Model Boat Mayhem
The Shipyard ( Dry Dock ): Builds & Questions => Pleasure boats, Sports, Race, Power and Leisure Boats: => Topic started by: craig dickson on January 10, 2014, 07:35:25 pm
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Hi folks
I am interested to know if you use a float chamber in your fuel supply system.
Whilst I understand the potential benefits in theory of using a float chamber to deliver a constant level (or head) of fuel and even feed pressure to the carb during the duration of a race - do such devices deliver in reality?
Are they reliable?
Are there any pitfalls to think about if using one in race conditions?
If you use one, is it necessary to have your fuel tank under pressure from the tuned pipe?
I ask these questions because I have never used a float chamber. I have no option for radio adjustment on the main carb needle. So when I run my boat full of fuel it runs on the rich side until the tank drains to about half full. My tank is pressurised from the tuned pipe currently.
Your advice and comments will be much appreciated. Thank you in advance of any replies :-)
Craig :-))
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Most (if not all) of the FSRV multiboats run with a float chamber. Check out Dave Marles Prestwich web site or Ian's Boats and to get really technical they run RC programmed trim tabs as well plus variable length tuned pipes. If they can vary it they will change it on the run!
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Hi craig
Yes a float chamber will deliver a constant head of fuel to the carb, and act as a "sump" when the main fuel tank is running near empty, which it should ideally be when nearing race end. You need to pressure feed the tank then the float chamber will be constantly full of fuel so there will be no deviance in the fuel head.
Pmc and Ian's boats do great ones. Pay more for the best.
Also with float chambers, there is no concern with the fuel tank pick up picking up air when low or cornering or accelerating, as the float chamber acts as a perfect sump.
Garry
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Thank you for your advice and comments guys :-))
I will be ordering one over the weekend. The prospect of having my engine running at its best on a full tank of fuel and not getting too lean as it empties is an appealing one. :-)
Cheers
Craig
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dont hold me to this but dont think they make a big difference on sports engine !!
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Float chambers are pointless on a pressure system as the apparent (gravity) head in the fuel tank isn't making any difference to the mixture. What is is the capacity of the system when it leaves the tank ie. how much fuel is tank pressure shoving down the tube into the carb and out of the jet.
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So when I run my boat full of fuel it runs on the rich side until the tank drains to about half full
Would a chicken hopper system help?
Regards Ian.
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Thanks for your further interesting comments folks. Just for information my current set up is just a sports engine (SC46) with fuel tank pressured from the tuned pipe. My fuel tank is about 4" high. And if I set the main needle for peak speed when the tank is low on fuel, once filled up the engine runs noticeably too rich. To the extent that I estimate it is running about 5% (or more) slower on a full tank.
So that leads me to believe that the difference in fuel level in the tank does indeed affect the volume supplied to the engine (please correct me if I am wrong). With a vented tank the fuel is still under a constant pressure, that being atmospheric pressure. With a tank pressured from the tuned pipe, my logic suggests a similar situation the only difference being that the pressure is higher but yet still constant assuming full throttle conditions. And therefore comparing pressured and non pressured tanks I believe that the fuel (header) level will still have an impact upon the rate of fuel supplied to the engine albeit perhaps less of an impact in relative terms with the pressured supply. Again please tell me if I am misunderstanding this.
Ian, what is a "chicken hopper system"? {:-{
On another note, an alternative remedy would be to have a remote needle for servo adjustment in running. However I am not keen on that as I want to keep things as simple as possible with as little to go wrong.
Thank you all again for your interest and comments in respect of this topic.
Cheers
Craig :-))
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Float chambers are pointless on a pressure system as the apparent (gravity) head in the fuel tank isn't making any difference to the mixture. What is is the capacity of the system when it leaves the tank ie. how much fuel is tank pressure shoving down the tube into the carb and out of the jet.
If a float chamber is pointless on a pressurised fuel tank, are you saying that they work well on a vented tank? That raises a question for me:
The float chambers I have ordered both take their supply to the chamber via a nipple at the top of the chamber. When the fuel level in the main vented supply tank drops below the level of that nipple, without pressure how does the float chamber get a reliable supply of fuel?
