THE ROLE OF THE

Power Train Applications 101
lem. However, microscopic particles will invariably be produced from wear and cause
erosion. The oil filter is another design feature that will help with the overall process.
Engine oil lubricates rotating parts in the crankcase of a vehicle as it is applied
to crankshaft journals, rods, and bearings. During testing and design of oil coolers,
thermocouples are placed at the engine sump at the bottom of the crank to measure
temperature of the oil. In many engines, turbulators are placed in the oil pan to
spread the oil on the reciprocating members of the engine. The oil pump will send
oil through the filter into the galleries and across critical parts such as the camshaft
bearings. Oil from holes in the main journals will move through passageways inside
the crankshaft to exit holes in the rod journals to lubricate the rod bearings and
connecting rods. Passageways through the rods carry oil from the rod bearings to
the rod–piston connections and lubricate the contacting surfaces between the piston
rings. Interior surfaces of the cylinders are lubricated from oil coming from the rod
bearings. There it will be a barrier between the piston rings and cylinder walls before
oil returns to the sump.
7.6 engIne oIl grouPs
Table 7.1 shows engine oil groups from the API. Within the five oil groups, group
I is base stock from fractionally distilled petroleum with some further refinement.
Group II is base stock as in group I, but it has been hydrocracked for purification.
Group III base stock will have a higher viscosity index than group II due to further
hydrocracking. Group IV oils contain polyalphaolefins and group V oils are a group
used to catch any oil not described by one of the other groups. The synthetic oils
discussed later are contained in this latter group.
7.7 engIne oIl grades
Engine oil is graded based on an SAE coding system based on kinematic viscosity.
The grades range from 0 to 60 in increments of 10. An oil grade with a “W” behind
it designates a winter or cold-start grade; this means that the oil will have a better
flow viscosity at lower temperatures. For single-grade oils, viscosity is measured at
100°C and the range that the viscosity falls in is called its weight. A single-grade
oil’s viscosity will increase on a logarithmic scale as the temperature increases.
A specific motor oil will have a high viscosity when cold and a lower viscosity
when hot. The viscosity difference in single-grade oils will be too large to provide
adequate protection for today’s vehicles. Viscosity improvers such as polybutene are
added to decrease the difference between the two extremes. It is these additives that
designate a multigrade oil, which will have the viscosity of the base number when
cold and the viscosity of the second number when hot. Multigrade oils have two
grade designations. For instance, the “30” in the 5W30 grade means that the viscosity
of this oil at 100°C corresponds to the viscosity of a single-grade 30 oil at the
same temperature. The first number, “5,” is associated with the winter measurement
and is not rated at a single temperature. A multigrade with a first number of 5 can be
utilized as well as a single-grade 5. SAE procedures test a 5W oil at –30°C; a 0W oil
is tested at –35°C and a 10W oil is tested at –25°C on a cold crank simulator.