Rich, thanks for the spreadsheets.

Shane and Pete got me digging and doing my homework, but yours is better than what I found.

You're right guys; thanks for the enlightenment. Sometimes Age shows up alone.

Personally I like that 393.7 inch rod!

Thanks again, Guys.

 

To answer your question, I don't have an equation for acceleration. I could come up with one, but there are too many variables. Basically, my spreadsheet calculates the piston motion geometrically. It outputs a distance from TDC for each degree of revolution. Now, assuming an rpm, it can calculate the velocity at each of these points (velocity is change in distance with respect to time). These velocity calculations are also computed and outputted for each degree of rotation. Again, based on the inputted rpm value, the acceleration values are calculated (acceleration is change in velocity with respect to time).

Now, it should only make sense that since the longer rod dwells at TDC longer, it's changes in velocity are smaller and therefore it's MAX acceleration will be smaller.

Here's a little trivia for you, though. Even though MAX acceleration in the downward direction occurs at TDC, MAX acceleration in the upward direction DOES NOT occur at BDC. It has to do with the fact that the rod angularity is changing enough at BDC (straightening up) to offset the motion of the crank. It offsets this motion just enough to make peak acceleration at the bottom of the stroke happen at about 25-35 degrees Before and After BDC. It all has to do with the geometry of the system.

One other little tidbit most people don't realize. During the downward stroke, the piston travels further during the first 90 degrees of rotation than it does in the last 90 degrees of rotation.
In other words, with a 3.48" stroke, the piston travels about 2.0" from TDC down to 90 degrees ATDC. Then, the piston only travels the remaining 1.48" during the next 90 degrees of rotation to BDC.

Anyway, just a couple of things to make you think... Hopefully everyone got the spreadsheet who requested it. If you have any questions, just let me know.

Shane


HEY RICH, I just saw your spreadsheet..... who came up with that. I know I e-mailed you mine a while back. Did you create that one yourself? Looks really good, man......

 

Oops re: BDC. Of couse that's right about peak piston acceleration near BDC.

Rich Krause

 

81ZMouse, I can't run your spreadsheet (don't have Excel). It looks like it calculates wrist pin position as a function of crank angle in increments of one degree. Regardless of how it works, the values you posted agree very well with the values I have calculated.
Now we need to determine how much max acceleration the components of the reciprocating assembly can handle. With that, I think I'll hand things over to the engineers.

 

I know this is an old thread but........

For reading pleasure you recieve a special bonus prize, the unadulterated pure formula for a slider crank mechanism derived by yours truly. don't ask how many hours this afternoon i wasted on this instead of taking notes in class.....

xdot = (stroke/2)*omega[-sin(theta) + cos(theta)*tan(beta)]

beta = Pi -arcsin[(stroke/2)*(1/rod)*sin(theta)]

beta is the rod angle in terms of crank angle.
xdot=piston velocity
Theta= crank angle
omega= angular velocity in radians per second (RPM*2*Pi/60)

Once i figured out that i needed to use (stroke/2) for the length of the link instead of the stroke, DOH!, it matches up with the interated spreadsheets posted above and the formula i got from the below book.



AND

Heres a formula for piston velocity from a book i've been reading, "Internal Combustion Engines" 2nd edition by Richard Stone (SAE Press)

xdot ~ = -(stroke/2)*omega*[sin(theta)+.5*(stroke/2)*(1/rod)*sin(2*theta)]

This formula assumes that [(stroke/2)/rod]^2 is less than .1 and therefore you can ignore [(stroke/2)/rod]^4 and higher power terms back in the position (x) equation that xdot is derivated from.

 

Man, this has been gone over so many times. Peak piston speed is a function of R/S, RPM and Stroke. Average piston speed of course in unaffected by rod length. Piston speed peaks around 72-77 degrees depending on how much rod you are running at a particular stroke. As CompAirflow says most of the ultrahigh perf stuff has short rods.

Short rod engines will lock the smaller heads up earlier due to their higher pressure drop and I think as CompAirFlow said do need a slightly bigger port. They also have more blow down time as they linger near BDC longer than the long rod engines do.

