DIY_EFI Digest V5 #952

yahoo dhunt16 at yahoo.com
Thu Jan 3 19:54:59 GMT 2002


Date: Wed, 19 Dec 2001 05:28:46 -0800 (PST)
From: skulte <skulte at skulte.com>
Subject: Re: Maximum Piston Speed

> > the primary problem with high piston speed on US designed engines.  As
it
> > turns out the maximum piston speed is at the same point that maximum
> torque
> > is being transferred to the crankshaft.

Not really. Max torque is a combination of cylinder pressure and crank
angle. You should see max cyl pressure somewhere near 10 degrees after
TDC. Max crank angle will be closer to 90 degrees, where the pressure has
gone down by orders of magnitude. Max instantaneuous piston speed will
always be roughly halfway between TDC and BDC. That's not what you should
be concerned about. The max acceleration forces put the force on the
rotating assembly, which is what you should  be designing for, based on
RPM and torque expected. The piston speed shouldn't have anything to do
with ring sealing. It's the flutter at the top of the stroke, as the
piston begins to come back down. Just my 2 cents..

- ------------------
Andris Skulte
Skulte Performance Designs
http://www.skulte.com
Z28tt-89 IROC T56 DFI Twin Turbo

Actually, I said maximum torque.  Your assertion that maximum cylinder
pressure goes down by orders of magnitude is quite incorrect.  For example,
if the maximum pressure is at 10 degrees ATDC for a small block Chevy with a
10:1 compression ratio, 100 % filling the compression volume at TDC is ABOUT
4.3 cu. in.  At 10 degrees past TDC the volume is about 7.6 cu. in. in the
cylinder and 4.3 cu. in. in the non-active volume.  Or a compression of
about 3 to 1.  At 90 degrees down the compression ratio is about 2 to 1.
The volume has changed by about 50 per cent between the two locations and
the pressure is not changed all that much.  You could argue that there is
some pressure loss due to gas cooling, but that discounts the fact that the
mixture is still burning.

Yes the cylinder pressure is higher at TDC to about 35 degrees after TDC but
the pressure isn't translated into torque because of the angle of the crank.
For example, consider a cylinder pressure of 400 PSI at 10 degrees and
100PSI at 90 degrees.  Transferred torque for each is

cyl pressure * sin(angle) * piston area * stroke

so for a 350 chevy the instantaneous torque at each level is:

400 * sin(10) * 12.256 * 3.48/12 = 246 ft lbs

100 * sin(90) * 12.256 * 3.48 = 355 ft lbs

In fact the 4:1 temperature drop isn't realistic it's much less than that
(as noted above.)

In fact, that is the problem.  The piston sealing is a function of piston
top pressure.  The piston rings are pushed into the cylinder wall by
combustion pressure.  So the max piston speed is at 90 degrees ATDC and
maximum torque is at fairly close to the same area.

Piston speed is probably more important to me because I'm more impressed
with an engine that gives good performance at 100,000 miles not at the
quarter mile mark.  If I were working for 1/4 mile, I would concentrate on
strong cranks and rods, because you can make easily make all the power an
engine can hold for just a few miles.

Consider these numbers for a top fueler at 8000 RPM one of these engines
will run for a total of about 5 seconds at full speed before a rebuild.  In
5 seconds the engine turns about 670 revolutions at FULL LOAD.  For a total
of about 15,000 revolutions per rebuild.  Many of these engines (more after
the oil down rules) don't make the 14,000 revolution mark.  On the other
hand your average OTR diesel will make 400 horsepower at 2,300 RPM for about
5,000 hours or about 600 million revolutions.  Millions of the diesel
revolutions will be at full power.

Piston speed is important to these numbers because the !@#$% that gets past
the rings is especially destructive to the engine.   I also forget to change
the engine at regular intervals.  I also let the cars set for long periods
(for example, the Camaro and GN are put up in the winter and I haven't
driven the van since the end of boating season)  For these reasons, a good
seal over a long life is very important to me.

The difference between a good ring seal might be .2 seconds (20Hp) in the
quarter mile, and 50,000 miles on the road.

dh




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