Not long ago, there was a lot of talk from carmakers about switching to all EV fleets, and finally letting go of the jumping, oily pistons that got them where they are today. But now a lot of carmakers are walking that pledge back, and realizing that they may be dealing with combustion engines for a bit longer after all. Gasoline is so energy-dense, it’s hard to quit it, after all. In light of this, it’s interesting to see how combustion engine development is being pushed forward, especially when it comes to really strange stuff like what’s seen in this Porsche patent application for a six-stroke engine.
Yes, six-strokes! Or, as the patent application confusingly titles it, “METHOD FOR A COMBUSTION MACHINE WITH TWO TIMES THREE STROKES.” Why the hell did they call it that way instead of saying “six strokes?” Is there some technical reason I’m missing?
I suppose a four-stroke engine takes two rotations of the crankshaft, and a two-stroke takes one, so this six-stroke one takes three rotations. Maybe it’s because of that? I’m really not sure.
The engine described in the patent application is pretty interesting. Here’s how the patent abstract describes what’s going on:
A method for a combustion engine has a working cycle of three revolutions of the crankshaft. The method includes: feeding a fuel mixture into a combustion chamber of a cylinder while moving a piston from a second top dead-center to a first bottom dead- center; compressing an air-fuel mixture ni the combustion chamber while moving the piston from the first bottom dead-center to a first top dead-center; burning the air-fuel mixture while moving the piston from the first top dead-center to a second bottom dead- center; compressing a gas mixture ni the combustion chamber while moving the piston from the second bottom dead-center to the first top dead-center; burning the gas mixture while moving the piston from the first top dead-center to the first bottom dead-center; and expelling the gas mixture from the combustion chamber while moving the piston from the first bottom dead-center to the second top dead-center.
That’s uh, kinda tricky to follow, so let’s just simplify what they’re talking about into a description of strokes, because that’s at the root of all of this. First, let’s refresh what a four-cycle engine does:
Okay, now what this patent application proposes is this:
So, what they’re doing is adding on an extra compression stroke after the power/combustion stroke, and then another power/combustion stroke, then exhaust to clear everything out.
Essentially, it seems like it’s compressing and combusting the same charge of fuel and air twice. Is there that much unburnt fuel after combustion in a modern engine, enough to get some usable benefit from burning everything twice? Maybe?
The extra compression and power stroke have a different top- and bottom-dead-center than the first compression and power strokes because the piston is on a crankshaft that rotates within a planetary gearset, so it’s also a variable-compression engine.
You can see the crankshaft and piston planetary gear design in the patent drawings:
That’s a lot of manufacturing complexity to get that extra power stroke.
There have been six-stroke engines before, starting with Samuel Griffin’s 1883 design, which you likely know as the Kilmarnock engine you use to run your private power plant:
There were also some six-cycle engines that included a water-injection cycle, but this Porsche concept feels very different from all of those. Frankly, I’m not at all sure how much benefit an engine like this could have over a conventional four-cycle; it must get some more power and efficiency, sure, but enough to justify all the extra complexity and potential points of failure?
But I’m not an engineer! And who knows why Porsche patented this, it could just be to prevent anyone else from getting any fancy six-stroke ideas. You know how patents work.
I guess we’ll just have to see what happens, like we always do, eventually.
Regarding the Kilmarnock: I love that the “Smellie & Co.” organization is touting their engines’ “absence of noxious vapors”.
Going to be that guy, but since it’s in my line of work:
This is not a patent just yet. It’s a publication of the application Porsche filed. That said, it has been allowed and the issue fee paid so it’s just a matter of the actual patent issuing.
Sometimes, large companies will file patent applications on things their teams have worked on that may be of marginal value because there are rewards for the engineers for getting a patent, and/or because the engineers like having their names on a patent. Maybe that’s the case here?
This is just the number-of-blades-on-a-razor all over again.
Porsche have only gone to six because Gillette went to five.
Ferrari will now go up to seven, forcing Lamborghini up to eight.
I imagine Honda are either working on fractions of a stroke or will blindly insist that four is best.
It wasn’t hard for me to find another article explaining why they call it 2×3 stroke, which also expanded on the possible benefits of such a design
https://interestingengineering.com/innovation/porsche-6-stroke-engine-innovation
https://www.roadandtrack.com/news/a62258759/porsche-six-stroke-combustion-engine-patent-documents/
I think this is just one of those corporate patent applications that have little to no intention of being utilized in an actual product. Seems like a much better idea for VC than Nissan’s Mickey Mouse implementation and more fully utilized with maybe some thinking about Atkinson or even a multi-expansion steam engine thrown in as inspiration. Potential advantages in economy and emissions are great, but what about noise from those spur gears, friction, rpm capability, torque, balance, and anticipated long term durability? Obvious questions aside, I love this kind of stuff and I hope they build a test model and release a video of the running.
So, a hit-and-miss engine?
I had six strokes trying to figure this out.
Comment #2:
The ultimate EGR System?
Pro: maybe it would be very fuel-efficient and wouldn’t require a catalytic converter.
Con: holy adding many parts and 1000 gear teeth to the engine, batman!
Better this than catalyzing unburnt fuel. Even a few percent of thermal efficiency gained would be huge!
I drove my three-cylinder, two-stroke SAAB 96 to work today and 3×2=6, so there we have it: prior art. It’s right there in the math.
I think how it goes is: Intake Upcharge Compression Upcharge Ignition Exhaust
Jason, you need to remember; to a German engineer, extra complexity and points of failure are a defining feature, not a problem to be overcome!
And a way to charge 50% more!