Variable Compression for Rotary
 

I was reading articles about Variable Compression engine designed by SAAB. And it turns out that by lowering the compression ratio during high load/WOT, chance of detonation is lowered, and torque numbers can increase.
The application also mention of charging the incoming air, in this article it was supercharger.

The article: http://www.edmunds.com/news/innovati...7/article.html

Although that article is for piston engine, I think that this technology of varying the compression ratio depending on load and temperature of the chamber would help rotary engine produce more power and lower chance of detonation. Specifically in FD, turbocharged rotary engine.

Read it, and lets discuss...

I'm confused by your statement. We have no valves, no con-rods or cylinder head per se, so how could you change the CR in the rotary?

I would imagine it could be handled easily with some sort of solenoid computer controlled compression release....

An interesting idea.

How about just turn down the boost? Wouldnt that pretty much lower compression?

To be useful the compression ratio has to vary mechanically and the rotary has no means of doing this.

The Saab system is pretty ingenius. Imagine the cylinders and head in one peice. Imagine the head/cylinder liners in one piece as the lid on a cigar box and the engine block as the rest of the cigar box. The cylinders are attached to the lid but the crank is attached to the box, so by cracking open the lid slightly you have in effect pulled the head away from the crankshaft/pistons and therefore lowered the compression ratio. The Saab system works exactly like this. The head and cylinders literally pivot inside the engine block and with a hydraulic actuator the amount of "tilt" on the head and therefore compression ratio can be varied on the fly.

There is no elegant way to change compression ratio in a rotary on the fly.

No. Compression ratio is a mechanical property of the engine. Boost has nothing to do with it.

Yeah I did a research project on that Saab engine just last year. It is a pretty unique idea, it's just that they were having problems with the seal between the head and the block I think.

It would seem to be difficult in making a VC rotary. But maybe someone crazy enough would try it. :)

Sure it does. Spitting an apex seal out through the exhaust will mechanically change your compression ratio in a hurry. :D

You could change the compression ratio on the fly if you could change the size of the indent on the rotor on the fly. Good luck getting that to work.

On a somewhat related note, you could "miller cycle" (do a google search if you don't know what a miller cycle motor is) in a rotary by modifying the intake port. Doesn't the Renisis have "high" intake ports?

I doubt anyone here cares much SAABs variable compression or the Miller cycle because they are about getting more efficiency than getting more horsepower.

How about using the Spark Plug holes? perhaps by moving the trailing plug in or out of position mechanically depending on the temperature of chamber, and actual compression.

Detonation only happens during high load, usually WOT. Thus chamber must be really hot, and the compression may exceed the 10:1 or whatever the standard is.
The higher the pressure of the chamber, the easier the mixture is to explode even without assist from spark plug.
So if we lower the pressure just a little bit, it will prevent detonation from happening.
Maybe someone could help with the calculation to how much this has to varies.
For example:
650cc per rotor
650cc/3 = 216.67cc per chamber
10:1 ratio means at spark plug area = 21.667cc
Now compression test usually is 100psi, but I don't know how it relates it here...
And how much more cc to lower ratio.
......?

https://www.rx7club.com//attachment....postid=2770245

This chart may or not apply to our engine, I think rotary has its own detonation limit chart that mazda could provide....

This chart taken from this website:
http://www.prodrive.com/uploads/Bene...20of%20VCR.pdf

Or, can we have a little hole near sparkplug area to make sure that the compression ratio follows the detonation limit, probably works just like pop-off valve, but for 100psi or so...

Is it possible this is what KDR have?

Probably not.