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4 strokes Fuel - Presurize or Not

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Kit Morpeth08/01/2009 14:20:00
52 forum posts
17 photos

Having read the article in RCME that encouraged 4 stroke IC fliers to not pressurize the fuel tank, this all seemed fine, however; If the pressure line is not connected to the exhaust (the latter being blanked off) the question remains "What to do with the line to the tank?" This was not covered in the article.

If it is closed then the tank is effectively hydraulically locked and surely fuel starvation would result. If it is left open then when the model changes attitude fuel is going to be lost through what is now a breather tube.

It would seem that some sort of one way valve is needed that allows air in but prevents fuel coming out. Am I right or wrong?

batcho9908/01/2009 14:36:00
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347 forum posts
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You can get NRV's for fuel lines, YS make some good ones. Although in all the years i've been using four strokes, i've always had the exhaust connected to the tank. It's an absolute must with my YS. In my experience, four strokes run better with a bit of back pressure from the exhaust. Surely thats why they have a nipple on the exhaust?

Which issue is this article in? I'll have to read it.

Hamish08/01/2009 15:25:00
642 forum posts
47 photos

Run my OS 4 stroke with no pressurisation.  The line that would normally go to the exhaust is left open in the engine bay.

Initial start I pressurise by a short length of tube to the vent pipe.  Once it has run for a while it will start without pressurisation.

Its an old engine and its the easiest engine I have for starting and the best for reliability

260 Flyer08/01/2009 23:36:00
523 forum posts
1 photos

How I would do it.

/sites/3/images/member_albums/31653/tank.jpg


Unless you are in a deadstick vertical dive fuel cannot escape.
Simon Chaddock09/01/2009 00:29:00
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5716 forum posts
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And if you are in a dead stick vertical dive the loss of a bit of fuel is the least of your worries!

Hamish09/01/2009 08:00:00
642 forum posts
47 photos

Thanks for the drawing Rob.  It now dawns on why fuel is spilling out of my model when carrying it nose down.  If I could take the vent pipe to the rear end of the tank this would stop this although to be honest it is not a major problem.

Edit

This solution would also prevent the loss ,as suggested by Simon. As a tight fist'ed Scot,  the loss of any fuel is of major concern

260 Flyer09/01/2009 08:13:00
523 forum posts
1 photos

Just make sure the clunk cant get tangled up in the pipework.

Iif you dive fast enough the fuel will be effectivly weightless, so it won't come out!

Hamish09/01/2009 08:15:00
642 forum posts
47 photos
Thanks for the advise.  I will just need to bite the bullet and accept this loss of fuel.  Could I fit a catch tank?
Kit Morpeth10/01/2009 10:13:00
52 forum posts
17 photos

The article is in the January issue of RCME and can be found on page 86 by Brian Winch

Thanks for all the useful comments guys. Much appreciated 

Engine Doctor11/01/2009 12:16:00
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Hi Kit. From my own experience ,the decision wether to pressurise a tank or not depends on various things, such as type/make of four stroke engine being used ,postion of tank compared to carb and I believe the type of model and flying you intend do . A lot of modern performance four strokes have larger bore carbs to allow easier passage for air into the cylinder so as to produce more power.. This usually means a lower air speed past the spray bar/fuel jet and a poorer venturi effect.;so pressurised fuel helps maintain reliability by forcing the fuel into the carb and not having to rely on the venturi effect .Older engines have smaller carbs and lower power output but will draw fuel better due to the smaller bore of the carb which in turn means a higher airspeed past the fuel jet /spray bar , producing a stronger venturi effect. . Some of the older two strokes also had large bore carbs fitted and suffered engine cuts for the same reasons . A simple cure was to fit a restricter ring in the carb to increase the speed of the airflow past the spray bar thus enhancing the venturi effect. Laser engines dont use any exhaust pressure and draw fuel very strongly but are designed for reliability in flght rather than maximum available power . A similar sized YS although a more complex engine , will produce a lot more power but MUST have a pressurised fuel system. Pumped engines are another way of increasing the reliability of an engine without having to worry where the tank is fitted ;as the pump ,not a venturi,will do all the work necessary to supply fuel the fuel jet. All the different systems require different plumbing requiremnts  ie tank breathers/open  or no breather/ sealed system . Try following the maunfactures advice for the engine and then experiment to find out wich system suites your model and type of flying. With regard to the open fuel pipe to the tank on an unpressurised system , once the engine has been started the inflow of air to replace the fuel being drawn into the engine is sufficient stop any fuel from flowing out. just make sure that the breather pipe ends outside the  model .Fitting a one way valve could  cause fuel stavation as a small amount of pressure is reqiured to over come the valve. Good luck and let us all know what you finally decide and how you get on.

Kit Morpeth11/01/2009 12:48:00
52 forum posts
17 photos

Thanks engine doctor for a very comprehensive reply.

The target model is a Glens Extra and the engine is a Laser 80 4 stroke. I was very leery about fitting it upside down although aesthetically this would be best. I think now I shall act on all this advice, fit the engine upside down but with the tank as low as I can practically get it and a breather pipe to outside the bottom of the cowl and no pressurisation. I shall let you know how it pans out

KIt

Braddock, VC11/01/2009 16:28:00
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1654 forum posts
82 photos

I have to say that the article by the wizard of Oz was the biggest load of twaddle I've ever read from him, how on earth he made some of the assumptions beggars belief. I particularly refer to his description of how the pressure is maintained in the tank after the engine has stopped. (incidentally I have three sizes of saito and the manufacturer insists that they need exhaust pressurisation, who would you believe, w.o.o. or gen saito?)

