The petrol engine is nearly 150 years old. It’s been the subject of continuous development since, and much of what Porsche has announced in its new six-stroke is built on innovation from the past. Some things never change.
Induction, compression, combustion, exhaust. That’s how four-stroke petrol engines work. They suck in gas, squeeze it, light it with a spark so that it explodes, and then vent the burnt gasses. The power comes. The ‘stroke’ comes from the cylinder going down, up, down, up at each stage. And each stage is an equal length.
The new Porsche six-stroke engine runs induction, compression, combustion, compression, combustion and exhaust. It also has strokes of different lengths.
If you look at the diagram from the Porsche Patent, the most obvious difference is the gear ring, which is where the regular crank would be. It’s the path the concentric gears take, which varies the length of the stroke.
This is known as a hypocycloidal engine. The Henry Ford Museum of American Innovation in Michigan houses a hypocycloidal steam engine, which was patented in 1802 by Matthew Murray in Leeds, England. The engine has separate top-dead and bottom-dead centre positions for the two compression and combustion strokes. The Porsche Patent describes it as “two times three-stroke”.
The key to the unequal strokes is the planetary gear, which travels around an annulus, a connecting ring, using a toothed mechanism because the point at which the piston is attached isn’t central to the planetary gear. So, while the crankshaft rotates conventionally, the centre of the connecting element rotates around it, which means the alternate strokes start and stop in different places.
Given the force of the piston, the engineering must be precise as it adds extra scope for failure and vibration over a conventional four-stroke.
Two plus four equals six
The variable length of the stroke means that the induction system for the new Porsche engine is similar to both a four-stroke and a two-stroke. In a two-stroke, the down stroke is used for both exhaust and induction because the cylinder doubles up as the valve mechanism with the ports on the side of the cylinder wall. As the cylinder goes down, it uncovers the ports in the side and lets the gasses out and in.
The Porsche six-stroke does the same thing with induction. As well as having an exhaust and inlet with valves on the top of the cylinder as in a conventional four-stroke it has an induction port, which becomes uncovered at the bottom of the power stroke.
With the vacuum from the burnt fuel in the cylinder providing assistance, the new mixture from the side port is sucked in. As the cylinder rises for the second compression stroke, the port is blocked off, and the burnt gas is vented through the exhaust valve in the conventional way.
What the six-stroke does not do that a two-stroke does is use the area below the piston to compress the fuel-air mixture and force it into the chamber. What’s going on below the cylinder is so complex in this engine that you’d not want to disrupt the lubrication with a flow of fuel through it.
This engine doesn’t suck
The process of sucking out the burnt fuel is called scavenging and is very important to the efficiency of the engine. One thing that makes the Porsche six-stroke unusual is that it doesn’t scavenge everything after the first combustion stroke. Instead, it mixes the burnt fuel with the new mixture from the side port. The advantage of this is that the unburnt fuel is brought in with the burnt fuel from the first combustion stroke. With a second go-around, the engine can use this for power instead of wasting it.
The exhaust valve in the diagram has an extra set of lobes, which open for longer but not as wide as the inlet valves. It also has a second set of ports at the bottom of the cylinder wall, which allows exhaust gas to escape while a new mixture is added from the inlet valve at the top, again a little like a two-stroke or Mazda’s Skyactiv-X.
The two most significant upsides of the six-stroke are less unburnt fuel and that two in six strokes are power strokes rather than one in four. Both of these things should make the engine more efficient.
One problem with the design is vibration. The hypocycloidal path is generated by the planetary gear having an eccentric motion, which is not ideal. Piston engines naturally have some vibration element as they have vertical motion changing direction, which can be balanced by having two cylinders in harmony. But with this engine, the unequal strokes won’t balance out against each other. Combining vibration with the complexity of the toothed gears will require a good maintenance regime, particularly with regard to lubrication.
An interesting feature of the Porsche announcement is that it describes the optimal configuration using the cylinders in multiples of three. While the maverick thought might lead to a nine-cylinder engine, the force on the crank, the length needed, and the vibration level will likely make that impractical.
A 12-cylinder is similarly troubled. Instead, a horizontally opposed six-cylinder is most likely. Indeed, some things never change.
