Dynamic CR: pressure development / intake closing

[80Stang]

This is a topic that applies to all engines and puzzles me in a spesific way, so it applies to our sixes as well. I'm looking for some more information from you gurus for my own work and at the same time hope this thread comes somewhat useful to more people. Maybe you can catch my mind-flow from this:

Pre-question: How do you calculate dynamic CR?

Simple...yes there are calculators in the net and so on. I personally use the one on the Keith Black website. But...

Core question #1: How do you determine when pressure starts to develop in the chamber?

Yes yes, take the intake valve closure point and use the calculator. Ahs, not so simple. The pressure starts to develop before the intake is closed, and usually you consider the intake closed in means of flow before it actually is closed.

Remember the pre-question.

ok, we can calculate the pressure development after the intake fully closed, but shouldn't we also consider how much of it developed before the intake closed?

Core question #2: How to determine how much pressure developed before the intake totally closed?

At least you'll need to know the flow characteristics from the chamber/bore to the intake manifold and maybe some other things to determine this, like any pulses from the intake manifold from other cylinders etc...

I'm a simple man and not at all genius in this field or in mathmatics or flow technology. Still, I do build engines for my customers, plan and test different combinations, learning from my previous experiences to build better ones in the future. There is a lot going on with planning CR, cams, timing, displacement effect on cam choices/rev ranges/torque etc etc. Basically I use the KB calculator with a certain method determining the intake closure point for the calculator for dynamic CR, thus my build-ups are planned the same way and that way comparable to each other. Then I aim the dynamic CR to certain range using that calculator, knowing exactly how the previous combinations worked (as they are dynoed in an engine dyno) and learning from that. So far the combos have been good and they are getting better, but I'm very eager to learn more.

Roller cams are much more exact in the way they open and close the valves. They are much sharper, of course. I can pretty reliably use the intake closure value at .006" lift with the calculator, and get consistent calculations between cam manufactureres and different cam grinds. With flat tappet cams the cam closes usually pretty sharply to some point, and then seems to slowly close to the rest and any low lift measurement like .006" will give you very low dynamic CR. That is why the .050" lift value is given as that is more comparable between different types of cams.

 

[Stubby]

If you are going that deep, you should purchase a book titled, "The Scientific Design Of Induction and Exhaust Systems".

 

[80Stang]

Yep thanks, I can read books and that book sure seems interesting. More:

Scientific stuff, like the one Stubby recommended:

Dynamics of Combustion Systems (Hardcover)
by A. K. Oppenheim (Author)

A little lighter, probably not going very deep in detail into the question I asked but still might be interesting to read:

Racing Engine Builder's HandbookHP1492: How to Build Winning Drag, Circle Track, Marine and Road RacingEngines (Paperback)
by Tom Monroe (Author)

How to Choose Camshafts and Time Them for Maximum Power (Speedpro) (Paperback)
by Des Hammill (Author)

But has any member here thought of the matter of how to calculate the dynamic CR a little more accurately?

 

[THE FRENCHTOWN FLYER]

But has any member here thought of the matter of how to calculate the dynamic CR a little more accurately?

I can tell you how it is done at the Ford Engine Development labs:

The combustion chambers of the engine are machined to accept surface-mount pressure transducers. Then the engine is run through a highly complex and rigorous set of dynamometer tests with lots of proprietary computer programs to analyze the results. A series of pressure-vs-crank angle plots and tables are obtained which optimize spark timing (among other parameters) and give cylinder pressure vs piston location. An almost endless array of variables (e.g., cam timing, EGR concentration, coolant temp., fuel injector timing, etc., etc) can be studied.

Once you have this data you can look a single point to get static CR.

For example, let's say for a specific set of running conditions, say 4000 RPM, WOT, LBT fuel, MBT spark, 0% EGR, stabilized temps, we see 300 PSI in the cylinder at 20 degrees BTDC just prior to ignition. Then you can calculate the dynamic CR using the Gas Laws.

You may be getting an understanding of just how difficult it is to precisely answer your question and why Ford and others employ dozens of bright young engineers to look at such data, optimize all the factors and produce the best engine possible. The other two legs of this triangle are to do it as cheaply as possible and with as much reliability as possible.

All the modifications we discuss on this website are merely feeble attempts to duplicate these efforts and bias the results more in favor of our specific needs of our personal cars and trucks.

Great question though.

 

[80Stang]

Thanks for your answer, Flyer.

Yes this is a complicated thing and we can't get a precise answer anyways. That's why I asked if anybody had a practise to calculate a little more accurately.

Currently I'm going forward with looks to the past; how the previously built combinations worked and with what values. Given the pump gas we have I think I'm on the map pretty well. Its just a little complicated with flat tappet cams but with rollers its way easier and needs less quesswork.

While we approach the limits of pump gas (I have run a SBF at 11.65CR safely with current pump gas, and I know of others running +12CR) it just feels proper to calculate a tad more precisely.

 

[XPC66]

I think your questions are predicated on a certain amount of myth. That is not a slight, but more an observation from seeing the same old best fit assumptions that circulate around the industry.

I find calculating DCR is pretty accurate using inlet closing. The excuse people like to use for compression happening before closing is that:

-there has been a rarefaction boost on opening (This ignores the fact the exhaust valve is still open and does nothing to increase density as a result);

-the inlet valve opening is insufficient to cause decompression on the rising stroke.

The fact is that if you use advertised timing, it is closing at 0.006" not slammed shut. Decompression will occur up to that point.

Calculating the DCR does give a very precise indication of cylinder pressure. You can test this yourself by comparing the compression test actual figure with the calculated one from DCR. The cylinder pressure obeys the gas laws. The calculation itself is simple trigonometry. I use it in reverse for finding the cam events on unknown profiles.

As has been stated, there are calibration engineers who spend a long time tweeking bits 'n bobs.