Variable Compression??

[michaelmurray]

I know this is a lot more complex than variable valve lift, but the performance potential from variable compression ratio is amazing. Imagine idling the engine at 22:1 (great fuel economy); running under slight load and 18:1; increase load @ 30 psi and drop to 16:1; then with your twin sequential turbos at 100 psi drop to 12:1. Imagine a daily driver triple staged turbos @ 250 psi. These numbers are for example, but what the VCR allows is for more boost=power while maintaining correct cylinder pressures.



Engines today have a constant compression ratio. When an engine is turbo/supercharged, the CR must be lowered to prevent high cylinder pressures and, for gas engines, to prevent detonation. However, when the CR is lowered the engine doesn't operate as effeciently under light load or idling. Also, the CR can't be lowered too much because the engine won't start. Therefore, the engine can only safely run at a certain amount of boost. It now becomes clear why VCR would be such a breakthrough.



There are several methods integrating VCR. Some require new head designs, new block designs, or both. The system that I find has the most potential for integration into our Cummins engines is at www.alvar-engine.se This would either require modifying a 12V head or needing to design a new head. A timing chain to drive the upper crank with phase changing capabilities would be required as well. This is not a trivial design or modification, but the power possibilities are truly out of this world!

 

[nickleinonen]

hydraulically actuated valves would solve one part of the problem. you would need a screw of some sort to change the volume. we had a single cylinder lister hand cranker at school that had a screw in the head that could be turned from starting position to running position. all the way in for higher compresson to start it easier [that was kinda hard to start without using the decompression lever, but i did it!] and once it was running, you quickly unscrewed the screw to the stop to allow it to run under load better.



with the variable valves, you could control the compression by controlling when the ex valve closed, but that would not give you power, just a shorter stroke...



hmm, maybe in 2010 when it will be time to buy my 3rd truck [2nd will be a 04 2500/3500 HD H. O. ] dc will have the hydraulic valves. woo hoo, with some computer tinkering, that will allow for a proper compression brake!

 

[dmurdock]

i remember reading in popular mechanics about 10 yrs ago about an engine (they called it a wobble-plate) that had 5 pistons in a circle (if you imagine our pistons in a line, these were in a circle, with a circular head). Instead of running perpindicular to the pistons travel, the crank ran in the same direction as the pistons, straight up thru the center of the head. The engine was mounted "on its side" so the head was toward the front of the vehicle. At the rear (opposite of the head) of the engine, there was a circular plate, like a flywheel, except when rotated it "wobbled" (it wasn't at a right angle to the crank). Each connecting rod connected to the edge of the wobble platge with what would look like a c-clamp, basically the way you would hold a dinner plate. On the upper and lower sides of the "clamp" were rollers, so the plate could spin while held by all the connecting rods. So, you have a plate with a crank thru the middle and running in the same direction are 5 pistons which connect to the edge of the plate. As the plate spun on the crank, it wobbled, forcing the pistons in and out. What was cool was that the crank could be slid forward and backwards moving the relative position of the pistons closer or farther away to the head. This changed the compression ratio. The amount of "wobble" in the plate could also be altered, changing the "stroke", thus changing the displacement!. The picture I saw had one of these motors mounted in an audi, and supposedly it ran. . but i've never heard anything about it again. A simple design which allowed for variable compression ratio and displacement is really cool, just don't know if we'll see one until forced to by emissions concerns.

 

[nickleinonen]

they use that design in hydraulics all the time for pumps and motors. you angle the swash plate to change the stroke of the pump/motor to change the speed and pressure. . i think the standard type of automotive a/c compressor uses that style too. .

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[dmurdock]

yes, that's pretty much the design. . only thing they added was the ability to slide the "crank" in and out to change compression...

 

[kchristopher]

SAAB came out with one a few years ago. They attached the cylinder to the head, then split the crankcase halfway up the water jacket. Using a hinge on one side, tilting the top of the motor effectively increases the combustion area and decreases the compression ratio.