Carburretor Thoughts

[Roger B]

The next piece was the throttle control. This was mostly a drill, hacksaw and file job based on the carb I made for the 3cc vertical engine. When it was assembled I had another suction trial.

<a href="https://www.youtube.com/watch?v=ZIJ_X8WyEco" target="_blank">http://www.youtube.com/watch?v=ZIJ_X8WyEco</a>

[Vixen]

Hi Roger,

The petit SU is coming together very quickly.

How closely do you think your (lung) suction matches the aspiration of an engine? Could you create a more constant suction with Henry the Vacuum cleaner or the inlet of an air compressor?

Mike

[Admiral_dk]

I don't think I could make a diaphragm in this size but a piston is possible.

That was partly a tongue in cheek and partly a serious comment about the diaphragm - and like you I can't see it made in a much smaller size as the thickness and maintaining strength and flexibility get really complicated. The original diaphragms are rubber casted around a woven piece of "cloth" .....

George - I'm sure that the other slides etc. for the Japanese carburetors were made - but I wouldn't have a clue about how to get them outside Japan .... oh and I do know this from the original workshop manuals - where I can see that the US models has different parts (not only Jets) to those here in DK, OZ, etc.

George made some valid questions and I have thought a bit about it ....
It is possible to get accurate flowmeters for the air in the required amount (the fuel is almost more in Chris old line of work with pico liters) - but is it also in a reasonable price range ?

Fuel in an experiment could be measured with a small diameter fuel hose filled all the way to the Jet and an open end (not in fuel). Have a constant airflow empty the line and measure how long it takes ....
Repeat experiment with many different flowrates and plot into a graph.

This would be simpler if one has access to some automatic measuring equipment (I presume that automakers (or fuel injection companies) has something along these lines ....

Per

[Roger B]

Thank you all for your interest   

George,

I did try and make a float chamber for my first engine but as you say getting enough force on the needle to close it was a problem. ETW noted the problem and recommended using a lever system to increase the force on the needle.

I found a couple of pictures of Lee Root’s engine. There seem to be two electrical connections to the injection unit, one to what looks like an electromagnet and one to what is probably a throttle position sensor.

Looking at the video the fuel pressure pump seems to be separate from the engine, the fuel connection is on one of the engine mounts.
If I was doing this I think I would make some sort of electromagnetic shuttle valve that would deliver a defined quantity of fuel per pulse and then control the fuel quantity by varying the pulse rate.

I agree about sharing knowledge and always try to do so. This usually results in me sharing how much I don’t know 

Mike,

I think that my suction is probably much higher volume but lower pressure than my engines, Henry will be much the same. To avoid having to quantify this I will use the engine being driven by an electric motor as the test vacuum source.

Per,

My initial set up attempts will have to be much more practical. At these small sizes the measuring devices will have a significant disturbing influence on the system, suitable ones are also not cheap. A basic differential pressure instrument like this would probably have too much internal volume.

http://www.drucksensorik.org/products/en/pressure-gauge-HMG/HMG1/HMG1-1000-mbar.html

Small volume U tube manometers will probably be influenced too much by capillary forces.

[Vixen]

Hello Roger,

I know this topic is about CD carbs but you touched on PI in your last post

If I was doing this I think I would make some sort of electromagnetic shuttle valve that would deliver a defined quantity of fuel per pulse and then control the fuel quantity by varying the pulse rate.

You may already know that the diesel garage heaters, discussed of the forum awhile ago, have such an delivery pump. The dosing pump on my heater delivers 0.02ml per pulse. At the lowest heat setting the pulse rate is 1.6 HZ which delivers 0,1152 litres per hour to the spray jet in the heaters combustion chamber. This is still much higher flow than your engine would need but these dosing pumps (made in China) are/were cheap. Cheap enough to take apart, copy or modify, reduce the stroke, perhaps. Sorry to distract you.

Mike

[Admiral_dk]

My initial set up attempts will have to be much more practical.

I thought that my idea using a clear fuel tube with an internal diameter off perhaps 1mm. a meter long or so would be cheap, easy, depentable + easy to calibrate - was just such a device, for measuring the fuel use at a give flow through the carburator. With such a small diameter, the fuel will not back out after opening the end with the sucktion from the carb and measuring the time it takes to empty => you will get an accurate measure.

