Join Date: Mar 2006
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Liquid and Vapor systems:
The difference between liquid and vapor systems is where the system turns the liquid propane to vapor. In a vapor system, the liquid uses the area in the top of the tank to turn the liquid to vapor as the vapor is taken away at the top of the tank. In a liquid system you have liquid propane introduced to a vaporizer and regulated from there. Using a liquid system you usually route heated coolant to the vaporizer to supply the energy to vaporize the liquid propane.
The big difference between proportional and constant valving is how the propane is introduced to the system. The proportional system starts a pre-set pressure at a pre-set boost and grows with the boost to a maximum psi. A constant system will introduce a set amount of propane at a pre-set boost psi. Since you can use more propane as your injection volume increases with rpm, and the constant rate will not grow, many users will use several constant stages to inject more propane as the diesel fuel volume grows with the boost pressure. These are known as stages and you can build as many as you want in a constant rate system. 2-stage and 3-stage systems are most common, introducing one constant rate at a lower boost pressure, then introducing one or two more constant rates as the boost pressure increases.
A constant type system will kick in at a boost level but will not grow or lessen with more or less boost pressure. The size of the orifice you choose to run along with the pressure you set at the regulator will determine the amount of propane that is bled into the system. As boost and fuel delivery increases you have the opportunity to add more propane, so some people have gone with a dual stage valving (or three stage) where a lesser orifice bleeds in at a lower pressure and a second orifice is introduced at a higher level. Again, the more propane introduced without detonation the more power you will create. Some systems call themselves variable when they are not. The Powershot 2000 is a truly variable system. Some systems deliver the propane with a tube very close to the impeller that will pull more as it spools and draws more restriction in the intake, but they are just varying the amount of propane drawn through the staged orifice. It will provide a little variance, but nothing near enough to mirror a true variable system where the boost will equate to different flow amounts.
Consumption of propane and max flow can be adjusted individually, but are related to each other. Ex: If you start the bleed in at 5psi, then adjust the flow to a maximum amount, if you change the bleed in pressure to 3 psi you would also increase the max flow rate. A pre-set 3psi bleed in setting will come in sooner, probably use more propane since you are coming in at a lower boost setting which you will hit more often, and will top off at a higher max since it starts earlier and that will allow it to reach a higher level. A proportional system will start at your pre-set level and (more or less) match the boost reading pound for pound (or very close) as the boost increases. Of course, your application is also to be considered. If you are an RV'er pulling a 5'er you will hit 5psi of boost quite regularly and a higher bleed in setting will allow the tank to last longer. If you are looking for fuel mileage you will want to bleed in just a tad below your cruising boost pressure so you will introduce just a trickle of propane to make an efficient combustion. For performance settings you would probably set bleed in at the least amount possible to yield the most possible propane at all settings.
Your max propane pressure will be limited to the max pressure your tank can vaporize, and that is why there is so much talk of the different types of systems and how fast they can supply vaporized propane to the engine induction system. As propane vaporizes it uses the surrounding heat in the transfer from liquid to gas. When it gets colder outside you will have less available heat than when it is warm, but propane still vaporizes until -44° (although at a slower rate). 110° will yield 230 lbs of pressure, 100° is 196lbs, 70° is 127 lbs, 0° is 31 lbs. Liquid feed systems will generally use heated engine coolant to supply a heat source to the vaporizer, then regulate the pressure from there.
A Powershot2000 is a proportional vapor system. The variable boost propane injection, and the easy adjustment of the system provides a smoother power band and more drivable feel.
Another consideration is where the regulated propane enters. In the PS2000 system, the regulator acts as the orifice and the line running to the engine compartment only carries the amount of propane that is going to be injected. There are no orifices or "tips" on the hose. A staged liquid system will flow full pressure liquid to the engine compartment where the vaporizer is usually mounted (to be near the coolant flow) and will need orifices or tips to adjust the flow rate past the pressure regulator.
Also, there are different types of tanks. Tanks are a personal choice and are largely dependant on your truck and how you use it. There are safety features built in to each tank and using them for applications not intended would not be wise. Liquid tanks for liquid applications, vapor tanks for vapor applications, and in all applications a safe, vented location.
Some regulations to be aware of: No part of the tank can hang below the lowest part of the truck. This can be interpreted as the lowest skid plate (tranny) or the actual lowest section (differential) depending on who's reading the code. If the tank is enclosed in the bed with a canopy or in a tool/work box it must be in a sealed container air tight to the inside of the bed and vented to fresh air outside the enclosure. OPD valves have to be placed on all tanks, but ASME tanks (usually horizontal tanks referred to as RV, Fork Lift, or Fuel tanks) will generally have a dedicated supply valve separate from the fill valve. DOT tanks (think of BBQ style horizontal tanks) generally use the fill valve for the supply valve which can restrict flow. Using two DOT tanks in parallel will allow a better supply. Your tank mounting must support the weight of the tank when filled times 3. Ex: a mount capable of holding 150 lbs must support a filled tank that weighs 50lbs.
Some good information here at least, but as always opinions from those who have never used it. One guy gave you some solid first hand information and he had for 5 years. Maybe more will come along.