Tunnel Temperature Limits

[paul330]

I have searched the forums and elsewhere on the net but cannot find this answer.

I have insulated the front, floor and behind the (stainless steel) shuttle boxes. I also intend to insulate the scat tube. However, I still wish to monitor the tunnel temperature so have fitted a spare OAT probe and hooked it up to my Skyview (it works!)

The question I have is: what would be a good range of temperatures for caution and warning?

 

[Bill.Peyton]

That is a very good question. I considered doing the same thing. The real variable is the heat ducts. I know that on my piper, those ducts also run down the center section, and when the heat valve is on, they are almost to hot to touch. When the heat is off, they are cool. I would guess that there would be a wide range and it would be difficult to establish a specific high temp limit.
Rather than installing the probe in the heat duct, I decided to install two over-temp sensors in the cowling at the firewall. I am going to start with 250 deg F. The sensors are simply on/off bi-metal elements that feed my efis. They do not read the temp, just a binary input.

 

[Wayne Gillispie]

I have temp labels on the following with max temp noted after a nice hot summer: mech fuel pump-225*F; alternator-300*F(due to #1 exh radiant heat); magnetos-225*F. I have one bi-metallic snap switch near lower cowl outlet. It has been set to open at 250*F for the last 75 hrs, but just reset it to 240*F. I had no LED indicating overtemp this summer, but want it to warn me of an engine fire along with my ff max set at 27 gph.

Back to your question. I just use my hands on the tunnel. Hot water temp= 120-140*F. If my tunnel feels hotter than that without heat on, I know something is wrong. With heat on in the winter areas of my tunnel definitely get hotter than that. I have no worries because my fuel temperature is less than 30*F if my heat is on. Fuel pump and transducer stays cool with fuel. Flap motor is not used enough to overheat. I have my flap positioner board away from scat.

I would recommend cowl outlet temp thermistor or snap type switch over tunnel temp as you can feel tunnel temp while flying. If you have an engine fire you may not realize it until it burns through tunnel bottom.

 

[paul330]

Thanks for your input, Wayne. However, the "feels a bit hot" on the side of the tunnel doesn't really help.

Maybe I should simplify my question. What I am trying to avoid is (obviously) vapor lock. There will be a temperature at sea level where vapour lock becomes an issue. I would see that as my WARNING level since any thing above this would mean vapor lock is possible. Since this temperature will reduce with pressure (and hence altitude), there will be a lower temperaature at, say, 8-10k. I would envisage using this temperature as a CAUTION level ie vapor lock may be an issue depending on altitude.

So, the real question is - can someone give me those 2 temperatures?

 

[douglassmt]

Complex?

What you are describing is really a fairly complex heat transfer problem. As I understand it, you are concerned that vapor lock could occur in flight due to high temps in the tunnel. We all know that vapor lock occurs after engine shutdown (at least and for certain in the injector lines on top of the engine), hence the hot start procedure that purges vapors from the fuel injector lines and replaces them with cold fuel. However, in flight, the cold fuel is always moving through the lines in the tunnel, with the exception of the portion of your fuel line from the tank that isn't being used, upstream of the fuel valve. Even then, as soon as you switch your fuel valve, you immediately introduce cold fuel and purge any vapor that might have been generated - which would probably not even be noticed by the engine.

It's my guess that the most common cause of vapor lock is not just temp, but stagnant fuel that allows the fuel to absorb enough heat to create vapor in a long portion of a skinny line - causing vapor lock because compressible vapors don't "pump" as good as mostly incompressible liquids. Think about the heat transfer in an efficient radiator - slow down and spread out the liquid to transfer more heat. The solution is to forcefully purge the injector lines with cold fuel. Since this is what is happening all the time in flight, I would guess that the temps would have to be extremely high in the tunnel to cause vapor lock there. It seems safe to say that if the temps in the tunnel are lower than the temps just above the engine after shut down then vapor lock shouldn't occur in the tunnel. Vapor lock doesn't occur during flight in the injector lines over the engine, so it shouldn't occur in the tunnel. There are lots of complicating factors such as the fat lines carrying fuel in the tunnel at a much lower linear flow rate than in the injector lines, diameter of lines, steel lines vs. aluminum and so forth, but I feel pretty safe making these off the cuff conclusions.

Now that isn't too say that your tunnel should just be a little cooler than the top of the engine. I think most of us have found that it can be, if not cool to the touch in all conditions, at least touchable without fear of burning yourself.

My $0.02

 

[Wesael]

I would guess that the temps would have to be extremely high in the tunnel to cause vapor lock there. It seems safe to say that if the temps in the tunnel are lower than the temps just above the engine then vapor lock shouldn't occur in the tunnel. Vapor lock doesn't occur during flight in the injector lines over the engine, so it shouldn't occur in the tunnel. There are lots of complicating factors such as the fat lines carrying fuel in the tunnel at a much lower mass flow rate than in the injector lines, diameter of lines, steel lines vs. aluminum and so forth, but I feel pretty safe making these off the cuff conclusions.

My $0.02

Fuel under High pressure (20+ psi) will NOT vapor lock as easily as fuel at low pressure (in tunnel area).

I would not make the assumption that vapor lock on top of the engine has any corrilation to vapor lock in the tunnel.

