Dynamic C/R discusion

[afterburn 549]

Have we had a discussion on adjusted or dynamic C/R ?
I have as of late been studying it a little.
It seems i do not know everything yet !
I knew cam timing is a big consideration, but smaller things have their due too.
Like minute changes in rod lengths .
I have always laughed at some high schooler talking about his
bored .040 over engine thinking ..yea ok LOL
Turns out there is a LOT of nuance going on in the internal combustion engine.
I guess I am still chasing Smoky Yunick.

[Eagledriver]

We have indeed had that discussion. I think it's been in regards to fuel to use and pistons to use with various cam profiles. IMO the idea of dynamic C/R is misunderstood in many of these discussions. The idea is that if you have an aggressive cam overlap, then some of the air will flow back out of the intake valve while on the compression stroke. While this would lend itself to thinking that the dynamic C/R will be lower than the Static C/R, the logic is flawed. If air was incompressible like water, the logic would hold. But air is compressible, the aggressive overlap actually allows the cylinder to fill with more air than a less aggressive cam at high RPM. If the concept of dynamic C/R were accurate the performance of the engine would be poor since the engine would get less air into it than an engine with a milder cam.

A good measure of how much air gets into the cylinder is the torque the engine produces. You will find similar torque numbers (at different RPM) between engines of similar size regardless of cam selection. The 72 911s has more torque than the 911t therefore it fills it's cylinders pretty much as well as the 911t even though it would have a much lower dynamic C/R.

-Andy

[afterburn 549]

Aha, that is a good point. !
I understand that volumetric air flow is reason behind overlap at some point it becomes continuous , thats the desired effect anyway .
I am wondering about the little things that we do not normally consider.....
perhaps slight changes in rod length ,
or, perhaps slight changes and shapes in the squish band .

[wwest]

Look into the original version of the Atkinson cycle.

Then the way Toyota implements it, delaying the intake valve closing, in the Prius, etc.

Physical CR of 13:1, effective CR of 10:1

Awaiting an Atkinson/Toyota cycle engine with DI.

18:1 physical CR, 14:1 effective. Mazda SkyActive DI engines are already running 14:1 physical CR.

Variable intake valve timing should allow 18:1 for low volume cylinder fills and drop back to 14:1 at WOT, FULL volume cylinder fills.

Wankel seems ideal for variable intake valve timing.

[afterburn 549]

Variables.
i know Variable valve timing has been around for awhile and will improve..
I am guessing variable stoke should have surfaced by know and surprised it has not.!
The piston engine is a mass of moving parts and for sure a Wankle or other some version would lend itself to a adjustable pwr stroke better.