Mazda revives the rotary engine, but not in the way you might expect
After ending mass production of the rotary engine in 2012, Mazda has brought the technology back. However, this isn't the high-revving sports car engine of the past. Instead, the new rotary engine serves a modern purpose: acting as a compact generator to power an electric vehicle's battery system.
The return of the rotary engine in the MX-30 e-Skyactiv R-EV marks a significant shift in how Mazda utilizes this unique architecture. While the previous 13B Renesis engine found in the RX-8 was a 654 cm3 two-rotor unit designed for direct propulsion, the new system features a single 830 cm3 rotor. This engine is specifically engineered to function as a generator, allowing for a compact design that integrates seamlessly with an electric motor and decelerator. By utilizing aluminium for the side housing instead of the iron used in the Renesis, engineers successfully reduced the engine's weight by more than 15 kg.
Efficiency was the primary hurdle for the rotary engine's revival, addressed here through several technical innovations. The most impactful change is the adoption of direct fuel injection. In older rotary designs, fuel injected into the ports often failed to combust fully, leading to wasted fuel and higher emissions. Direct injection ensures the air-fuel mixture reaches the main combustion area more effectively and allows for better vaporisation at lower temperatures. This prevents the need for excess fuel injection, directly improving both output and fuel economy.
Further efficiency gains are achieved through an exhaust gas recirculation (EGR) system, which operates primarily at low engine speeds and loads. This system helps mitigate the cooling losses inherent in the rotary engine's combustion chamber geometry, which typically has a larger surface area than a reciprocating engine. To ensure durability, Mazda also upgraded the apex seals—the components that keep the combustion chambers airtight—to a width of 2.5 mm. Additionally, the housing surfaces now feature a ceramic coating applied via high-velocity oxygen fuel, which significantly reduces wear and frictional resistance compared to previous iterations of the technology.
