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| Technical Highlights |
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At the onset of the Americanization of the Merkur XR4Ti,
the power that be at the Ford Motor Company made two things perfectly clear; the
engineers would not be permitted to "screw-up" the car when the U.S.-spec
turbo-four engine was substituted for the German-built V-6, and the resulting hybrid
would not go on sale until it was right.
The noise, vibration and harshness (NVH) specialists went
to work with these mandates and did things that had never before been done in America.
Their labors paid off handsomely, and the four-cylinder Merkur is a model of civility
from idle to redline.
The cause of concern centered on the second-order vibration
that is inherent to all four-cylinder in-line engines. Balance shafts and other anti-shake
schemes were not within the scope of the Merkur program, so the Ford engineers had to work
outside the engine. Their two-pronged attack was aimed at minimizing the vibration of
the engine's external components and reducing the magnitude of the shaking forces that
are transmitted into the body structure.
Every engine-mounted accessory is like a tuning fork: when
excited by an external force, it will vibrate at its own natural frequency. If this
natural frequency coincides with the frequency of excitation forces from the engine, the
result is an annoying level of NVH. Knowing that 7500 rpm was the upper limit of the
engines operation, the Ford engineers quickly calculated that all accessories should have
a natural frequency at or above 250 hertz, if possible, to minimize harshness through the
entire rev range. An attempt was made to raise the natural frequencies of the alternator,
the air-conditioning compressor, the power-steering pump, the turbocharger's waste-gate
actuator, and intake manifold toward the 250-kertz goal by making stiffer mounting brackets
and lighter cantilevered masses. In the case of the intake manifold, a completely new
design was needed (the old one vibrated at 170 to 200 hertz); nonetheless, management
eagerly gave the go-ahead, because the original two-peice, four-runner manifold has been
a prime source of harshness in Thunderbirds and Cougars. The new manifold is still a
ram-tuned design, but now the runners are arranged in a square, rather than a linear,
pattern. Several internal and external reinforcing ribs have also been added to stiffen
the structure.
Once the engine and its accessories were calmed down to
acceptable levels, the mount experts were called in for consultation. Very soft rubber
pads were specified to limit the transmissibility of the remaining structure, but this
posed two new concerns: controlling the engine's roll and limiting the extent of its
movement over large bumps. To manage roll with the soft mounts, outrigger-type brackets
were developed to give the engine the widest practical mounting base. To damp out gross
engine excursions over bumps, special hydraulic mounts were specified for use between
the outrigger brackets and the body. These steel-and-rubber assemblies (made by Paulstra,
a French firm) contain chambers filled with hydraulic fluid; the engine's motion is
damped as the fluid is forced through internal orifices.
Although the hydraulic engine mounts are new to Ford products in
the American market, the engineers are quick to point out that there's really no Buck
Rogers here. Their success is simply a result of painstaking attention to detail. In other
words, this is one more case where the little things made the biggest difference in
transforming a good car into a great one.
--Don Sherman |
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