Generally, a good "rule of thumb" is to pre-heat the engine when ambient temps fall below 32 deg F (0 deg C). And, there are some (me included) that feel that it is wise to preheat the engine whenever temps are below 60 deg F (15 deg C) as the viscosity begins to increase rapidly at temps below 60 F, even with the multi-viscosity 15W-50 that I suggest is best. It is not only to reduce wear but a cold engine with thicker oil takes more battery power and my Sport Cub has a tiny battery that is marginal even at 80 degrees F. So I need to do everything prudent to insure the engine starts within the first full rotation.
In addition to the oil viscosity, the piston to cylinder clearance is important. The cylinder and pistons are different materials with a significant difference in the rate of expansion with temperature, so at colder temps the pistons are "tight" and subject to abnormal wear with cold starts.
Finally, fuel needs to be vaporized in order for the cylinders to fire, and cold gasoline is more difficult to vaporize than warm fuel. So, warming the gascolator (sediment filter just in front of carb) and the carburetor tends to help the engine fire on the first crank.
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| Gascolator inside engine cowl |
I have read many forums and internet articles about preheating. Like everything, there are lots of opinions. So, the engineer in me just had to do a bit of evaluation and experimenting.
One type of preheating is a "built in" system of electrical resistance heaters. Silicone heating pads glued to the oil pan and heating rings placed around the cylinders or heating rods screwed into the cylinder heat temp sensor threads. These are convenient and efficient, but they heat by conduction, so by design some parts of the engine heat up faster than others----some claim that this can lead to condensation and rust. This is especially problematic if engine oil temp rises faster than the cylinders as the hot (now moist) air rises in the engine, the moisture condenses on the cold upper engine parts. I think if you fly the plane after using these systems, all will be well. The only downside of these is cost, weight and perhaps maintenance. Biggest advantage is easy operation and the system is always with the plane. E-Z Heat, Tanis, Wolverine, and Reiff brands are the popular names.
Another type of preheating system uses propane or other fuel to produce heat from a flame, heating air and blowing it or allowing it to flow thru convection into the cowling space/s. These can produce massive amounts of heat and can heat the engine rapidly--maybe too rapidly. And, of course they pose a serious fire hazard. Red Dragon (Flame Engineering) and Northern Companion seem to be the most common for small aircraft. Both of these are "portable" as the Red Dragon can be run using small stove cylinders.
The third type of preheating system operates as a "cowling environment control" system or as a preheating system. These systems are meant to run almost continuously (thermostatically controlled) to maintain the air temperature that surrounds the engine--the theory being the engine can't tell that the air outside the cowling is cold--the engine thinks it is in a warm climate. The best example of this type of system is Hornet heaters--very well designed and sophisticated technology---safe enough to run continuously unattended. The downside is they are very low capacity so they take a long time to bring temps up. Another example is the systems from Aerotherm--the Deuce and the various Alien models. Aerotherm systems have enough heat output to be used practically as either a preheating system or to be thermostatically controlled for continuous operation. The innovation of the Aerotherm systems is they conserve/optimize energy by recirculating the air thru the engine compartment. Neither of these brands are inexpensive, but both are well engineered.
I liked the Aerotherm concept and interestingly, my FBO at KTHV has an Alien system available for rent at $20 per use. So I gave it a try.
Here I have the system hooked up to Sierra Charlie, blowing into the left air inlet and sucking from the right inlet. The fan is hanging from a strap straddling the propeller spinner.
This worked as expected--raising the cylinder head temp by about 30 degrees F in less than on hour.
The Aerotherm system has a thermostat that is affected by intact air---so when running, the outlet air temp was a high 180 degree F, but when the inlet air exceeded 70 F, the heat was turned off, reducing outlet temp.
One disappointment was that my oil temp did not rise signficantly. So, I tried using the system to blow air into the exhaust outlet section of the cowl---blowing air directly onto the oil pan, and toward the carburetor. This produced a good result raising oil temp about 30 degrees in less than an hour, while maintaining the 30 degree rise of the cylinders.
