water on engine

[shdw]

If the engine was hot, I would think splashing cold water on it would really harm the metal.

Turns out water is a poor way to rapidly cool metal. In fact, when metal is treated using a rapid cooling process, the liquid used is typically some form of oil. Water evaporates too rapidly to cause any significant change in temperature. At least not to the extent needed to cause structural deformation (heat treatment) in the metal.

 

[inigo88]

Turns out water is a poor way to rapidly cool metal. In fact, when metal is treated using a rapid cooling process, the liquid used is typically some form of oil. Water evaporates too rapidly to cause any significant change in temperature. At least not to the extent needed to cause structural deformation (heat treatment) in the metal.

From both personal welding experience and again in an engineering materials science course, I learned that water is actually one of the most efficient quenching methods. In fact it cools steel so rapidly that it can lead to microscopic cracking and distortion of the part. I believe it's because water has such a high specific heat capacity (c) compared with metal - it takes a lot of thermal energy to change the water temperature while it takes very little to change the temperature of the metal (q=mc⌂T). As you'll see below, oil is actually used because it cools the part at a slower rate. While it's true that water evaporates, just add more water. If you're quenching metal it's assumed that you're dunking a relatively small part in a bucket containing an abundance of water.

Quenching metals is a progression; the first step is soaking the metal, i.e. heating it to the required temperature. Soaking can be done by air (air furnace), or a bath. The soaking time in air furnaces should be 1 to 2 minutes for each millimeter of cross-section. For a bath the time can range a little higher. The recommended time allotment in salt or lead baths is 0 to 6 minutes. Uneven heating or overheating should be avoided at all cost. Most materials are heated from anywhere to 815 to 900 °C (1,500 to 1,650 °F).
The next item on the progression list is the cooling of the part. Water is one of the most efficient quenching media where maximum hardness is acquired, but there is a small chance that it may cause distortion and tiny cracking. When hardness can be sacrificed, whale, cottonseed and mineral oils are used. These often tend to oxidize and form a sludge, which consequently lowers the efficiency. The quenching velocity (cooling rate) of oil is much less than water. Intermediate rates between water and oil can be obtained with water containing 10-30% Ucon, a substance with an inverse solubility which therefore deposits on the object to slow the rate of cooling.
To minimize distortion, long cylindrical workpieces are quenched vertically; flat workpieces are quenched on edge; and thick sections should enter the bath first. To prevent steam bubbles the bath is agitated.

(Source)

The reason quenching steel and cast iron is so preferred has to do with the molecular composition of the metals (which are mostly iron with a tiny bit of carbon impurity). Hot steel (we're talking 1500 degrees F or more) called Austenite, has a Face Centered Cubic (FCC) crystalline structure. But if you dunk it in a bucket of water the molecules on the outside of the part rapidly rearrange themselves into a Body Centered Tetragonal (BCT) crystalline structure into a needle like composition called Martensite (not to be confused with Martinaire ), which is incredibly strong but brittle. So you end up with regular steel on the inside (called Pearlite) with all its normal strength properties, and then a hard brittle shell of Martensite on the outside.

Of course aircraft and automotive engine blocks and cylinder heads are made out of Aluminum these days. Quenching aluminum is usually done as part of the "Solution Heat Treatment" process, where impurities (like copper) are added and grown in the "slip planes" along the grain boundaries in the aluminum. They essentially act like glue and prevent the aluminum from slipping along those planes, making it stronger. This website has an awesome explanation of the whole process, and this is how the strongest aluminum alloys used in aircraft (those in the 7000 range) get their strength. For instance, to solution heat treat 7075 sheet aluminum, they heat it to 900 degrees F and then dunk it into a cold water bath!

Long story short, the aluminum jugs and crank case on any aircraft engine have been heat treated, and this heat treatment happens around 700-900 deg F depending on the process used. If you fly through a raincloud at full climb power in a C172, your CHT will be around 400 degrees F or less - so clearly you're not doing anything that hasn't already been done to that engine.

That being said, a one time heat treatment at the factory is far different than continuous cycles of heating and cooling (which equals expanding and contracting) being applied over and over. A sucky thing about aluminum is that unlike steel, it has NO minimum fatigue stress. This means that every cycle (takeoff and landing), even the small forces applied to the metal cause the molecules to slip along those "slip planes" and cause microscopic stress fractures to form. These stress fractures never get better, so the aluminum just keeps fatiguing more and more until it finally fails from the cumulative effect of those many thousands of cycles. (In comparison, steel simply "doesn't notice" any stress applied to it unless it exceeds the minimum limit.)

So looking at an aluminum engine, every time it runs it heats up and the metal expands. Then every time it cools the metal contracts. Dump a bunch of cold water on it (whether in a rainstorm or out of a bucket) and it's going to contract MUCH faster than it should have and you're going to get more fatigue stress and more of those microscopic fractures.

Obviously your engine isn't going to immediately start cracking like glass before your eyes, but logically stress fractures will shorten the life of the part and could eventually it could lead to a cracked crankcase or cylinder head. Whether you've shortened the life of your engine by hours, or days, or years, I have no idea.

But considering the engine survived that initial Solution Heat Treatment process at the factory, which is far worse than anything you could do, I wouldn't lose too much sleep over it.

 

[Autothrust Blue]

The procedure in the POH says to run it up 1200RPM and lean to max RPM and then keep it leaned out for ground ops. Works pretty well for me.

For me, that's way too rich.

 

[inigo88]

For me, that's way too rich.

If you turn the mixture knob a quarter turn to the left, it should die.

 

[Autothrust Blue]

If you turn the mixture knob a quarter turn to the left, it should die.

That will also preclude taking off (or even running up) with an inappropriate mixture setting for the power being demanded.

I mean, not that I ever experienced roughness/low power, thought "that's odd" then shoved the red knob home...

 

[will_fly_for_bandwith]

Is the engine block in aircraft a battery negative like in a car? Perhaps the HT leads from the mags have small cracks in the plastic/rubber/fabric that when wet allow the lead to ground out (I had this happen in an old truck).

 

[shdw]

From both personal welding experience and again in an engineering materials science course, I learned that water is actually one of the most efficient quenching methods.

Well I haven't had time to soak this in yet, thank you for setting me straight. I must not have used enough water when attempting this process in college. Heated a 3"x10" cylindrical piece of steel to 850 C for some 3 or 4 hours as I recall. Then soaked it in a filled 5 gallon bucket. Only the outer edges (~1 mm) showed change under our microscope. It was mentioned that water was a poor medium. Apparently not.