Six-stroke engine from Mazda

[Micael]

I appreciate Mazda’s continued unorthodox approach to engines.

Mazda Patents Bizarre Six-Stroke Gas Engine That Separates Hydrogen From Carbon
Evan WilliamsAug 25, 2025

2025 Mazda EZ-60 (11)-1 Mazda
Mazda has never been afraid of some truly odd engine designs. It keeps trying to make the Wankel work. It has a gas engine with compression ignition. And the company has made 14:1 compression gas engines that somehow run on regular unleaded. Then there are the more normal oddballs, like the Miller cycle Millenia or the 1.8-liter V6 in the MX-3.

This time, though, Mazda has outdone itself. The company just obtained a patent for a six stroke engine. What does it do with the extra strokes, you're wondering? It separates the hydrogen and carbon in gasoline so it can burn just the hydrogen, thus taking the carbon out of the system. Easy, right? No, not easy. And it gets even wilder from there.

Gas Goes In, Carbon Doesn't Come Out

2010 Mazda Mazdaspeed 3 Mazda
The patent is called "fuel reforming system for vehicle," which does almost nothing to describe the magic going on here. Mazda's engineers describe it as a way to recover carbon, improve thermal efficiency, and give you a car that runs carbon-neutral.

It works like this: The engine makes hydrogen from gasoline using its own heat and a catalyst. It then burns that hydrogen and stores the carbon to be removed later. The result is that you can burn gasoline with zero CO2, at least most of the time.

The engine only stores small amounts of hydrogen, so it doesn't need complex tanks. If there isn't enough hydrogen ready for use, it can just run the old-fashioned way on gasoline until hydrogen is ready. It could save the internal combustion engine from eventual extinction, but there's just one problem. This is pretty much the most complex solution from Mazda we've ever seen. And that's saying something.

Old And Busted: Four Stroke. New Hotness: Six StrokeMazda H2 Engine Patent (2)

A normal engine works like this: In cycle one (aka the intake stroke), air is pulled in on a piston down stroke, and fuel is added. Cycle two (compression stroke) sees the piston move up, compressing the mix so that the spark plug can ignite it and push it down (the power stroke) through cycle three. Cycle four, the exhaust stroke, pushes out the exhaust gases as the piston moves up. And the process repeats from there.

This engine has those four, with some changes, and two extras. On the first cycle, air is pulled like normal. It can also open an exhaust valve to pull exhaust air in, for simpler EGR.

The next two cycles are standard – compression followed by power. Cycle four is called the re-compression stroke. In this cycle, the exhaust air is pushed out through a different valve. This valve routes the exhaust air through what's called a decomposer.

Mazda H2 Engine Patent (3) Mazda/USPTO
Mazda's decomposer is like a catalytic converter without expensive metals. Right in front of it is a fuel injector that squirts gasoline into the hot exhaust air.

The hot exhaust and fuel mixture enters the reformer and the carbon – pure carbon, it's not CO2 yet – sticks to the catalyst. Gasoline is a mix of hydrocarbons like octane (C8H18). The reformer separates and stores both.

A carbon recovery unit holds the carbon that comes out of the reaction. It would be emptied or removed when you go in for service. The carbon that comes out can be used in steel or for pigments (and other uses) or simply stored.

Cycle five, the re-expansion cycle, pulls the remaining air back into the cylinder. Then it's pushed out through the exhaust valve in cycle six, which is cycle four in a normal engine.

Mazda's patent includes some variations. Two of them use slightly different methods in the catalyst, and one uses a four-stroke cycle where the fourth is a "scavenging cycle" that has the intake, exhaust, and reformer valves all open at the same time. Think of it like an even more complex two-stroke engine.

Extremely Complex And Possibly Much Less Efficient

Turbocharged Mazda 13B Rotary Engine Mazda
The engine would be much more complex than a current internal combustion engine. Adding an entire extra valve path means more parts and a complicated cylinder head design. The extra valve and the exhaust valve would need to be opened and closed by an actuator, not a camshaft.

Every cylinder could share a single reformer, but then the engineers need to make sure one cylinder doesn't pull all the air. The reformer and its passages would also need to be able to handle the heat of combustion. Then there are the three extra injectors and a small hydrogen storage tank.

Mazda H2 Engine Patent (4) Mazda/USPTO
It would also need a serious amount of carbon storage. Gasoline is mostly carbon by weight, with a gallon of gas having around 5.5 pounds of it. So for every 15-gallon tank of gas on a Mazda CX-5, you would get 82.3 pounds of carbon. That's about five one-gallon jugs of carbon. Dealer service departments will love it. However, its also about 300 pounds of CO2 that doesn't come out of the tailpipe, which is the real reason for all of this.

What about efficiency? It's hard to find a fuel economy figure for hydrogen combustion, because it's rare. One of the most notable was the 2007 BMW Hydrogen 7. The car's 6.0-liter V12 ran on gasoline or liquid hydrogen, returning 125 miles on 18 pounds of hydrogen. Crunch some numbers and you get 6.9 miles per pound. With around 0.8 pounds of hydrogen per gallon of gas, that's 8.6 miles per gallon. A gas 760Li returns around 15 mpg.

The idea is amazing, and certainly interesting. But the implementation is a nightmare, and its actual efficiency is questionable. In other words, it's perfect for Mazda, so perhaps we'll see some sort of production version in the years to come.

https://carbuzz.com/mazda-6-stroke-hydr ... on-engine/

[Craiglxviii]

Now they just need to build Wankel design around it, and shoehorn it into a coupe that can rev to 20,000rpm…

[Nik_SpeakerToCats]

FWIW, perhaps more applicable to eg semi-portable and 'skid' generators etc that, for *reasons* do not run on diesel...

Regarding the Wankel, wasn't there a recent sorta-variant that was 'Inside Out' and lacked the tip-seal / lube issues of a 'trad' Wankel ??

[rtoldman]

the carbon should be converted to diamonds

sooo gas goes in one end and micro diamonds and water sprinkle out the tailpipe. More commonly known as Unicorn farts

[kdahm]

Wonder what the actual efficiency number are for the onboard hydrogen generating system compared to loading hydrogen to the vehicle from an offsite petroleum-to-hydrogen plant. Bet it's significantly less, even with catalyst chemistry.

Or, you know, generate the hydrogen by splitting water with power provided by solar panels....