I was impressed enough that I was ready to place an order, but I wanted longer hoses to allow sucking air from one end (exhaust) of the engine while blowing into the other end (air inlet port/s) or vice versa. The owner of Aerotherm kinda discouraged the idea and told me to just hook up the system and let it run continuously 24/7. Hmmm... I have a lot of experience with bimetal thermostats, and while they are safe most of the time, I have seem enough of them fail in the on position (especially 110 volt) that I was not comfortable running the system 24/7. (Pretty sure system has a non resettable thermal fuse that does provide overheat protection.) So I am probably just a bit of a worry wart. (I think the Hornet heaters with redundant safety devices and low enough output, are probably OK to run 24/7.)
There is a trade off running a heating system 24/7. Theoretically, the engine remains above the air dewpoint, but what often is overlooked is that rust/corrosion occurs more easily at higher temps. A cold engine can allow condensation in warm moist air, but cold air is comparably much drier---so a cold plane in cold air may not see as much rust/corrosion as a warm plane in warm air. This reduced corrosion applies to that caused by acids in the oil film.
What I really wanted was a system (not necessary to be portable) to preheat my plane in about 2-3 hours. And, I liked the idea of heating the engine's air environment, applying heat to the engine thru convection over the idea of heating the engine directly thru conduction using resistance heaters attached to the oil pan and cylinders.
The other design feature I wanted was the ability to regulate heat input and air flow to achieve the optimum heating process for various ambient temps. So, I wanted a variable output heating element and a variable speed fan. So my project began.
I must say, the project ended up taking me back to my engineering background using reference materials related to heat transfer, thermodynamics, fluid mechanics and basic HVAC principles along with motor speed control technology.
The forums led me to a neat little "little buddy" 900 watt "cab" heater produced by Phillips and Temro, a well respected producer of heating accessories. This little unit is designed safety tested by CSA to be used to run all night heating the inside of a truck cab or car interior. Two of these in "parallel" side by side in the air stream would produce around 5000 BTU's per hour while using less than 15 amps. This gave me the variable heat output, running one or both.
I then looked around the shop and decided to "re-purpose" and old inline propane heater. I took out the propane nozzle and stuff, leaving a robust cylindrical housing (perfect 8" duct diameter) with a high CFM axial fan and a decent electrical junction enclosure. I built a platform for the heaters, to be attached to the housing and enclosed downstream in a standard 8" diameter heating duct section. (So far $180 invested.)
Next came the tedious process of fabricating the duct work to interface with the plane using readily available and inexpensive HVAC and dryer duct hardware--pretty much all from the local hardware store. Lots of measurements and thinking as to how to transport and assemble.To hold everything, I re-purposed some old metal stack chairs--cutting legs to get desired height and welding platforms to support and hold things in place. (My trusty MIG welder, recip saw and 4 inch grinder.)
My initial testing indicated that the high CFM fan was too much---the resulting outlet air temp was lower than I wanted. So, I decided to use a three speed Vornado fan for addition air supply. After lots of study regarding speed control of shaded pole ac motors, I decided against trying to slow down the propane heater fan with a triac type ceiling fan control.. Better to use a motor designed to run a different speeds. Here is the setup for blowing air into the front.
This by the way will work very well to quickly heat up the cockpit and instruments.
Here is the setup for blowing air into the rear of the cowl.
On a cold 35 degree F day (cold for me) I tested the system. The ideal set up was using medium Vornado fan speed and both heaters, producing about a 55 degree F temperature rise with 100 CFM flow blowing at the equivalent of about a 6 mph breeze.
By the way, a good rule of thumb formula is Watts = CFM x 1/3 of Temp Gain. 1800 watts = 100 x (54/3) . 1800 watts is around 5500 BTU's per hour.
