Just read a tidbit in CAR magazine that VW is playing with a variable compression connecting rod for its diesels. It didn’t go into detail about how it works, but the sketch shows a connecting rod whose top (where it connects to the wrist pin) is a rocker connected to the connecting rod body with a pin, and a small piston integrated into the connecting rod moves the rocker up & down, changing the distance between the centerline of the wristpin and the centerline of the crank arm. I’ve never seen any idea like this before, and I’m intrigued. This kind of thing could have applications for improving gas mileage in gas engines too.
Looks like the same thing.
I’m intrigued.
Saab did variable compression way back in 2000.
But Saab did it a different way;
Given that production engines are being released with 11:1 and higher compression ratios, some that can run on 87 octane, I think the complication is unwarranted.
Gasoline diesels are in development right now. Some with turbo and supercharging systems to vary pressure.
I guess that the Lotus effort at varible compression ratio
http://www.lotuscars.com/engineering/case-study-omnivore-research-engine
was a bust. It looked good in the animation anyway.
http://news.pickuptrucks.com/2009/04/mce-5-variable-compression-gas-engine.html
I like this one the best-notice the lack of power robbing side skirt and the pure reciprocating motion.
I was unable to get the Lotus animation, but these all sound interesting.
As intrigued as I am with new technologies, I have to wonder whether changing the compression ratio has any real advantage over simply changing the pressures (via turbocharging or supercharging). The added complexity might not be worth it.
Until this thread, I was unfamiliar with attempts to change the geometric compression ratio. I’m glad I posted. It’s an interesting subject to me.
Changing pressure changes the amount of oxygen there is for the fuel to oxidize. It is not related to the thermal efficiency of the engine (the amount of mechanical energy you can derive from heat energy). To increase efficiency, you need to have high compression ratio.
A gasoline engine can operate at high compression ratio during low load with a low risk of detonation because the amount of fuel, thus the amount of heat that’s generated is relatively low. During high load operation, the amount of heat increases, which can ignite pockets of air fuel mixture due to cylinder pressure rather than the advancing flame front. This is when the engine needs to have a low compression ratio.
For over a hundred years, an engine’s compression ratio has been compromised between low and high load operation. And just like variable valve timing, variable compression ratio is another way to optimize the engine’s efficiency for low load operation while protecting it for high load operation.
One thing I liked about the porsche design was that it was a passive system. No external control mechanism. It reacts to forces at work on the parts during operation.
It will be interesting to see how it shakes out…
From chunkyazian’s article:
MCE-5 says a 2.2-liter engine using its Variable Compression Ratio intelligent technology could generate 330 horsepower at 4,000 rpm and 395 pounds-feet of torque at only 2,000 rpm while returning 21/25 mpg city/highway.
Not stated, but I assume this is turbocharged. (?)
Much of the effect can be had by varying the point of intake valve closing (close later to reduce effective compression).
And that’s being done already.
Well, varying the intake valve timing can allow more volume to be drawn in… and varying the exhaust valves can eliminate the need for EGR systems (they’re doing that too. And it is true that varying the intake valve time can, by doing that, increase pressures in the chambers.
Turbos and superchargers do that too. They enhance the pressure in the chamber, allowing more bang for the cubic inch.
But the one thing that interests me about variable connecting rod lengths is that it can reduce combustion pressures as well, beyond what simply closing the throttle plate does. I don’t know how far this can go in reducing fuel use under low load conditions, but the subject interests me.
The argument could be made that an engine will use exactly the amount of fuel it needs under low load conditions, whether it’s done by changing the geometric compression ratio, changing the cylinder pressures by NOT using forced induction when unnecessary (which is why turbos improve mileage if used for that purpose), or even shutting down half the cylinders (an idea that I hope has passed). Or even some combination of all of the above. But it’s more complex than that. It also involves how much of the energy stored in the fuel you can effectively covert to torque, and that involves exactly how the flame propagates, crank/cylinder/compression geometries, and even operation son the exhaust side of the equation.
I’d love to work in a lab playing with these ideas. It’s fun to do the theory, analyze it on a good design software, and then actually build it and test it. I’ve done this with countless designs over the years, never car related (most of what I’ve done is aircraft related), and I have never failed to learn from the testing.
The variable connecting rod is an interesting idea but I could see several problems with it.
One is the obvious complexity; meaning more $$$$.
Two is what happens when someone neglects the oil change interval; which we all know never happens… 
@mountain bike, it isn’t just about pressure. You have to draw the entire Otto cycle on a pv diagram and take the path integral to … Let’s just say that as you decrease the volume at the end of the compression stroke, you get a higher pressure increase for the same amount of heat (combustion of fuel). Therefore, you get more mechanical work from the fuel that you burn
“when you further decrease the volume at the end of the compression stroke, you get a higher pressure increase”
Chunky, that IS about pressure!
Pressure after ignition. For the same amount of fuel, higher the compression ratio, higher the pressure increase after combustion. That’s what we are striving for because it is the pressure increase that’s useful. However, pressurizing the intake has no effect on thermal efficiency. It only allows more fuel to be burnt
True, but it ain’t that simple. How the combustion pressure is utilized is a major part of what counts. That comes from the crank and stroke geometries as much as anything else.
I agree that pressurizing the intake has no effect on thermal efficiency in the technical sense that the entire amount of stored energy will be converted to heat energy in both cases, but the resulting of expansion of the greater volume of expanding gasses will produce more torque than not pressurizing the intake.
Honestly, I think we’re just using different words to say the same things.
I remain wishing I could play with these variable stroke engines in a lab. I always enjoyed testing out theories and learning how well they actually work… or sometimes don’t! 
A single cylinder variable compression engine is used for measuring fuel octane.
The stroke is fixed but the cylinder and head can be moved up and down relative to the crankshaft.
The lower the cylinder/head the higher the compression.
With ignition timing, rpm, temperature etc. fixed compression is raised until spark knock occurs.
Settings are compared to reference fuels.
That IS interesting to learn, circuitsmith. Things always seem so obvious after someone else teaches them to me. Thank you for the post.
@circuitsmith A single cylinder variable compression engine is used for measuring fuel octane. The stroke is fixed but the cylinder and head can be moved up and down relative to the crankshaft. The lower the cylinder/head the higher the compression. With ignition timing, rpm, temperature etc. fixed compression is raised until spark knock occurs. Settings are compared to reference fuels.
Another way to get adjustable compression is to have a contrapiston in the head that can be moved up and down with a jackscrew.
This is a “diesel” model airplane engine, never popular in the U.S. but used a lot in England where nitromethane for glow engine fuel is very hard to obtain.
It burns a mix of kerosene, ether, a small amount of amyl nitrate, and castor oil for lube. The jackscrew in the head is adjusted for maximum power, much like the spark advance of gasoline engines is. An overcompressed engine behaves a lot like a gas engine with the spark too advanced.
There is no glow plug in this engine, ignition is entirely due to compression. Unlike auto diesels, the fuel is not injected into the cylinder, but is mixed with the intake air using a carburetor.