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The Effects of Engine Compartment Heat
By Tony Bingelis (originally published in EAA Sport Aviation, December 1984)
DURING MID-AUGUST in Austin (Texas, that is), the temperatures hovered daily around 100° F and peaked out one day at 106° F. I flew my Emeraude that day. Man, was it hot! But the outdoor air was nowhere near as hot as that air rising out of the oil inspection door opening when I opened it after the flight. It got me to thinking about the problems that sort of heat had previously given me in my Rolls Royce powered installation (notice how I slipped in that ego bit?) and the precautions I had better take with my Falco engine installation.
Lately, a lot of builders have been learning that their new, high powered (anything over 100 hp), closely cowled airplanes seem to be developing more and more unexpected engine operation problems and other engine related problems. Welcome to the club.
Experienced builders and aircraft manufacturers alike have long been painfully aware that heat induced engine compartment problems were difficult to isolate and solve. Persistent instances of vapor lock in fuel Systems, failures of magnetos, alternators (or generators) and, yes, even batteries and vacuum pumps; have often been traced to excessive localized engine compartment heat.
How hot it really gets in the engine compartment came to me as a surprise. I had never before thought in terms of specific temperatures outside of being subconsciously aware that it did get hot in there.
How Hot Does It Get?
Well, sir, if I am to believe what I see, it gets hot enough inside the engine compartment to boil water, or more to the point, fuel . . . and then some.
While prowling the flight line during Oshkosh '84, I came across an uncowled Lycoming engine installation that caught my eye. It had several Temperature Indicator Plates stuck on it in various places around the engine compartment.
The builder had apparently been concerned with his temperatures at the firewall, fuel pump and at other strategic locations on the engine. An excellent idea. I'll bet he was surprised, though, to learn that most areas were recording searing temperatures of 340° F, higher in some places. Even if .you challenge the accuracy of these readings, temperatures around 300° F are HOT.
Note: Please be advised that I don't know of a source outlet for these Temperature Indicators. However, they are manufactured, under patent, by a company called WAHL of Los Angeles, CA. The sticker element is called a Temp Plate Recorder. The #240 plate visible in the photo apparently records temperatures of 310° F to 340° F in 10 degree increments.
Each stick-on Temperature Indicator is fabricated with 4 small circular openings that will turn black at some pre-calibrated temperature. In checking several of the Temperature Indicator Plates, I could see that most of the circles labeled 340° F had turned black in that particular engine installation. Sure, that's hot. That seems very hot. I would assume, however, that during engine operations and flight, the temperatures are lower than that due to the constant flow of cooling air through the compartment. Still; there are always stagnant areas where the temperatures can easily reach those mentioned. Besides that, an engine shutdown ends the flow of cooling air, and there is always a significant heat rise . . . until the engine finally cools down. Furthermore, temperatures in the 340° F range are easily reached and exceeded in areas adjacent to the exhaust pipes. These pipes radiate temperatures in the range of 1200° F, possibly higher.
Can you imagine what temperatures like that could do to your battery? To the fuel in your lines, pumps and gascolator? To the ducting and even to your hardware? And how about plastic tubes? When you think it all through, you begin to realize that most engine compartment problems are, indeed, heat related.
Almost any engine installation can be improved and benefit from a fresh review of its heat sensitive installations, so why not take another look at yours?
Heat Sensitive Materials
You have probably read that you should not use common fiber-insert type lock nuts (AN365) where they might be subjected to high temperatures. Now you know why they shouldn't be used in essential applications in the engine compartment. Since engine compartment temperatures often exceed 300° F, that is no place for a nut that is satisfactory for use only to temperatures not exceeding 250° F. Guess what can happen to the fiber locking feature. You should instead use the all-metal high temperature type nuts for all important engine compartment installations.
How about ducting? You know they have that black (CAT) ducting and the red (SCAT) ducting. Not only is the price different, the temperatures each is capable of withstanding differs, too. The red ducting, capable of withstanding temperatures from -80° F to something over 450° F, costs about twice as much as the black ducting which cannot tolerate temperatures higher than 300° F. Be sure you are using the red stuff for your carburetor heat box ducts, the fuel injector air duct and any duct that is directly connected to a heat muff.
Well, need we say anything about plastic hoses and lines inside the engine compartment? I would not recommend their use outside of, perhaps, a vent line not directly exposed to a heat source. Certainly do not use a plastic hose for your engine breather line. Efficient venting of the engine is critical and you don't want some plastic hose collapsing and creating an awkward engine operational situation.
Most FAA inspectors will not approve the use of plastic hoses for fuel and oil lines. I know of one builder who was able to get the inspector to O.K. his installation by slipping a Firesleeve over the line. However, I don't know that that is such a good idea either.
Show me a hot battery and I will show you one that is not long for this world. When a battery is exposed to temperatures much over 100° F, its longevity is jeopardized.