I wonder why our esteemed editor let that slip by - it's reminiscent of alex whitakers comment that there is only one british engine manufacturer in existence (PAW according to AW), completely ignoring laser, rcv and wren turbines. It leaves their credibility in tatters in my opinion. But perhaps that doesn't matter.

batcho9911/01/2009 20:45:00
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347 forum posts
16 photos

John, still not read the article. But, my YS set up allows pressure to be stored in the tank after it has stopped, days after! That will have alot to do with the tank plumbing and NRV fitted. I always release the pressure though, unless I forget, as to not damage the tank. All my other four bangers are the same set up as yours, with exhaust pressure.

Braddock, VC12/01/2009 13:12:00
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1654 forum posts
82 photos

batcho99, hwrote that the temperature of the silencer would maintain the pressure in the tank, as if the pressure wouldnt equalise via the big hole at the rear of the silencer.IIRC th whole idea was to prevent fuel syphoning into the engine.

A few years back he wrote advising how to stop the fuel syphoningby using a header tank with a rudimentary non return valve enclosed within.

TBH I don't enjoy any of his articles as I find most of them at odds with my own views.

Howard Tomlin13/01/2009 22:30:00
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69 forum posts
Kit Morpeth wrote (see)

Thanks engine doctor for a very comprehensive reply.

The target model is a Glens Extra and the engine is a Laser 80 4 stroke. I was very leery about fitting it upside down although aesthetically this would be best. I think now I shall act on all this advice, fit the engine upside down but with the tank as low as I can practically get it and a breather pipe to outside the bottom of the cowl and no pressurisation. I shall let you know how it pans out

KIt


i'm running a laser 90 in a graupner extra (patty wagstaff jobby), its inverted and absolutely has to have pressurisation as it wont run well (if at all without it).

 I've had a shed load of trouble making it work, the carb is way way lower than the bottom of the tank as the engine is inverted, this leads to it running ok on the ground and then goes rich in the air!! the complete opposite of what you would expect.

 I've tried rotating the spray bar to control the amount of fuel that gets drawn in to the carb and thats made little difference. The only real way to tune it is to have it running really lean on the ground and fingers crossed it doesnt go rich in the air.

 I'm seriously thinking of doing away with the aesthetics and mounting the bl**dy thing sideways!

 If your glen is as heavy as my extra then i think you may find the 80 a little underpowered too, sorry to break it to you...

Braddock, VC16/01/2009 06:56:45
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1654 forum posts
82 photos
Couple of things, for info re Lasers I'd phone the factory you'll get the best of advice there as many of the problems associated with inverted lasers have surfaced time and again.
One solution to a problem hilighted in howard's response is the seemingly incredible situation of lean ground run and rich in the air.
This is a cooling issue IMHO, in any tightly cowled engine the air passing over the engine is going to be heated and is going to expand some say by 100% others make it even more. On an extra, like on my WM midget mustang, an inverted laser is going to have a small amount of its cylinder head sticking out of the cooling air passage into the prop draft. Behind the cylinder head, when the plane is flying will be a low pressure air zone compared to the sides and front of the cylinder head. The carbie sits smack in this zone and I experienced all the problems that howard mentioned. I cured it by fitting one of the silicon exhaust deflectors to the carb, holding it on with a tie wrap. The open end sat out in the breeze, proud of the cylinder head; I also cut the end at about 45 degrees with the angled face facing forward. This plane was one of the few I've flow where the throttle was almost always fully open in flight as I used to like doing pylon turns, it really never faltered after that mod.
With regard to the syphoning, my FF9 has a stop switch and when this was activated the throttle barrel closed completely thus effectively closing the needle valve and eradicating any tendency to syphon.
The engine was the laser 80 and the model was close to 7.5 lbs in old money, I wouldn't bother with a breather, follow laser's instructions and point the vent into the prop draft and you should have no problems. I used an OS 4s plug and prosynth 10% fuel and that model was not underpowered. 
Howard Tomlin18/01/2009 06:58:23
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69 forum posts
John,
 
I'm going to try your suggestion, it is very tightly cowled so this might help.
 
Are you using the grey rubber tubes with the corrugated bend in them used to angle the exhaust away from the plane?
 
Thanks for your suggestion.
 
Howard
Braddock, VC18/01/2009 14:42:20
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1654 forum posts
82 photos
That's the one howard, I held it on with a thin cable tie and amazingly it never fell off .
Just ensure the bore of the exhaust deflector as JP call them is equal to or greater than the hole in the throttle barrel.
Let us know how you get on.
Steve Hargreaves - Moderator20/01/2009 14:55:10
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Moderator
6763 forum posts
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Just to add my 2ps worth I always pressurise the fuel tank for both 2 & 4 strokes as I find I get a more consistent engine run. After all we're not talking about a huge amount of pressure here forcing the fuel into the engine....just a low positive pressure to help the fuel on its way.
 
A positive pressure is better than a negative pressure in my book!!!
Tom Doyle21/01/2009 22:27:48
65 forum posts
I'm not entering the debate on whether or not to presurise 4 strokes, I always follow manufacturers advice. But I would like to comment on John G's remark that RCVs are British made. I heard recently, from a reliable source, that they are now made in the same factory as SC, ASP and Magnum.
 
Tom D

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