WINTER 2014 | MARINE TECHNICIAN TODAY
15
T
he ongoing march to achieve
more technologically advanced
marine engines continues and
certainly the modern high performance
marine engine exemplifies that quest.
The competition between GM, Ford and
Volvo has benefited you and me: the
improvement in all aspects of these
engines is easily quantifiable in terms of
horsepower and torque as well as fuel
efficiency and endurance. Recognizing
how vastly improved these marine engines are to their predecessors,
it should not surprise anyone that advances in the lubricants for
these engines have also facilitated quantum leaps in performance.
Any oil, properly rated for use in a high performance marine engine,
is a remarkable lubricant regardless of the base oil used. In this
article, I will compare synthetic engine oil to petroleum engine oil and
draw some conclusions and make some recommendations. Previous
articles have established fundamentals of lubrication and how oil is
made, so if you haven’t read those, a review might be in order. I am
writing this article assuming you have read the preceding articles.
THERMAL STABILITY
How well does the oil hold viscosity as temperature increases?
This is reflected in the Viscosity Index (VI) rating, with a higher
number indicating greater stability. Petroleum oils rarely exceed
100 on the Viscosity Index while some synthetics rate higher than
180. Oils that maintain rated viscosity – instead of thinning out
at higher temperatures – perform better in your engine. Thin oil
will reduce film strength and result in higher wear rates of critical
engine parts.
Thermal Stability Advantage:
Strong for Synthetic Oil
Higher viscosity index (VI) liquids are less responsive to
temperature extremes. At 0°F, the VI 95 petroleum oil is thicker
(measured in centistokes, a dynamic measure of resistance-
to-flow) than the synthetic oil with a VI of 150. On the hot side
at 210°F, the VI 150 synthetic maintains viscosity better than
the VI 95 petroleum oil that thins out more easily.
TEMPERATURE RANGE
What are the highest and lowest temperatures the oil can tolerate
and still provide proper lubrication, during continuous or intermittent
duty? This range is established by measuring the pour point (lowest
temperature the oil will pour) and the highest temperature at which
the oil can hold sufficient viscosity in order to provide lubrication.
Chart Two demonstrates the superior performance of synthetic oil
vs petroleum oil. Group III hydro-cracked synthetics are not on the
graph and it is important to note that the Group III synthetics will have
similar cold flow performance but significantly less high temperature
performance compared to Polyalphaolefins (PAO). PAO and Dibasic
Acid Esters are the primary chemicals used in engine and drive line
oils. A strong temperature range is paramount to providing proper
lubrication, especially in severe duty or extreme temperatures.
Temperature Range Advantage:
Very Strong for Synthetic Oil
OXIDATION STABILITY
Howwell does theoil resist oxidationandsludge formation?Asoil oxidizes,
it thickens (viscosity increases) and deposits sludge in the engine. Sludge
may eventually clog critical oil passages, preventing necessary oil from
reaching vital engine parts. This causes excessive wear and, eventually,
failure of various engine parts. Synthetics are inert, meaning there are no
polar sites (having positive or negative polarity) and simply do not react
with oxygen. Petroleum oils are highly polar (mostly positive polarity)
and readily react with oxygen. To counter this reactivity, petroleum oils
are treated with anti-oxidation additives. When oils are operating in
the intermittent range (temporarily outside the normal operating band)
they are susceptible to higher rates of oxidation. Unfortunately, today’s
engines are forcing oils to operate routinely at 230°F to 250°F. This puts
the petroleum oils in a range of temperatures that causes increased use
of the anti-oxidants in the additive package and shortens the life of the
oil. PAO or Ester based synthetics are in the normal operating band for
temperatures in excess of 330°F and suffer little or no oxidation. This is
one of the reasons you hear of mechanics reporting how clean engines
with synthetics are, even those with high mileage.
Oxidation Stability Advantage:
Very Strong for Synthetics
SYNTHETIC vs PETROLEUM
By DanWatson
CHART ONE: VISCOSITY INDEX (VI)
SAE 50 / ISO 220
5790
VI:95
VI:150
25.9
19.3
2750
0
O
F
210
O
F
TEMPERATURE
VISCOSITY, cSt
TECH TALK
.
