This is related to the Miller Cycle question I posted a few weeks ago.


Here's the question:

You have a piston on a 4-cycle engine.

It's on it's compression stroke.

If the intake valve is closed it will build pressure.

If the intake is OPEN, it will still be able to build pressure, only not as fast, because as the piston is trying build pressure the intake is letting it back out again.

Now, there is 14.7psi of atmospheric pressure trying to keep the combustion mixture from coming back out.


What I'm trying to figure out is how much pressure the piston is generating at the valve as it tries to force the combustion mix back out the intake valve.

I know that I'll need to calculate the volume of the cylinder and the surface area of the piston, as well as how much air can flow out past the valve.

The problem is, this involves both Calculus, and it's been WAY too long, and it involves TIME, because this is a provess that isn't instantaneous.

Anyway, the math is over my head. Simple quadratic equations I can handle. Differentiation and Integration, not so much.


BTW, I don't need precise answers, although that would be nice. Rule of thumb is going to be enough at this point.


If it helps, the basis of this question is this:

If I want to use hold the intake valve open on the compression stroke, past BDC, how much boost (on top of the 14.7lbs of atmoshere), do I need; in order to prevent the combustion mix from blowing back into the intake manifold while the piston comes up?

 

Hmm..A LOT of variables for a precise answer.

 

There are dynamic factors involved in such a calculation and looking at this statically won't cut the mustard.

So basically what you want to know is the pressure differentials... cylinder to exhaust port to intake port, per given degree of crank angle? And at what rpm?

What exactly are you looking to find with this? I'm assuming you want to find an IVC event for cam selection??

-Mindgame

 

I really like the idea of the Miller cycle.

In a Miller engine, you hold the intake open past BDC on the compression stroke, because it's more efficient to use a supercharger to begin the compression process. You then close the valve at a certain point, usually between 20 & 30 percent of piston travel, to let the piston compress the mix. That gives you a lower dynamic compression ratio but a full power stroke.


The key of course, at least for me, is knowing how much pressure you need in the manifold to prevent backflow.

 

Don't see where that differs so much from an otto cycle engine. Most HP cams have IVC events of 60º ABDC and better.
The DCR is a function of IVC closure... so it is "lower" than what? And define "full power stroke".

The second paragraph is easy... intake port pressure > cylinder pressure and exhaust port pressure. You're assuming that backflow is undesirable at all engine speeds...

-Mindgame

 

That still doesn't tell me how to figure out how much pressure the piston is generating as it comes up. (Which would tell me how much boost I need to keep it in the cylinder and out of the intake manifold.)

 

As soon as intake runner flow stops, the pressure in the cylinder is going to be identical to the pressure in the intake manifold (assuming flow has truly stopped, and there is no inertia continuing to force the air charge into the cylinder). Any upward movement of the piston is going to create a pressure in the cylinder that is higher than the pressure in the intake manifold. Flow will reverse immediately.

I don't think if is possible to stop reversion, unless you can take advantage of intake runner tuning to keep a pressure wave briefly on the valve opening to buy you a few more degrees of crank rotation.

 

There's the static factor in all of it (that part's not too difficult to calculate) then there's the dynamic. That one is reaaal tough because it involves wave modeling. As Fred mentioned, inertia will continue cylinder fill even after the piston starts accelerating away from BDC. The fill rate is different at all rpm... that's what VE shows us. There's always a little reversion at lower engine speeds. The later the IVC, the greater the degree of reversal.

I'm still trying to figure out exactly what you're after here. If you want to gather information for camshaft selection, then why not work with someone who uses Dynomation or another wave simulation software. What you are looking to generate is a pressure to crank angle diagram. This will show you where to place the IVC. It's total engine simulation that you're after. Getting accurate info into the program is the toughest part.

-Mindgame

 

Engine simulators like EA Pro show intake pressures dynamically. You could vary boost and cam timing to get the numbers you want, I believe. The software uses some pretty sophisticated modelling. IMO, it's easier to just find software that gives you what you want rather than writing it. You might assume I'm not a software writer.




You'd be correct.

 

The hell you say!



OS, do you have any experience with that new software that's out? Something "Sim"... man I had it on the tip of my finger just a moment ago. Anyways.. any experience with it?

edit: Dyno Sim... how tough was that!

-Mindgame

 

Not yet, but there is a Virtual Engine Master competition using Dyno Sim which I want to enter, so I'd better get it soon.

 

Bring it on Jon!

No.... haven't really considered that... til just now! Sounds like fun.

-Mindgame

 

This would be a supercharged engine.

 

I thought I covered that.

I want to know how much pressure is being generated by the piston as it comes up on it's compression stroke, SO THAT I can know the right amount of boost I need to run to keep the combustion mix in the cylinder until the valve closes.

 

What difference does that make? My statement is valid for NA and S/C setups.... when the flow stops, the pressure is the same on both sides of the valve... by definition. You only get flow when there is a pressure difference. Put 20# of boost on an engine, and you have 34.7 PSIa in the intake manifold, and 34.7 PSIa in the cylinder, when there is no flow. When the piuston moves up, it increases the pressure in the cylinder and moves the air back up the intake runner.

While intake runner "tuning" is generally not of significance in a blown motor, there are still cyclic events and pressure waves operating, whether there is a blower on it or not.