Chapter 5 Fundamentals of Engine Construction and Operation 81
due to the tighter-than-normal clearances between moving
parts. An excessively hot engine tends to ping, diesel, or be
hard to start. It may also be hot enough to melt the exhaust
valves. This is usually called valve burning.
Some of the engine heat is removed by the exhaust
gases and some radiates out of the engine block and heads.
The rest of the heat must be removed by the cooling system.
The amount of heat removed must be controlled so that the
engine does not run cooler than its normal operating tem-
perature. An engine that runs too cold wastes fuel, drives
poorly, pollutes the air, and quickly wears out.
All cooling systems remove excess heat from the
engine and transfer it to the surrounding air. The two main
kinds of cooling systems are liquid cooling and direct air
cooling. In liquid cooling, the heat is absorbed by a liquid,
which then transfers it to the air. In direct air cooling, the
heat is directly transferred to the air.
Liquid Cooling
All cars and light trucks manufactured today have a
liquid cooling system. The engine block and cylinder head
has many internal passages through which coolant circu-
lates. The coolant is moved through the cooling system by
a pump. As the coolant circulates through the passages,
it picks up heat from the surrounding metal. The coolant
then flows from the engine into the radiator. As the cool-
ant travels through the radiator, heat is transferred from the
coolant to the air passing through the radiator. A fan draws
air through the radiator at low speeds. At higher speeds,
the movement of the vehicle forces air through the radia-
tor. The cooled liquid then returns to the engine to pick up
more heat. Figure 5-14 shows a typical cooling system.
Engine Coolant. The engine coolant is the medium of
heat transfer. It must be able to absorb and release heat
without damaging any cooling system parts. Engine coolant
Figure 5-14. Components of a typical cooling system. As coolant circulates through the engine, it picks up heat. This heat is trans-
ferred to the air by the radiator.
Lower (return)
radiator hose
Engine oil
cooler lines
Coolant pump
Oil filter
Heater hoses
In-tank heat
exchanger
In-tank heat
exchanger
Fan and motor
Radiator
Heater core
Coolant first flows through
the heads and then block
Transaxle fluid
cooler lines
Thermostat
housing
Upper radiator
hose