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 quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
.
The situation where rapid cooling arises is on
descent
Descent may refer to:
As a noun Genealogy and inheritance
* Common descent, concept in evolutionary biology
* Kinship, one of the major concepts of cultural anthropology
**Pedigree chart or family tree
**Ancestry
**Lineal descendant
**Heritage
** ...
from altitude. In this condition, less
power
Power may refer to:
Common meanings
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power, a type of energy
* Power (social and political), the ability to influence people or events
Math ...
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 fittings, ...
,
excessively worn
or cracked
pistons
A piston is an engineering component of engines and pumps.
Piston(s) may also refer to:
Science and technology
* Misnomer for a hydraulic cylinder
* Piston (optics)
* Piston (subcellular structure)
* Piston valve
* Fire piston, an ancient device ...
and cracked
cylinders
A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infinite ...
and
cylinder heads.
Analysis
While the subject is very controversial, some believe shock cooling, as commonly explained, is nothing but a
myth
Myth is a genre of folklore consisting primarily of narratives that play a fundamental role in a society. For scholars, this is very different from the vernacular usage of the term "myth" that refers to a belief that is not true. Instead, the ...
. 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 a Function (mathematics), function that gives the probabilities of occurrence of possible events for an Experiment (probability theory), experiment. It is a mathematical descri ...
between engines. Doubts have been raised regarding the plausibility of achieving the temperature reduction in
cylinder heads during flight that is purported to cause damage in most 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.
Complicating the analysis, high operating temperatures in of themselves, can contribute to excessive component wear and damage, which is typically associated with "shock cooling".
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.
Spoilers on the wings may also be deployed to lose lift without having to reduce engine power substantially, slowing the rate of engine cooling.
Notes and references
{{DEFAULTSORT:Shock Cooling (Engines)
Aircraft engines