Topic: Nitro Marine Engines-fuel supply??? Float Chamber? (Read 15592 times)

craig dickson · « **on:** January 10, 2014, 07:35:25 pm »

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

bj · « **Reply #1 on:** January 10, 2014, 08:54:58 pm »

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!

Brigadair · « **Reply #2 on:** January 10, 2014, 10:11:51 pm »

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

craig dickson · « **Reply #3 on:** January 11, 2014, 05:53:31 pm »

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

tmbc · « **Reply #4 on:** January 14, 2014, 08:40:43 am »

dont hold me to this but dont think they make a big difference on sports engine !!

hydrobob · « **Reply #5 on:** January 14, 2014, 11:06:29 am »

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.

Circlip · « **Reply #6 on:** January 14, 2014, 11:43:46 am »

Quote

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.

craig dickson · « **Reply #7 on:** January 14, 2014, 06:25:56 pm »

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

craig dickson · « **Reply #8 on:** January 14, 2014, 06:36:03 pm »

Quote from: hydrobob on January 14, 2014, 11:06:29 am

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

hydrobob · « **Reply #9 on:** January 14, 2014, 07:17:20 pm »

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

craig dickson · « **Reply #10 on:** January 14, 2014, 07:34:51 pm »

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

Brigadair · « **Reply #11 on:** January 14, 2014, 08:14:01 pm »

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,

hydrobob · « **Reply #12 on:** January 15, 2014, 10:37:16 am »

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

craig dickson · « **Reply #13 on:** January 15, 2014, 06:45:47 pm »

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

Brigadair · « **Reply #14 on:** January 15, 2014, 08:23:53 pm »

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.

craig dickson · « **Reply #15 on:** January 15, 2014, 08:49:39 pm »

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

craig dickson · « **Reply #16 on:** January 21, 2014, 08:18:20 pm »

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.

Craig

Brigadair · « **Reply #17 on:** January 21, 2014, 08:55:24 pm »

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

craig dickson · « **Reply #18 on:** January 21, 2014, 09:03:56 pm »

Quote from: Brigadair on January 21, 2014, 08:55:24 pm

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

Brigadair · « **Reply #19 on:** January 21, 2014, 09:25:23 pm »

Sorry, you are correct craig. I am the plank!

craig dickson · « **Reply #20 on:** January 21, 2014, 09:29:10 pm »

Quote from: Brigadair on January 21, 2014, 09:25:23 pm

Sorry, you are correct craig. I am the plank!

I forgive you my son

Thy will be done

tmbc · « **Reply #21 on:** January 22, 2014, 10:35:10 pm »

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

tmbc · « **Reply #22 on:** January 22, 2014, 10:37:13 pm »

on a serious point i thought float chambers were meant to be as low as possible in the boat

tmbc · « **Reply #23 on:** January 23, 2014, 08:30:03 am »

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 !

vnkiwi · « **Reply #24 on:** January 23, 2014, 09:19:17 am »

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.

cheers

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Author Topic: Nitro Marine Engines-fuel supply??? Float Chamber? (Read 15592 times)