| Two years ago, in response to a
question about the longevity of Yanmar diesel engines, I wrote
that I had serious reservations about the extensive use of cast
aluminum components on those engines. Up to that time, we hadn't
heard of any serious problems with the engines, but then, they
were fairly new on the market at that time.
I now have the answer, along with
documented, photographic evidence of how well those aluminum
parts are holding up.
My original doubts were founded
on other engine company's attempts to use cast aluminum. First
came Johnson & Towers, the remarketers of Detroit Diesels.
Some time in the late 1960's they tried using a cast aluminum
heat exchanger tank. In those days, aluminum alloys weren't
so good, and tank wide spread failures occurred. Nor did
it escape my attention that it was J&T who tried this, and
not General Motors. Seems the GM people knew better.
Not to learn from other's mistakes,
soon it was Perkins Engine Company who tried aluminum parts.
My own father was a victim of this debacle when both the combined
heat exchanger/exhaust manifolds on his engines failed. Replacement
cost was a whopping cost of $6,000 each!
Next, along came US Marine and
their marinization of the Japanese Hino diesel. Same story,
a combined aluminum heat exchanger/exhaust manifold casting
at an equally horrendous cost to replace.
Not to be outdone, now comes Yanmar
with the very same combined aluminum heat exchanger/exhaust
manifold castings. Shown in the photos below are the very predictable
results. In this case, both castings on both engines failed
at about the same time.
 |
 |
Galvanic
corrosion inside heat
exchanger tank. |
Attempts
to patch holes from exterior proved futile. |
So what's wrong with using aluminum
parts? Dissimilar metals. Here we had three engine companies
that tried to brass heat exchanger bundles into an aluminum
tank. Brass and aluminum are far apart on the scale of electrical
potential of metals. Hence, the aluminum becomes anodic to the
brass, just like the zincs you use on your shafts are anodic
to stainless steel. It doesn't take long for them to disappear.
Even though the cooling system
is filled with an anticorrosive coolant, that is not going to
stop galvanic corrosion.
Bottom line: The effort to make
diesel engines lighter by using aluminum components for the
cooling system isn't going to work. And, of course, these parts
didn't finally fail until the engine was long out of warranty,
so the boat owner had to foot the bill.
Fortunately, there is a better
way to reduce the weight of cast iron tanks. Most other manufacturers
are using formed brass plate tanks. Brass tanks, brass tube
bundles, no problem. And the cost is a lot less than a
machined casting.
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