Shock cooling (engines)

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.

Damage from shock cooling most commonly manifest itself as stuck valves [http://avweb.com/news/maint/182883-1.html] and some have attributed cracked cylinders to shock cooling as well.

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 through a simple gauge or a bar-graph style graphical display. Spoilers on the wings or thrust reversal may also be deployed to lose lift without having to reduce engine power substantially.

Kas Thomas, a respected aviation engine expert, believes that "shock-cooling is not a major contributor to cylinder head cracking".

While the subject is controversial and hotly debated, many 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 distribution between engines. Equally, it has been pointed out that 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 that damage usually associated with shock cooling is actually caused by rapid throttle changes where fuel that has been supercooled during high altitude flight is introduced into a very hot engine cylinder during descent, where rich of peak (ROP) operation is considered the norm. In fact, it is well established this behavior 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.

External links

* [http://www.avweb.com/news/maint/182883-1.html "Shock Cooling: Myth or Reality?" at AvWeb.com]