Rockerblock I.C.--Something a little different-

[MJM460]

Hi Brian, I can see the problem, I wonder if I can help you straighten it out.

The first part is correct, about the piston at bottom dead centre causing the decrease in pressure that draws in the air.  However, the volume drawn into the engine by the piston does not change when the throttle plate is closed, only its density.  The velocity increase is only local around the edge of the throttle plate. 

Because of the lower density, the mass of air drawn is less, and hence the quantity of air in the cylinder is limited to burning less mass of fuel, and producing less power.  This is just what we want when the throttle plate is closed, though only to the idle stop if we want the engine to continue to run.  When the throttle plate is opened, the pressure drop across the throttle plate is reduced due to more flow area available around the plate, but the total volume of air, hence velocity all the rest of the way to the cylinder is not changed.  However with less pressure drop across the plate, the density is higher so we have more mass of air flowing, we can burn more fuel, and produce more power.  This is what we wanted when we opened the throttle.

The Venturi is a separate and quite different effect.  Because of the flow area reduction in the Venturi throat, the velocity of the fixed volume of air must increase compared with the velocity just before or after the throat, and conservation of energy (that basic law of physics) requires that the pressure decrease.  The job of the carby is to adjust the ratio of fuel to air so that the fuel mass is adjusted proportional to the air mass flow, as well as staying in the flammable range which it does quite well.

Now the carby throat has a fixed area, and hence the pressure decreases in a manner dependant on engine speed, as engine speed obviously does change the volume of air per second.  And the pressure decrease is what is used to draw in more fuel, just what is needed when we want the engine to speed up.  Of course the lower pressure does again mean the density is lower, but the pressure change is proportional to the velocity squared, so we do get more pressure reduction in the carby throat as the velocity increases which the demand valve harnesses to provide more fuel.  The physics is such that the air:fuel ratio stays roughly acceptable over an adequate range of engine speed.  (Not sure how it goes if you are trying for 30,000 rpm, it probably means you don't get a very low "idle" speed.). You then adjust the needle valve for best position over the range you want.  That bit is definitely in your court.

Now if you have a throttle plate followed by the Venturi, and you have to top of the demand valve open to atmosphere, the throttle plate and the Venturi are working in series but in opposite directions, and I can assure you that the throttle plate will win.  Your path to getting a running engine will be frustrating and endless.

The solution is to make the venturi and demand valve quite separate from the throttle plate.  This is done by connecting the top (atmospheric) connection of the demand valve to the engine inlet after the throttle plate.  It is best not too close, and this may be best achieved by connecting it on the manifold after the venturi.  The difference will be a fraction of a turn of the needle valve.  The requirement is that this reference pressure tapping be in a location with flow pattern not to much upset by the throttle plate movement, means not in the turbulent flow around the edge of the throttle plate, and it connects to the top of your demand valve.

A Closer look at the demand valve will show that it is actually a differential pressure controller, controlling the pressure difference between above and below the diaphragm. You need to connect the top side to a place where the difference is not dependent on the throttle plate position.

I hope that makes sense and helps, even if it is a bit heavy reading.

MJM460

[Brian Rupnow]

My regulator and gauge arrived, and as I suspected, they don't have any way to hook up to a propane bottle. This is the gauge and regulator recommended in the Jerry Howell "propane demand valve" kit.  However, I resorted to "do it myselfry" and worked with an empty propane torch and nozzle to make what I needed. The torch already has a shut off valve which I could use. The gauge screws into one port on the regulator. I hunted around in my box of fittings and found 3 other brass fittings that had the correct threads, and silver soldered the ends I needed onto them. I haven't leak tested anything yet, but it looks good.

[Brian Rupnow]

That pressure gauge is 0 to 60 psi. Am I impressed? No, not really. I am flying semi blind on this entire Propane thing. The information in the "kit" from Jerry Howell is not up to my expectations. As I understand things so far, the engine should operate on about 1 psi. However--I suppose that Propane is a gas, the same basic nature as air, and the gauge and the regulator probably won't know the difference. If I don't blow myself up in the process, I am going to fight this thru to a running engine. So far, I'm out $81 for the regulator and gauge. If it all doesn't work as I had hoped, I can still buy a tank of Propane with a "fixed value" regulator to run campstoves at Walmart for $26.---Brian

[Dave Otto]

Brian

I find that 5 to 10 PSI works just fine; just order a 0-30 psi gage and replace the one you have. No need to trash the regulator.

Dave

[MJM460]

Hi Brian,

I am expecting that the combination of your demand valve and the carburettor needle valve will control that final reduction to 1 psi.  The demand valve just needs a relatively constant inlet pressure, not just a valve restriction on the inlet.  However, because the demand valve is not adjustable, it is expecting a certain range of pressure.  Pity they can't tell you what it is.  Higher pressure from your bottle regulator over the area of the demand valve needle orifice adds to the spring force to close the valve when the engine needs less fuel, but might prevent the demand valve opening enough or at all if the inlet pressure is too high.  If the regulator pressure is too low, the demand valve might not close securely under spring alone, but more likely just not enough fuel pressure when the engine is revved up.

