Where Engine Dreams Meet Possibilities: The SKYACTIV-Z Challenge and Mazda’s Internal Combustion-Focused Electrification Strategy

While the move toward electrification for carbon neutrality intensifies across the automotive industry, Mazda remains committed to exploring the potential of internal combustion engines. We sat down with Masahisa Yamakawa and Michiharu Kawano, two leading figures in Mazda’s internal combustion engine development, to understand this approach.

They share the evolution of the revolutionary fuel-efficient SKYACTIV engine and reveal insights about the SKYACTIV-Z engine currently under development. Our conversation explores how internal combustion engines can remain relevant in the age of electrification, including the exciting potential for carbon-negative internal combustion engines that actually clean the air the more they're driven. Read on to discover Mazda’s vision for brand-defining cars that balance environmental responsibility with the joy of driving.

Around 2005, I was assigned by Mitsuo Hitomi (currently Executive Fellow Innovation of Mazda), the General Manager of the Powertrain Advanced Development Department at that time, to conduct technical research and experiments that would later lead to SKYACTIV-G. Hitomi narrowed down factors that improve engine efficiency (thermal efficiency) to seven control factors, including compression ratio, air fuel ratio, and combustion period. It was my job to verify these factors. Among the seven factors, we confirmed that compression ratio had a particularly large impact on thermal efficiency, and decided to use this as our breakthrough point in pursuing a low-fuel-consumption engine that could compete with hybrids.

Internal combustion engines traditionally waste over half their potential energy as heat and friction. Mazda identified the seven control factors for these energy losses and has systematically pursued the perfect combustion process, pushing the boundaries of thermal efficiency in their quest for the ideal engine.

When we consider Mazda's size and areas of expertise, exploring the possibilities of internal combustion engines before taking steps toward electrification was the most logical path. We also believed that if we could design a good engine, it would give us a significant advantage when combining them with motors. When Hitomi explained this direction to us engineers, we were convinced that this was the right approach and could devote ourselves to engine development.

Masahisa Yamakawa. Joined Mazda in 1986. In 2005, moved from an established role in mass production development for gasoline engines to take part in the development of SKYACTIV-G. Currently affiliated with Mazda's Technical Research Center, teaching and conducting research at Hiroshima University in areas such as combustion and catalysts to meet strict exhaust gas regulations.

The compression ratio represents how much air or air-fuel mix (gaseous state of mixed gasoline and air) is compressed within the cylinder. Higher compression ratios improve engine thermal efficiency, leading to better output and fuel economy, but they also cause “knocking”—a phenomenon where fuel burns unevenly. To avoid this, conventional naturally aspirated gasoline engines typically set compression ratios at around 10 to 11:1, but with SKYACTIV-G, we increased it to 14:1.

The SKYACTIV-G, released in 2010. After debuting with a 1.3L version in the Demio, the lineup expanded to include 1.5L and 2.0L versions.

That’s right. It was said that if you increased the compression ratio to around 12, 13:1, knocking would become so severe it’d damage the engine. So initially, I wasn’t enthusiastic about this experiment! But Hitomi said we should just give it a go at 15:1 and see what happens. We designed pistons that could withstand high compression ratios and monitored their performance. That’s when we made a surprising discovery. Theoretically, to avoid knocking when running an engine at a compression ratio of 15:1, torque should decrease by 30-40%. Yet when we assessed this with an actual engine, the decrease was only about 12%, which was a level we were confident could be managed. After repeating experiments for another month or two, we began to see ways to effectively avoid knocking as well, and SKYACTIV-G development began moving toward mass production.

It was around this time when I joined the company. I can still remember how I felt when I saw the entire department pulling together to create one single engine. And I’ll never forget that initial test run of the very first SKYACTIV-G prototype. The lab fired up the engine and, when it worked first time, called the development department to let us know it was running smoothly. The entire floor erupted with a celebratory "Woah!" I was still a new employee at the time, but I got goosebumps thinking about what an amazing engine had just been created.

Michiharu Kawano. Joined Mazda in 2005. After working in the Powertrain Advanced Development Department and Powertrain Technology Development Department, joined Engine Performance Development Department in 2009 and became involved in developing SKYACTIV-G/D/X for mass production. From 2018, works in the MBD Innovation Department while also collaborating with MCF Electric Drive, a joint venture established by Mazda for battery EV motor development. An expert engineer, his years of experience in analysis and simulation have been essential for engine development.

