Mazda hits a hydrogen powered six

THE SECOND six-stroke petrol engine may be just around the corner as Mazda follows Porsche and patents a novel reciprocating petrol power plant that is claimed to produce zero carbon dioxide tailpipe emissions… most of the time.

It is totally different to the Porsche six-stroke that adds extra compression and power strokes to the traditional four-stroke. Instead, Mazda uses heat and a catalyst within the engine to convert petroleum into hydrogen for fuel.

The Hiroshima-based Japanese manufacturer has a reputation for unusual engine designs and the six-stroke hydrogen burner recently patented is a classic example.

Other Mazda engine oddities include a supercharged two-stroke engine for EV range extension that can switch between spark ignition and compression ignition for maximum power or efficiency.

It also designed a Skyactiv-X engine that uses a stratified charge combustion technique and compression ignition, similar to a diesel engine for improved efficiency and of course the legendary rotary.

The hydrogen six-stroke may find its way into production as buyer resistance to EVs in many countries is driven by their limitations: charging times, range anxiety, towing limits and the lack of infrastructure.

Mazda’s six-stroke could provide an environmentally friendly zero tailpipe emission answer to the above.

A report in hotcars.com says the new engine design “utilises a much more complicated combustion pattern with six strokes instead of four, but those two additional strokes make a huge difference in the resulting carbon dioxide emissions coming out of the tailpipe”.

“This new engine could be the solution the industry has been waiting for; a way to continually utilize fossil fuels with a cleaner-burning engine, at least in terms of tailpipe emissions,” it added.

But the report clarified this saying, “The new Mazda six-stroke engine is nothing more than a filed patent with the US Trademark and Patent Office… but the idea is intriguing”.

“The patent is called ‘fuel reforming system for vehicle,’ which doesn’t do the engine justice, especially considering what it could mean to the industry,” the article continued.

“In a nutshell, this engine is built to make hydrogen from gasoline, using heat and a catalyst built within the engine’s systems.

“It is a way to recover carbon, improve thermal efficiency, and result in a vehicle that runs carbon neutral.”

The design can be correctly described as both a petrol engine and hydrogen engine as it burns both using the petrol as fuel and separating the hydrogen from the carbon then using the hydrogen for the combustion process.

This results in CO2-free exhaust emissions… most of the time.

hotcars.com says only a small amount of hydrogen is stored, which means the engine doesn’t need the same complex tanks that are used in a hydrogen-only vehicle.

“When there isn’t enough hydrogen ready for the engine, the new Mazda six-stroke engine can run on petrol until there’s enough hydrogen to do the job,” it clarified.

“During this process, the emissions would include carbon dioxide.”

Delving deeper into the engine’s technology, we find the six-stroke engine has the same four strokes as a regular engine but adds two more.

During the first cycle, air is pulled into the chamber, which is a normal first stroke. The next two strokes are also traditional in nature: compression and power, but things change when you get to the fourth stroke.

During the fourth stroke (or cycle), which is called the re-compression stroke, the exhaust air is pushed through a different valve, which sends the air through a decomposer.

The decomposer works like a catalytic converter but doesn’t have expensive metals in it.

In front of the decomposer is a fuel injector that introduces petrol into the hot exhaust air.

The mixture of exhaust air and fuel enters the reformer, and the carbon sticks to the catalyst. This is the separation process, sending hydrogen to its small storage tank and the carbon to the carbon recovery unit.

Once the separation is completed, the fifth stroke occurs, which is called re-expansion. During this stroke, the remaining air re-enters the cylinder, which is then pushed out of the exhaust valve in stroke number six which is the normal fourth stroke for most engines.

Any excess stored carbon in the storage tank will be emptied periodically for re-use at routine servicing. 

hotcars.com says the six-stroke engine is much more complex than current four-stroke engines, has more moving parts, more technology, and a much more complex method of burning fuel all of which increases maintenance costs and leads to the potential for more costly repairs.

By contrast, EVs have fewer moving parts than petrol engines, resulting in lower maintenance costs, “tempered by their above-mentioned limitations which spurred Mazda to develop six-stroke technology,” the report concluded.