I wonder if the demand valve is just designed for the standard bar-b-que regulator.  I had a look at mine and they don't seem to be labelled with a set pressure.  Typical consumer stuff, probably not very high.

I suspect your regulator gauge is really only needed to show you if the regulator is faulty, and not closing properly.  I agree with you about the range, but I don't think it is required for really fine adjustment.  If the engine runs well, it is set ok.  You will have no doubt if the regulator does not close properly when the engine stops, as your gauge will show full bottle pressure if the regulator does not close tightly.  In summer it will still be full scale.  Your demand valve however will close very tightly in this case.

MJM460

[Jasonb]

Low pressure BBQ regulators here are about 37mbar, thats 0.037bar or about 0.5psi

Pressure gauge range is less important than the range of the regulator, if reg is 4bar about 60psi also then it will be fiddly to adjust by small amounts. 2 or 4 bar regulators are more what you use on torches etc where you need to flow larger amounts of gas

[MJM460]

Thanks Jason, that makes sense.

MJM460

[Brian Rupnow]

So, here we have it, set up and ready to try. Immediately above the blue propane tank is the shut off valve that was part of the original torch head. Above that is the adjustable regulator which has a gauge in one port, and a plug in the opposite port. Above the regulator is a fitting which I made up from a brass fitting with a 1/8" npt thread and a custom made brass extension silver soldered to it , which fits into my neoprene gas line. The neoprene gas line runs over to the "gas demand valve", and from the gas demand valve up to the fuel inlet on the carburetor. I have tested all of my connections with liquid dish soap, and have hopefully sealed them all, with the help of some Teflon plumbers tape on each threaded fitting. Before I get really crazy and start modifying carburetors, I will try it with the carburetor "as is" and see what happens. I am doing this with my door and window open down in my office to get a bit of cross draft going and disperse any build up of propane in my office.---Brian

[Jasonb]

Just remember that propane is heavier than air so if you are DOWN in a basement any gas leaks won't make their way up and out. But having doors and windows open will help with getting rid of exhaust gasses

[Brian Rupnow]

Jason--My basement is a walk-out so I do have a door in my office which opens to directly outside. So--No joy at the moment. I have opened the needle valve two additional turns from where the carb was running happily on liquid fuel, I have checked for spark, and I can see the propane pressure gauge registering anything up to 10 psi. ( I have tried numerous different psi settings, all less than 10 psi). Engine doesn't want to fire at all. I have choked it with my finger completely covering the air intake, but it makes no difference. I pulled the sparkplug and checked for spark, and it has lots of spark. I don't want to modify the purchased carburetor that is currently on the engine. I do have an extra home built carburetor around, and if I can't get my engine to fire at all on propane I may re-jet the carb I have (a matter of enlarging the fuel discharge nozzle) to see if that helps.

[Jasonb]

Are you getting any smell of propane out of the engine? Unlike fuel it is not easy to tell if the gas is flowing and there is no tell tale liquid to flood the engine. I've only fiddled a bit with someone elses engine on a demand regularor and it was hard to tell if gas was being drawn into the engine let alone how much.

[Dave Otto]

You should be able hear and see the diaphragm moving through the vent hole in the demand valve, during the intake stroke. Keep in mind the fuel mixture when running on propane is very touchy and will most likely require quite a bit of fussing around until you find what works. It is very easy to flood an engine with propane.

Dave

[Brian Rupnow]

Somebody on another forum who claims to have expertise in using Propane is suggesting a different carburetor. He is suggesting a carburetor with no needle valve to adjust mixture, just a simple pick up tube. The tube itself creates a restriction in the main airflow to create the venturi effect which will signal the gas demand valve for propane flow.The drawing shows a very simple carburetor with no needle valve. I don't show a throttle plate in this drawing, but a throttle plate is simple enough to add. (As per the solid model.) The existing carburetor is held into the brass elbow with some light Loctite. I would remove the existing carb and put this one in it's place.

[MJM460]

Hi Brian, I guess it's not unusual for it not to fire first time on a totally unfamiliar fuel, I am sure it will run when you get the right adjustments.  But a few minor points.

Don't block the atmospheric vent of the demand valve with your clamp.  Can't see it in the photo.

The needle valve provides an extra fine adjustment for the air fuel mix, not provided by the alternative.  Also provides a little back pressure which should help the demand valve be more positive in its action and control.

Remember the 50% extra area in both the needle valve and the spray bar outlet.

Hard to clear the fuel if you flood the engine, as the demand valve will open as soon as you turn the engine over.  Might have to shut the needle valve while you turn it over a few times.  But my job was always to prevent ignition, so I can't offer much comment on that part of the problem.

Which side is the engine on the solid model?

MJM460

[Brian Rupnow]