cycle. The engine has undergone its own number of improvements over the years, with Volkswagen pointing out that the 2025 Tiguan benefits from even more intensive application of the Miller cycle. The numbers tell the story, as the ’24 Tiguan hit the road with 184 horsepower and an EPA-rated 23 mpg city/30 mpg highway/26 mpg combined, while the updated engine makes 201 horses and reports to the EPA on 25/32/28.