If anyone has really figured out why some engines corrode and others dont, we havent been favored with the explanation. Were told by engine shops we periodically contact that engine corrosion has reached epidemic proportions and that more owners than ever who should have reusable cams and cylinders at overhaul are seeing them trashed by rust instead.
This trend has ignited a dual-pronged interest in engine corrosion control. One is aimed at improved oils and additives-we reported on that topic in the April 2006 issue of The Aviation Consumer-and the other is whats best defined as active corrosion prevention by gadgets called aerators or engine dehydrators. Theoretically, these work by pumping dry air into the engine and cylinders to flush out acidic moisture-laden gasses that would otherwise condense when the engines metal innards cool. Engine dehydrators claim to turn the engines insides from a steam bath into the Mojave Desert. But do they actually work?Three Versions
We found three engine aerators on the market, including a new offering from Tanis Aircraft Services, which is well known for its engine pre-heater products. Two of the products-and the only type we tried-use chemical drying agents to provide the dry air. A more sophisticated model has an electric condenser element similar to a household dehumidifier. That model is made by Aircraft Components, Inc., which also makes the chemical-based Engine Saver model we tested here.
The Engine Saver ($285 directly from the company) has been on the market the longest and youve probably seen the ads for it. Its the one that looks like a two-liter soda bottle strapped to a metal base with plastic hoses for pumping dried air into the engine. The principle couldnt be simpler. The bottle is filled with a couple of pounds of silica gel beads of the same sort used in the cylinder desiccator plugs we evaluated in our April report. The beads are clear but a small percentage dyed blue turn pink when theyre too saturated to absorb more moisture. The material is thus known as indicating gel.
An electric pump in the base of the Engine Saver drives a small volume of air through the desiccant material and into the engines oil breather pipe. According to specs supplied with the Engine Saver, the pump runs for an hour to initially flush the engine of moist vapor then runs periodically to keep the engine flooded with dry air. It has onboard temperature and humidity sensors to control the cycling, typically between seven and 15 minutes on-depending on ambient humidity-and 15 minutes off.
Why not just run it continuously? Because chemical driers like the silica gel used in the Engine Saver can absorb only so much water before becoming saturated and ineffective. They can be reactivated by baking the drier in a 275-degree oven for a few hours. But given the need to dump the desiccant-slippery little beads that roll everywhere except where you want them to go-into a baking pan, the less baking, the better. Cycling the drier is a compromise between high volumes of the driest possible air and longer desiccant life.
The Tanis Engine Dehydrator-$425-works on the same principle, although the design and operation is different. The silica gel is contained in a fiber net bag and placed in the bottom of what appears to be a sturdy plastic tackle box with a double lid. The drier material is placed so it blocks inflow and outflow, forcing air driven by a fan through the silica. The Tanis dehydrator is a recirculator; it blows dry air into the oil filler pipe and draws it out of the oil breather at the back of the engine.
Tanis uses a much higher volume of air than the Engine Saver gadget does and also has larger hoses, which we found easier to attach to the engine oil breather line, but a bit more difficult to snake into the oil filler. Although high flow dries the engine more rapidly-and our data showed this-it also tanks the drier sooner, meaning youll have to bake the stuff to restore it more often.
Tanis solution to this quite clever: a built-in heater under a grate upon which the bag of drier sits. The idea seems to be this-it wasnt clear in the instructions-while youre out flying, you flip the devices switch to the “reactivate” position and the heater runs while youre away. When you return, flip it back to the blower position and resume the cycle. The Tanis box also has a digital indicator in the lid, displaying the relative humidity and temperature of the recirculating airflow through the engine.Setup
We set up the driers as instructed and ran each for 24 hours, using the Continental TSIO-360 in our Mooney as the lab rat. Before attaching the engine driers, we flew the airplane and heated the oil to normal operating temperature. To measure performance, we used five Lascar digital dataloggers which measure temperature, humidity and dewpoint. One kept track of ambient conditions, two sampled the air from a cylinder on each side, one was in the exhaust pipe and a fifth deduced crankshaft conditions through the oil filler pipe.
Setting up the engine driers is easy enough. The Engine Saver has a 1/4-inch diameter plastic hose fitted with a foam flange washer. The idea is to force the tube up into the oil breather line past the point where the line has an ice vent to prevent it from dumping the engines oil supply due to the venturi effect. But we immediately ran into a problem. Our oil breather line has a quick turn and the washer just wouldnt get past it. So we settled for taping up the vent, with the air supply line and its flange washer well sealed, but not deep into the breather pipe.
One other problem: While snaking around the pipes gooey inside diameter, it picked up enough oily gunk to plug the narrow tubes end and the pump wouldnt blow the stoppage completely clear. Neither of these are showstoppers, but some fiddling may be necessary to get these devices working correctly.
When we plugged the pump in, it started and, as advertised, ran for an hour to flush the engine. It then shut down and reverted to cyclic operation, as explained above. The pump is noisy enough to hear through the hangar doors, but not noisy enough to spook the neighbors.
The Tanis works a little differently than the Engine Saver does. Its an outgrowth of another product Tanis sells, the Engine Aerator, which is nothing but a fan that evacuates the crankcase of vapor and mist, but without any air drying. In fact, the Engine Dehydrator appears to have the same high-volume fan that the Aerator uses. To be fair, Tanis tells us the product is still work in progress and it plans improvements before the final product is fielded.
