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But anyway, here's a primer on how gaseous fuel systems work:
First of all, bottled fuels come in many flavors, but we usually put them in two categories... propane, and natural gas. Since we're working with propane...
Propane comes to us in bottles, and inside that bottle, is liquid, and gas. Propane goes from gaseous state to liquid at some point below about -35F. When you have a bottle of propane at -35 F, you can pretty much just unscrew the lid and look in at the propane. (not advised, btw).
Propane starts to evaporate above -30F... but only so long as the ambient PRESSURE allows it. That means, if you have it enclosed in a bottle, you can raise the temperature... and the pressure in the bottle will rise, but it will NOT leave the liquid state.
At 90F, the pressure in that bottle will be pushing 200psi.. but it will NOT evaporate, because it's contained inside that bottle.
Now, if you remove the pressure... open the valve, one of two things will come out. If the valve is up on the top of the tank, it'll spit out propane gas. If the valve is picking up fuel from the BOTTOM of the tank (inside the liquid) it'll shoot out a geyser of liquid propane, which will, once clearing the orfice, rapidly turn to gaseous state and expand... and in the process, absorb LOTS of heat (you'll see frost). Again, this is NOT an advisable experiment, because you'll have one helluva dangerous condition.
Anyway...
In order to get that fuel into an engine in a way that it's controllable, there's several things that must be done. First, is assure that it's in a GASEOUS state. The Impco J is NOT a regulator- it's much more than that. It is a CONVERTER and FUEL CONTROLLER...
First, fuel enters through the fuel inlet, and on the Impco J, it can come in directly at tank pressure, and can do so in either gaseous or liquid state. Many fuel controllers cannot do this.
Second, it has to make sure that it BECOMES gaseous. It has an expansion volume, and a heat-source path that will FORCE the fuel to go from liquid to gaseous state. Remember the -30f concern? Yeah, there's places where it gets so cold that liquid fuel will NOT evaporate to a gaseous state... nor will it generate enough pressure to push gaseous fuel... and LIQUID propane WILL NOT burn properly when sent into the intake of an engine... so there's a passageway in the Impco J that you can run engine coolant THROUGH, which will warm the fuel enough to evaporate.
Next... the fuel system must METER the fuel to the engine... you can't just lay a pipe in the intake, turn on some fuel, and expect it to run worth a hoot, unless you're gonna sit there with your thumb over the end of the gas pipe and feed it a little at a time based on load.
What there is, though, is a very sensitive valve in there... one that responds to a very, very, very slight vacuum signal... so slight, that they don't measure it in PSI... they measure it in INCHES OF WATER COLUMN... that means, the amount of pressure it takes to lift a column of water one inch. If you're not familiar with taking this measurement, do a web search for something called a MANOMETER... you can make one with a clear piece of plastic hose and a bucket of... water... (food coloring makes it easier to see!)
Now... one inch of water column, is about ≈ 0.03609psi.
So what the Impco J-series converter does, is that it measures engine demand using an engine vacuum signal... but not from the manifold... it measures it from the VENTURI... (just like the main jet in the venturi of a liquid fuel carb)... and compares it to AMBIENT ATMOSPHERIC PRESSURE (just like the vent on a liquid fuel carbeurator's bowl), and meters out a certain amount of fuel based on the pressure DIFFERENCE.
It does this with a sensitive polymer diaphragm, with a light spring (yours is blue), and a lever, on a pivot, with a bias adjustment acting on one side of the lever, and a needle valve controlling fuel flow on the other.
When a little bit of vacuum is applied to the signal port of the J-converter, it literally senses the vacuum, and feeds a 'sip' of propane to the engine. As the vacuum signal increases, it sends more.
Oh... and when the vacuum signal stops... it SHUTS OFF the fuel.
So a fuel controller is a regulator... but a SELF SENSING regulator, but different... instead of taking a pressurized gas and knocking it down to some lower pressure (like... a 5000psi oxygen tank, getting knocked down to say, 25psi), it knocks it down to some pressure LOWER than atmospheric pressure... meaning, that 200psi propane pressure is regulated at 1.5 INCHES OF MERCURY LOWER than atmospheric pressure. It's working UPSIDE DOWN.
