Porsche Patents Six-Stroke Engine
Just when you thought Porsche was phasing out internal combustion the Stuttgart sports car maker has decided to re-invent the whole process.
Porsche has now developed a six-stroke combustion system which adds an additional compression and power stroke to increase efficiency and horsepower. If the tried and tested four-stroke Otto cycle can be best remembered by the order ‘suck, squeeze, bang, blow’ then Porsche’s method is ‘suck, squeeze, bang, squeeze, bang, blow’.
A patent application described as a “method for combustion machine with two times three strokes” reveals a crankshaft that spins inside a ring on planetary gears. As the shaft rotates the pistons have two different top-dead-center and bottom-dead-center positions. Variable compression is also created as a result. Adding the extra strokes should deliver a more complete fuel burn for better power, efficiency and reduced emissions.
The idea of a six-stroke engine has been around since Samuel Griffin devised a system in 1883, while more recently Bruce Crower came up with a six-stroke diesel that added water injection as a fifth stroke and a steam flash as the sixth.
So far it would appear that the benefits of six-stroke combustion have been outweighed by the complexity and cost of the designs, but perhaps Porsche could use it to cling onto combustion a while longer in conjunction with ramping up production of carbon neutral e-fuels.
Not clear to me how this works. Does it compress the exhaust gas and ignite again??
Yes
… but with less compression, for some reason.
But the article notes that “the benefits of six-stroke combustion have been outweighed by the complexity”. Achieving that reduced compression and stroke on the extra two cycles requires planetary gearing for the crankshaft which seems to make the whole thing more of a “science fair project” than a product for the real world.
This is pretty interesting. Plus it has a couple turbos on the picture above. It looks interesting if it sees production.
This has me wondering if a 2 stage air compressor pump would advance anything if you added a couple spark plugs and a carburetor?
Sort of like a two or three stage steam engine — steam is pushed into the first small high pressure cylinder, then exhausted into the next much larger cylinder. Apparently after the third time (on a large marine engine) the steam doesn’t have enough pressure to be worth doing it again. On a compressor it’s the opposite — the large cylinder exhausts into a smaller one, boosting the pressure. Not sure which way trying to use the exhaust for the second stage would work. On a modern engine there shouldn’t be enough fuel in the exhaust to actually fire again, and there wouldn’t be enough oxygen. At the least you’d have to inject some air — like Mazda did to make the rotary pass emissions in the early years (injected air into a cast iron “thermal reactor” where any unburnt fuel burned). Mazda didn’t try to recover any power from that unburnt fuel. There shouldn’t be enough to make it worth the effort. Injecting a small amount of fuel and air in the exhaust to burn again probably isn’t worth the added complexity either (compared to the “six stroke” idea). You’d need something like a four cylinder block with eight cylinders… four “normal” size and four small ones, all on the same crank. I can think of a couple ways to configure that to save space and reduce complexity, but that’s a lot of added moving parts. More efficient to capture wasted exhaust energy with a turbo…
so basically egr but with extra steps
This is not a new design or idea, except for the 2 stoke port. The Atkinson cycle engine pre-dates the Porsche design by 140+ years but achieves the same varying stroke length for better fuel efficiency, the design is used to this day in the Prius and other hybrid vehicles.