How to Warm a Cold Engine

By Kas Thomas

TBO ADVISOR, Nov/Dec 1998

Getting an engine started in cold weather has never been much fun. In 1937, when my father was stationed in Alaska in the Army, the standard trick was to use a generous amount of fuel dilution (i.e., raw gasoline added straight to the engine’s oil system, mixed with the oil) and “Armstrong prelube”—turning the propeller over (and over, and over . . .) by hand—to get airplanes started. In really cold weather, the surefire solution was to drain an airplane’s sump the minute the engine shut down, bring the warm oil into a heated hangar, and keep it warm until needed again. It turns out that this is not such a bad idea, energetically speaking. The specific heat of oil is at least four to five times the specific heat of the metals in an engine, which means the same energy that produces a 10 F temperature rise in a pound of oil will (theoretically) produce a 40 F rise in a pound of engine. As luck would have it, a typical aircraft engine (dry) outweighs its oil supply by about five to one. What it means is that if you could transfer all of the excess energy in 80 pounds of 50-above-ambient oil to a 400-pound engine, you would expect to raise the engine’s temperature by 40 F—which isn’t bad. (All these numbers are ballpark figures, of course, and assume loss-free heat transfer, etc. etc.)

But there’s no longer any reason to fuss with fuel dilution or heating oil in a hangar. Nowadays, the modern, safe way to heat an engine is with electricity. And there are more products than ever to help you with the task.

What’s that? You say you don’t have access to 110-volt AC? Don’t rule out an electric heating system. For less than $50, you can buy an inverter that plugs into any car cigaret-lighter, producing 110-volt juice that you can use for any purpose whatsoever (as long as you don’t need more than 300 watts or so): running computers and modems, powering up an electric weed-eater, or—yes—preheating an engine.

So really, all you need is access to a car cigarette lighter. If you’re out of town at a strange airport, you can just use your rental car to sit there and idle for awhile as the engine warms up.

Is it realistic to think you can actually do this? Can you hope to get a meaningful temperature rise in, say, one hour of preheating using a car’s electrical system, hooked up (through a cheap inverter) to a Reiff or Tanis preheat system? Let’s try to calculate it. If we assume that 1,000 BTUs are needed to achieve a 10 F temperature rise in a 400-lb engine (which is probably not that far off), and if we know that a kilowatt-hour is roughly 3,500 BTU (which it is), then we should expect to find that a Reiff Hotbandd/Hotpadd system drawing 360 watts (50 watts per cylinder hotband and 160 watts for the sump heater) produces around a 13 F rise in the overall temperature of our O-320 after running the rental car for an hour.

This compares favorably to real-world data posted on the Reiff web site (http://www.execpc.com/reiff). The Reiff folks actually took hourly measurements of a Piper Cherokee’s O-320, equipped with the Hotbandd/Hotpadd system as described above, and what they found (under 22 F ambient conditions, with the engine tightly blanketed) was that after one hour, the cylinders had scored a 14 temperature rise. (The oil scored a 38 temp rise.)

Lord Kelvin is vindicated.

Cold War Rhetoric

It’s no secret that in the last few years the market for preheat systems has essentially become a two-player market, with upstart Reiff Corp. of Delafield, WI vying for pre-eminence over established front-runner Tanis Aircraft Services of Glenwood, MN. Reiff originally established a beach-head in this competitive market with a line of thermostatically controlled sump heaters. Then, a year or so ago, it announced a unique cylinder-base “hot band” heater that puts BTUs into each jug from the bottom up. A war of words began over which system is better: the traditional Tanis system (which not only heats the sump but individual cylinders with small heating elements that fit in unused cylinder-head-temp wells) or the Reiff Hotpadd/Hotbandd system. It’s not clear to us that any one system is so vastly superior to the other that there has to be a P.R. meltdown over the whole issue. Both companies make quality products (FAA-PMA approved) and charge a fair price. Both firms’ products get the job done. The question is how and where you like your heat delivered to the engine. In one sense, that shouldn’t matter too much since heat applied at any point on a metal object tends to diffuse to far-distant points rather quickly. If you could keep heat loss to zero (and could wait a suitable period of time), you could heat an entire engine with a $20 clothes iron clamped to one propeller blade.

But that brings up an important point, which is that no matter what type of preheat system you use, it’s a very good idea to cloak the plane’s nose with a tight-fitting blanket-like cover. (That goes for the prop blades, too, which are like giant cooling fins attached to the crankshaft.) Reiff and Tanis both offer covers; be sure to consult their web sites for details (see URLs at end of article).

Reiff argues that its Hotbandd heating elements—which clamp around the base of each cylinder, in the unfinned area near the hold-down flange—heat more effectively than Tanis units because the heat is applied around the entire circumference of the cylinder, rather than being supplied in pinpoint fashion to one small part of the enormous, heat-sink-like cylinder head. In point of fact, though, both units (the base band and the Tanis CHT plug) are 50-watt elements, and after a suitable amount of time you get very nearly the same cylinder temperature rise out of either unit (as Lord Kelvin would have predicted). Testing confirms this: After just 60 minutes the two units produced identical temp-rises in tests done by Reiff. (Reiff, incidentally, disputes some of the data offered by Tanis because of the way Tanis measures cylinder temperature rise, by taking a measurement only a few inches away from the heating element. See the Reiff web site for a discussion of this.)