Previously, team members were divided by vehicle and engine type, but with SKYACTIV-G, many gasoline engine developers in the Powertrain Development Division temporarily stopped their other work to focus on this. As we continued development, I became convinced that this would be a great engine.

Almost all car manufacturers reached out to see if we’d be open to sharing technology. It shows how significant it was to be able to control the compression ratio of internal combustion engines. I even had people come up to me and say how much they envied Mazda's commitment to internal combustion engine technology.

At that time, a lot of car manufacturers were allocating resources to hybrids. Even now, when networking with other manufacturers, we still hear how impressed they were with Mazda for focusing on compression ratio.

True. But at the same time, diesel had many strengths like good fuel economy and acceleration performance. We believed there was still room for improvement, just like with gasoline engines. Our development concept was to create a diesel engine that was both environmentally friendly and could rev smoothly to high RPMs (Revolutions Per Minutes).

The SKYACTIV-D, announced in 2012. While diesel engines were common in passenger cars in Europe, in Japan they were associated with trucks. SKYACTIV-D transformed this image with agile performance and drivability.

Again, compression ratio was a significant factor. A diesel engine works by igniting diesel fuel using heat from highly compressed air in the cylinder. A higher compression ratio is better for making the air hot, so typical diesel engines had compression ratios of around 17 to 18:1. But with SKYACTIV-D, we deliberately lowered the compression ratio to achieve better environmental performance and rev to high RPMs comparable with a gasoline engine.

SKYACTIV was a project that fused the characteristics and strengths of both engines: giving diesel engines the clean combustion of low compression ratio gasoline engines, and giving gasoline engines the fuel efficiency benefits of high compression ratio diesel engines.

The SKYACTIV-X, released in 2017. It addressed gasoline compression ignition technology and developed an unprecedented combustion method. Behind these innovations was advanced simulation technology.

There’s a gasoline engine combustion method known as Homogeneous Charge Compression Ignition (HCCI), where gasoline is combusted through self-ignition by compressing air, similar to diesel engines. It was considered to be more efficient and produce cleaner exhaust gases than conventional combustion methods, but no manufacturer had been able to commercialize it. The reason? Gasoline is harder to ignite than diesel.

That’s right. So Mazda developed a method called Spark Plug Controlled Compression Ignition (SPCCI) that uses spark plugs to induce compression ignition. But, the more advanced the technology becomes, the more time development takes.

Achieving proper SPCCI combustion was estimated to require 16,000 times more hours than SKYACTIV-G, and was considered impossible to complete using conventional methods. However, with SKYACTIV-X, we’d began using combustion models to advance high-speed measurement and data processing, and developing AI machine learning technology. This meant we were able to keep development time to less than one-third of SKYACTIV-G.

Thanks in part to this analysis and simulation technology, it was possible to have highly efficient combustion with less fuel from low to high RPM, making SKYACTIV-X an internal combustion engine that comprehensively combines the benefits of both diesel and gasoline engines.

As a niche player, Mazda has limited human resources. Using computer calculations to compensate for this is one of the reasons for our focus on simulation technology. Compared to the time it takes to prototype parts and run tests, it saves a lot of time and effort. Also, when trying to verify gas flow during intake and exhaust, experimental measurements can show numerical results, but they can't fully capture exactly what phenomena are occurring inside the engine. That's why visualizing phenomena through simulation has become an important feature of our analysis team.

Analytical models allow for simulated experiments under multiple conditions, while also visualizing the phenomena occurring inside the engine.

We engineers excel at thinking about things theoretically, and once a phenomenon is visualized, our imagination expands with more ideas for improvements and problem-solving. We work as three parts of the same body, with the analysis team supporting ideation and collaborating with the experiment team and design team. It’s a system we’ve gradually developed since the SKYACTIV-G era to create new technologies. Mazda was probably the first car manufacturer to use simulation as an established part of the process.

Yamakawa pictured with Mitsuo Hitomi (left) in 2014. Hitomi built the foundation for SKYACTIV engine development and led the project. For a niche player like Mazda, recognizing the importance of data analysis was significant.

With SKYACTIV-Z, we're evolving the compression ignition combustion technology using the SPCCI method that we commercialized in the previous model, aiming for an engine that can combust with even leaner fuel mixtures. We're also planning to incorporate new heat insulation technology. This technology converts heat that would normally escape from the engine into power, further improving thermal efficiency.

