I can never resist a good oddball engine, and this one’s pedigree is impressive. See if you can bend your mind around it.
Most of you reading this probably have at least a basic understanding of how an internal combustion engine works. Today’s four-stroke suck-squish-bang-blow concept was first demonstrated successfully by Nicolaus Otto and Eugen Langen in 1876. It involves a piston moving down to draw charge air (and fuel) into the cylinder, followed by an up stroke that compresses this mixture. Then ignition creates an explosion that drives the piston down, imparting work that turns the crankshaft, before a final upstroke forces the exhaust out.
Just a few years later, Scotsman Dugald Clark reckoned he might double the power and improve smoothness by halving the number of strokes, and by 1894 the modern two-stroke engine began to take shape. It used the end of the down stroke and the beginning of the up stroke for a combined (if somewhat hurried) simultaneous exhaust and intake process via scavenging ports in the bottom of the cylinder walls, followed by compression and the ignition/power stroke. “Same work with half the cylinders!” was the selling point. Auto Union’s brand DKW even badged its cars “3=6.”
Nasty emissions, however, were the downside to that smooth, dense power. Early engines that routed intake air through the crankcase required the lubricating oil be mixed with the fuel, resulting in the blue smoke that Lawn Boy mowers and outboard boat motors still made until the early 2000s.
Best of Both Worlds?
On September 12, 2024, Porsche and the Technical University of Cluj-Napoca in Romania jointly received patent No. US 2024/0301817 Al, “Method for A Combustion Machine with Two Times Three Strokes.” It basically looks like a conventional piston engine, except it uses a complicated geared crankshaft arrangement that allows each piston to reach two top- and bottom-dead-center positions over the course of three crankshaft revolutions, which in turn provide for two back-to-back power strokes bracketed by more conventional intake-compression and exhaust strokes. Think of it as a four-stroke with an extra two-stroke revolution inserted in the middle.
The lowest bottom-dead-center position exposes scavenging ports that enable the insertion of that bonus power pulse. These ports allow fresh, probably pressurized intake air to force the exhaust out, through scavenging ports on the opposite side of the cylinder and/or through the conventional overhead exhaust valve(s). If employed, the cam actuating the overhead exhaust valve would need to have a very short duration, closing in time to allow the fresh mixture to begin compressing as soon as the piston covers the scavenging ports. And that’s how this “six-stroke” engine achieves power pulses in two consecutive revolutions, followed by a conventional exhaust/intake revolution.
How Might a Six-Stroke Sound?
The short answer is smoother than a four-stroke, with a character that will ultimately depend on the total cylinder count and layout. A single cylinder might produce sort of an inverse waltz 3/4 cadence: one, TWO, THREE, one, TWO, THREE … On this front, the patent notes, “The number of cylinders is preferably a multiple of three,” and that “it can be an inline engine, V engine, W engine, or horizontally opposed engine.” Beyond the power-pulse cadence, with some air entering and leaving the cylinder via conventional poppet valves and some through scavenging ports, the intake and exhaust note character may sound peculiar, too.
“Eccentric” Crankshaft Design
That complex geared crankshaft is the key to allowing the piston to reach a higher top-dead-center position twice and a lower bottom-dead-center just once in three revolutions. Instead of bolting the connecting rods directly to the crankshaft, they bolt to a “big end” that’s sandwiched between two planet wheels, each enmeshed with a ring gear, or annulus. The crankshaft passes through the center of the planet wheel, but the big-end connecting-rod mount is offset from the crankshaft throw’s centerline. This forces the connecting rod’s lower end onto a hypocycloidal path (picture a flower pattern your old Spirograph might have made) that alters the piston’s top-/bottom-dead-center positions. The patent suggests the planet wheel should be three-fifths the diameter of the annulus and that the speed ratio of the planet wheel relative to the crankshaft should be two to three.
But Wait, There’s More!
In case that crankshaft setup sounded too simple, the patent also covers a variable-compression option that rotates the annulus in such a way as to alter the crank/big-end eccentricity. This relocates the extremes of the piston’s top- and bottom-dead-center locations, which alters swept volume, compression, and scavenge-port area. If you’re worrying about a moving annulus handling combustion forces, don’t. The crankshaft still mounts in traditional main bearings that aren’t shown in the patent drawings.
Potential Benefits
Getting 33 percent more power pulses from the same number of revolutions could pay some specific output dividends, but that’s by no means a foregone conclusion. The operation of an inline-six six-stroke (6=9) engine would likely approach V-12 levels of smoothness. And perhaps that smoothness in a single or three-cylinder arrangement could make this a compelling range-extender engine.
Potential Pitfalls
- Friction: The crankshaft-rotating assembly looks like a huge challenge to lubricate, as oil flowing through the crank would have to lubricate the planet wheel, and through that, the connecting-rod bearing. And that’s before we start to compute the friction between the two planets and two annuluses per cylinder.
- Speed Limit: The rotational inertia of the crank assembly and the fact that the exhaust valves must open more often (making it run hotter, potentially requiring heavier sodium-filled valves) suggest this design might not be able to rev fast enough to realize its full power-density promise.
- Emissions: Even without mixing lubricating oil into the fuel, the piston rings will necessarily drag trace amounts of it over those scavenging ports, introducing more hydrocarbon emissions. Plus, any time intake air is used to push out exhaust air, there’s no way of ensuring no excess air remains in the combustion charge, so a lean-NOx catalyst would be required.
- Packaging: Sandwiching a connecting rod between two planet gears while leaving room for a crankshaft counterweight and a main bearing seems like a recipe for a long crankshaft.
- Assembly: We can envision no easy way to cast and machine this crankshaft and then bolt the planet gears and connecting rods to it, forcing an expensive assembled crank.
What Are the Six-Stroke’s Prospects?
I’d love to share Porsche’s own claims for this engine’s output, efficiency, and future development/production prospects. But sadly, representatives of both Porsche and TUCN declined to comment further. So I consulted with my former Chrysler-engineering big-boss and MotorTrend Car of the Year guest judge Chris Theodore to formulate the above ruminations and concerns. We concur: The headwinds facing any “oddball engine” concept are stiff, but at least this one has a big name behind it and requires vastly less wholesale retooling than, say, the Astron Aerospace Rotary or the Tomahawk TTX Turbine. Color us both fascinated—just don’t hit us up for venture capital investment.
Photos By US Patent Office