Craig
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I'm just having a look at some of the fuel systems I used in model aircraft...control line variety so the tricky bit there is centrifugal force. That's were chicken hopper tanks are (were) used and they are briefly 2 tanks in one with the small one that feeds the engine fed from a larger one connected to the tank. I'll enlarge on this later as I've had rather an epic couple of days and my brain hurts!
Bob
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Hi Bob
Thank you for your further input. And do please offer your further thoughts in due course as they will be interesting.
In respect of my longer comments above, do you think my thoughts on the principles (applied to a boat situation) are sound? Or would you suggest otherwise? :-))
Cheers
Craig
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Interesting comments by all.
I think with no float chamber, the critical fuel pressure at the carb intake will vary (irrespective of the carb suction.)
With a pressured tank, carb intake fuel pressure will depend on pipe pressure, higher at max throttle, lower at idle. A full tank of fuel will add to the pressure, diminishing as the level falls. With no tank pressure from pipe, ie atmospheric air pressure only, the only variable will be with tank fuel level, making engine run leaner as tank empties. So lots of variables!
With a float chamber properly set up, you should get a constant carb intake pressure throughout the race,
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After a gallon or 2 of tea I think a float chamber is probably a good idea. Thinking about fuel systems in 1/8th stockcars they run a small, baffled tank with a vertical feed pipe from a well to the outlet at the top of the tank and a pressurised from the exhaust. Enough pressure there to force the fuel upwards out of the tank but the whole system is very compact compared with our boats. Here is the nub of the gist I think. Given the length of the pressure pipe and the surface area of the fuel in a boat tank I don't think a pressure system actually works to any significant extent and are only fitted because that's what you do (I would be interested to know if anyone has any figures on in-tank pressures). So, float chamber/header tank (shades of chicken hopper) is probably the way to go although I think the ideal solution is fuel pump, header tank and return line. One thing that hasn't been mentioned (until now!) is the capacity of the system downstream of the tank to flow fuel and any back pressure.
Cheers
Bob....in the process of getting my coat
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Thank you Garry and Bob for your further input.
It now appears that the consensus of opinion (within these threads) is that float chambers are beneficial to achieving a more constant supply/pressure of fuel to the carb.
Bob, I don't know how much extra pressure results from the feed off from the tuned pipe, although on my engine it means that the main needle on the carb is set at about 3/4 turn out compared to 2+ turns when the tank is vented with no extra pressure.
Garry you said that with a float chamber properly set up you should get a constant carb intake pressure throughout the race.
Consider the float chamber in operating conditions: Assume there is a constant pressurised supply of fuel to the top intake of the chamber. If the float initially is below the valve cut off point, then presumably that pressure feeds its way right to the carburettor as in that state there is no restriction. At the point the fuel in the chamber rises sufficiently to lift the float to seal the intake valve what impact does that have on carb intake pressure? Does it momentarily drop off? Would that interrupt the running?
My unqualified thoughts are that after the system has settled down, the float chamber must surely enter a kind of equilibrium state where by the cut off valve is semi sealed (float constantly near the top of the chamber) allowing just enough fuel in to replace that which is consumed by the engine.
Forgive me if I am over cooking this topic folks! Being new to the concept of float chambers it has got my mind thinking. If one works well on my boat and even if it only delivers a 5% overall improvement in race laps, that can make the difference between winning a race and not winning.
I will let you know how I get on once I get my chamber and test it.
Cheers
Craig
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Craig
No your analysis go the float chamber is incorrect if I have understood your comments correctly.
If the fuel level in the float chamber is say half way, (don't forget the float chamber is not a sealed unit, it has an air vent in the lid), you may have overlooked this not having had one, the fuel will trickle in through the top feed until the chamber is full and the viton valve stops further ingress. Agreed there will be a minor minimal variance in float chamber level and corresponding fuel pressure to carb. Under race conditions. The float chamber will operate at full most of the time with the level in it varying by just a fraction of a mm as the engine draws fuel from it at the same rate as the float chamber is fed from the fuel tank (fuel under pressure).