Unfortunately, this also means, that when you applied 5psi to the back side of the diaphram, you applied 138 times MORE pressure than the diaphragm was designed to operate. It's probably ripped open pretty bad now... but who knows... they're pretty tough.
What you probably had, was an improper fuel signal connection at the engine... combined with fuel lines that were still full of air (meaning... not purged)... and therefore, when cranking, if it WAS actually drawing fuel, it simply had not gotten to the engine yet.
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Gaseous fuel systems aren't complicated... in many ways, they're no different than liquid... the biggest issue people have with understanding them, is frequently just the fact that they're holding onto what they know of liquid carbeurators 'too much'.
Here's some side-notes for general grey-matter digestion... grab your notepads, and keep your eyes on the chalkboard while I draw:
Liquid Gasoline doesn't burn. Gasoline VAPORS burn. When you dump raw gas into an engine, it won't run worth crap- it belches unburned gasoline out the exhaust pipe... only the vapors burn... it must evaporate, then mix with (oxygen containing) air in order to burn.
When you run an engine on propane, you don't feed it liquid propane, because it won't burn in liquid form... it has to evaporate, in order to mix with oxygen and become flammable.
If you put liquid propane in a very cold refrigerator, it will stay in liquid form forever. That temperature would be around -40F or so. As it warms up, it tends to evaporate... and just like water on a stove, will boil, and when enclosed, it will generate pressure. IF contained, it will stay liquid... just like contained water will stay liquid at 250F, under PRESSURE.
If you put gasoline in a can in a cold refrigerator, it will stay in liquid form basically forever. Gasoline isn't a simple compound, it's a complicated mixture of various hydrocarbons, and if you're running blended fuels, you'll have carbohydrates in there too (ethanol).
As temperature rises, liquid fuels eventually reach a point where they start to evaporate... and if you keep raising the temperature, they'll evaporate faster... they'll boil like water... and if you enclose them, they'll generate pressure, just like steam in a kettle.
Since gasoline is a MIX of compounds, SOME of them (aka "lighter fractions") will evaporate off at a lower temperature than others. Lightest ones evaporate off first, leaving the 'heavier' ones to evaporate later. Add to this that the 'fractions' are actually of different densities, they'll 'settle' and wind up in 'layers' in the can, with the 'lightest' ones on top... so if they haven't evaporated off fast enough, now they'll depart sooner, and you'll pour them off first, leaving the heavier stuff for later.
So this is why you can visit your local gas pump, fill a 2-gallon can, and set it on your garage floor, only to find that the lawn mower doesn't run worth crap, and the stuff smells nasty three months later.
Add to the mixture a different type of molecule... a Carbohydrate... Ethanol. It doesn't actually 'mix' with the Hydrocarbons... it's more 'in suspension', at least, when well mixed. It likes to settle out... just like the different layers of hydrocarbons.
Carbohydrates, however, like to absorb moisture. If there's air in contact, those carbohydrates will act as sponges... little dessicant packages (you know- SILICA, DO NOT EAT) extracting moisture from the air, and since the moisture is heavy, it falls down to the bottom of the fuel can, and again, creates a layer that gets trapped from air by the hydrocarbons above. Water in gasoline.
The liquid fuel carbeurator's concept is crude... you have airflow being drawn by cylinders, the air flows through a venturi (a narrow spot), whose restriction causes air velocity to accellerate through the narrow point, but slow down on the back side. On the back side, exists a lower-pressure atmosphere... aka, a slight vacuum. There's a straw that dips down into a bowl of liquid. The pressure difference (vacuum) draws fuel up the straw, into the intake airflow, where the droplets mostly evaporate (we hope, anyway), and mix with airflow, go into the cylinders, and hopefully burn.
