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Internal combustion engines convert fuel into mechanical energy and a lot of heat. When the heat of an engine is not controlled, by the cooling system, things get out of hand very quickly. At what temperature is an engine considered overheated and what happens when it is?

Coolant flow and temperature regulation

Anyone who has ever stood near a running engine knows of the heat produced.  Heat is energy from the fuel used, a byproduct of the combustion process.  Within limits, heat is good and engines are optimized to run at a given temperature.  Most modern engines run between 200 and 225 degrees Fahrenheit, with a 195 degree thermostat.  Below this temperature range, the engine is less efficient and sludge may build up.

Engine coolant, also called antifreeze, is used to remove heat from an engine.  Coolant is a mixture of water and other chemical compounds.  Water removes heat, ethylene or propylene glycol prevents freezing and other compounds prevent corrosion.  Without a precise mixture of water and coolant, there will be problems. 

coolant flows through an engine and removes heat

Coolant, circulating through the engine, carries excess heat to the radiator.  The radiator is a heat exchanger where air reduces coolant temperature.  The reduced-temperature coolant is again circulated through the engine by the water pump.  A thermostat opens and closes to regulate coolant flow and control engine temperature.  As temperature gets too low, the thermostat closes, blocking coolant flow.  As temperature rises, the thermostat allows additional coolant to flow.  Cooling fans may also switch on and off to assist the cooling system, in regulating engine temperature.

The role of air flow in the cooling system

When a vehicle is in motion, air naturally flows through the radiator removing heat.  When a vehicle sits still, such as at a stop light, the lack of motion results in much less air flow.  Vehicles use cooling fans, of many different types, to help move air through the radiator when needed.  Older vehicles normally used engine-driven fans.  These fans often bolted to the water pump and were turned by a belt.  They required a good deal of energy to rotate and were difficult to regulate.

Engineers soon saw an advantage to reducing the air moved by the fan, when it was not required.  Many early fans were designed to flex or reduce their pitch as engine speed increased.  Later, thermostatically controlled fan clutches were used.  These clutches allowed the fan to slip when the engine was cool and turn faster as temperature increased. 

Today, most vehicles use electrically controlled fans.  Some vehicles use one and others two of these fans.  Sensors in the engine and air conditioner input to a computer, which varies the speed of the fans or turns them on or off.  This gives far more control over engine temperature and allow air conditioner pressures to be much more closely regulated.  It also adds a great deal more complexity to the system and makes diagnosis far more difficult.

For instance, fan speed is critical.  Simply looking at a fan, to determine it is turning, means very little.  Fans may be rotating and the engine may still overheat, if fan speed is incorrect.  Fans may also turn too fast, if an air conditioner problem occurs.  An overcharged air conditioner may command higher fan speed, resulting in a check engine light for reduced engine temperature.

How temperature affects an engine 

The temperature indicated by the temperature gauge or light, is an average engine temperature.  In reality, the temperature in the lower crankcase is considerably lower than the cylinder head temperature.  Combustion takes place in the cylinder heads, so their temperature is greater than the rest of the engine.  There are often multiple sensors for engine temperature.  For instance, the dash gauge may have a sensor and the engine computer may have another.

Engine temperature is a major factor in engine control.  Fuel mixture, idle speed, fan control and transmission shift points are just a few of the systems that rely on engine temperature.

Why does metal expand when it is heated

Metal components, in an engine, expand as temperature increases.  Metal expands because energy (heat) causes the molecules, in the metal, to vibrate faster and take up more space.  The more the heat the more the metal tends to expand.  Clearance, in an engine, is the space allowed for thermal expansion and lubrication.

For an example, the crankshaft in an engine is slightly smaller than the bearings, in which it turns.  This space or clearance is filled with oil, from the lubricating system.  As the crankshaft and bearings expand, the clearance becomes less.  Should they expand too much, there would be no place for the lubricant to enter.  Lack of lubrication will cause major engine problems, very quickly.  Engine clearance is closely matched to engine oil viscosity.  If the oil is too thick, improper lubrication results.

Metals, used in an engine, expand at different rates

At normal temperature, this expansion is factored in.  As temperature increases, different expansion rates create problems.  For instance, the coefficient of thermal expansion for cast iron is less than six.  The coefficient of thermal expansion for aluminum is over 13  This means aluminum expands a lot more than cast iron, at the same temperature.

Metals expand at different rates when heated 

As temperature of an engine exceeds 230 degrees Fahrenheit, the engine is overheated.  Above 245 degrees Fahrenheit, damage may occur.  As heat continues to increase, the different rates of thermal expansion cause metal to distort.  Because aluminum expands much more than cast iron, an engine, with aluminum cylinder heads, may have a real problems.

expansion of metal in an engine causes damage in an overheat

The cylinder head is tightly bolted to the engine block. Steel head bolts do not expand as much as the aluminum cylinder head.  As the head expands, the head gasket may be crushed.  The damaged head gasket may allow coolant to enter the cylinders or compression-pressure to enter the coolant.  Coolant entering the combustion chamber reduces lubrication on the cylinder walls.  Decreased lubrication results in increased friction and even more heat.  As the coolant exits the cylinder, it travels with the exhaust.  Coolant in the exhaust quickly contaminates the oxygen sensors and damages the catalytic converter.  These expensive components may be destroyed.

Combustion pressure entering the cooling system will drastically increase the temperature of the coolant.  This is why an engine with a blown head gasket causes continued overheating.  The excessive pressure in the cooling system also damages other components such as radiators and heater cores.

Combustion byproducts are also very corrosive.  Driving with a damaged head gasket can drastically increase system corrosion.  This may quickly cause the cost of repair to exceed the value of the vehicle.  Damage caused by a single overheat, may show up days, weeks or even months later.  Once the head gasket fails, it will contribute to continuing and worse overheating.

In part two of this article what happens to an engine, when it overheats will be examined in detail.

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