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Basically, to use that irritating in-word, engine
lubrication is simple, and consequently boring.
So I intend to treat the subject 'complicatedly',
which may not be an in-word, but makes life far more interesting!
So, to take a quick look at the simple picture; the oil keeps moving
parts apart, reducing friction and carrying away heat. Where there
is metal-to-metal contact there are chemicals in the oil to reduce
the damage. Because the internal combustion process is always less
than perfect, some soot is produced and this must be washed off
the pistons and rings by the oil, so it has a cleaning or detergent
function.
The trouble is, all this is just as true for Henry Ford's original
Model T engine as it is for any high performance engine. So where
is the difference? The Model T, with 10b.h.p. / litre at 2,000 revs
per minute and a single underhead
camshaft, was filled with a thick, greenish liquid from somewhere
near the bottom of the distillation columns on the Pennsylvania
oilfields. It did a vague tour of the internals by guesswork (there
was no oil pump) at a temperature
around 50 C, and lasted for 1,000 miles. On the plus side, some
of the impurities acted as anti-wear and detergent chemicals. They
didn't work very well, but it was better than nothing. The engine
wore out in around 20,000 miles, but even ordinary people, not just
amateur rally drivers, were happy to put up with this.
The difference begins with the first turn of the key. The modern
high-pressure pump would cavitate on the old heavy monogrades, starving
the bearings for a vital couple of seconds, even in warm weather.
Likewise, cam lobes would suffer as the sluggish oil found it's
way along narrow oil ways to the valve gear. The turbo bearing ('if
fitted', as the handbook says) already spinning fast, would also
starve, and when it got going, how long would it be before the heat
soak-back fried the primitive oil into a lump of carbon?
(This was a problem even with 'modern' oils only 15 years ago.)
So, a good oil must be quite low in viscosity even in the cold,
so that it gets around the engine in a fraction of a second on start-up.
On the other hand, it must protect engine components (piston rings
for example) at temperatures up to 300 C without evaporating or
carbonising, and maintain oil pressure.
Unmodified thin oils simply can't manage this balancing act. The
answer is to use a mixture of thin oil and temperature-sensitive
polymer, so as the thin oil gets even thinner with increasing temperatures
as the engine warms up, the polymer expands and fights back, keeping
the viscosity at a reasonable level to hold oil pressure and film
thickness on the bearings. This is called a multi-grade.
But this is all too basic!
What I have just written was and is relevant to a 1958 Morris Minor.
The questions that high performance engine owners need to ask are
"Will this thin base oil evaporate and be drawn into the intake
manifold (via the closed circuit crankcase ventilation), leading
to combustion chamber deposits and de-activated catalysts?"
and "Will the polymer shear down at high engine revolutions
and high temperatures, causing low oil pressure and component
wear?" and "Will it carbonise on the turbo bearing?"
These are 21st century questions which cannot be answered by a basic
1990's approach.
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