logansc

logansc

Well Known Member
Like a lot of fuel injected engines, mine is a bit of a challenge to start hot (IO 540K). The other day I had occasion to take the top cowl off while the engine was quite hot and even with a purge valve and a considerable period on the electric fuel pump, 3 or 4 of the spider lines were very hot.

Got me to wondering if sleeving them with a suitable insulator would be a good idea. Since the famed TS of TSF is building his RV-7 in my hanger (helped him with his flap enclosure and floor boards this weekend), I asked him if he had a solution. He said he'd look into it and already has a sample or two coming for us to examine.

I can't do much on it until November because I'm leaving the country for a month to do some motorcycle riding in Italy this weekend, but I thought I'd best ask if anyone has ever done anything like this and what the results were. Since I've never seen an engine with fire sleeved spider lines, I'm guessing there may be reasons not to do it, but honestly I don't know.

Any ideas?


Lee...
 
I don't think insulating the lines to the injector nozzles will help with starting. The lines will heat soak during shutdown, as the insulation only reduces the thermal conductivity and with no fuel flowing to cool the lines they will end up a short time after shutdown at the same temp as the compartment. If you flow enough fuel in those lines during starting you will likely have a flooded engine. Note the hot start procedure is mixture lean until the engine is running.
 
I don't know if it would make any difference but be aware that covering these fuel tubes will make it difficult, or impossible to inspect them for cracks. They are supposed to be inspected every 50 hours for correct attachment (Adel clamps per SB 342G), chafing and cracking. These lines usually crack at the juction where the tube is silver soldered to the machined flare fitting. The most common failure is actually not a failure of the tubing itself, it is cracking of the solder, or weld which allows the tubing to leak or even seperate from the flair fitting. Cracking of the solder/weld usually propagares fairly slowly and can be caught during inspection before a problem occurs.

I have never seen flex lines installed in this location, but Tom may be able to make you some nice firesleeved flex lines. After all this is EXPERIMENTAL aviation.
 
I thought about wrapping mine with fiberfax and covering the fiberfax with heavy aluminum tape to reduce the rumbling and ocaisonal backfiring during hot taxi after landing. Running the boost pump usually cures that though.
 
About a minute after shutdown you will hear a skoosh noise. That's the fuel boiling in the injector lines. If you really insulate the heck out of them that will happen in two minutes. Same difference. Don't do it.
 
Lee, it sounds like the Rocket would be a good candidate for EFII. Stop by LHW some day and check out the EFII installation on Scotty's -7.
 
Lee's Rocket does have a purge valve, but has 2 mags. Anxiously waiting on the PMags. I know he's been on the list to get them.
Tom
 
I routinely fly in 110 degree OAT and my Rocket has no purge valve, no firesleeves, 2 mags with no impulse, no slick start...

Hot starts are NO problem.

Heat shields may help with some circumstances, but the injector lines have such small thermal mass that they are going to conduct a bunch of heat off the heads in just minutes after shutdown. Cant see radiant shields helping anything towards hot starts.
 
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I have wrapped my spider lines with heat rejection tape. No difference. I still have to keep my idle around 1000 RPM when taxiing after landing on a hot day. Otherwise, the engine will sputter and stop because the fuel in the little metal spider lines is boiling. I have the Airflow Performance FI with purge valve, so hot starts are not a big deal. The electric pump circulates cooler fuel up to the flow divider and the engine seems to like to start with a little priming and full throttle until it catches.

IMG_6250-M.jpg
 
Changing the mass of the injector lines, by putting insulation on them, could change the natural frequency, creating resonant vibrations that are not normally seen. If this happens, fatigue cracking could occur. There could also be some aerodynamic forces that are transmitted into the lines because of larger cross sections in the slipstream.

It seems like risk for no gain to me.
 
The heat rejection tape is very lightweight and is probably negligible compared to the mass of the spider lines. It is just adhesive tape about 1" wide and is simply wrapped around the lines between the existing adel clamps. I have taken it on and off once before and I did thoroughly check the lines at annual. It is very solid and I doubt that it would contribute any risk to the lines.
 
Lots of useful information here from a wide variety of perspectives. Thanks for checking in everyone. I'll take a careful look at my next steps. I don't suppose anyone has any empirical data on fuel (spider) line temperatures with and without some sort of fire sleeving/insulation?


Lee...
 
To say insulation would not work would mean the space shuttle should have burned up on every re-entry :D
 
I think we are all correct.
I talked with a thermal dynamics engineer---from the same compay that make our firesleeve---and alot of other insulating goodies. He says that "the trouble with insulation is that it does not prevent the problem, just delays it. In a heat "soak" application, the more insulation you add (to a certain point), the longer the short term prevention is."
In thinking about what he said, I can come up with a solution, but gee, its involved. Its puzzling to me that some examples of the same engine have hot start issues, and some dont. (Saw a Piper Arrow the other day that seemed to take FOREVER to hot start.)
Lee's Rocket has always been alittle hard to hot start, so yes its puzzling. In addition, the wear and tear on the starter just exaborates the problem. In his case (IO540K) I was told the 2 aft injection lines were hot, compared to the other 4. So, my thought was to "prime" those 2 cylinders with cool fuel from the boost pump. But, I think we need to verify that those cylinders are the issue, which means some elaborate changes for testing.
Tom
 
Hate to open this can of worms again, but what is Lee's "hot start procedure", and are the results consistent, or is it all over the place?
 
What Mags?

Also related is what make and type of magnetos. Bendix or Slicks? if Slick mags what model is the LH side, a 6393 retard breaker or otherwise?
 
Guys: This is a useful discussion I think, and I'm right in the middle of it. However, at the moment I am also right in the middle of Rome and about to go out to a very nice dinner reservation! Renting a motorcycle in the next few days and riding until the end of the month. I'll try and pick this up when I get back. I got some very good advice on hot start procedures for a VAF source and I will do a bit of a scientific testing with them when I return.

I do have Slicks with the advance on the left mag. I usually find that mine starts most reliably with full throttle in ICO and with the boost pump on. it's a bit of a raucous start though and I don't really like it. The advice I got should get me around most of that. We'll see how it works in November!

Thanks to all for checking in...


Lee...
 
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bruceh said:
The heat rejection tape is very lightweight and is probably negligible compared to the mass of the spider lines. It is just adhesive tape about 1" wide and is simply wrapped around the lines between the existing adel clamps. I have taken it on and off once before and I did thoroughly check the lines at annual. It is very solid and I doubt that it would contribute any risk to the lines.
Click to expand...

Bruce,
I know its an experimental, but I am solidly in tune with Alex on the resonance issue. I know I have seen documentation from either Lycoming or the FAA about not messing with the injector tubes. Just my 2 cents.
 
I'd be interested in seeing any applicable information.

IHMO, if anything the tape on the lines would be dampening any resonance.
 
I've been reading aviation stuff for a long time. Some of it even sticks :rolleyes: even though the source is long lost from me.
Tried to search a little today. (vacation time)
There is of course the SB-342 for inspection of mounting every 100 hrs which is the output of Lycoming experience with the cracking issue.
I found a couple of older ADs prior to SB-342 related to increased mounting points which would raise the natural frequency of the system (by shortening the "strings" between hard points). Adding mass such as insulation reduces the natural frequency, moving it closer to the operation frequency of the engine (not the direction desired).

Hard part is that my dim memory says there is a similar recommendation "somewhere" to not add any mass to the pushrod tubes (like more injection tube Adels :rolleyes:) but I can't find any docs on the net related to that.
 
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