stroke/compression/rod length /cam timeing info

[grumpyvette]

DOING RESEARCH I FOUND THIS! INTERESTING? http://victorylibrary.com/mopar/cam-tech-c.htm http://victorylibrary.com/mopar/rod-tech-c.htm

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heres some more articals you might find of interest http://www.aros.net/~rbuck/rick/rodstudy.htm

http://www.stahlheaders.com/Lit_Rod%20Length.htm

http://madlab.me.utexas.edu/~yspae/thesis/thesis/node46.html

http://victorylibrary.com/mopar/rod-tech-c.htm

http://www.bergen.org/ETTC/Development/TechPrep/Theory.html

AND

[Guest super280z]

Grumpy, thanks a million for posting the link to liberty. i've been looking for that it for allmost 4 months since we upgraded our computer and lost all my links. that guy really knows his mopar stuff and on top of that he's pretty cool too. good info.

[Peternell]

THANK YOU Grumpy!!

[blueovalz]

I'll 3rd that. Thanks again. I feel even better with my 289 vs the 302 now (considering the intake set-up and other very important perimeters now).

[pparaska]

Great Stuff, Grumpy!

 

BTW, I put together an Excel (office 2000) spreadsheet for the equations for V/P index (covered at http://victorylibrary.com/mopar/cam-tech-c.htm ) and put it up on my site. I checked it against their numbers and got their results. It's here on my site (excel file).

[Owen]

Yes! Some more reading material for my long flight to Japan!

Thanks,

Owen

[grumpyvette]

here someone elses ideas may help you see this stuff from another angle.

 

The effect of connecting rod length on torque production throughout the power stroke in an internal combustion piston engine

So one day while I wasn't doing my math homework, I realized I almost knew how

to figure this stuff out for real. And, for some strange reason, I tried to.

With the classes I was taking when I came up with this, it's not like I was

hurting for math problems or anything, so maybe I'm just a glutton for

punishment. Or maybe my brain was stimulated by the thought-provoking class

discussions. Or maybe I'm just a misunderstood genius. (Hey, knock that off

over there.) Probably, it was the really good coffee I've been drinking

lately. Some day, maybe, I'll understand how to integrate a multivariable

function, and I'll finish this by hand once and for all. Or maybe someone will

plug my little formula into some big bucks math program which will do it for

me. But for now, I'll punch the numbers into my handy dandy TI-82, and it'll

do a sort of half-assed numerical integration and show me pretty graphs and

stuff when I ask it to do a definite integral.

 

So first, I needed to figure out piston motion in terms of crank rotation.

That's this one:

 

To get piston position for theta:

 

r = stroke radius

c = connecting rod length

b = bore radius

theta = degrees of crankshaft rotation away from TDC

y = piston's distance from TDC

 

y = - r cos theta - SQRT(c^2 - (r sin theta)^2) + c + r

 

 

 

And to get volume inside the cylinder:

 

V = y(pi b^2) + volume above piston at TDC

 

So now we can calculate total cylinder volume for theta, which we need to

figure out nRT (as in, PV=nRT). Figure out a way to mount a pressure

transducer in the combustion chamber of the engine under development. Drill a

hole into the combustion chamber, use some super spiffy spark plug with one

built in, whatever. Then track crankshaft position with a toothed wheel and

pickup or some such thingy. Then run the engine with a known setup (stroke,

bore, con rod length, fuel, temp) and track pressure in the cylinder for

theta. You'll end up with a scatter plot that looks something like this, but

includes pressure data from the intake and exhaust strokes too (from -360 to

360 instead of just -180 to 180 like in this diagram):

 

 

 

The whole cycle looks something like this:

 

 

 

Run this test every 500 rpm or so from the bottom of the range of real power

to just short of where you think stuff will come flying through the engine cases.

Test a couple of different fuels while you're at it in case some burn faster than

others. You might need that data later. So now you have scatter plots showing

pressure at theta for a known stroke/bore/rod, but that doesn't do anything for

you yet, because the piston motion, and therefore volume @ theta, isn't the

same for a different rod length, which is what you're testing. Remember

Pressure times Volume = number of moles times the gas law constant 'R' times

Temperature in degrees Kelvin... PV = nRT. Good luck counting the number

of moles of gas that are in the cylinder, or traclomg the temperature in terms of

theta. Plus with all the reactions going on, there's no way I'm ever in this

lifetime going to be able to figure out how to calculate nRT at every instant

from first principles. But from the scatter plot, we have pressure as a function

of theta. From the piston motion function we have volume as a function of

theta. If you perform a gazillionth degree regression sum of the points of the

plot and find the equation of that curve then divide by volume for every theta,

you have nRT as a function of theta, which for a known rpm (time) through the

various equations noted gives pressure as a function of theta for any connecting

rod length running at that rpm, and since you tested every 500rpm, you also

have a function for nRT based on RPM. There are a lot of factors I'm electing

to neglect, like, oh, friction (such a trivial thing, really...), piston motion's effect

on port flow (number of moles) and charge turbulence (burn rate, probably), but

this should be close enough to get an idea of what's going on in there.

