Author Topic: Carburetion Video & Discussion  (Read 3058 times)

Offline cfellows

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Carburetion Video & Discussion
« on: July 08, 2014, 04:26:28 AM »
The past couple of days I've been experimenting with carburetion on my vertical single 4 stroke.  In this video I've mounted the carburetor on the fuel tank and connected it to the engine with a relative thick urethane tube. 

<a href="http://www.youtube.com/watch?v=C8AYDaoRpHU" target="_blank">http://www.youtube.com/watch?v=C8AYDaoRpHU</a>

I've discovered that the carburetor doesn't do a very good job of vaporizing the fuel.  A lot of liquid fuel, probably in the form of spray or droplets gets into the fuel/air mix.  I'm guessing that most model engine carburetors perform equally poorly demonstrating the problems with trying to scale down full size carburetors.

On the plus side, this video shows that the carburetor mounted on the fuel tank can be located remotely from the engine and the farther below the intake the better.  This seems to reduce the amount of liquid fuel that finds it's way to the engine.  It also makes the engine run smoother at low speeds.

It seems to me the solution to good, low speed performance is to get the fuel in as fine a vapor as possible.  The vapor fuel tank does a pretty good job of that, but is too sensitive to external temperatures as well as fuel level and, for me at least, it's difficult to get a rich enough mixture.  It might be interesting to see if I can find an ultrasonic transducer like those used in portable humidifiers.

Another option I'm working on is a carburetor inside the gas tank such that the excess liquid fuel would be reabsorbed by the fuel in the tank.  The idea would be to create a vapor of fine fuel fog in the space above the fuel level and draw off the vapor into a mixer much like the vapor fuel tank.

Anybody got any other ideas for producing a fine fuel vapor with minimal liquid fuel in the mix?

Chuck
So many projects, so little time...

Offline John Hill

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Re: Carburetion Video & Discussion
« Reply #1 on: July 08, 2014, 04:36:42 AM »
Heat the fuel?  A candle flame heating the fuel line until the engine got hot enough to use the exhaust for heat?

Offline PStechPaul

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Re: Carburetion Video & Discussion
« Reply #2 on: July 08, 2014, 04:53:51 AM »
What about a pumped atomizer as found in spray bottles?
 
Perhaps it would be better to use a pressurized liquid fuel such as propane or butane, which would eliminate the pesky carburetor and perhaps be safer to transport and use at model engine shows. You could probably get a lot of run time on a small bottle as used in lighters.
 
From a purist viewpoint, models of actual gasoline powered engines might need to run on the original fuel, but the same engine will run on propane or even wood gas with only slight modification. Such engines have been around since the 1800s but when gasoline became dirt cheap around 1900 most engines used that because it was much more convenient, especially for vehicles.
http://en.wikipedia.org/wiki/Wood_gas

Online Roger B

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Re: Carburetion Video & Discussion
« Reply #3 on: July 08, 2014, 07:59:04 AM »
The original carburettors were used to produce a vapour from liquid fuels which was then fed to the mixing valve of gas engines as a substitute for town gas. There were various designs, typically a flat round tank with a spiral structure inside supporting a textile wick. This was supposed to ensure that there was always a saturated vapour that could be sucked into the mixing valve.

Gas, Gasoline, And Oil-Engines by Gardner D Hiscox  gives some details.

http://www.amazon.com/Gas-Gasoline-And-Oil-Engines/dp/1559182032

or as a PDF

https://ia700508.us.archive.org/2/items/gasgasolineoilen00hisciala/gasgasolineoilen00hisciala.pdf

The simple vapour tanks probably do not have enough surface area to produce a constant saturated vapour. Separating the vapourisation and the mixing may work better.
Best regards

Roger

Offline Graham Meek

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Re: Carburetion Video & Discussion
« Reply #4 on: July 08, 2014, 08:49:21 AM »
I am thinking the fuel is condensing out of the air/fuel mixture and forming droplets on the wall of the tube. It is one of the reasons why early inlet manifolds were heated to improve atomisation. Have you tried shortening the tube to a bare minimum? A longer inlet duct does however improve the torque characteristics of an engine.

My best regards
Gray,

Offline lakc

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Re: Carburetion Video & Discussion
« Reply #5 on: July 08, 2014, 12:23:48 PM »
A hunting idle is almost always an indicator of running lean. You can see that especially as it flares up when it runs out of fuel at the end.
Getting all vapor is the holy grail of carburation, and of course, impractical to achieve in real life. :( Industry has gone to high pressure direct fuel injection to get rid of the "mixture drop out" issue and promote the best possible atomization of fuel. I believe since gasoline is a mixture of different compounds, there is not a single vapor point for it to become truly gaseous, just progressively finer droplets of liquid suspended in air. Much of it will flash to a vapor like state when it hits the hot intake valve and combustion chamber, so the trick with carburation is to deliver the same amount of fuel consistently, and with the proper amount of air to combust it all.
The trick to get vaporization is lower the pressure, often using a booster venturi, and breaking up the fuel flow with tiny air droplets sucked in the fuel path with a "calibrated leak" via air bleeds and emulsion tubes.
That long intake tube is actually promoting fuel separation, and loosing your pressure available to draw and vaporize fuel from the carb.
Jeff

Offline cfellows

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Re: Carburetion Video & Discussion
« Reply #6 on: July 09, 2014, 05:40:07 AM »
So, I've decided to put together some bits to let me use propane fuel, at least for a benchmark.  But first, here's how my day ended...