I will admit that measuring the air flow is a lot more complicated - unless you can fill the air sucked out off the carb into a "ballon" for an exact time too.
There are flow meters that has no moveable parts, but likely not cheap - though modern flowmeters for measuring hot water for heating appartments, to calculate the size of the bill are cheap. They can work a number of different ways - one being Ultra Sound.

Sorry I'm just thinking out loud ....

[Rick Doane]

Hello Roger:

One of  the tools we use in two stroke boat racing is a powered flow meter to "guestimate" the needle valve opening based on the load (prop pitch and diameter).  Initial runs are performed and tuned by ear to get proper needle settings, and when one likes the power curve and after the boat returns to the bank, the carburetor/needle valve is connected to this meter and air is supplied by twin pumps, through a  blood pressure gauge, and powered by a regulated voltage supply.  This establishes a base line for subsequent runs.  Minor needle tuning is performed based on the atmospheric conditions for the time of day etc.  The difference with this meter and say a mass flow meter is that the associated number one obtains from their setup is based on the back pressure of the system.  Accurate(?) and reproducible flow rates can be established based on orifice size vs. mmHg.

One question I have had reading through this and other posts is, does everyone run the same fuel in their engines and has any consideration been given to the various stochiometric variables involved with each type fuel?

Please excuse any ignorance on my behalf.

Best Regards...…….Rick

[Roger B]

Lots to think about 

Mike,

I have done some rough calculations based on the Root engine:

The Root engine is around 6ml per cylinder.  The density of air is around 1.2kg/m3 so the air charge will be 7.2mg. For a mixture of 14-1 this will require a maximum 0.5mg of fuel. The fuel density is around 0.8kg/dm3 = 0.8mg/mm3 so ~0.6mm3 of fuel will be required per stroke.

This is a 4 cylinder 4 stroke so a maximum 1.2mm3 of fuel per rev will be required. A combustion heater pump delivers 0.02ml = 20mm3 per stroke so one pump stoke will supply the fuel for 16.7 revolutions of the engine. At 6000 rpm this would require 360 strokes per minute or a rate of 6Hz for full output. At idle this would be significantly less, maybe 20-30 strokes per minute. With a small accumulator between the pump and the injector nozzle to smooth out the flow and a reasonably large inlet manifold to even out the mixture that might work.

The control system would need to take into account rpm (not neccessary with my mechanical system), throttle position and ideally manifold pressure. I think I will leave that project to someone else 

Per,

I'm not sure I actually need to know what the actual flows and pressures are  The load on the piston just has to match the engine the carb is fitted to which as long as you can turn the engine at operating speed with an external motor should be ok to do. The fuel flow also just has to match the engine and using the moveable jet it again should be feasible to find a working needle profile. It's the difference detween making a piston to fit a cylinder by trying it, boring/laping it a bit and trying again and accurately measuring the piston and cylinder to achieve the desired tolerance.

Rick,

An interesting technique   

There are several fuel options all with diferent requirements. I am running pump petrol (gasoline) or one of the alkylate variants. Some people add a little oil on four stroke engines. Two stroke engines have a significant proportion of oil so more fuel is required. Glow plug engines use alchohol as the fuel basis.This again requires more fuel in relation to petrol (gas). I have run my engines on cooking alchohol instead of petrol which requires a significant opening of the needle valve/increase in injection pump stroke. Model compression ignition engines also have an additional proportion of ether.

I would guess that for nominally equivalent size engines a glowplug two stroke would require twice the fuel flow of a four stroke spark ignition engine.

Keep the thoughts coming 

[Vixen]

Mike,

I have done some rough calculations based on the Root engine:
The control system would need to take into account rpm (not neccessary with my mechanical system), throttle position and ideally manifold pressure. I think I will leave that project to someone else 
Keep the thoughts coming 

I thought of the diesel burner pump as the metered fuel supply to your mechanical high pressure injector pump. But as you say, the development of a multi parameter, computer controlled ECU is best left to someone else.