With its larger volume, it can flush the crankcase quickly and the instructions direct you to do just that. Insert the dehydrators output hose into the oil filler neck and blow all the nasty stuff out the breather and suck it out through the breather pipe. The Engine Dehydrators hoses easily slip into both the filler and the breather pipe and the fan runs with an imperceptible hum.
The dehydrators instructions arent clear on whether the product is intended for continuous use or just to dry the engine out quickly.Trials
Before installing the engine dryers, we collected some control data by operating the engine and letting it cool normally, with no dry air to help things along. The graphs on page nine show the results of this test. As any Continental engine owner knows, the oil filler cap condenses a lot of moisture and becomes a gooey and often corroded mess. Presumably, conditions are no better inside the crankcase.
We werent surprised that in our TSIO-360, the oil filler area remained at nearly 100 percent humidity 24 hours after the engine was shut down and that dewpoint and temperature were in lockstep. These are the ideal conditions for condensation and corrosion.
Conditions inside the cylinders appear to be less harsh. Just after shutdown, cylinder humidity decreases rapidly, then rises slowly, roughly tracking ambient humidity conditions, but in a more damped pattern without the periodic spikes as the day progresses.
In our tests without the engine dehydrators installed, the temperature dewpoint spread in the cylinders was never less than 10 degrees, so there was no condensation. Thats not to say, however, that there couldnt be, especially in colder climates with lower nighttime temperatures.
We hooked up and ran both devices in our TSIO-360 and in a Lycoming O-235 for 24 hours. The Engine Saver dramatically reduced crankcase humidity in both, but more important, it depressed the dewpoint and increased the temperature/dewpoint spread to as much as 40 degrees but never less than 20 degrees. In other words, we would have needed nighttime temperatures in the low 50s to see any condensation, unlikely during high summer in Florida.
In colder climes with lower nighttime temperatures, dewpoints are likely to be lower because of lower humidity, so the Engine Savers performance should be better and it will run less, since the pump is controlled by ambient humidity.
Our data showed an interesting sawtooth pattern with max and min humidity spikes between about 50 percent and 20 percent. Aircraft Components Duane Groves told us this was evidence of the pump cycling, as its supposed to. He surmised that the highside humidity spikes were due to air leaks in the engine but we think thats inevitable.
In any case, when smoothed, the data revealed average dewpoints in the high 40s to low 50s. It took the Engine Saver about four hours to wring the engine out, after which it maintained the values easily. Groves told us the silica gel would last between 30 and 60 days at this rate of cycling before it needed regeneration.
We found little evidence that the Engine Saver keeps cylinders drier, however. Twelve hours into the test, the temperature/dewpoint spreads in the cylinders were 15 degrees without the Engine Saver running, with humidity in the mid-60s. With the Saver running for 12 hours, the spreads were still about 15 degrees with humidity in the mid-60 percent range. The Engine Savers literature suggests that dry air from the crankcase gets past the rings and into the cylinder but were skeptical of this claim, given our trial results.
Measuring the performance of the Tanis Engine Dehydrator proved more difficult. Because it actively blows air into the engine through the oil fill pipe, there was no room there for our sensor, so we constructed a T-fitting to sample air coming out of crankcase breather, surmising that if the air is dry at that point, its probably dry in the crankcase, too.
With its higher airflow, the Tanis device dried the engine more quickly than the Engine Saver did and, as shown in the data graphs, it delivered similar depressed dewpoint values, all good stuff.
But even with the higher flow, it didnt dry the cylinders in any measurable way. In our view, neither device is capable of driving enough dry air past the piston rings to help much. In fairness, we dont think this is all that important. Although cylinder corrosion is a concern, the big worry is camshafts and, to a lesser extent, crankshafts. Our control data suggests that conditions inside the cylinders are less likely to cause condensation than on the crankcase side, which is, by comparison, sopping wet.
We see two things in the Tanis product that we think could be improved. One, its airflow is too high, in our view. It blows the crankcase clear of gasses quickly-thats good-but after that, a gentler airflow would work just as well and the silica gel would last longer. In the Florida humidity, the gel was saturated after 24 hours.
The self heater in the Tanis seems to address this-it runs at just over 200 degrees and dried the gel back to its blue condition in about 12 hours. Although its easy enough to bake the material, we think its better to do that as infrequently as possible. Thats especially true if you have long-term, unattended storage in mind.Conclusion
Do these things really work? Yes, we think our tests confirm that they do. The leap of faith is that our moisture measurements in the engine openings reasonably represent conditions inside the engine and that the dry air finds its way into the motors nether regions effectively enough to eliminate condensation. We think this assumption is reasonable.
Aircraft Components claims that its Engine Saver reduces the possibility of rust and “helps reach TBO.” We think this is a fair claim and not hype, given the products performance. The thing may look like a high school science project, but it works as claimed.
Which of the two to buy? For long term, unattended storage, we would pick the Engine Saver. Its inexpensive and the low flow rate means the silica gel needs less regeneration. If you can attend the airplane often and have a higher budget, the Tanis is not a bad choice. The self-regeneration of the silica is a nice touch and less of a bother than dumping all those beads into a pot to bake them periodically, as with the Engine Saver. But in our view, the Tanis could be improved with a lower airflow rate on the fan. Well look for that in a re-designed version of the product, which Tanis may offer.
The other option-not tested for this round-is ACIs Black Max. At $450, it uses a thermal electric drier with an automatic defrost cycle to deliver dry air and can thus run indefinitely without regular maintenance. If it delivers air as dry as the other two units do, it should be just as effective.