Understanding how cold weather affects an aircraft engine
By Gary Schmidt
Probably no single law of physics has a broader and more powerful impact on the operation of mechanical apparatus than temperature. For those responsible for keeping machines running smoothly, "hot" is a very familiar subject. Heat comes from converting fuel to energy and powerplants deliver a lot of heat. We are required to design and build our machines to deal with very hot temperatures. Cold on the other hand doesn't get nearly as much attention. Obviously it doesn't affect a lot of the folks who live in areas where it never gets cold. But aircraft operations cannot be restricted to regions where cold temps are not a factor.
So what are the problems relating to cold, or in a more scientific sense, an absence of heat? For aircraft owners there are three primary issues. The most significant is getting the power plants up and running. The other two are the airframe and avionics. In the following discussion, we will focus on the powerplant.
Aluminum and steel expand and contract at different rates
Dealing with temperature changes is not a simple matter. Cold affects the various parts of an aircraft engine in different ways. Metals contract at different rates changing tolerances. Lubricants lose their viscosity creating friction and wear issues for moving parts. Plastics and rubber parts become brittle. Even the elements such as moisture contained in the air that surrounds all parts behave differently affecting the internal combustion process.
Understanding the physics of temperature and how it affects various elements is the first step in knowing the best way to deal with cold for an internal combustion engine. Tanis Aircraft Services has conducted ongoing testing in various areas concerning cold weather aircraft operations. The following information is based on that testing:
We all know that a drop in temperature causes most matter to contract. Naturally this causes a change in the tolerances of parts. The fact that an aircraft engine uses both aluminum and steel creates a huge problem because the co-efficient of thermal expansion between steel and aluminum is almost double. But how does that specifically affect an engine? Tests show a steel crankshaft within an aluminum crankcase at -14 F has a reduction in tolerance of more than .001. Minimum manufacturer's tolerances are as low as .0015. The result is a tolerance that may not allow enough room for cold oil to flow to lubricate the parts.
Wrist pins are another problem. A wrist pin floats in the piston. At normal temperatures the pin will float freely. Tests at -15 F show virtually no float. Interestingly, when the wrist pin was thoroughly cleaned it did move freely, indicating that the thin film of oil on the parts actually caused the problem. In any event, the consequence is that if the pin at the down stroke extends to the limit (i.e. cylinder wall) at the up stroke it will scuff the smaller diameter wall at the choke point. That fact is the major reason pilots must be reminded that hand proping a cold engine can cause damage.
Once combustion occurs, another unexpected process starts in the cold cylinder. Combustion produces a 1,200 to 1,400 F flame in direct contact with the aluminum piston, which produces exponential growth of the piston while the cylinder wall warms and grows more slowly. The result is potentially more piston and ring scuffing. Essentially it is the reverse of "shock cooling," commonly perceived as potentially damaging.
Oil is dramatically affected by cold
Automotive lubricants have of course received the major engineering focus in the world of engines and cold weather. The characteristics of oil and grease change dramatically in very cold temperatures. Years ago the commonly used single weight oils performed best at 50 F and warmer. However, when the temperatures dropped below 0 F, an engine needed to have heat applied to the oil. Then came multiviscosity oils that seemed to solve everything. Sure the transmission and bearings were a little stiff but the engine started and if it started you could force everything else to go.
In aviation, however, that is an extremely costly and dangerous operating practice. One 20W50 oil on the market has a "pour temp" of -29 F. That means you could tip a cup of oil upside down and it wouldn't fall out.
Another cold temp viscosity measurement offered by one oil company is the length of time it takes for the oil to travel from the sump to rockers and lifters. Its 15W50 oil touted a time of less than 19 seconds at -20 F. Considering that tests show that multiviscosity oils, when warm, are prone to drain off of surfaces leaving little oil for lubrication, the piston will have at least five strokes before oil reaches the part.
Tests conducted in the Tanis lab revealed that multiviscosity oil is subject to the same effects of cold as single weight oils, only not to the same degree. That means that 15W50 oil does not flow as freely when it's cold as one might expect. Here is a simple test that you could conduct in your own shop. Take about 5 ounces of oil, place it in a cup and pour into another container through a 1/4-inch hole. You will find a dramatic difference in the flow time and quantity when the oil was cooled. In the Tanis test, fresh oil at 80 F, 4 ounces of oil flowed in 37 seconds. At -12 F it took more than nine minutes for 4 ounces to pour from one container to another. Less than a quarter of an ounce poured through the hole in the first 30 seconds.
Another interesting result of the test was that used oil didn't flow as well as fresh oil. The assumption is that the impurities in the oil collect the congealed oil creating "globs" that do not flow smoothly through the 1/4-inch hole.
Other lubricating issues relate to heavier weight oils and greases used in gearboxes and transmissions in helicopters. For example, the oil in the Robinson 44 gearbox has the consistency of peanut butter at -20 F. Although the engine has enough power to rotate the rotor, significant stress is created on the shaft, bearings, and gears to create unnecessary wear.
Yet another issue is the combustion process. Water is a natural by-product of combustion. Burning a pound of fuel creates about a cup of water. If a spark plug fires once or twice it creates a small bit of moisture. In a cold soaked engine, that moisture will condense on the plug as frost and that spells the end of that plug firing again until the frost is gone.
Finally, there is the issue of priming. Because fuel does not vaporize well in cold temperatures, pilots have a tendency to overprime. This extra fuel can wash off any oil that hasn't already drained off the cylinder walls.
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