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
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
.
The situation where rapid cooling arises is on
descent from altitude. In this condition, less
power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may ...
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
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
,
cracked pistons and
cracked cylinders.
Analysis
While the subject is very controversial, some believe shock cooling, as commonly explained, is nothing but a
myth
Myth is a folklore genre consisting of narratives that play a fundamental role in a society, such as foundational tales or origin myths. Since "myth" is widely used to imply that a story is not objectively true, the identification of a narrati ...
. 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
probability distribution
In probability theory and statistics, a probability distribution is the mathematical function that gives the probabilities of occurrence of different possible outcomes for an experiment. It is a mathematical description of a random phenomeno ...
between engines. Equally, it has been pointed out the rate
cylinder head
In an internal combustion engine, the cylinder head (often abbreviated to simply "head") sits above the cylinder (engine), cylinders and forms the roof of the combustion chamber.
In sidevalve engines, the head is a simple sheet of metal; whereas ...
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
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
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.
Detection and prevention
A single cylinder head temperature (CHT)
sensor, 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
Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's thrust for it to act against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft ...
may also be deployed to lose lift without having to reduce engine power substantially, slowing the rate of engine cooling.
Notes and references
External links
''Shock Cooling: Myth or Reality?'' at AvWeb.com''GAMI Injectors'' at GAMI.com for LOP operationImage of a graphical bar-graph engine analyzer
''How to Avoid Sudden Cooling of Your Engine'' at Lycoming
{{DEFAULTSORT:Shock Cooling (Engines)
Aircraft engines