Date:         Mon, 30 Sep 2002 02:51:29 EDT
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      On I4 Engine Vibration
Content-Type: text/plain; charset="US-ASCII"

I've now seen several posts suggesting that the vibration of the I4 engine is
a function of mounting angle: 0 degrees, 15 degrees, 50 degrees, ...
poppycock! The vibrational characteristics are a function of the primary
oscillator (engine) and the coupling to the media (body). Now poor coupling
modes can lead to a bottleneck for certain frequencies and those frequencies
can be mechanically amplified (made easier for human detection) by
resonances. Often resonances are hard to calculate and must be measured for a
given application.

The key difference in vibration for any mounting angle is in the details of
the motor mount used!

Now the diesel vanagon was originally developed by VW for the German
PostOffice. Its design was optimized for a city transit commercial vehicle.
The level of vibration for the diesel was reasonably managed in the design.
Now the diesel was a 1.6L thumper, the majority of the GJ conversion engines
were 1.8L, the Audi 3A, the ABA engine and the TIICO SA unit are 2.0L. None
of these engines used balance shaft technology to neutralize 2nd and 4th
order vibrations. The intensity of vibration (attributed to unbalanced 2nd
and higher order moments) increases with the square of the displacement, so 2
liter much worse than 1.6. When Porsche designed the 944 engine the design
target was 2.5L and no non-industrial 4 had been built for 6K and higher revs
at a size greater than 2.2L. (See the tech papers archive on Alistair Bell's
web site). They had to add balance shafts and had to pay Mitsubishi patent
royalties.

Now, Volvo, SAAB, MBZ and others have built smooth running large displacement
I4's (up to 2.0L) without balance shafts. In this premium market, vibration
was unacceptable. The solution lies in the approach to vibration dampening -
active or passive.

VW relies on passive rubber elastomers and bushings to suspend the I4 diesel.
But as I have noted, the vibration amplitude increases with displacement. For
the GJ cars, VW uses a combination of passive and active mounts (hydrolastic
isolators). The larger Audi's with the smoother 5 cylinder engines use
hydrolastic mounts, as do Volvo, SAAB and MBZ.

Now the technology of these mounts is quite interesting. There are very
similar to shock absorbers tuned for very modest travel (less than +/- 1 cm).
After considerable study, I have modified the Vanagon engine carrier to
accept the SAAB hydrolastic mount. I have engineered the revised mount to use
the same intersection points as the stock vanagon engine attachment mounts.
These mounts also include mechanical stops to keep the engine located should
the mount fail. I have also incorporated the mount heat shield approach as
used on the Audi 5000T. Typical lifetime of these liquid filled mounts is
50,000 miles (only).

I will report of the differences in the vibration spectrum when I get the
time to mount them. Until then, the vibration is a function of the mount
(includes the carriers or sub-frame mounting) not the mounting angle!

Frank Grunthaner