Admittedly, I have little knowledge on how compression ratios in engines are changed. This idea is inspired by [Ned_Ludd]'s idea for a deck spacer plate with squish tabs [link].
Ok, so take the sparkplug out of the center of the head, and instead put a sparkplug on the right side of the head and one on the left. Now, make a hole in the piston with a metal insert in the center of the head that fits accordingly. You can tune it so that the engine has a specific compression ratio when the metal "tab" is as low as it can be without getting banged by the piston. But wait, why does this insert have to be fixed into place? It doesn't. A linear electric motor [edit: linear actuator] (one per each cylinder) could lift this tab up and down to vary the compression ratio (if my logic is not mistaken). When you lift it up, it seems to me the compression ratio will go down. When you push it down, it seems to me the compression ratio would go up. There would be some kind of very thick and strong washer attached to the top of the insert to prevent the motor from jamming it too far down so the piston slaps it.
Now the two sparkplugs on either side of the head would rob them of their optimal location, but if the motor has a large bore they would actually most likely make ignition and combustion more efficient.-- acurafan07, Jul 30 2007 Deck Spacer Plate With Squish Tabs Deck_20spacer_20pla..._22squish_20tabs_22Ned_Ludd's inspiring idea [acurafan07, Jul 30 2007] I would use linear actuators instead of linear motors.-- BJS, Jul 30 2007 There, I made an edit.-- acurafan07, Jul 30 2007 Yeah. I guess I'll vote for this because it's so simple. But it seems like I've heard of something pretty much the same as this that already exists. I don't feel like doing a search for it though.
I bet you could keep the spark plug in the center on the movable part, but I don't know if you would really want to.
The moving part could even be on the side of the cylinder, but I don't think you would want that.-- BJS, Jul 30 2007 In theory this is fine, but I think in practice you would not be able to create a sealed movable area capable of pushing against the cylinder pressure 100 times per second (6000rpm). That's why we still have cam shafts and why my suggestion of how to achieve this uses that low-tech approach.-- marklar, Jul 30 2007 Thanks for the vote of confidence but, though it generated a whole train of useful thought, my squish tab idea wasn't a very good one in itself!
Neither the plug position nor the sealing situation need be a problem. What you're looking at is a circular face that can be withdrawn to form a recess, thus varying the volume of the combustion chamber and, hence, the compression ratio. The mechanism that springs to mind is simply a second, small piston with its crown facing downward - or roughly downward as it may be positioned in any part of the combustion chamber. Piston rings and a worm-drive-and-screw-thread mechanism should solve both the sealing and pressure problems.
As for the plug position, you don't have the problem you thought you had, for what better location for the sparking plug than precisely the crown of the small piston? No reason it can't be done: mount the plug on the adjustable face.-- Ned_Ludd, Jul 30 2007 // worm-drive-and-screw-thread mechanism // That operates 100 times per second?-- marklar, Jul 30 2007 No, it varies the combustion chamber volume slowly, as needed. There's no percentage in moving it back and forth at the same rate as the piston.-- Ned_Ludd, Jul 30 2007 //and why my suggestion of how to achieve this uses that low-tech approach.// And what is that?
I guess one of the main benefits of this would be in turbocharged/ supercharged applications. In turbocharged engines, the compression could be, say 12.5:1 starting out off idle (it would be direct injection) while slowly reducing as the boost came in (and cutting lag down significantly). In supercharged engines (mainly twin-screw and roots-type) this would be able to make the compression very high at high rpms when the supercharger is no longer boosting at its peak.-- acurafan07, Jul 30 2007 The "tab" would still have to move moderately quickly--I would estimate my car's engine will drop from redline to idle within a second or two when the throttle is released. But that's not too fast, really, given the total travel of the tab is probably on the order of millimeters.
Wouldn't work on engines that have high compression to start with though. On, say, a F1 engine, the piston basically touches the head already, leaving no room for any projections.
You might end up with very odd and sub-optimal airflow as the tab projected into the cylinder, and cooling would be tricky, but I can see the advantages outweighing this.-- 5th Earth, Jul 31 2007 //Wouldn't work on engines that have high compression to start with though. On, say, a F1 engine, the piston basically touches the head already, leaving no room for any projections.// Yes it would, but instead of protruding into the cylinder to create a higher compression ratio, it would retract into the head to create a lower compression ratio.-- BJS, Aug 01 2007 Maybe I got the wrong idea about this idea, I assumed it was a way of making a more powerful Miller cycle engine, in which case the cylinder volume needs to change between the intake and power strokes. If the idea is just to change the compression ratio during the usage of the engine, you can simply add more or less fuel, which almost all modern computer controlled engines do anyway.
[acurafan07] The low-tech solution I suggested was to have a second cam shaft which could be moved to open the valves more or less.-- marklar, Aug 01 2007 //you can simply add more or less fuel
Not at all the same thing as changing the compression ratio. Changing the compression ratio will require you to also change the fuel-air mixture, but doing the latter alone will not duplicate the effects of the former.
//a second cam shaft which could be moved to open the valves more or less
Also not the same thing as changing the compression ratio. Actually, I'm not sure why you would want to do this at all--I always assumed that you wanted to use the highest valve lift possible, within mechanical limits.-- 5th Earth, Aug 01 2007 Ok, I think I finally understand the point of this, I thought it was something more complicated. The effect that's being aimed for here can be done (on flat non-V engines) by moving the block relative to the drive shaft. In fact, you could modify an existing engine to do that with a rubber seal and adjustable clamps.-- marklar, Aug 02 2007 that'd be kind of handy on a dual-fuel car as ethanol can use higher compression ratios that straight petrol-- random_patenter_syndrome_victim, Aug 07 2009 For a linear engine, if the connecting shaft in the middle connects to one cylinder with a left-hand thread and the other a right-hand thread, then you can vary your CR reasonably easily, though not quickly.
About dual-fuels, yeah... but why not make it the other way around?: an engine optimized for the "new" common fuels, then use *diesel* when they aren't available, in "limp to nearest new gas-station" mode.-- FlyingToaster, Aug 07 2009 random, halfbakery