Mazda has once again turned to unconventional thinking in its pursuit of cleaner combustion. The Japanese automaker, which builds on a solid reputation of daring powertrain experiments, has filed a patent for a six-stroke internal combustion engine that doesn’t emit any local CO2. So no, it is not running on e-fuels. Ready for something completely different?
Mazda’s revolutionary engine basically runs on hydrogen. But in a very peculiar way. Inside the cylinders, the injected gasoline is reformulated, and the trapped hydrogen, on one hand, is extracted for combustion while the carbon, on the other hand, is captured in solid form and afterwards deposited.
Mazda’s contrarian history
Mazda has often zigged where others zag. It spent decades trying to perfect the rotary engine, despite persistent efficiency and reliability drawbacks. More recently, it introduced a spark-controlled compression ignition engine and production gasoline engines with compression ratios as high as 14:1, capable of running on regular unleaded.
This new six-stroke concept may be Mazda’s boldest yet. Where Porsche’s six-stroke patent, published last year, sought to extract more energy by adding an extra compression and expansion cycle, Mazda’s system aims to tackle emissions directly by splitting that gasoline into hydrogen and carbon during the combustion process itself.
How the system works
The engine retains the familiar four-stroke cycle—intake, compression, combustion, and exhaust—but adds two more stages. On the fourth stroke, instead of venting exhaust gases straight into the atmosphere, the engine redirects them through a special valve into a decomposer unit.
There, an injector sprays additional gasoline into the hot exhaust stream. Heat and a catalyst split the hydrocarbon molecules, sending hydrogen into a small onboard storage chamber and trapping carbon in a recovery unit.
The fifth stroke, called re-expansion, draws gases back into the cylinder for further work, while the sixth and final stroke vents them out. In principle, this system enables the engine to operate primarily on hydrogen generated from its own fuel, producing power without emitting carbon dioxide.
But what happens to the captured carbon? It is stored for later removal during servicing. Mazda’s filings also suggest further applications for the carbon, such as use in steelmaking, pigments, or other industrial solutions.
Benefits and complications
On paper, the idea is compelling. Vehicles could continue to rely on existing gasoline infrastructure, thereby avoiding the high costs and logistical challenges associated with not only EV charging networks but also hydrogen refueling. The car would emit little or no CO2 from its tailpipe, without requiring drivers to change their habits fundamentally.
But the challenges are daunting. Adding a decomposer, extra injectors, new valve pathways, and electronic actuators makes the design far more complex than a conventional engine.
Each liter of gasoline contains about 0.66 kilograms of carbon. For a 57-liter tank, that means around 37 kilograms of solid carbon to store between fill-ups. The logistics around emptying and recycling that carbon can’t be underestimated, and don’t align well with the aforementioned servicing intervals.
Fuel economy is another unknown. Past hydrogen combustion projects have yielded disappointing results, as the energy process is very inefficient. Mazda’s design might suffer similar drawbacks.
Less is more
For Mazda, which has spent decades developing engines most competitors would never consider, the filing with the US Patent and Trademark Office underscores its refusal to give up on internal combustion in the wake of battling CO2 emissions, even as the rest of the industry accelerates toward battery power.
It is also a matter of investment and cost. If the Japanese carmaker could crack the code, it would benefit greatly and refrain from burning cash into the research, development, and marketing of electric vehicles.
But legislators are clear. The European framework for banning the sale of carbon-intensive transport means that not a single puff of CO2 is allowed. And as combustion engines consistently emit some emissions from lubricants, they can’t comply fully. Engineering curiosity or a commercial gamechanger? Either way, the six-stroke combustion engine is a showcase of how less CO2 can lead to more creativity.
