Preheat

wrote:

Sounds like he had a defective heater. They shouldn't be gettings
things above 212F which would be needed to boil off water or acid.


Matt


Water vapor will slowly come off at much lower temperatures. All it
needs is to condense on something cooler than the oil. I don't recall
if he had crankcase heating only, or if he had cylinder heat too. I
recall the jug sitting above his desk at work, very thoroughly
corroded on the bottom in one winter.

The original post said the water and acid boiled off, not evaporated.

A cylinder that is being used once in a while won't corrode as the oil
film from the last flight will protect it for at least a couple of
weeks. If he let his airplane sit the entire winter, then he had no
need to use an engine heater!


Matt

With minimum allowable bearing clearances, some engines have ZERO
clearance at about -10 degrees F. Cylinder choke and rapid heating of
the piston on cold start may also lead to piston scuffing. See
www.tcmlink.com for Continental's recommendations.

wrote:
Getting oil to flow easily is only one part of the problem. As you note,
this can be "solved" with a multi-viscosity oil.

But another important factor is the clearance between the pistons and
cylinders (or, more specifically, between the steel and aluminum parts of
the engine). Because of the differential expansion/contraction of the
different metals, at cold temperatures, clearance may be non-existent, so
there is markedly increased wear until the engine gets above some critical
temperature.


The differential expansion of aluminum vs steel is 6 microinches per
inch per degree F. Minimum bearing clearance is about .002 inch which
would take roughly 140 degrees of chilling to fully close up.
Probably even greater temps are needed for the cylinders to close up.
It is certainly a significant factor, but the reduced main and rod
bearing outflows (which goes as the cube of the clearance and
inversely as the viscosity) is also a reason there isn't much spray
inside a cold crankcase. All of these factors work against you,
especially on a new or fresh majored engine having minimum clearances.

Multi vis oils certainly help. Conversely having a summer weight oil
greatly increases the need for thorough preheating.

I use 32 degrees as a cutoff with 15W50, higher if it has been sitting
for more than a week. Below that I get an oil pump cavitation whine
on initial start on an O-320 E2D.

On Fri, 12 Nov 2004 18:56:18 GMT, wrote:


On 11-Nov-2004, Matt Whiting wrote:

A friend corroded the cylinders on his Cardinal from continuous
heating in just one winter. The water/acid boiled off the oil and
condensed in the cylinders on the bottom side. I understand Tanis
says that won't happen since they also have cylinder heat, but I'd be
more than a little scared of it still happening. If nothing else
minimize the preheat as much as convenient.

When I start using the Tanis heater I leave it on all the time. I
have both the block and cylinder head heaters. The entire front end
of the Deb gets wrapped up in two layers of the new high efficiency
blankets. There is not a part on the engine where you can comfortably
lay your hand. I've been doing this now for about 12 years and over a
1000 hours of engine time.


Sounds like he had a defective heater. They shouldn't be gettings
things above 212F which would be needed to boil off water or acid.

To boil it off, yes but it will slowly vaporize at lower temperatures.



Strictly speaking, as you point out, the term "boiled off" was probably in
error. However, the point that moisture will be released as oil is heated
is not. If water is being held in suspension, heating the oil and the air
in the sump will cause some of that water to evaporate, and it may then

If you heat only the sump there will be condensation.
If the whole front end is well insulated the cylinders should get
warm, but probably not warm enough to prevent condensation.

condense on cold surfaces. It's like when you blow on cold glass. The
moisture in your breath will condense and fog the glass, but your body temp
is surely well below 212 F. The acid he speaks of is probably dissolved in
the suspended water.

The acid is from the combustion products. I'd guess, mainly SO2 in
water. At 40C you won't find much condensation.

The first place to check is the top of the dip stick. If you have
visible water droplets there, don't leave the heater on except as
needed.

Nor would I leave it on if I didn't fly every week. At 40 to 50C the
oil's ability to "cling" is reduced greatly. Leave it more than two
weeks and I think you'll be starting on a dry cam.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com