Shock cooling (engines)
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.
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 distribution 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 temperatures. 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.
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 may also be deployed to lose lift without having to reduce engine power substantially, slowing the rate of engine cooling.
Notes and references
- Shock Cooling: Myth or Reality? at AvWeb.com
- GAMI Injectors at GAMI.com for LOP operation
- Image of a graphical bar-graph engine analyzer