Model Engine Carburetion & Fuel Systems

[cfellows]

I wanted to discuss fuel injection for a moment and I'm looking for any information that experienced or knowledgeable members might have. I know from looking at Find Hansen's model hot tube engines that the speed is controlled by varying the amount of fuel injected, but there doesn't seem to be any control of the amount of air inducted. I do know that he injects the fuel near or slightly after bottom dead center on the intake stroke, so how is it that the fuel mixture always ignites despite the fact that the ratio of fuel to air changes with the amount fuel injected?

Chuck

 

[cfellows]

Do you have a choked carburettor - where the carb is restricted, to help draw air, part way into the jet tube, to break up and reduce the amount of fuel - to atomise the spray - like in a conventional carburettor.

It may pay to actually make a 2 carb set up - a properly set up IDLE SPEED only carb, and a second one that cuts in to provide full power.

There is also a way of metering fuel, that it's simple friction / turbulence flow control, of a loose screw, in a thread, and the fuel has to go through the thread path - it's a simple fuel control - the amount of looseness in the threads and the amount of turns the fuel has to go through.

This is a great way to meter a small single speed carburettor.

Hmm, sounds interesting. I'll have to see if I can translate your description into a drawing and a model.

Chuck

 

[tornitore45]

One other thing that sometimes improves difficult combustion is to open the spark plug gap considerably.

That is the problem I had with my Upshur Twin, the first engine I made.
Only after increasing the factory gap it started and kept going.

 

[mu38&Bg#]

Find Hansen's engines are compression ignition injected diesels. Diesels do not have throttles (most of the time). Diesel injection is not like the fuel injection in your car. There is plenty of reference material on the topic.

Greg

 

[kuhncw]

For a naturally aspirated diesel, air flow depends on engine speed. There is always more air than the fuel needs until fuel rate is increased to the point where the engine begins to smoke.

At least that is how I remember it.

Regards,

Chuck

 

[BronxFigs]

Thank you for the links re: Propane Demand Valves, explanatory articles, etc. Much appreciated.

Frank

 

[TinyTool]

Hmm, sounds interesting. I'll have to see if I can translate your description into a drawing and a model.

Chuck

The screw thread fuel flow controller, is really a combination of two things.

1. Think of a drain bolt, say about 20mm OD, in the bottom of a petrol tank, and the bolt has loosened off a little bit, and the petrol in the tank is seeping down through the threads, and dripping onto the ground.

Think of the constant flow rate of the fuel - leaking through the thread - not the dripping from the end of the bolt.

That is the basic principle.

2. How this is applicable to a fine but constant flow of fuel for an engine, is to think of a longer smaller bolt - say a brass one, 1/8" OD and about 2" long, screwed into a block of brass, with fuel on one side - from a tank and a carburettor on the other side, sucking the fuel up, that does weep through.

So assuming that there is 20 turns per inch on the thread and the thread - being a common brass thread, is a little on the loose side.

Well there is wicking of the petrol and we shall more or less discount that - but every time a liquid (or gas) has to change direction or the longer the distance it has to rub against surfaces to get from A to B, the more energy is used in getting the liquid through the conduit.

Think inlet tract with small holes and square corners and internal turbulence etc...

So with that 40 turns of thread, seeping X CC of fuel per hour - by winding the screw out, it reduces the amount of friction and turbulence it has, that soaks up the flow rate energy and so as the screw gets to 35, then 30 then 25 and then 20 etc., the flow rate of the fuel increases.

It's a brilliant system of fuel metering that I invented, but it's really only good on fixed carburettors on single speed engines, or in having multiples of the carburettors... to have a REV range...

You really MUST have absolutely clean fuel as the passages are so small they will clog up fairly easily.

But it allows for very accurate mixture control - a screw to choke the incoming air and then a screw to meter the fuel....

It's main advantage is that it's piss easy to make and adjust and it's dead reliable.

And you don't need to drill jets or hone needles etc...

And it's brilliant for constant speed / constant load engines...

It's much like having a single fixed jet, with a long tube extending below the carburettor, and having a tank that can be lowered or raised to control the fuel flow into the engine....

But having a screw thread adjustment for the tank height...

X vacuum + y Fuel head + Z gravity -

It best features are that it's simple - assuming a 10mm ID inlet tract throat - a 10 x 1mm threaded bolt to act as a butterfly near the start of the inlet tract to control the vacuum in the inlet - coming from the top into the inlet, and then a 3mm x 0.5 mm threaded screw, going into a hole with the same thread, which hooks into a fuel tank, and the screw can be adjusted from the top of the carb.

 

[Cogsy]

1. Think of a drain bolt, say about 20mm OD, in the bottom of a petrol tank, and the bolt has loosened off a little bit, and the petrol in the tank is seeping down through the threads, and dripping onto the ground.

But surely in your drain bolt analogy above, the amount of fuel in the tank would alter the pressure that the fuel is being pushed through the threads and hence the flow rate. Maybe with a constant pressure fuel pump this could be overcome.

 

[mu38&Bg#]

Right, the effects of gravity or fuel pressure are present regardless of the metering device. That's why carbs have floats or demand valves.

Greg

 

[Nerdz]

Speaking of fuel injection, why not use an Aquarium Bubbler to help atomize gas (ie like Jan Ridders Tank)

I found a low pressure fuel pump on ebay:

http://www.ebay.com/itm/Super-Auto-...Parts_Accessories&hash=item5d382db506&vxp=mtr

 

[mu38&Bg#]

Carburetors are supposed to meter fuel in a relatively fixed proportion by mass to the air flow required by the engine. There are two main problems with this. First, a venturi is a volume metering device so the fuel supplied is not proportional to mass flow. This is why various speed and throttle compensating circuits are used. Second, the carburetor is sensitive to fuel pressure, which is why a float or other fuel pressure regulator is used. If you want a carb that can handle temperature, altitude, and various engine operating conditions(cold start, acceleration) things get very complicated.

Model engines with low flow velocities, low compression pressures, etc. only exasperate carb tuning issues.

Production model engines for RC use simple carbs and run well, but they do not run with no load so flow velocities are much higher and compression ratios are also higher to make power. Smaller RC engines generally use methanol based fuel which has a much wider combustible air to fuel ratio, and makes tuning much less critical. Methanol requires compatible oils like castor or alcohol blending synthetics.

Greg

 

[johnny1320]

Hi Chuck, maybe a miniature version of the SU carb would be an idea, butterfly throttle with a vacuum piston controlled needle valve. might be a challenge to build depending on the scale.