In addition, as for carb back pressure this is in my view unlikely to be significant or relevant as the pulses (carb intake pressure variations) will vary during each single revolution of the engine. However the carb is designed to draw fuel from the pick up over even a short time period, say 0.1 second when the engine will have completed 33 revolutions on an engine doing 20000rpm which is not untypical of a glow engine.
This thread definitely gets the brain working! Interesting comments.
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Garry
Thank you very much. I did not realise that the float chamber is vented. Now it all makes perfect sense!
The principle of the float chamber is much more simple than I imagined.
Cheers
Craig :-)) :-))
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An update folks...>
Well I got my float chamber and it is now installed. Until testing I do not know whether it is properly set up so to speak or whether it will deliver better more consistent running. One thing for sure is that I will have to completely alter and reset my carb needles {:-{ Only time will tell.
(http://i734.photobucket.com/albums/ww350/DICKSONUK8/Crusader/Chamberinstalled.jpg) (http://s734.photobucket.com/user/DICKSONUK8/media/Crusader/Chamberinstalled.jpg.html)
Craig :-))
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Neat job done craig. I think you will find the new set up will give you an edge by keeping the boat running on optimum needle settings throughout the race. Engine will no longer lean out as tank lowers and run slow and rich at start of race as his has done last year. You will need to make a minor adjustment to carb, ie lean the main needle out. Previously, the carb was set for a fuel feed under pipe pressure, now it will be just gravity fed from the float chamber.
Adjustment is likely to be minor, a quarter a of a turn of so.
Like the mount, and float chamber fuels level looks good.
Look forward to seeing it tested soon.
Garry
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Engine will no longer lean out as tank lowers and run slow and rich at start of race as his has done last year. You will need to make a minor adjustment to carb, ie lean the main needle out. Previously, the carb was set for a fuel feed under pipe pressure, now it will be just gravity fed from the float chamber.
Garry
Garry, thank you for the compliments on my installation. You said I need to lean out the main needle??? Surely with no pressure from the float chamber it will need the opposite ie richer setting???
Forgive me if I am being a bit of a plank!! {:-{ :(( <*<
Craig
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Sorry, you are correct craig. I am the plank! %% :police: :police:
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Sorry, you are correct craig. I am the plank! %% :police: :police:
I forgive you my son <*< <*< <*<
Thy will be done ;)
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{-) {-) {-) {-) all this messing about ! {-) {-) %% i would give up now craig you know you don't stand a chance ! them pesky juniors will be ganging up on you ! and ill sneek past the lot of you and win O0 O0 O0 O0 ok2
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on a serious point i thought float chambers were meant to be as low as possible in the boat {:-{
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one other point if your "gravity feeding" from the float chamber why not gravity feed from the tank take the pressure off and use it as a breather like other do ! {:-{ {:-{ {:-{
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My take on Float chambers and carbies.
The object of the exersize is to maintain a constant flow of fuel to the carby, equiped in this case with a spray bar, regulated by a needle valve.
To do this, we require a constant flow of fuel at constant pressure, at a given rpm.
with a float chamber we can maintain a constant head of fuel as applied to the spray bar.
Without this, and only a free to air vented fuel tank, the head varies as the fuel level drops, and any level below the spray bar will not flow under gravity to the spray bar, therefore the whole of the tank should be above this level, resulting in a large flow reducing to zero.
To get around this, we pressurize the fuel tank, enabling the tank to be mainly below the spray bar, as the pressure pushes the fuel out of the tank, upto the carbie.
Using the pressurized system, the flow to the spray bar
- if above will be pressure plus a head factor due to gravity trying to push the fuel out of the tank.
- if below will be pressure less a head factor due to gravity trying to keep the fuel in the tank.
Only the header tank supplies a constant head of fuel.