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Put on your funny hat, and take your seat- it's time for Physics 173 : Adiabatics and Combined Gas Law
One of the physics challenges which occur, is described well in the Combined Gas Law... which is... the relationship of pressure, volume, and temperature. Many refer to it as Boyle's Law, but there's a few other pieces to the puzzle beyond what Boyle stipulated... so we call it Combined Gas Law... and I can summarize it by the example of air compressors and air tools:
When your air compressor is working, the compressor head gets hot. When you run an air tool, the tool gets COLD. Why? Well, you're taking air, at ambient temperature, and you're cramming it into a smaller space. Let's say that at ambient temperature, that air consists of a certain amount of thermal energy. By compressing it to say, half it's original space, you take the material AND it's thermal energy, and cram it into HALF the space. Now, that means twice the thermal energy is crammed into half the space. That doesn't mean temperature doubles, because the way we measure temperature is 'maligned' with physics a bit... but the result is a tank full of air at much higher pressure... and the tank is warm. Of course, you let the tank sit, and it's internal heat radiates out... eventually, the air inside is at a much higher pressure, but back at ambient temperature.
Now, when you run an electric die grinder, they get hot. The motor is carrying electricity, which is converted to magnetic energy which spins gears which turn the grinding bit chewing into your metal. When you're running one that runs on compressed air, though... the air escaping isn't hot... it's COLD... cold enough that I often find myself taking off the leather work glove and putting on a warmer one. Reason why... is the opposite of what the compressor was doing... the air is going from high pressure to low pressure... it's expanding, and in order to do that, it must ABSORB HEAT from the environment to fully expand. That's what makes it cold.
The nice thing, is when you're working in a naturally hot environment, the tool stays cool... the bearings stay cool... it ALL stays cool... and the tool runs GREAT! The bad thing is, the tool can get too cold... moisture in the compressed air system will cause frost or even freeze a mechanism, and the little air motor will lose performance as it gets colder, because the compressed air's force will drop off because there's insufficient HEAT to make the air expand in the turbine, blah blah blah... ZZZZZZZZzzzzzzzzzzzzzzzzzzzzzzzzzzzzz....
<bell rings> Test on Tuesday! Class Dismissed!
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Back to wrenching on engines...
So the air coming through the venturi, because pressure is low, the temperature FALLS. Notice ice on the outside of the manifold? Yep. Now, imagine you're a droplet of some mix of hydrocarbons and carbohydrates, mixed with water, and you've just been sucked up from a bowl into a rapidly moving chilly windstorm... what in the world would make you WANT to evaporate? Aside from the lower pressure, not too damned much. Fortunately, as you get warmer, the lighter fractions will evap off, but the heavy stuff is still clung to that tiny fragment of ice that formed the moment you came up out of that main jet... yep... the water in the gas froze... oh, and there was some moisture in the airstream that turned into ice as it went through that venturi too... so it was SNOWING in that windstorm you got dragged into...
But after you bounced off that valve, into the chamber, and the door slammed shut, you got a little warmer, and then pressure came way up. The lighter fractions ignited, but the rest of you... well, it was over before it began, the next door opened, and you got spit out into the muffler, with whatever other debris and oil residue that was bouncing around in the cylinder, as half-charred pyroclastic-looking goo.
Suffice to say, when an engine is running, it isn't exactly a 'perfect' system.
Once the engine has warmed up, it'll run much, much better, but still, there's a venturi, a pressure drop, blah blah blah, so those liquid fuel droplets ain't gonna completely burn, and by the way, there's several different 'flavors' of stuff in that gasoline you buy, along with water, so some of it just ain't gonna burn. If you were to sit down with a sharp pencil and calculate out all the different fractions' molecular structures, and figure out exactly how many parts air you need, for a given mix of fractions, then you might get fairly close to a burn that results in something low in noxious crap and carbon monoxide, but you can throw all that theory straight into the ashcan the moment you put in a carbohydrate... and did I mention that the 'mixture' all settles out? You could drive down the road with a full tank of well-settled fuel, and the actual mixture will change several times over in just a few miles, because the fuel has all settled into 'layers'. Oh, and the water that the ethanol absorbed? It ain't combustible... it's the equivalent of 'filler' in a fast-food burger... takes up space, has no positive nutritional value... but it sure as heck can freeze to the intake or valves, and wreak havoc with other things. Liquid fuel engines need a certain amount of heat in the intake and carbeurator to start and run reliably... and to actually get started, they usually need substantially higher quantities of 'liquid' put into them, in order to get enough 'lighter' fractions to actually evaporate. That's what the CHOKE is for, right?
