ENGINE COOLING SYSTEMS
Principles, Types, Components, and Maintenance of Cooling
Systems in Internal Combustion Engines
,Topics covered in this chapter
Engine cooling systems
Common faults and their diagnosis and repair
Blocked radiator core
Combustion gas leakage into coolant
Coolant loss
Antifreeze and its composition
Vehicle interior heating and air-conditioning systems
Introduction
During combustion, when the engine is operating at full throttle, the maximum
temperature reached by the burning gases may be as high as 1500-2000oC.
The expansion of the gases during the power stroke lowers their temperature
considerably, but during the exhaust stroke the gas temperature may still be
approximately 800oC as shown by Fig 1.
All the engine components with which these hot gases come into contact will absorb
heat from them in proportion to:
Flue gas temperature;
Area of surface exposed to the flue gases
Duration of the exposure.
OVERHEATING
For all these reasons the heat will raise the temperature of the engine components.
If the temperature of the exhaust gas is above red heat it will be above the melting
point of metals such as aluminium, from which the pistons are made.
Unless steps are taken to reduce these temperatures a number of serious problems
could arise:
The combustion chamber walls, piston crown, the upper-end of the cylinder, and
the region of the exhaust port are exposed to the hottest gases and will therefore
reach the highest temperatures. This will create distortion, causing a leakage of
1
, gas, water, or oil. It may even cause the valve to burn or the cylinder head to
crack and as a consequence there will be a loss of power output.
The oil film will be burnt, causing excessive carbon to form. The loss of
lubrication of the piston and rings will cause excessive wear or the piston to
seize in the cylinder.
Power output will be reduced because the incoming mixture will become heated,
so reducing its density. It may also cause detonation (this is an uncontrolled
explosion in the cylinder), making it necessary to reduce the compression ratio.
Some part of the surface of the combustion chamber could become hot enough
to ignite the incoming charge before the spark occurs (called pre-ignition),
which could cause serious damage to the engine if allowed to continue.
For these reasons the engine must be provided with a system of cooling, so that it can
be maintained at its most efficient practicable operating temperature. This means that
the average temperature of the cylinder walls should not exceed about 250oC, whereas
the actual temperature of the gases in the cylinder during combustion may reach 10
times this figure. One of the other things to remember is that the engine should not be
run too cool, as this would reduce thermal efficiency (this is how good the engine is at
converting heat into mechanical power), increase fuel consumption and oil dilution,
and cause wear and corrosion of the engine.
2
, Figure 1: Engine operating temperature ranges Figure 2: Heat is transferred by
conduction from the
valve stem to the valve guide. Both
objects are solid.
Heat transfer
3
Principles, Types, Components, and Maintenance of Cooling
Systems in Internal Combustion Engines
,Topics covered in this chapter
Engine cooling systems
Common faults and their diagnosis and repair
Blocked radiator core
Combustion gas leakage into coolant
Coolant loss
Antifreeze and its composition
Vehicle interior heating and air-conditioning systems
Introduction
During combustion, when the engine is operating at full throttle, the maximum
temperature reached by the burning gases may be as high as 1500-2000oC.
The expansion of the gases during the power stroke lowers their temperature
considerably, but during the exhaust stroke the gas temperature may still be
approximately 800oC as shown by Fig 1.
All the engine components with which these hot gases come into contact will absorb
heat from them in proportion to:
Flue gas temperature;
Area of surface exposed to the flue gases
Duration of the exposure.
OVERHEATING
For all these reasons the heat will raise the temperature of the engine components.
If the temperature of the exhaust gas is above red heat it will be above the melting
point of metals such as aluminium, from which the pistons are made.
Unless steps are taken to reduce these temperatures a number of serious problems
could arise:
The combustion chamber walls, piston crown, the upper-end of the cylinder, and
the region of the exhaust port are exposed to the hottest gases and will therefore
reach the highest temperatures. This will create distortion, causing a leakage of
1
, gas, water, or oil. It may even cause the valve to burn or the cylinder head to
crack and as a consequence there will be a loss of power output.
The oil film will be burnt, causing excessive carbon to form. The loss of
lubrication of the piston and rings will cause excessive wear or the piston to
seize in the cylinder.
Power output will be reduced because the incoming mixture will become heated,
so reducing its density. It may also cause detonation (this is an uncontrolled
explosion in the cylinder), making it necessary to reduce the compression ratio.
Some part of the surface of the combustion chamber could become hot enough
to ignite the incoming charge before the spark occurs (called pre-ignition),
which could cause serious damage to the engine if allowed to continue.
For these reasons the engine must be provided with a system of cooling, so that it can
be maintained at its most efficient practicable operating temperature. This means that
the average temperature of the cylinder walls should not exceed about 250oC, whereas
the actual temperature of the gases in the cylinder during combustion may reach 10
times this figure. One of the other things to remember is that the engine should not be
run too cool, as this would reduce thermal efficiency (this is how good the engine is at
converting heat into mechanical power), increase fuel consumption and oil dilution,
and cause wear and corrosion of the engine.
2
, Figure 1: Engine operating temperature ranges Figure 2: Heat is transferred by
conduction from the
valve stem to the valve guide. Both
objects are solid.
Heat transfer
3
