WAY COOL: HERE’S WHAT YOU NEED TO KNOW ABOUT YOUR ENGINE’S COOLING SYSTEM AND THE COOLANT THAT FLOWS THROUGH IT.
Engine Cooling System
The tubes on the closed cooling side of
this heat exchanger were fouled by a film that required solvent
flushing, after which the change in color was clear to see
Most marine diesel engines rely on what’s
known as a closed cooling system. As with an automobile engine, the
heat of combustion is absorbed by antifreeze (or, more accurately,
coolant). In a car, that heat is then released airborne, via a radiator;
with a sailboat, it’s transferred to the ocean via a heat exchanger.
Closed cooling systems, which typically
operate at between 180 and 195 degrees F, offer several important
advantages over open cooling systems. The latter allow seawater to flow
through the iron engine, the predictable result of which is corrosion.
Additionally, open cooling systems must operate at a cooler and less
efficient temperature, typically around 150 degrees F, to prevent salt
from precipitating out of the seawater and depositing itself within the
cooling system’s passages, where it acts as an insulator and reduces the
heat transfer efficiency. This comparatively cool operation promotes
the formation of carbon within cylinders.
Perhaps the most important advantage of a closed
cooling system is the pressure under which it operates, typically
between 6 and 15 psi. The pressure is important for a couple of reasons:
First, while coolant already has a higher boiling point than ordinary
water, the pressure further increases the boiling point of the coolant
(for each pound of pressure applied, the boiling point rises about 3
degrees F). Second, and even more important, the increased pressure
reduces the occurrence of cavitation within the cooling system.
Cavitation, the rapid creation and violent collapse of vacuum bubbles,
can be destructive, as each implosion can scour away metal from the
cooling passages.
Unless you
purchase it pre-mixed, coolant should be mixed in equal parts with
distilled water. (Distilled water contains no minerals, which can be
deposited inside the closed cooling system.) How often should you
replace coolant? Many engine manufacturers specify two-year intervals.
However, depending upon the cost of replacement, sending the coolant to a
lab for analysis, may make more sense, especially if it extends the
period to three or four years. (Some extended-life coolant is rated for
six years or 14,000 hours, but it should also be analyzed annually.)
Alternatively,
coolant test strips can be used for field analysis, although they yield
less definitive results. When you do replace or top up coolant, avoid
mixing brands and types, and stay with those recommended by the engine
manufacturer; at the very least, use coolant designed specifically for
diesel engines. Every few months, when the system’s cold, closely
inspect the pressure cap and neck for corrosion, gasket failure or other
defects. When doing so, check the coolant as well. It should look clean
and bright; green, blue or orange; and never rusty.
Now back to that chronically overheating engine. The
culprit turned out to be a little-known phenomenon wherein a film forms
inside the closed coolant side of the heat exchanger. The result of a
previous overheating episode, the film inhibited heat transfer, which
led to the chronic high-speed overheating. Flushing the heat exchanger
with a proprietary solvent removed the film and put the temperature
gauge back in the green. The moral of this story is twofold: One,
systematic analysis will eventually lead you to the cause of the
problem. And two, once overheated, coolant should always be replaced and
the closed cooling system flushed to prevent future overheating.
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