For data purpose I put a thermister in the tunnel on a flight with OAT at the surface around 98F and after a cruise of 2+ hrs the tunnel temp stablised at 140F.

I would like to reduce this temp some way in an effort to cool the cockpit.

It seem like there is a awful lot of radiant heat that is being dissipated into my body on hot days

 

[jchang10]

For data purpose I put a thermister in the tunnel on a flight with OAT at the surface around 98F and after a cruise of 2+ hrs the tunnel temp stablised at 140F.

I would like to reduce this temp some way in an effort to cool the cockpit.

It seem like there is a awful lot of radiant heat that is being dissipated into my body on hot days

I noticed the same thing. With cabin heat OFF, the hottest part of the tunnel is the area closest to the firewall. My firewall is also pretty hot, though i have an insulating blanket on the inside. I happened to be wearing flip-flops one day and noticed that I could not keep my bare feet on the firewall.

The 2 sources of heat are from the cabin heat selectors on the firewall and the cowl air exit area. I wonder how much each of those contribute.

There is minimal personal discomfort, as i already have the firewall insulated. My feet are fine. If cabin heat is ON, i feel the tunnel cover getting heated.

For me, the main issue with these hot areas is I think it contributes to heating up the panel area behind the instrument panel. I have noticed at times the metal panel itself can get intensely hot. I routinely fly with the fresh air duct pointed up underneath the panel which immediately cools temps down.

I already have a computer fan blowing up on top of the glare shield. I hope they sell a higher CFM fan to help.

 

[Wesael]

I have the firewall insulated with 1" fiberglass board as well as insulation under all the floorboards and insulation on the floor of the tunnel area.

As a test I disconnected the hot air ducts from the muffler to the cabin heat selectors and was not able to decern any reduction in heat radiated inside the cockpit however, I did not have the temp probe in the tunnel for accurate compairison.

I think most of the heat is getting picked up from some other source than the selectors. This has been talked about in quite a few other threads and forums but I still have not found definative data on it.


It will all be ok again in a few months when the OAT's go south but I still would like to find a way to reduce the load on the air conditioner.

 

[N427EF]

Tunnel Temperature

Since you specifically stated that your purpose of keeping tunnel heat down is to prevent vapor lock, I can offer the following.
While a warmer tunnel may contribute to vapor lock further down stream,by pre-heating the fuel, it is unlikely to make a large contribution to a vapor lock condition. Your chief culprit in vapor lock scenarios is the mechanical fuel pump, or as I call it the "fuel boiler".
Lengthy discussions on vapor lock issues are found elsewhere in the forum.
Having built my fuselage during a time when the tunnel heat issue was discussed in more than one forum, I doubled the tunnel floor for 2" insulation as well as firewall insulation between firewall and tunnel space.
Like others, I have not measured the temps but can easily rest my bare leg or hand on any surface of the tunnel structure at any time.
Since I am burning mogas I am acutely aware of vapor lock prevention.
I do have a temperature probe installed on the lower part of the firewall just outside of the tunnel wall in part as an instant fire alarm and the limit is set at 200F. As you can see in the pic I generally see it around 150F to 165F.
I have tested mogas at 101F for take off and continued climb to 12000 feet with the electrical pump shut off at 1000 feet and no adverse effect on engine operation.
Vapor lock issues need to be addressed in another part of the fuel system if you are serious about using mogas. If you plan on using avgas just follow the plans and you'll be fine.
The 3rd picture shows a fuel vapor sniffer installed in the tunnel but could probably accomodate a temperature sensor. As I see it there is not much value in knowing the temperature in the tunnel.

 

[paul330]

Maybe I have misunderstood. I thought that high tunnel temperatures had been implicated in incidents of engine failure due to vapor lock - hence the VANS SB concerning tunnel temperatures.

If this is not the case then (since I will be running Avgas) it merely becomes a matter of comfort. In which case I shall simply monitor the temperature to see how effective my heat mitigation strategies are.

Thanks for the input.

 

[douglassmt]

Misunderstand

Fuel under High pressure (20+ psi) will NOT vapor lock as easily as fuel at low pressure (in tunnel area).

I would not make the assumption that vapor lock on top of the engine has any corrilation to vapor lock in the tunnel.

For data purpose I put a thermister in the tunnel on a flight with OAT at the surface around 98F and after a cruise of 2+ hrs the tunnel temp stablised at 140F.

I would like to reduce this temp some way in an effort to cool the cockpit.

It seem like there is a awful lot of radiant heat that is being dissipated into my body on hot days

I wasn't saying there is a comparison. I was using the example of how we know vapor lock occurs when fuel is stagnant, in a high temp environment, in little tubes and not under pressure. None of those conditions occur in the tunnel.

 

[Don at Airflow]

Avgas boiling point

Avgas boils at 130 degrees F at sea level. At 10,000 ft it's boiling point is around 120 degrees F. This assumes that there is no pressure drop in the supply plumbing.

Don

 

[N427EF]

Avgas boils at 130 degrees F at sea level. At 10,000 ft it's boiling point is around 120 degrees F. This assumes that there is no pressure drop in the supply plumbing.

Another reason I installed as many of the fuel supply system components
as possible on the cool side of the firewall and by all means, do what you can to keep the tunnel temps as low as possible.