CompAirflow, I have never heard of PST cams that big on the intake side at 290-305! I know the good ones I've seen were around 278 or so max including the one that had the mph record. Maybe they were all more big port small cam rather than the big cam small port people?

Short rods also give immense packaging dividends allowing much better intakes under some classes set hood height or scoop heights. They also allow for much shorter pushrods which is a big deal with really high rpm engines. I've seen many types of PST deals here too and they all run the short deck and short rods. I think a few guys actually tried to run a real sbc at the beginning and it didn't last long with the short deck and canted valve stuff there.

 

 

Where is a good place to reasearch different style blocks available..... or ones that have been tried before and didn't workout worth a pile of beens and were discontinued, haha. I'm very interested in whats been tried already since most of the new stuff is secret if its giving the user an advantage.


-brent

 

Thats one of a couple things i was thinking about last night. I was certain it had been tried before. What are the drawbacks that keep it from being used in a production engine? I can see that the machineing would be more complex because the lifters angle would need to closer to horizontal and a L shaped rocker used. Is there more friction and wear with the lifters being closer to horizontal?

-Brent

 

I think the biggest drawback to a higher cam position is a longer cam drive, and maybe interference with some intake manifolds. I don't think lifter angle plays any part. Boxer engines have used hydraulic lifters, solids, rollers, etc. Roller lifters probably cause the most lifter bore wear because thay can side load the lifter. Flat lifters don't generate much side force. If the pushrods are in line with the lifter body, loads are almost completely axial (some Mopars excepted).

The LS family has a place in the block above the cam that could possibly be used for a second cam without much design changing.
I still like 2 cams in the block: top one working 2 intakes with very short pushrods, and the lower one working the exhaust(s). You could have variable phasing in one or both, and DOD. You could even do it with just 2-valves. The phasing and more revability would be the advantages, IMO. Unfortunately, GM doesn't listen to me.

There's been a lot of noise recently about the 3-valve head for the so-called "Gen IV" engine. According to WARD'S Autoworld, Alan Hayman, manager-advanced concept group in GM Powertrain's Advanced Powertrain unit (boy, that's quite a title), would not confirm a production launch for the 3-valve, but said it would come after the 'Gen IV" is introduced sometime in 2004.

The 3-valve head is for V6 and V8's and because it used standard lifter position, Displacement on Demand (DOD) can be used. Only a few machining operations to the block are necessary, and the cost for a 3-valve pushrod configuraton is about 1/3 the cost of gong to a 4-valve OHC, at least in a V6. It's worth 10-15% more hp, according to Hayman.

If you haven't seen the drawings, the 3-valve uses 2 intake valves, a central spark plug and an exhaust valve across the bore from the intakes. There is a very short transfer pushrod and a transfer arm. The two intakes are actuated by a y-shaped rocker arm.

To quote Hayman: "The 3-valve is a way in the future. There are some surprises in store for the 2-valve (small-block V8), and we're not talking 10-15 hp."

Should be interesting the next year or so.

Unstable Bob still might get his Dominion running before GM hits the road with the 3-valve.

 

OldStroker,

""Short rod engines ....linger near TDC longer than the long rod engines do.

How does that work? Doesn't the "linger time" approach zero as the rod length approaches zero? I've always thought that longer rods make the piston "linger" longer at TDC. Isn't it just trig?""

Yeah I meant BDC! I edited the post. Now you biotches are going to pick apart everything I say! Anyway good work since I never noticed it at that time. All rod and crank assemblies dwell or linger more at BDC never TDC whether the rods are "long" or not. An infinitely long rod would be equal and have simple sinusoidal motion. Longer rods move slower at TDC only as compared to shorter rods with the same stroke.

It is just trig like so much of this and yet people can't understand and think they will change a rod and net 70 hp or something.

 

Yeah, many folks are looking for that "magic bullet" that will make lots of hp, improve fuel economy, driveability and probably their sex life, last forever, and not cost an arm and a leg. It's like the "magic cam" that does everything much better than any OEM cam. Almost every forum has that question, or some form of it almost daily. IMO, a lot of people have made a lot of money selling those magic beans...oops, cams. Barnum had a point.

No offense intended, EK.

Jon