From: eliot@stalfos.engr.washington.edu (eliot)
Subject: Re: flat engines
Organization: clearer than blir
Date: Fri, 16 Oct 1992 23:30:38 GMT

In article <1992Oct16.221643.18573@newsgate.sps.mot.com> mark@wdcwdc.sps.mot.com (Mark Shaw) writes:

thanks for explaining the complex manufacturing process of a flat engine.

>Aside from the balance (which is not always true -- look at how a 4-cylinder
>Lycoming shakes!) and a possible lower CG there is little to offer with
>flat designs.  And these are secondary performance issues.  

what is a "Lycoming"?  sorry, i've never heard of it.  as for
vibrations, it is probably appropriate for me to repost the original
article in its entirety here, and i'll let you critique it.

eliot

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[Guess what guys?  I dug up a 1977 issue of CAR that answers the question
 and has even more stuff.  Written of course by LJK Setright]

The inline 5 is perfectly balanced in terms of even firing, primary
and secondary forces and couples (the significant modes of
vibrations).  It is in these respects better balanced than the inline
4, which suffers from an unbalanced secondary couple, which is felt as
the familiar 4 cylinder buzz.  

However, the inline 5 suffers from an "oblated conical couple" which I
gather to be a longitudinal shake along the crankshaft axis.  From the
same article Setright writes that "Audi has shown that the straight
five can be made to work tolerably in a fwd configuration, because the
integrated transmission system provides a really long base for the
bearers to support the engine's cyclically variable wobbling."

Some more interesting comments:

horizontally opposed engines:

"open [the vee angle] out to 180 deg and you get the most perfect V of all,
a flat one:  with horizontally opposed cylinders, all problems of vibrations
disappear.  Horizontally opposed engines are the best kind to have, whether
they number 2 cylinders, 4, 6 or even 12; but they are very expensive.."


inline sixes:

"the inline six is perfectly balanced, both as to primary and secondary
forces and all couples, the only theoretical blot on its copybook being
a 6th order vibration so tiny it can always be ignored."


flat sixes:

"With adjacent cranks set apart by 180 degrees the flat six is
delightful, and the late lamented Covair was proof that such an
apparatus need not be as costly as when Porsche make it.
Nevertheless, it is no better balanced than the much cheaper inline
6."


[lots written on the different kinds of V6s, including Lancia's narrow
angle V.  but too much for me to type in, so I will skip it for now]


Mitsubishi's balance shafts:

"it is possible that they [went] too far, for not only did they try to
eliminate the secondary vibrations in the plane of the cylinders
(originating from the reciprocating masses, and fairly easily
nullified by supplementary balance shafts as Lanchester showed ages
ago), they also tried to balance out vibrations in the plane of
crankshaft rotation which originate from combustion forces
successively applied twice per revolution and thus in phase with the
inertial seconday forces.  to get rid of the latter, which increase as
the square of engine speed, two diametrically opposed counter-balance
shafts are all that are necessary.."

[gory details on how to do it edited]

"even this refinement does not deal with the rolling moment
originating from successive explosions.."

[..]

"Here at the end is merely another compromise to replace the one with
which they began, before adding the shafts; and it is an insoluble
problem, because the combustion moment varies with engine load.
mitsubishi tried to find a setting which would give the best results
in the operational mode most frequently encountered, a part throttle
mid range condition.  on the overrun or under full load the system
does not work correctly, and no other permutation of the devices
chosen can ever be right all the time."

"it hardly seems worth bothering: the inline 4 is a sow's ear, from
which a silk purse cannot be made - not even artificial silk. "


eliot