Re: fizzicks question

["Jouko Haapanen" <joukoh@vtoy.fi>]

Hello again guys/gals, did we switch from the infinitely accelerating
pistons to this?  Let's look for some partial differential equations next
time.  ;^)

Torque, the turning effort of the crankshaft, is independant of engine
speed.  Its value depends on the pressure on the pistons, the piston area,
and the length of the piston stroke.  Since the bore/stroke dimensions are
fixed, any increase in mean effective pressure (mep) means an increase in
torque and vice versa.  Thus, the mep curve and the torque curve follow a
similar curve on a performance graph.

If the torque of the engine remains constant, the bhp will increase in
direct proportion to the speed.  The brake mean effective pressure (bmep)
falls off as the rpm inreases due to the inability of the engine to inhale a
full charge in the diminishing time of the intake stroke.  Frictional losses
increase with higher rpm as well.

To confuse the issue even more, this time I've dug out a book full of BTU's,
inches, feet, centimeters et al, all mixed up in a mumble jumble.  It ssems
that the other books with more modern SI references are at work..

One can measure the mep on a test bed, calculated from the power, and thus
get bmep (brake mean effective pressure).  This is a value that allows for
mech losses etc, but is good for comparing designs.

bmep in psi:
bmep=(33000 x bhp) / (l x a x n)
where l-stroke in feet, a-piston area in sq.in., n-no. of power strokes/min.

torque in lbf/ ft
torque=(bmep x swept volume in cm3)/2473

therefore,
torque=(((33000 x bhp) / (l x a x n)) x swept volume)/2473

or,
bhp=rpm x torque / 5 250
where torque in lbf/ft

oh yeah, one more thing, take the ft.lb or lbf/ft torque, multiply by 1.356
and you'll have something meaningful...
The torque in Nm seems so much simpler today, I don't remember how I
survived school with two bloody systems of measurement.  I guess that the
horsepower, being 550 ft lbf/sec, could be converted to kW by multiplication
by 0.746 too.

I must be rambling, it's midnight, and I'm hitting the sack.

Jouko Haapanen
Pori, Finland
83 HP41C
84 HP15C