A more important issue from a practical standpoint is the fact that Tanis probes sit in the CHT boss in each cylinder head. If you have a CHT probe in that spot already, Tanis offers a couple of workarounds, but they entail extra expense. One relatively cheap workaround is to take CHT readings from spark plug gasket-style thermocouples (leaving the original wells free), but this is hardly an ideal solution since—for one thing—plug-gasket thermocouples tend to read about 75 F higher than probes located in the threaded boss.

Tanis, aware of these shortcomings, recently announced a rocker-cover-gasket heating element (see photo), available in nine different shapes to fit all of the popular Lycoming and Continental (and some radial) engine models. Whether the wires coming out of these gaskets will survive hundreds of hours of throbbing vibration, no one knows (yet); but it’s certainly possible to moot that issue by simply removing the heated gaskets during warmer months of the year (replacing them with plain gaskets) and only use the heated ones in winter.

Sump Heat

Both Reiff and Tanis offer sump heat. The Tanis way is to put either one or two 50-watt cylinder thermo-plugs on the oilpan itself. Reiff prefers to cement a flexible silicone heat pad (held on with high-temperature RTV) to the pan and control it with a thermostat. The Reiff pads have a 10-watt-per- square-inch energy density and are sized according to the type of engine so that a maximum oil temp-rise of 100 F is not exceeded. The 10-watt/sq.in. number is significant in that the metal surface temperature never gets above 200 F (according to Reiff), which means there is no chance of thermocracking the oil on the other side. Some competitive sump heaters put out 18 or more watts per square inch, which can bring the metal to 300–400 (definitely not good for the oil). The Reiff thermostat is set to cycle at 150 F, for added safety and convenience (i.e., you can leave it plugged in 24 hours a day, seven days a week).

There has been a lot of discussion on the internet about whether it’s good to leave electric preheaters plugged in on a 24/7 basis, some people going so far as to suggest that thermostat-controlled systems—by cycling up and down in temperature—can cause a kind of greenhouse effect in the crankcase, with beads of (corrosive) moisture condensing on steel parts, etc. No one seems to have any hard evidence on this score, however, so we consider the issue moot. As long as the temperature inside the engine is above the dew point of the air, who cares?

The other criticism often aimed (obliquely, to be sure) at Reiff involves cylinder-wall oil runoff. Supposedly, critics say, warm oil can easily run off the barrel I.D. in a Hotbandd-heated cylinder, leaving bare metal. Reiff counters that the temperature inside the barrel of a Hotbandded cylinder on a cold day is no more than the OAT on a spring day in Florida. Yet you don’t hear plane-owners in the Sun Belt complaining about oil runoff as their planes bake on the hot ramp for weeks at a time.

We see no merit in any of the arguments we’ve heard against Reiff systems. The Reiff way seems fine to us.

Other Goodies

As mentioned above, Reiff and Tanis both offer engine covers. They also offer prop and prop-spinner covers. This is a very important add-on, since testing has shown that heat loss through the prop blades can be substantial. To that, we’d add one other thought, which is that one of the more common types of cold-engine damage we’ve seen over the years involves poor lubrication at the front of the crankshaft, specifically at the thrust bearing. (Lycoming IO-720, TIO-541, and TIGO-541 engines are particularly vulnerable.) Keeping prop covers on while preheating should help get this area warm and keep it better- lubricated from startup to takeoff. If there’s one thing you don’t want to have to buy at major overhaul time, it’s a new crankshaft for that IO-720 (or TIO-541) engine.

Of course, no matter how warm the engine is, it’s not apt to crank easily if the battery is frozen. Reiff and Tanis offer help here in the form of a thermostatically controlled battery heater. Reiff feels its heater is the best because the thermostat is placed inside the battery box, not on cold metal outside the box. The whole issue is moot, though, if your battery is located in the engine compartment, since the entire compartment (and everything in it) will be kept warm if you preheat with a cover on. Battery heaters are really for tailcone-battery aircraft like Mooneys, Cherokees, and older Cessna 182s.

Tanis scores a point for originality with its latest ancillary item: an engine pickling kit. The PKL-4 is for four-cylinder engines and kit PKL-6 is for six-cylinder engines. Each kit contains cylinder dehydrator plugs, desiccant bags, streamers, warning sign, and the proper preservative oils. The cylinder oil comes in a spray can ready to spray into your cylinders, while the MIL-C-6529 crankcase preservative oil is added to your own mineral oil to make the proper preservative mix. The price is $97 for the PKL-4 kit and $146 for PKL-6.

Conclusion

We don’t intend to take sides in the Great Preheater Debate; to us, calories are calories. (One does get the impression at times that the best way to preheat a cold engine might be to bring Bob Reiff in direct contact with Peter Tanis and stand well back.) There’s no question that a good deal of piston scuffing, thrust-bearing scuffing, and rocker-compartment mayhem occurs each year as a direct result of pilots starting their ice-cold (OAT 32 or less) engines without benefit of preheat. We’ve seen the damage and know it exists. Prevention, fortunately, comes in many forms. Tanis and Reiff both offer excellent solutions at moderate prices. (Reiff is definitely less expensive.) There’s no longer any reason to mess around with oil dilution, propane, car exhaust piped into the cowling, etc. Let electrons do the work.

For more information, phone Reiff Corp. at (262) 968–2342, (www.ReiffPreheat.com), or Tanis Aircraft Services at (320) 634–4772 (www.tanair.com).


To contact Kas Thomas at TBO ADVISOR:

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