People wonder why carbeurated engines have so fallen out of favor...
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Back to Propane:
Propane is a Hydrocarbon. C3H8 to be exact. In common atmosphere, it boils at -43.6F degrees, it's a byproduct of petroleum production... both natural gas and liquid petroleum refining. It is captured and compressed to become liquifed.
Because it's chemical makeup is very, very simple, it's easy to compute a very clean burn. Since it's manufactured and transported in sealed containers, it stays very, very clean, doesn't absorb moisture, and since it's all the same, it cannot 'settle'. Since it boils at a very low temperature, there's no need to add enrichenment to make up for only a small part of the fuel wanting to light... it all does... no questions asked. That also means the mix is always the same, and thus, the chemical process is simple and easy.
Getting that gaseous fuel into the airstream is easy. The liquid fuel engine uses demand detection... it places the fuel introduction point just downstream of the venturi... and uses the vacuum at that point to 'draw' fuel in based on demand... after all, the throttle plate is restricting airflow to 'throttle' the engine, so as that plate moves, airflow through venturi changes, and with it, suction of fuel.
To draw a gaseous fuel, we do exactly the same thing. Instead of a 'main jet' protruding into the 'venturi', we call a gaseous feedpoint a 'stub', because it's just a tube... and the pressure difference 'sucks' gaseous fuel in... but there's one little catch: The gaseous fuel is already under pressure... so anything hooked to it is just gonna flow gas constantly, which ain't gonna work. We need something else!
Enter the demand regulator... aka 'zero governor' or 'fuel controller', and it does it all... and it's amazingly simple... it's a regulator, just like the one on your air compressor, one on your oxy torch tank, or on the pressure line coming to your house from the water main. It regulates the high-pressure gas source to a given pressure of the outlet.
Let's say you've got a compressor set up to run from 115 to 125psi. Your air tool is rated to 90psi... your car tires 36psi. You install a regulator in the line, and the regulator's got a spring pushing on a diaphram, and in the middle is a needle valve... much like the one that controls fuel level in a liquid carb. When pressure comes in through the needle valve, it flows through and pushes on the back side of the diaphram, which overcomes the spring's tension, and closes the needle valve.
Let's say that diaphram has one square inch of surface in contact with the incoming pressure source, and the spring holding the needle valve open is only strong enough to hold back the diaphram 'till diaphram pressure reaches reaches 40lbs... now the incoming air will only flow 'till the valve shuts off at 40psi. On the same chamber is an outlet port going to your air hose. If air exits the hose, pressure behind diaphram drops, and the needle valve opens... pressure rises 'till the needle valve cuts off incoming pressure.
Simple, eh?
Now, let's say you have a T-handle pushing on one end of that spring, so you can INCREASE the spring's force, thus, it takes 50lbs of force to close the needle valve. Now you have a 50psi setpoint for your 'adjustable' regulator... and no matter how much or little demand is, you'll always get regulated output, as long as the valve can actually 'flow' that much air.
But for gaseous fuels, we have a different need... the regulated pressure needs to be BELOW atmospheric pressure. That means, grab that regulator's spring adjustment knob, and crank it down, down, down... below 10... now zero. The regulator will cut off all flow... completely... even with hose disconnected, because there's insufficient pressure (even at zero) to overcome the diaphram and spring.
BUT... if you put your mouth on the outlet, and suck on it... you'll get airflow. Why? Because your outlet pressure is LOWER than the regulator's setpoint!
Hook this to your engine's carbeurator STUB, and now you have a gaseous fuel system.
Oh, and did I mention that the propane is clean, and predictable in it's chemistry? There's a reason why they run so rediculously clean... and that big tank you have... if you don't use ANY of that fuel, it will last indefinitely... not an iota of degradation from sitting.
And the engines start nice. You gotta have it set up right, but the engines start nice. No choke or enrichment needed... there's no evaporation required, it all mixes with air immediately... no water in the fuel, no unburned crap or additional carb heat needed... Just crank it over. It fires right up.
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