That's my take - right or wrong. %)
:-X
cheers
:-))
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nice job craig :-)) as a side note what paint have you used inside your boat as it looks nice a neat all the same collor without resin and glass showing :}
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Hi Folks
Thank you for your further interesting replies. Just to pick up on what was said recently:
tmbc - Mark are you racing in the A-Class this year? If so I may need to do modification 2 and reinforce the sides of my hull! %) O0 O0 You said you thought that float chambers should be as low as possible in the hull. That not being so with my installation, what effect do you think I might suffer?
You also asked why not remove the pressure pipe and have the main tank vented? My reasoning is that with the float chamber as shown pressure will be essential to ensure fuel supply to the top nipple of the chamber. If not using a float chamber, I could run the main tank vented as you suggested. But that still leaves the issue of the fuel mixture leaning out as the tank empties.
vnkwi Your take on float chambers made a lot of sense to me. Thank you very much for the logical explanation.
gw84the2nd In answer to your question: When the boat was built last year I applied two coats of polyester resin with yellow pigment added. As well as making the inside look a lot neater it gives the big advantage of sealing all those fibres which otherwise trap oil and muck. Not only does it make it easier to wipe clean inside, it more importantly allows easy preparation of the surface should extra components need glassing in. The float alloy chamber mount was fixed in with fibreglass paste and yellow pigment added. Once set I applied a coat of clear resin to seal the fibres.
Out of interest, there is in my opinion one factor that will be critical as to whether this installation is successful or not. And that is the main tank maintaining at all times sufficient pressure to make the fuel flow into the top of the chamber. This boat/engine has to date never stalled. If the engine does stop during slow running then this may be the issue.
Cheers
Craig
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that would be telling !!!! :-)) :-)) :-))
{-) {-) {-) {-) {-) {-) {-)
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that would be telling !!!! :-)) :-)) :-))
{-) {-) {-) {-) {-) {-) {-)
Don't be shy Mark........>
It is okay to say, "I don't know"
%% %% %% %%
:-))
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i heard it on the grape vine ! :D :D :D :D :D
i was just told to mount a float chamber as low as possible on the right hand side of a challenger i had ! didnt bother took it out and ran with out it !
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Hi Craig
I wish you luck with your float chamber. I tried one many years ago, but didn't get on with it, as it kept jamming and flooding - but that was an earlier version, so yours may perform faultlessly!
Unless you are using a very large bore carb (for all out speed), a standard carb usually has enough suction at the venturi to allow the fuel to be drawn from quite a distance. I have found with the smaller engines I have used, that there is no need to have a float chamber.
One of the reasons I would use a float chamber, is to minimise the movements of a heavy mass of fuel in a big tank. When accelerating, the fuel is pushed back towards the pick up (at the rear of the tank) and vice-versa when cornering. As can be imagined, with a 90 engine halfway through the race, the fuel sloshing about (technical term :-)) in the tank can vary the amount provided at the carb enough to make the mixture "unpredictabe".
However, if you are going to use one, then the best setup is:
1. mount it so that the fuel level in the chamber, (ie. the height of the fuel when the valve cuts it off) is the same height as the carb spraybar, (as has already been mentioned).
2. mount it as close to the carb as possible and keep the delivery tube length short (fuel filter BEFORE the chamber to pick up muck before it jams the float valve)
3. If possible, use an anti-vibration mount to minimise frothing.
4. mount it absolutely vertical to the normal running angle of the hull.
If you are running a pressurised system (another thing I never found reliable), you may find that it runs rich when the tank is full, and leans off when getting empty (the air in the tank compresses more easily than the fuel), so you may still need a third channel for mixture control.
Another thing I see from your photo, is that you are using the standard carb throttle arm (similar to an OS arm). This is a piece of thin steel with a Z bend in it. The bends are a weak point and will snap, especially if used with a solid rod from servo to carb. Make yourself a replacement out of 1.5mm brass sheet, silver soldered to a brass boss (with grub screw). This will last forever!