I think you are heading in the right direction with your self regulating, self metering CD carb experiments.

Got to it.

Mike

[Rick Doane]

Good Morning Roger:

"I would guess that for nominally equivalent size engines a glowplug two stroke would require twice the fuel flow of a four stroke spark ignition engine."

You are correct.  The amount of oxygen in the alcohol and nitromethane fuels replaces the amount of air required to about a 5.7:1 A/F as compared to a 14.7:1 for gasoline.  Also, if any other petrol mix is used, such as E85, will require recalculation of the A/F ratio.  Synthetic oils are used in the 8 to 17 % range and contain no usable burnable compounds.  Of course, the less oil, the more oxygen = bigger bang.  And that leads sometimes to more parts.

I think you are doing a great job in this research.  Following along.

Regards...…Rick

[Admiral_dk]

I'm not sure I actually need to know what the actual flows and pressures are

True to a point or seen another way - it's a question of from which angle you start to solve a problem with many unknowns ....

I can certainly see the idea of using an external motor to turn the "real engine" to the desired RPM. That could be used to make the Vacuum Piston open the whole way and check that it goes up and down as desired with the RPM's as the first thing on the list.

To me it will then be interesting to know / measure how much Air and Fuel goes through at those RPM's in order to see how much I'm away from the mark that would make it run - but I can also see the point in using an external Fuel metering device as found in many of the latest model airplane engines and just adjust it to make it run as it should @ max RPM first. That would allow me you to do it the next steps as you would have done in our youth 

[Vixen]

To me it will then be interesting to know / measure how much Air and Fuel goes through at those RPM's in order to see how much I'm away from the mark that would make it run - but I can also see the point in using an external Fuel metering device as found in many of the latest model airplane engines and just adjust it to make it run as it should @ max RPM first. That would allow me you to do it the next steps as you would have done in our youth 

Per, can you provide a link to these modern external fuel metering devices you refer to? It is easy to loose track of the latest airplane engine developments

Mike

[Admiral_dk]

Sorry Mike but I haven't seen them for model engines.

The reason I kind of know about them are twofold. One from an old proffession of mine, as I in my youth worked for Navitronic - a company that specialized in equipment for making Seamaps, amd in order to make acurate soundings you need to know the Speed Off Sound where you're measuring. We (among many other products) made a "Fish" that was lowered  into the water and it measured the SoS + temperature for every fathom (aprox 2m.) you lowered it. One of my old colleague that I talked to many years later told me that he was approached by Kamstrup who makes some very advanced Joule Meters today. These meters use some of the same priciples, but much more advanced and they are placed around the heating water tube and the return colder water tube. This allows them to measure how much energy you took out of the system and wirelesly report that to the supply company as the measure temperature and flow amount down to centi liters.

Some off the other informations are from old "Das Motorrad" magazines I don't have anymore and Internet sources I have forgot, but mostly online Motorcycle Mags and their linked tech info + some YouTube videos - again I do not remember the exact source.

But as mentioned - I would use the trick with the clear fuel tube for test purposes. I got the idea from seeing airbubbles in the fuel lines.

Per

[Roger B]

Next up was the jet sealing block. This could have been cut from a piece of 3mm plate or turned from some 10mm square bar. As the bores were important I decided on the latter. The seating for the O ring was bored, one again, using a 3mm end mill. As I had to remove the cover and piston to drill the fixing holes I have added a picture of the anti rotation pin. I followed Amsbury's thoughts and didn't turn a register on the body for the cover, a mistake  , for the next body there will be one. I put it all back together including the fun of re-centring the cover and fitted it to the two cylinder engine.

[Roger B]

I set the engine up with the starter battery to try the carb. It cranks at around 2800 rpm with 12V at 11A and the carb piston responded well 

<a href="https://www.youtube.com/watch?v=VXN49TcK34Y" target="_blank">http://www.youtube.com/watch?v=VXN49TcK34Y</a>

To try the full rev range I will have to borrow  24v ~20A power supply from work but looks good so far 

I measured the needle of the needle of the carb I have been using to get a base point for this needle. The jet is about 0.5mm (drilled) and the needle was around 0.42mm.