Chapter 5 Fundamentals of Engine Construction and Operation 85
the cooling system during service. Pockets of air in the cool-
ing system can block coolant flow, resulting in hot spots or
engine overheating.
Engine Belt. An engine belt drives the coolant pump
and other engine accessories. The belt is called a serpentine
belt because it winds around several pulleys, Figure 5-22.
It is driven by a pulley on the front of the crankshaft. The
belt must be in good condition and properly adjusted. Most
engines have an automatic tensioner to keep the serpentine
belt properly adjusted.
Excessive tightness in the belt places a heavy load on
the bearings in the coolant pump and possibly the crank-
shaft bearings. This will cause premature wear. Looseness
in the belt permits slippage. This can reduce the speed of
the coolant pump, alternator, and belt-driven fan. As a
result, the engine may overheat or the battery may become
discharged. An excessively loose belt will have a tendency
to whip or flap, which can cut hoses or cause intermittent
loading of the bearings in the coolant pump.
Direct Air Cooling
Direct air cooling is a method of directly transferring
engine heat to the surrounding air. It uses a fan to force air
around the cylinders and cylinder heads, which are the hot-
test parts of the engine. The fan is driven by the crankshaft
via a belt. The cylinders and heads of an air cooled engine
are made with fins to present a larger heat transfer surface.
The engine is surrounded by a sheet metal shroud to direct
more air over the hottest engine parts. Air-cooled engines
are no longer used on cars and light trucks.
Lubrication System
The lubrication system circulates engine oil to internal
engine parts. Engine oil serves several purposes. It must be
delivered to the right place at the right time and in the proper
quantity. The lubrication system may have no apparent effect
on driveability, but it is a vital part of the engine and some
defects in it can result in driveability symptoms. Also, some
driveability problems can affect the lubrication system.
The various moving parts of the engine are subjected
to differing levels of stress. The amount of oil needed var-
ies between engine parts. Some parts of the engine are
lubricated by pressurized oil, while others are lubricated by
splashing oil. Figure 5-23 shows a typical pressure/splash
lubrication system.
The camshaft and crankshaft bearings are pressure lubri-
cated. Also, hydraulic lifters must have a supply of pressurized
oil to work properly. On some engines, pressurized oil drips
through nozzles onto moving parts such as timing gears.
Some engine parts can be lubricated by oil that is
splashed onto them. Oil is thrown upward by the rotation of
the crankshaft. This oil strikes the engine cylinder walls, pis-
ton skirts, and piston pins. Some splashed oil also reaches
the camshaft and lubricates the cam lobes and valve lifters.
Engine Oil
The engine oil, often called motor oil, provides lubrica-
tion between moving parts. Oil is made to do a good job of
lubrication, sealing, cooling, shock absorption, and clean-
ing. It is also formulated to prevent sludge formation.
Figure 5-22. Engines use one or two serpentine belts to drive all of the accessories. On this hybrid vehicle, a starter/generator is used
to restart the engine by driving the belt to turn the crankshaft. (Daimler)
Coolant pump
Serpentine belt
Crankshaft
Starter / generator
Hydraulic belt tensioner
A/C compressor