Shock cooling (engines)
Encyclopedia
Shock cooling refers to the theory that damage to engines (particularly air-cooled aviation piston engines
) may occur because of an excessively rapid decrease in temperature
.
The situation where rapid cooling arises is on descent
from altitude. In this condition, less power
is demanded of the engine (it is throttled back) so it is developing much less heat. In a descent, the plane's airspeed increases, simultaneously increasing the cooling rate of the engine. As metals expand and contract under temperature changes, dimensional changes in the engine may exceed tolerance limits.
.
between engines. Equally, it has been pointed out the rate cylinder head
temperatures drop off after a normal engine shutdown is often much faster than the usual rates deemed to present a shock cooling risk. Furthermore, others believe damage usually associated with shock cooling is actually caused by rapid throttle changes where fuel, which has been supercooled during high altitude flight, is introduced into a very hot engine cylinder during descent, where rich of peak (as opposed to lean of peak) operation is considered the norm, thus causing higher operating temperature
s. It is well established, high operating temperatures in of themselves, can contribute to excessive component wear and damage, which is typically associated with "shock cooling". Given the available data, it strongly suggests "shock cooling" is nothing but a myth, at least in the context as commonly explained. Nonetheless, the topic will remain highly controversial and surely continue to be hotly debated well into the future.
Kas Thomas, a respected aviation engine expert and author believes, "shock-cooling is not a major contributor to cylinder head cracking".
, or in more sophisticated installations, an array of sensors, one for each cylinder, may be employed to monitor the temperature and cooling rate of the engine. Usually a simple analog gauge or a more advanced graphical bar-graph display(see external links below for an image) is used to present information to pilots. Spoilers
on the wings or thrust reversal
may also be deployed to lose lift without having to reduce engine power substantially, slowing the rate of engine cooling.
Air-cooled engine
Air-cooled engines rely on the circulation of air directly over hot parts of the engine to cool them.-Introduction:Most modern internal combustion engines are cooled by a closed circuit carrying liquid coolant through channels in the engine block and cylinder head, where the coolant absorbs heat,...
) may occur because of an excessively rapid decrease in temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
.
The situation where rapid cooling arises is on descent
Descent (aircraft)
A descent during air travel is any portion where an aircraft decreases altitude, and is the opposite of an ascent or climb. Descents are an essential component of an approach to landing...
from altitude. In this condition, less power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
is demanded of the engine (it is throttled back) so it is developing much less heat. In a descent, the plane's airspeed increases, simultaneously increasing the cooling rate of the engine. As metals expand and contract under temperature changes, dimensional changes in the engine may exceed tolerance limits.
Manifestation
Damage from shock cooling is most commonly believed to manifest itself as stuck valves and cracked cylindersCylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
.
Analysis
While the subject is controversial and hotly debated, some believe shock cooling, as commonly explained, is nothing but a myth. This position is supported by the fact twin engine planes commonly experience ideal conditions for shock cooling during simulated, single engine failures, yet statistically show no difference in wear or damage distributionProbability distribution
In probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....
between engines. Equally, it has been pointed out the rate cylinder head
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
temperatures drop off after a normal engine shutdown is often much faster than the usual rates deemed to present a shock cooling risk. Furthermore, others believe damage usually associated with shock cooling is actually caused by rapid throttle changes where fuel, which has been supercooled during high altitude flight, is introduced into a very hot engine cylinder during descent, where rich of peak (as opposed to lean of peak) operation is considered the norm, thus causing higher operating temperature
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
s. It is well established, high operating temperatures in of themselves, can contribute to excessive component wear and damage, which is typically associated with "shock cooling". Given the available data, it strongly suggests "shock cooling" is nothing but a myth, at least in the context as commonly explained. Nonetheless, the topic will remain highly controversial and surely continue to be hotly debated well into the future.
Kas Thomas, a respected aviation engine expert and author believes, "shock-cooling is not a major contributor to cylinder head cracking".
Detection And Prevention
A single cylinder head temperature (CHT) sensorSensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
, or in more sophisticated installations, an array of sensors, one for each cylinder, may be employed to monitor the temperature and cooling rate of the engine. Usually a simple analog gauge or a more advanced graphical bar-graph display(see external links below for an image) is used to present information to pilots. Spoilers
Spoiler (aeronautics)
In aeronautics, a spoiler is a device intended to reduce lift in an aircraft. Spoilers are plates on the top surface of a wing which can be extended upward into the airflow and spoil it. By doing so, the spoiler creates a carefully controlled stall over the portion of the wing behind it, greatly...
on the wings or thrust reversal
Thrust reversal
Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's exhaust or changing of propeller pitch so that the thrust produced is directed forward, rather than aft. This acts against the forward travel of the aircraft, providing deceleration...
may also be deployed to lose lift without having to reduce engine power substantially, slowing the rate of engine cooling.
