From: rulon@ibms19.scri.fsu.edu (Phil Rulon)
Newsgroups: rec.autos.tech
Subject: Re: Balance shafts, engine vibrations, etc.
Date: 30 Sep 92 20:49:01 GMT
Organization: SCRI, Florida State University


sankar@Xenon.Stanford.EDU (Sriram Sankar) writes,

>I'm interested in some history information on the balance shaft technology.
>I'd like to know who invented it, and which cars today have it (and how long
>they have had it).  For example, I know that 1992 Saab 9000's and 1993 Honda
>Accords have it, but that's the extent of my knowledge.  Seems like many
>other cars must be having it too.  Also, are balance shafts something that
>only 4 cylinder engines have, or do some six cylinder engines have them too?
>For example, BMWs have 6 cylinders in line and they say its the most
>vibration free kind of engine.  Do they use balance shafts to reduce vibration?
>What about V-* engines?  Many others say that V-* engines are the most
>vibration free.  Finally, what is the source of vibration that balance
>shafts can handle?
>
>Hopefully, this message should start off an interesting discussion on
>engine vibration and technology to reduce it.

I'm delighted some one brought this up as it is a bit of a pet project of
mine at the moment.
  Mitsibushi holds a patent on one form of balance shaft technology and it is
used on many makes (Porsche 944 for example).  The idea is very old though.

  The reasons for using it depend on the configuration of the motor, but in all
cases it is used to counter-act an intrinsic vibration in the design.  Triples,
for example have a fundamental vibration in the pitch direction due to the fact
that while the forward piston is being accelerated upwards the rear piston is
accelerating downwards and vise-versa. This can be taken out with one shaft 
parallel to the crank with fly-weights on either end but this introduces a torque
vibration unless the balance shaft is concentric with the crank (mechanically
difficult). Two shafts turning in opposite directions  is closer to optimal.

  Fours (straight) have a better setup in the fundamental mode but still have
a second order tortional vibration which can be taken out with a shaft that is
not concentric with the crank (fortunately) but, since it is second order, the
best setup runs the shaft twice as fast as the crank.  Flat engines (VW, Porsche,
Subaru,etc) don't have this problem because each pair of cylinders (lateral) 
counteracts the torque introduced by the next.

  Straight sixes dont have any of these problems. Consider a six as two triples
180 degrees out of phase with each other and you can imagine why.  Straight
eights are the same.

  V-8s have a slight primary vibration but it is low and is usually taken out
with a fly-weight on the pulley end called a harmonic balancer.

  V-12s, well, what can I say?  Take the smoothness of a six and multiply it 
by two.  And then, of course there is the sound they make, Ahhh...

  The dynamics involved can be summarized by noting that the motion of the center
of mass of the piston and connecting rod system is not concentric with the crank
so no amount of crank-shaft fly-weights can counter-act it.

Other postings solicited...

P

Phil Rulon                                   rulon@mailer.scri.fsu.edu
Supercomputer Computations Research Institute
Florida State University
Tallahassee                                  audio  (904) 644 7008
Lines: 10