First we discovered that the lawn irrigation controller had stopped working.



After some diagnostics, I've decided the circuit board is toast so I'm replacing the whole box.  Then we discovered the water heater wasn't working.



It's gas and when I checked, the pilot light was out.  I could get it to reignite, but it wouldn't stay lit... obviously the thermocouple had died.  I went to Lowes where I bought it 3 years ago and they didn't stock the replacement part.  In fact, in today's throw-away-world, the main burner, the pilot light assembly with electronic igniter tip, and the thermocouple are all replaced as a single unit.  Luckily it's still under warranty so I'll go pick up a replacement assembly tomorrow.

Anyway, back to propane.  I ordered a pressure regulator that attaches direction to a standard propane bottle.

http://www.ebay.com/itm/270388811753?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Kind of pricey, but it looks like it will come in handy for all sorts of things.  I also started on the demand valve / mixer which was presented as a build article in Model Engine Builder, Issue #24. 



While this work could have been done in the lathe, it was a perfect candidate for my CNC mill.  It's made from a 1.75" square aluminum block, 0.85" thick.  The big depression is 1.5" diameter x 0.050" deep.  The next pocket is 0.5" diameter by 0.15" deep, followed by 0.332" diameter by 0.356" deep and finally, 0.221" diameter x 0.775" deep.  I used a 2 flute, 1/4" end mill for the first 2 pockets and a 1/8" end mill for the last two.  The innards for this demand valve are replacement parts for a Tecumseh carburetor, readily available on Ebay and other internet outlets.

Chuck
So many projects, so little time...

Offline lakc

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Re: Carburetion Video & Discussion
« Reply #7 on: July 09, 2014, 11:36:01 AM »
Bummer about the household projects :(
Not having the plans for that exact propane mixer, but aware of similar concepts, I am looking forward to this build.
Jeff

Offline Allen Smithee

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Re: Carburetion Video & Discussion
« Reply #8 on: July 09, 2014, 01:28:11 PM »
A couple of random points:

1. Intake manifolds aren't heated to aid atomisation; they're heated to prevent intake icing. Intake icing can occur whenever the dew point (humidity) gets close to the ambient temperature - the air temperature drops in the venturi when the pressure drops, and that causes water to condense out as ice. I've suffered carb icing in a Cessna 152 in Florida where the outside air temp was over 90F! I also had a persistent problem with icing on one of my kit cars where I had twin Webber 40DCOE carbs whose K&N filters were mounted outside the bonnet breathing outside air rather than engine-bay air from behind the radiator. When driver on any stretch of more than a couple of miles at high speed it would start misfiring, and that would get progressively worse as each of the four chokes iced-up until the engine cut completely. If it was left standing for a couple of minutes the heat-soak from the engine bay would melt the ice and all was fine again. It wasn't practicable to heat the manifold, so the problem was ultimately cured by baffling the underside of the filters to encourage them to breath engine-bay air.

2. High degrees of atomisation before the inlet valve may be esthetically pleasing and may improve fuel consumption, but it also significantly limits the available power for two reasons:

- The fuel occupies considerably more space in the inlet tracts as a vapour than as droplets, limiting the amount of air the engine can consume (at the risk of teaching people to suck eggs the power of an infernal combustor is ultimately a function of the amount of air that it can consume).

- If the fuel remains as droplets until entering the cylinder the latent heat of vapourisation cools the mixture when it does become vapour (before it burns), which has an intercooling effect that both increases charge density and offsets detonation (allowing a higher compression ratio to be used).

In the days when racing car engines still used carbs (back when Pontious was a pilot) "race" carbs were often explicitly designed to achieve a minimum droplet size in the cylinder rather than high levels of atomisation. I appreciate that in most of the cases being discussed here power isn't really a major concern, but if you do significantly improve atomisation you may need to drop the compression ratio (and even change ignition curves) to avoid detonation damage.

0.03 supplied,

AS
« Last Edit: July 09, 2014, 01:31:31 PM by Allen Smithee »
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Offline philjoe5

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Re: Carburetion Video & Discussion
« Reply #9 on: July 10, 2014, 04:30:56 PM »
Chuck,
Quote
The vapor fuel tank does a pretty good job of that, but is too sensitive to external temperatures as well as fuel level and, for me at least, it's difficult to get a rich enough mixture

This is an interesting thread for me because I had some carburetor troubles with my IC engine as well.  I started out with a vapor carb, but I had it sized too small so my engine ran for only short 10 second bursts.  Then I went to a carburetor supplied by Brian Rupnow which worked fine but every once in a while the engine would speed up so fast I had to shut it down.

I got to thinking about my original vapor carb and decided I needed a finer control on the air mixer and a constant surface area.  I came up with this design:



The air mixer has a series of staggered holes 0.050" diameter.

With the fuel tank 3/4 full I need to open the mixer to the maximum air, then as the engine runs I gradually close it off until I can't get the mixture any richer and the engine stops.  This takes about 20 minutes.

This performance suits me because the engine speed is almost constant at 800 +/- 100 rpm and a 20 minute run is more than enough for my needs.  I played around with vent diameters and air mixer diameters before I was satisfied.  The nice feature for me is that there doesn't appear to be any danger of overspeeding and with the spark somewhat retarded it runs as slowly as 600 rpm.

Cheers,
Phil

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