A battery installed on the firewall in the engine compartment has a tough life. Unless ventilated, it will be subjected to the very high temperatures previously mentioned. These temperatures are sufficient to make the battery boil. Needless to say, the battery life then drops as precipitously as penny stocks in a bear market.
A battery mounted on the firewall can and should be vented by ducting air into the battery case. This is fine while the engine is operating and the aircraft is in flight. However, what happens after engine shutdown? It could take quite a bit of time for the trapped residual heat to drop below 100° F . . . all the while the battery is suffering the damaging effects of the heat.
Hot Magnetos (I Mean Real Hot!)
It has become a standard practice to "ventilate" the magnetos. Many a magneto and engine failure has been charged directly to the malfunctioning of a magneto that had been operating in an environment too hot for it. That dead air space behind the baffles where both magnetos are situated becomes unbelievably hot.
A Cheerful Note: If you are a builder of a pusher, like the VariEze or Long-EZ, you will be pleased to learn that engine compartment heat will not be much of a problem in your installation because accessories are, for the most part, located in an area of incoming cool air.
Most magnetos can function and more or less survive up to certain critical temperatures. I understand that the Slick magnetos, for example, fade away when temperatures reach approximately 270° F while the Bendix mags fare somewhat better and can tolerate temperatures of up to 350° F . . . for a time.
Directing a blast of cooling air over both magnetos, therefore, is essential to their longevity. Short 5/8" to 1" diameter tubes projecting from the rear baffle and aimed directly at each magneto will do much to keep your magnetos operating properly. Do not omit this provision in your installation.
Some may balk at the idea of bleeding air through the baffles and look on it as a wasteful, drag-producing inefficient thing to do. Maybe so, but I'd be willing to give a little if it will insure that my magnetos don't suffer a heat stroke . . . wouldn't you.
Hot Alternators (Or Generators)
The cooling air blast tube for the magnetos works equally as well for an alternator or generator, wherever it may be located. Due to its location, however, ducting may have to be used to position the air outlet correctly. (Most generators have a small blast tube connection and all you need do is to connect it to a source of airflow with a length of ducting.) Be sure your alternator or generator does get its flow of cooling air even if you have to punch out another baffle opening.
By now you may begin thinking that your baffling installation is becoming more like Swiss Cheese than the ideal installation you first visualized . . . sorry about that.
Continental engine installations (65 hp - 100 hp) do not need an oil cooler but Lycomings do. An oil cooler should be considered as mandatory for all Lycoming engines (0-235s, O-320s on up). It should be mounted properly (see Figure 1) and located in an area where the air flow through the cooler is uninhibited. The oil cooler may be mounted up front in the inlet baffle (LHS) or in the rear baffle as shown. Some are even mounted on the firewall but require extensive ducting. At any rate, do not try to support the oil cooler with short bolts through only one of the flanges . . . it won't last.
The air exiting from the oil cooler will be hot and you should regard it as another unwelcome contributor to your engine compartment heat problem. Be sure that the hot air from the oil cooler doesn't blast directly on the gascolator or the auxiliary fuel pump, or for that matter, directly on a fuel line.
Oil cooler hoses should be protected with firesleeves. In this instance, you obviously are not trying to contain the heat in the hoses but are more concerned with safeguarding them in the event of an engine compartment fire.
Exhaust Systems Contribution
Here is a potential troublemaker. The sooner you can dump the hot exhaust gases overboard, the less of a heat problem you will have. Hence, short stacks equate with no engine compartment heat problems. On the other end of the scale we have the crossover exhaust system. We all know that the crossover exhaust system is reputed to be the most efficient that can be installed. The penalty you must pay, however, may give you cause to reconsider.
A crossover system, while marvelously efficient, does concentrate a lot more heat in the engine compartment because of the extra lengths of pipe crammed inside the cowling. These pipes, in conducting the engine exhaust gases (temperatures up to 1200° F) through the engine compartment, pass in close proximity to: the cowling, causing it to blister; closely skirt the crankcase, raising the oil temperature even more; pass the engine mount closely, inviting localized corrosion and, in passing through the area occupied by the fuel pump and gascolator, raising their temperatures even higher.
A lot of crossover systems have some of the pipes almost touching the cowling causing it to blister and become burned. The usual fix is to wedge in an asbestos or fire-resistant blanket between the pipes and the cowling. Others resort to wrapping the exhaust pipes to form a heat shield. The best solution, of course, is to keep your cowling as far away from the hot pipes as possible. An inch clearance and a good flow of air would help solve that problem.
One last note. Do not brace exhaust pipes against the aircraft structure with a rigid brace. Use a strap that is shock mounted to allow for the engine's movement and vibration in its shock mounts. A broken exhaust pipe in an engine compartment is a very real fire hazard.
While you're at it, see what else needs protection from engine compartment heat. How about those wires . . . ?