Heat in this example is transferred to the engine by way of convection. Since we are not recirculating, some heat is exhausted and lost. My calculations indicate the system heats the engine and oil at a rate of around 3000 BTU's per hour. Engine temp rises at a rate of about 1 degree F every two minutes. Would rise faster if I put an insulated cowl cover on. I used a simple meat thermometer inserted into the duct for measuring outlet temps.
On a day with ambient temps of 38-40 F, I ran the system pushing air thru the air intakes for one hour and raised the cylinder head temp gauge to 80 degrees F, then set up and pushed air thru the exhaust exit in the cowl, raising the oil temp to 70 F while maintaining 70 F cylinder head temp.
In colder temps, I would reduce the CFM, causing the air temp change to increase. On warmer days, I would increase the CFM and or reduce the heat input to cause the air temp change to decrease. The goal would be IMHO to maintain an air outlet temp from the system in the 90 to 105 F range. This seemed to be the optimum air temp to reach the goal of 70 F oil and cylinder temp.
I think it is also important to heat the cylinders and upper engine FIRST, followed by the heating of the oil. Heating the oil will drive out moisture from the oil. Hotter air above the oil will absorb the moisture and the warm upper engine will remain above the dew point temp--preventing condensation.
Obviously my homemade system is not portable. If I just wanted to buy a ready made system, I would buy the Aerotherm Deuce with longer hoses. The Aerotherm is portable. Now that I have my system for my hangar, if I need a portable system, I would use a Hornet and a cowl cover, using it to maintain temps NY plugging in right after I land at my destination.
The total cost for my heater was about $350, not counting the value of my time. But for the enjoyment of my "tinkering", with a value of $20 per hour on my time, it would have been more economical to just buy the Aerotherm with longer hoses, and making some sort of adaptor for hooking it up to the exhaust outlet in the cowl.
Now, here is the interesting part.
If you go and fly, the engine will get even hotter and more moisture will be driven from the oil, which is good.
But, whether you run the engine or not, whatever air is in the crankcase is likely fully saturated (100% relative humidity) and when the air cools, significant moisture will "fall out" of the air and return to the oil. As long as the engine parts are coated with oil, this is not too much of a problem which is why running the engine hard and often is good.
But, there is a way to significantly reduce the moisture in the air of the crankcase....open the oil fill cap and allow a "chimney effect" to occur, where hot moist air rises out of the filler neck and is replenished by cooler, drier air thru the crankcase vent. (The "smoke" often seen coming from the oil filler neck when checking oil after a flight is really water vapor.) An old timer suggested this and I was skeptical at first--but he was and is technically correct. My experiments showed significant air flow thru the crankcase and my calculations show a significant reduction in moisture! (In the range of a reduction from 90-100 RH in warm air down to 10-15% RH in cool air---MUCH less moisture precipitating back to the oil.)
Doing this after a preheat--without running the engine can remove 2 or 3 drops of water from the oil. Air at 70 F holds nearly four times the air capacity of 35-40 degree F air. And, air at 180 degrees F holds nearly 20 times the air capacity of 35-40 degree F air. (Holding a couple of teaspoons of water.) And tests indicate most engine crankcase are at 100% relative humidity at shutdown with 180 degree oil and air. So part of cold weather operation includes a procedure to allow the hot moist air to be replaced by cooler drier air, so as to reduce the amount of moisture put back into the oil. Open the oil fill for 10- 15 minutes right after shutdown as you hangar or tie down.
https://brownell.co.uk/datasheets/basics_humidity.pdf
Even with the drying procedure by opening the oil fill for a few minutes will still not remove all the moisture as the oil will stay hot for some time. Returning after a cold spell will often produce some melting "discharge" of oil/water from the crankcase breather during the pre-heat. This is a major benefit of preheating as this discharge is frozen and taking off without preheating and removal of it can lead to high crankcase pressure--causing oil to be blown past the seals during flight.
Steel is less likely to rust when cold and dry. So I prefer not to keep the engine warm when sitting in the hangar for long periods. I think the ideal would be to purge the crankcase with dry gaseous nitrogen after flight---more on this after some research.
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