Out of the seven control factors I mentioned earlier, heat insulation was the only unexplored area for improving thermal efficiency. While I can't reveal the details yet, we want to create an engine that boasts high thermal efficiency at any RPM or speed range, using Mazda's proprietary new technology. In conventional engines, meeting stricter exhaust gas regulations would typically result in a reduced output of about 30%, but SKYACTIV-Z is expected to maintain the same power output as before these regulations. The aim of SKYACTIV-Z is to create an internal combustion engine that achieves better fuel efficiency when running on its own while providing even greater synergistic effects when combined with motors.

The 2.5L inline 4-cylinder gasoline SKYACTIV-Z engine. Development of the engine continues to advance to meet stringent emission standards like Europe’s Euro 7 and North America’s LEV4 and Tier4, while delivering exceptional driving performance. This engine aims to provide customers with noticeably improved fuel efficiency and enhanced performance, all while maintaining affordability for mass-market vehicles.

Mazda’s vision is to commercialize engines that achieve high efficiency and clean emissions across all engine and vehicle speed ranges in real-world driving, without compromising on power output.

Set to debut in the next-generation CX-5 in 2027, the SKYACTIV-Z will be paired with Mazda's unique hybrid system. This innovative powertrain combines electrification with advanced combustion technology to deliver a balance between higher environmental performance and driving performance. The SKYACTIV-Z is positioned to become the cornerstone engine for Mazda in the age of electrification.

The choices for electrification are diverse, from battery EVs without internal combustion engines to strong hybrids, plug-in hybrids, and mild hybrids. At Mazda, we’re considering all options, taking a multi-solution approach to powertrain systems.

Mazda's expertise in internal combustion engines (ICE) delivers a multi-solution value proposition. Rather than a one-size-fits-all approach to environmental challenges, Mazda is tailoring solutions that respond to regional energy realities and diverse customer needs, wants, and lifestyles. This flexible approach enables Mazda to engage more drivers in efforts toward environmental sustainability.

That said, as things stand it's unlikely that all cars will be replaced by battery EVs. Which is why I believe we were right to persevere with the internal combustion engine. Whether generating electricity with an engine to turn the wheels or using a motor to complement the engine's power, hybrid systems require an (internal combustion) engine. Even with electrification, with the exception of battery EVs, we'll still be considering solutions based on internal combustion engines. With highly efficient internal combustion engines, we can make motors smaller, which should also help control electrification costs.

That’s right. We're already implementing efficiency initiatives, such as unifying all engine control software, but in recent years we've been promoting the use of a “model for a full vehicle ” approach. This involves modeling not just the engine but all the components of the car—combustion, control, aerodynamics, and so on—to determine which elements should be controlled and how to achieve the desired functions and performance. The result is more efficient function allocation decisions early in the design stage. With the advanced simulation technology we've developed, we can now achieve both faster development and economic efficiency.

While fossil fuels are currently the main source of energy, if we use carbon-neutral fuels we can significantly reduce CO2. We're also running experiments and testing of carbon capture technology that uses a substance called zeolite in the exhaust pipe to absorb CO2. Combining these approaches means that theoretically, carbon negativity is achievable.

Well, I’d say that’s the dream and it certainly has potential. For example, running a 2-liter engine at 1000 RPM means it's taking in and expelling approximately 1000 liters of air per minute. If we can create a mechanism that releases air with less CO2 than what was taken in, internal combustion engines wouldn't be eliminated, they'd be preferable. We could even have cars that act like air purifiers, cleaning the air as they drive through polluted areas. From this perspective, we could see a future where engine-powered vehicles are ultra-environmentally conscious mobility solutions.

I sometimes hear early-career team members express concern about the value of focusing our research exclusively on internal combustion engines. But in sharing my own experiences and mindset, I can inspire them to see how much potential there is yet to explore. As long as we can see the possibilities, we’ll continue to test the boundaries of this technology, which ultimately will lead to better value for our customers.

I also teach at Hiroshima University, and when I talk about cars that clean the air, the students’ eyes light up (laughs). One appeal of internal combustion engines is their use of liquid fuel, which delivers substantial power while keeping the weight low. This directly contributes to the joy of driving experience in a lightweight sports car like the Roadster. I hope that one day we'll live in an age when driving a Mazda car with an internal combustion engine gives customers a sense of pride that they're driving while cleaning the environment.

Yamakawa teaching in the engine laboratory at Hiroshima University. From April 2025, he will be based at Hiroshima University, engaged in further research on internal combustion engines and training engineers.

Mazda's engine laboratory. While stricter exhaust regulations are expected to reduce output by 30% in conventional engines, SKYACTIV-Z will maintain the same output as before these regulations.