Automotive

Porsche working on variable-compression engine

One of the holy grails ICE development is the creation of a viable variable-compression-ratio system (Image: Shutterstock)
One of the holy grails ICE development is the creation of a viable variable-compression-ratio system (Image: Shutterstock)
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Porsche and Hilite aren’t the first to attempt to make a variable compression engine, but the two firms seem to have struck upon a relatively simple, compact and workable method of doing it
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Porsche and Hilite aren’t the first to attempt to make a variable compression engine, but the two firms seem to have struck upon a relatively simple, compact and workable method of doing it
One of the holy grails ICE development is the creation of a viable variable-compression-ratio system (Image: Shutterstock)
2/2
One of the holy grails ICE development is the creation of a viable variable-compression-ratio system (Image: Shutterstock)

While the fashion in high-tech automotive developments might lean towards hybrids and electric vehicles at the moment, there’s still plenty of scope to improve the good old internal combustion engine and one of the holy grails of such development is the creation of a viable variable-compression-ratio system.

Now Porsche is working on just such an engine, revealed in the form of a newly-published patent, which will be able to alter its compression ratio. It’s partner in the project is engineering firm Hilite International, a leading maker of engine components including variable valve timing cam phasers.

Why is a variable compression system so desirable? Because, particularly when allied to a turbocharged engine, it has the potential to maximize economy and efficiency while simultaneously improving outright performance.

While turbos are great when it comes to allowing relatively small-capacity engines to produce giant-killing performance, they do require a compromise in terms of compression ratio. To accommodate the increased volume of intake charge that the turbo supplies at full boost, a turbo engine needs a relatively low compression ratio, which in turn means they can be lethargic when boost levels are low. Increasing the compression ratio towards that of a normally-aspirated engine helps that low-speed performance, but means peak boost needs to be restricted because forcing more air and fuel into the cylinder means the effective compression ratio is increased – if it gets too high, the result is detonation, which is usually just as bad for the engine as the name suggests.

By varying the compression ratio – in this case by putting the small-end bearing into an eccentric adjuster, automatically tilted by oil-pressure-activated rods either side of the con-rod – the idea is to be able to use relatively high compression ratios with low levels of boost, effectively making the engine behave like a small, normally-aspirated design. When needed, swiveling the eccentric adjuster moves the piston down a fraction reduces the compression ratio and allows the engine to cope with far greater levels of boost, resulting in massively increased power.

Porsche and Hilite aren’t the first to attempt to make a variable compression engine, but the two firms seem to have struck upon a relatively simple, compact and workable method of doing it
Porsche and Hilite aren’t the first to attempt to make a variable compression engine, but the two firms seem to have struck upon a relatively simple, compact and workable method of doing it

On the Porsche system seen here, a solenoid directs oil to one of two rods, rocking the eccentric small end bearing holder into either a "high compression" or "low compression" position. The technology is very similar to that used in oil pressure operated variable valve timing systems.

Porsche and Hilite aren’t the first to attempt to make a variable compression engine, but the two firms seem to have struck upon a relatively simple, compact and workable method of doing it. A huge question mark still hangs over whether the resulting engine will be cheap, effective and reliable enough to reach production, but it’s clearly a step in the right direction.

Finally, it’s worth noting that much of Porsche’s work is in the form of engineering consultancy, so the fact the firm’s name is on the design’s patent doesn’t mean the engine is being developed for a new Porsche-branded car; it could just as easily appear in a machine from another firm.

5 comments
Nik
Why not just adjust the compression volume with a reversed piston, like oil adjusted cam followers?
Expanded Viewpoint
This makes about as much sense as putting screen doors and bay windows on a submarine. Have these blokes done a calculation on things such as the result of increasing the reciprocating mass by such a large amount? Or are they planning on making the con rods out of Titanium? The counter balancing on the crankshaft will be very large indeed there, thus increasing the weight of the engine, and negating some of the level of power increase. How do they plan on powering the electric solenoids on each rod reliably at high RPM? How much extra power will the alternator have to put out to run those solenoids? What will the effects of the solenoids moving up and down 12K times a minute (2 X 6K RPM) be? How much juice will they need to overcome the forces of inertia at those kinds of speeds and stay open or closed? The longevity factor of the system and cost of repairing it when it does break alone should scare the heck out of anybody working on this project. And the increased amount of money needed to build that engine will easily be twice to 3 times as much. Probably much more than that though. Randy
Charles Ostman
I can appreciate the goal of the concept, but I'm a little bit skeptical of the implementation. If anything, this looks like an expensive repair waiting to happen.
The Skud
While noting the comments so far, I would point out that they would have spent a lot of their (or someone's) R&D money on this, so don't think they are not serious! I agree, the rapid reciprocating motion may affect the solenoids, they seem to cycle just as fast in a fuel-injection system, so that seems to be sorted. Magnetic or other switching seems to behave in a crankshaft sensing situation or ignition system, so that fixes the on-off worries as well. If they can pull it all off, it will help keep the I/C engine ahead of E/C for a while longer, and they will give the battery/capacitor storage scientists a breathing space for more gains as well!
warren52nz
I don't know why they don't just go with water injection. I played with W.I. for over a decade and it provides almost unbelievable results. Example: I had an RX7 FD3S "Batmobile" which runs 10 psi boost on 96 octane fuel as stock but put in a water injection system and ran it at 15 psi boost on 91 octane for years giving it a power kick of around 100 HP and no detonation. When it ran out of water the boost immediately dropped to 8 psi to prevent detonation. Simply add water and you're back in action. By the way, switching from 96 octane to 91 octane and doing nothing else increased the torque by 4% (measured). Low octane burns faster so it's like advancing your timing.
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