Mitsubishi is tackling incomplete fuel combustion in direct-injected engines with a newly patented piston crown design aimed at improving combustion, increasing power with less fuel and reducing soot and emissions. The concept is designed for engines such as the one in the 2026 Outlander and appears to rely primarily on a redesigned piston crown.
Direct Injection Poses Some Problems
In a direct-injected internal combustion engine, the fuel injector is positioned inside the combustion chamber, near the intake valve and spark plug. Unlike port-injected or throttle-body-injected engines, where fuel and air mix before entering the cylinder, direct injection requires gasoline to mix with incoming fresh air only moments before compression and combustion.
This process involves tumble flow, in which air enters through the intake valve and tumbles inside the cylinder. A specialized piston design helps swirl the air around and mix it with the fine gasoline mist delivered by the direct fuel injector.
Mitsubishi identified an issue in which some of the fuel can be pushed away from the spark plug. When the spark plug fires, that fuel may remain unburned, contributing to poorer fuel economy and increased emissions as the unburned fuel becomes fine particulates, or soot.
The automaker’s patent proposes a relatively simple solution: a redesigned piston crown featuring an inclined surface angled toward the fuel injector. The design positions gasoline droplets closer to the spark plug when combustion occurs.
How the New Piston Crown Works
Viewed from directly above the cylinder, the intake valve sits in the “northwest” corner while the exhaust valve is positioned “northeast.” The fuel injector is located due west, with the spark plug positioned above everything in the center of the combustion chamber.
The piston occupies nearly the entire circumference of the cylinder and includes depressions on its northwest and northeast sides to provide clearance for valve movement.
A conventional direct-injected piston uses a trough in the crown to capture and concentrate the fuel spray. Mitsubishi’s patented design replaces that trough with an inclined surface that runs from west to east across the piston crown.
The slope matches the angle at which the fuel injector is installed. Two raised edges positioned perpendicular to the slope help guide the fuel down the inclined surface.
Stabilizing Tumble Flow
Mitsubishi says the new piston design can better stabilize the tumble flow effect, helping produce more complete combustion when the spark plug fires.
The company also incorporates a small perforated covering for the spark plug. A pre-ignitor sends a compressed flame front through the perforations and into the cylinder.
Pre-chamber ignition has also been used on Maserati’s Nettuno V6 engine and the facelifted 2026 Jeep Grand Cherokee’s new turbocharged 2.0-liter engine.
Mitsubishi’s patented piston crown design therefore focuses on directing the fuel mixture closer to the ignition source while stabilizing airflow, with the stated goals of more complete combustion, improved fuel use, greater power and reduced soot and emissions.
