automotive internal combustion engine generates a tremendous amount
of heat. This heat is created when the gasoline and air mixture is
ignited in the combustion chamber. This ignition (explosion)
causes the piston to be forced down inside the engine, levering the
connecting rods, and turning the crankshaft, creating power.
temperatures around the combustion chamber can exceed 1000° F.
In order to prevent the overheating of the engine oil, cylinder
walls, pistons, valves, and other components by these extreme
temperatures, it is necessary to effectively dispose of the heat.
has been stated that a typical average-sized vehicle can generate
enough heat to keep a 5-room house comfortably warm during zero
degree weather (and I'm not talking about using the exhaust pipe).
1/3 of the heat in combustion is converted into power to drive the
vehicle and it's accessories.
Another 1/3 of the heat is carried off into the atmosphere through
the exhaust system.
remaining 1/3 must be removed from the engine by the cooling system.
The automotive engines have basically dumped the Air Cooled System
for the more effective Liquid Cooled System to handle the job.
In a liquid cooled system, heat is carried away by the use of a heat
absorbing coolant that circulates through the engine, especially
around the combustion chamber in the cylinder head area of the engine
block. The coolant is pumped through the engine, then after absorbing
the heat of combustion is circulated to the radiator where the heat
is transferred to the atmosphere. The cooled liquid is then
transferred back into the engine to repeat the process.
most vehicles, the coolant is circulated by the water pump, and the
thermostat controls the temperature. The thermostat is closed when
the engine is cold, allowing coolant to circulate ONLY in the engine
block, bypassing the thermostat and radiator. This allows the engine
to warm up faster and uniformly so that "hot spots" are
eliminated. When the warming coolant reaches the thermostat, the
thermostat will begin to open and allow coolant to pass to the
radiator. The hotter the coolant gets, the more the thermostat opens,
allowing more volume of water to pass to the radiator. The thermostat
also controls the length of time that the coolant remains in the
radiator so that the heat is dissipated effectively.
the thermostat to increase water flow because your vehicle is
overheating is dangerous to your engine and is NOT what you want to
do. Not only does the engine take longer to warm up, causing
excessive metal-to-metal wear, but once the engine does warm up it
can get too hot because the thermostat also controls the length of
time that the water is in the radiator so as to dissipate the heat to
Up until the late 1950's, liquid cooled systems did not operate
under pressure. This was primarily because these cars had much larger
radiators, more open air under the hoods allowing for more natural
heat dissipation, and richer fuel mixtures causing lower (and less
efficient) combustion temperatures. With the manufacture of
smaller vehicles, with smaller radiators, larger engines, and
emission controls along with the current use of unleaded fuels, more
efficient cooling efficiency became necessary.
The cure for this was to operate the cooling system under pressure,
thus isolating it from atmospheric pressure. A system under pressure
can handle higher temperatures, and offers a higher static boiling point.
liquids have a specific "boiling point", which is the
temperature at which the liquid begins to change to a gas. If
pressure is applied to the liquid, it must become hotter before it
can boil. Pure water in a cooling system will boil (at sea level)
at 212° F. At higher altitudes, atmospheric pressure is less
than at sea level. Example:
Water at 5,280 feet will boil at a mere 203° F.
cooling system that is less than 15 pounds of pressure however, will
now allow the water to reach nearly 250° F before it can boil.
Even at this temperature the water is able to circulate through the
engine, cooling parts that are at a much higher temperature without
the water boiling. As long as the coolant remains in liquid form it
can do it's job and transfer heat to the radiator so it can be
dissipated. Coolant that is boiling cannot transfer as much heat and
engine overheating is likely to occur if the coolant turns to a
gaseous state. Steam adjacent to a hot surface, such as a combustion
wall, can actually act as an insulator - thus preventing any heat
transfer to the coolant.
Pressurization of the system is achieved by a special radiator
filler neck and radiator pressure cap. The filler neck has an upper
and lower sealing seat with an overflow tube connection between them.
The lower seat is engaged by the pressure controlling valve on the
cap and the upper seat (in an open system) is engaged by a
spring metal diaphragm in the cap.
radiator pressure cap features a spring pressure relief valve which
closes off the lower sealing seat in the filler neck.
pressure relief valve allows pressure to build up to a specified
level; this permits excess pressure to escape through the overflow
tube when it exceeds the range of the pressure valve spring. This
valve protects the cooling system from damage from excessive
pressure. This pressure relief system also includes a vacuum relief
valve that allows air (in an open system) to enter as coolant
contracts. This prevents the radiator hoses and tanks from collapsing
from the partial vacuum that would be created if air was unable to enter.
Closed or Reservoir System vs the
"old style" Open System
One of the big disadvantages in the old open type pressurized system
is that as the system cools, air is allowed back through the overflow
tube. These systems are not totally filled with coolant because of
the potential for coolant loss through the overflow tube when the
coolant heats up and expands. As more coolant is lost through the
overflow, less coolant is left to do it's job within the engine.
Because of this, and that air can enter the system and reduce cooling
system efficiency, overheating can occur. Closed reservoir systems
were first used by car manufacturers in the early 1970's.
A closed or reservoir system has solved the problems listed above.
This system is different in that a special radiator cap and overflow
reservoir tank. Part of the radiator cap is a second sealing gasket
under the shell that contacts the upper sealing seat of the filler
neck. What was the overflow hose is now the connection between the
radiator and the "bottom" of the reservoir.
While the open pressurized system is filled to a point 2-3 inches
below the top of the radiator, the closed pressurized system is
filled completely with coolant and the reservoir is filled
approximately half full. When the engine is started and begins to
heat up, the coolant expands. As the coolant expands it is forced out
through the pressure valve of the radiator cap, through the overflow
tube, and into the reservoir. When the engine is turned off and
begins to cool, a partial vacuum is created in the radiator by the
contracting coolant. The upper sealing gasket in the pressure cap
will then allow the vacuum to draw the coolant back into the radiator
and engine from the reservoir. As you may have noticed, the actual
volume of coolant that displaces during heat-up and cool-down
transfer is minimal in most all cases.
Because of the coolant going back and forth between the radiator and
reservoir, practically all air is eliminated from the cooling system.
This pretty much guarantees that the engine block, heater core, and
radiator are full of coolant instead of air. This allows the most
efficient operation of the cooling system. Generally, on closed
systems, coolant is added only as required, and then it is added to
the reservoir, not the radiator.
The objective of this Web Page is to familiarize you with basic auto maintenance
- in some common emergencies - not to make you an expert in auto mechanics
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or form telling you to do this yourself. Your results may vary. If
something goes wrong, it is not my fault! These are just guidelines.