Finally - now you've got it all set up .... take the engine back out and put the starter belt in! {-)
Best wishes
Danny
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Hi Danny
Thank you very much for your exceptionally helpful contribution. You made a very sensible point about the position of the fuel filter and I will be doing as you suggested. The fuel level in my float chamber will be about same height as the carb spray bar, so I ticked that box correctly!
The only one thing I would question is where you mentioned that with pressure the engine will lean out as the tank empties. It certainly did with no float chamber. However bearing in mind that the actual float chamber has a pin hole air vent at its top, the fuel once in the float chamber is not pressurised from the main tank. Therefore I would expect hardly any change in fuel pressure to the carb irrespective of how much fuel is in the main tank.
You have a keen eye for detail Danny. The starter belt can be put in without removing the engine as it just slips in between the coupling and prop tube housing if the shaft is slid out.
Cheers
Craig :-))
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Hi Craig
Just goes to show - I'm never too old to learn! I didn't realise that there was a vent in the top of the float chamber! That makes it much clearer (I eventually gave up trying to work out the principles of a totally sealed system - doh!)
In that case you are correct and should have no problems with changing mixture. All the pressurised bit does is to deliver the fuel from the tank as far as the float chamber, after that its a pure suction feed to the carb, so should be nice and even all the time.
Don't know how true it is, but I was told that the oil in glow fuel "sticks" to the sides of silicon tube thus reducing its effective internal diameter. I always used to use one size bigger ID tube than I needed - just in case it was true! %)
Cheers
Danny
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Great thread this! Yes good idea on filter prior to float chamber Danny. Also bottom of float chamber (ie fuel exit) should be level ideally with carb needle, Venturi.
Key thing to remember is that the float chamber is vented in the lid, so a pressure tank will keep it full.
Good luck Craig, look forward to seeing you test it!
Garry
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I always used to set mine up so that the "full" level was roughly level with the spraybar.
On the basis that it works by providing a constant head of fuel (constant height differential between fuel and spraybar), I think - while it's running, it's not that critical - as long as it's level or above (so that you're not trying to suck fuel uphill). If it's higher, it changes the baseline needle setting slightly, but the key thing is that it's consistent. But, if you set bottom / exit level with the spraybar, it will start to drain into the carb when it stops - meaning firstly flooding, and secondly you need to refill the float chamber before you re-start.
The needle setting with a float chamber is usually (in my experience at least), in the same ballpark as suction feed. Absolutes are obviously subject to a few variables.
Because of the "fluid seeks its own level" principle, the float chamber ideally needs to be close to the needle, so that if the boat pitches or lists, the height differential between the two changes as little as possible.
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But, if you set bottom / exit level with the spraybar, it will start to drain into the carb when it stops - meaning firstly flooding, and secondly you need to refill the float chamber before you re-start.
That in my opinion is an excellent fine detail point and well reasoned. Thank you for that. I suspect that small details like that will make the difference between a good reliable set up and one that causes issues that could put people off the idea of a float chamber.
Craig :-))
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Hi folks
Just a quick update
Today I ran my crusader SC 46 engine with the new float chamber in situ.
The first thing I soon discovered was that it needed a good turn and a half on the main needle to keep it running on full throttle.
Once I had the main needle dialled in, the boat ran perfectly. Two key aspects were noticed:
The main needle setting was less critical and could be a notch or three either way without affecting performance.
Importantly, from running my fuel tank from full to empty,the engine ran exactly the same with no leaning out as the tank emptied.
You folks have provided good tips for me prior to my test run today and I thank you for that.
For the record I did fit my fuel filter such that it was before the float needle valve. Thank up you danny for that tip! :-))
Cheers
Craig
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hi m8
you mention you had to turn the main needle a turn and a half was that to richen or lean ? did the boat consume more or less fuel ?
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Sorry I should have explained. I had to richen the needle setting. I didn't notice any difference in the amount of fuel used but I would expect it to use slightly less with the float chamber. I say that because without the chamber and using a pressurised supply, I had to set the mixture on the rich side.
