Porsche was issued a patent for its six-stroke combustion engine on October 22, 2024. The company describes the new engine as having a “working cycle” of three complete crankshaft revolutions with two distinct top dead center (TDC) and two bottom dead center (BDC) piston heights. During the engine’s six strokes, the piston reaches TDC three times, the higher TDC twice, and peaking at the lowest TDC (OT”) once. Conversely, the lowest BDC occurs one time (UT”) between the higher TDC instances, with the two higher BDC instances (UT’) flanking the lower TDC instance. You can see the visualization below.
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In case you didn’t know, each movement between TDC and BDC is referred to as an engine stroke. However, it sometimes gets confusing when people use different terms like 2-cycle and 4-cycle when they are really referring to strokes.
While the newly-patented Porsche engine requires three revolutions of the crankshaft to complete its six strokes, I paid enough attention during small-engine repair class in 1980 to know that a four-stroke engine only needs two crankshaft revolutions to complete its cycle. It’s easier to understand if you can visualize what’s going on inside the engine, but I’ve tried to simplify Porsche’s complicated six-stroke engine below.
Is a six-stroke engine better than a four-stroke?
A six-stroke engine has the potential to create more power than a four-stroke since it can produce two power-strokes with three crankshaft revolutions. Compared to the four-stroke engine that produces one power stroke per two revolutions at 3,000 rpm, the four-stroke has 1,500 power strokes while the six-stroke would have 2,000.
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A four-stroke engine’s various strokes, in order, look like: Intake, compression, power, and exhaust. Porsche’s patented six-stroke engine design adds compression and a power stroke in the middle of the cycle: Intake, Compression(a), Power(a), Compression(b), Power(b), exhaust. Between Power(a) and Compression(b), with the piston at UT”, ports in the lower cylinder wall (11) become exposed during the piston’s lower BDC instance through which another air/fuel charge mixes with the spent gases from the previous power stroke, a process Porsche refers to as “Scavenging” in the patent filing.
Porsche’s six-stroke engine design isn’t without its disadvantages due to its additional moving parts. Recall the opening paragraph above with the discussion of varying TDC and BDC instances. That’s all accomplished by the crankshaft’s eccentric connection to a geared planet wheel rotating within a larger ring gear.
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It’s possible that the six-stroke could drive the four-stroke out of favor in much the same way the two-stroke engine became a thing of the past. After all, the two-stroke has more power potential in that it creates a power stroke during every crankshaft revolution, and it does so with fewer moving engine parts than the four-stroke.