 

Okay, so now we have a function for pressure in terms of degrees of crankshaft

rotation (theta). Because I forget how to put Greek symbols here, I'm using

the following:

 

d = theta

pi = pi, not p times i

Pressure = f(theta), so P = f(d), which is really a big crazy function, I'm

just going to call it 'P' and put it in another function which really is just

dying to be integrated symbolically, and you can't stop me!

 

torque = r*SQRT((P*pi*b^2)^2+(P*pi*b^2)*((r sin d)/SQRT(c^2-(r sin d)^2))^2)sin((90-d)+(cos^-1((r sin d)/c)))

 

 

 

Then find the definite integral of that function from 0 to 720 degrees (four

stroke), and just like magic, we have work performed for one

cycle.

 

Now take that engine and hook it up to a DC dyno or just spin the damned thing

over somehow (with the ignition off, no fuel in the fuel

system, and the throttle wide open) through the range of rpm and measure the

torque required just to overcome friction and pumping losses every few hundred

rpm. Let's just call that friction, since that's what it is, really. Now

here's the easy part:

 

rpm(Torque - friction)/(constant for whatever units of measurement you're

using... 5252 for English to find horsepower, or something else to get kilowatt

hours) = power put out, or output (after taxes and stuff)

 

Repeat for every 500rpm or so, do another regression sum to find the curve, do

the definite integral between the minimum engine speed you think the engine will

see on the track and redline, then do it again for different connecting rod lengths.

The one with the most area under the curve is the fastest engine, in my little

theoretical paper world, anyway. Of course, you could just put to graphs next to

each other and eyeball them too, like everyone already does anyway.

 

Hey, if the rod fits in the engine, it might even work.

 

 

(no, I'm not holding my breath either)

 

 

Drop me a line.

patrick@lifenet.com

 

Using the formulae above, I wrote an Excel spreadsheet to compare two sets of variables.

 

This subject came up on the motorcycle engine design list a while ago. I contributed to the discussion with this.

 

If you liked reading this, you might like reading about how Keihin FCR carburetors work, how to tune them using a dynamometer, and how to perform cylinder head airflow testing.

[Kevin Shasteen]

I really like what I've read so far...thanks for posting those sites (bout half way done reading the first site).

 

You know; for a Grumpy guy...you're okay.

 

BTW: "Piston Motion for Crankshaft Rotation" I've got on Excel & created a form for the numbers once generated...(last years project; whewww! glad that one's over). I'm not into the "Theta" thing but I did have to teach myslef sin, cosine ect.

 

Also; are you familiar w/the formula P-L-A-N/33,000? It's an older formula (older than me) but once figured will assist the user in determining Work/Hp/Torque...pretty cool stuff, IMHO.

 

Very good info: keep it coming. I'm not an engineer but I find myself thriving on automotive tech (substance..not hype) that explains the behavior of one engine in comparison to another engine.

 

Kevin,

(Yea,Still an Inliner)

[Kevin Shasteen]

Hey Grumpster (or anyone else who has an idea on the ideal cylinder pressures),

 

My question specifically deals w/the "V/P Index" on one of the sites you posted as good reading...and it was, & I thank you again for bringing that formula on the V/P Index to my attention: I found it quite interesting.

 

I've read many hot rod books & noticed that most articles concentrate on the "HP/Torque" figures of an engine but rarely if ever do I remember their builders stating what their [Cylinder Pressures] were for a specific build.

 

I've always thought that just because an engine sounds good (that lumpity lump sound of a V8 w/big cam) doesnt necessarily mean it's running efficiently for its intended purpose: which the V/P Index helps shed some light on that subject. Once I began understanding Dynamic Comp.Ratio I've found that to be equally as important as Static Comp.

 

The Question is: what actually is considered to be an appropriate cylinder pressure (high & lows) for a:

1) Typical Grocery Getter

2) Street/Strip Engine

3) Dedicated Race (Drag or Rally Car) Engine

4) Turbo/Supercharged as well

 

Appreciate any input from anyone on the subject.

 

Kevin,

(Yea,Still an Inliner)

[grumpyvette]

1) Typical Grocery Getter(130-150 psi)

2) Street/Strip Engine(150-180 psi)

3) Dedicated Race (Drag or Rally Car) Engine(160-200psi)

4) Turbo/Supercharged as well(100-130 psi)

 

all measurements taken with exhaust rocker removed to keep exhaust valve closed and intake valve loosened to get max compression. intake values need to be loose to remove valve overlap and cam timeing from the equation. the static compression ratio will vary with the engines use but the dynamic compression ratio at low cranking rpms will be about 9.5 to 1 in most non-supercharged engines that are properly camed, due to the cams durration and overlap necessary to run high rpms this means that up to about 13.5 to 1 static compression ratio is needed to get that 9.5 to 1 dynamic ratio!