Craig :-)
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Hi Folks
For anyone interested I uploaded a short video clip of my boat running yesterday on test with the float chamber after having re-set the main mixture needle on the carb. Okay I realise that this is only a bog standard sports engine so the power is limited (so I am aiming for reliability as my main priority):
http://www.youtube.com/watch?v=SvIXRmllXH4
Craig :-)
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Craig....I have ZERO knowledge of these type of engines....but see what appears to be a light BLUE exhaust haze.....in you video
Does this create any form of oil pollution on the water?...........Derek
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Hi Derek
I would suggest that all 2 stroke nitro and gasoline engines will emit some residual combustion products. I do not believe that it poses a pollution risk to the water in this instance. I would expect more oil (even if in relatively minute quantities) to enter the water via propeller shaft lubrication systems often seen on many model boats of all types of power plant. And I am not suggesting that even this is necessarily an issue.
Craig
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On the pollution issue - the engine running was using castor oil lubricants, not synthetic, and the methanol used in the fuel is burnt off by the high temperature combustion process, (with no other additives). So as far as internal combustion goes, these type of engines are probably as low a polluter as you can get.
Brigadair
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Hi Folks
For anyone interested I uploaded a short video clip of my boat running yesterday on test with the float chamber after having re-set the main mixture needle on the carb. Okay I realise that this is only a bog standard sports engine so the power is limited (so I am aiming for reliability as my main priority):
http://www.youtube.com/watch?v=SvIXRmllXH4
It has been said by some folks that float chambers have proved troublesome. If my set up proves reliable I would expect on say a 40 to 50 lap race to gain about 4 to 5 laps compared to running without the chamber.
However that extra component does give extra risk of a failure. If the float ball gets stuck (should the enclosure distort)....
If the float valve gets stuck...
If the pressure to the main tank fails (say an air leak?)....
The float chamber may be a curse! :(( :(( :((
Time will tell :-))
Craig
Craig :-)
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Agreed craig. Don't forget the float chamber on my webra powered challenger let me down towards the end of last season, and cost me a place. The brass nipple at the bottom exit sheared off!
Time will tell!
Lol!
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Well you can both have a good laugh at my new A class boat which has twin tanks for all sorts of not very good reasons!
Bob....from a dark workshop on a mountain.
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I would suggest that you dismantle the valve and clean the needle and guide after each outing. If fuel is left coating valve ... methanol evaporates, leaving neat castor. SHOULD wash off the next use. But if not, it could easily stick the needle causing no fuel, or flooding. The system does not operate on a very high pressure.
Just a thought.
Cheers
Danny
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Now what have you been up to Bob??? ok2
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Well you can both have a good laugh at my new A class boat which has twin tanks for all sorts of not very good reasons!
Bob....from a dark workshop on a mountain.
Bob.... could you upload a photo for us to see??
Craig :-))
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I would suggest that you dismantle the valve and clean the needle and guide after each outing. If fuel is left coating valve ... methanol evaporates, leaving neat castor. SHOULD wash off the next use. But if not, it could easily stick the needle causing no fuel, or flooding. The system does not operate on a very high pressure.
Just a thought.
Cheers
Danny
That sounds like good advice mate!
:-))
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It's not quite finished yet as the last piece of the jigsaw arrived only today. I'll get the IT manager (wife) to do a pic when the build has passed the embarrassment threshold. I will say that it will either be very quick or a complete fiasco....can't really see any middle ground.
Bob
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It's not quite finished yet as the last piece of the jigsaw arrived only today. I'll get the IT manager (wife) to do a pic when the build has passed the embarrassment threshold. I will say that it will either be very quick or a complete fiasco....can't really see any middle ground.
Bob
Was the last bit of the jigsaw the Float Chamber??? :D
Joking aside I much look forward to seeing a photo or few. Photos always speak louder than words. And your photo might reveal a hidden gem or two that we can learn from.
Cheers
Craig :-))
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I shall post the pics on the BMPRS build thread before I get told off for going of topic! Should be in the next couple of days.
Focused Bob