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From my experience with experimenting with coolant flow through the Model E converter mounted on my 1977 Pontiac, I found that running cooler coolant temperatures had a detrimental effect on my fuel consumption. On a different post about gas valve profiles, franzh confirmed that cooling the propane has the effect of making it denser, requiring a fuel mixture adjustment change. As the cruising fuel mixture is not adjustable on 425 mixers, I wanted to investigate this further. While searching for more information about converter temperatures, I came across two postings on www.eng-tips.com (http://www.eng-tips.com/viewthread.cfm?qid=47012 and http://www.eng-tips.com/viewthread.cfm?qid=53767). One of the interesting points I read in the qid=47012 posting is the following quote: Quote: The primary benefit to lowering the (propane) temperature is not the air charge (although that is a positive benefit) but that you reduce the formation and accumulation of hydrocarbon heavy ends, semi-solids, and paraffin’s. They tend to form at temperatures of 140 deg f. Some severe cases have resulted in vaporizer stoppage in as little as 30 hours of operation. I tried searching the internet for more information about converter temperatures and about the formation of hydrocarbon heavy ends. I did not find any collaborating information so I tried emailing some petrochemical companies and Impco once again. I received a reply from ExxonMobil and they write that their fuel department has the following answer: Quote: The ASTM D1835 special duty grade, GPA 2140 HD5 grade and Canadian CGSB grade 1 specification for auto propane all control the evaporative residue with D1837 max temperature at 95% evaporated or by max "butane and heavier" of 2.5 vol% by D2163 GC. The intent of these specs are to ensure that marginally heavier hydrocarbons (butanes, pentanes, hexanes etc.) that are present from fractionation are not present in amounts that would result in excessive collection in vapor withdrawal applications on multiple refills and multiple single plate distillations from vapor withdrawal. Auto propane applications are mostly liquid withdrawal, and heated regulator/vaporizers (commonly called "converters"), so this specification generally does not apply. Oily residues are controlled by the D2158 evaporated residue and oil stain. This is intended to limit the amount of oily residues that may be left behind at the point of vaporization, including in auto "converters". Sources include traces of compressor oils, pump lubricants, hose plasticizers and other materials that are invariably present at trace levels in all fuels. The specification limits are sufficient that this is generally not a problem in most applications, but could be an issue for equipment that is overly sensitive or severe. For example, most auto propane regulator/convertor usually have enough liquid propane transients to wash any evaporative residues into the engine where they combust along with the fuel (and any traces of engine oil from valve guides etc.). Some regulators have specific provisions to ensure this happens, such as mounting requirements that allow any residues to gravity drain to the converter exit, and no low point in the "dry gas hose" that connects the converter to the carburetor or inlet manifold. Some newer gas and liquid injection systems have had problems with residues, and recommend the use of solvent/detergent additives to control intake system deposits, similar to gasoline. These are generally solvent oil types, intended to keep a small amount of liquid present to continually flush small amounts of residues into the engine (very similar to solvent oils in current gasoline Deposit Control Additives). Detergent additives are not added to propane intended for general distribution,, as they are detrimental to most other LPG applications, as they are non volatile and collect at the point of vaporization, for example in a bbq tank or a low temperature vaporizer. They then followed up with the following message: Quote: Directionally lower temperatures will leave more liquid in the converter and tend to provide more liquid for "flushing" on startup. However there is more that could collect in an improperly installed dry gas hose with a low point, or there may not be sufficient heat in cold weather. Similarly, higher temperatures will vaporize more materials, but the smaller amount that remains will be thicker and more "baked on". The best way to go may be different with different converter designs and even driving cycles (for example the difference between a car with lots of cold starts and a forklift with 24/7 hot operation. May I suggest you consult with the individual converter manufacturer? I also contacted PetroCanada and they replied with the following answer: Quote: It would take more than the temperature experienced in the engine to convert propane to oil. If it were easy to convert propane to oil we would be doing this at refineries. When propane is certified at the refinery tankage it is tested for purity. Only dedicated lines can be used for propane and trucks that carry propane cannot carry liquid fuels. The only problem I have heard of in relation to oil residues in a propane vehicle relate to oil being deposited on the cylinder walls since is not washed away with the liquid fuel mixture as in a gaseous engine. I asked the technical department at Impco if the temperature of the water reaching the converter make a significant difference to the operation of a propane system. They replied with the following answer: Quote: Not a significant difference to speak of. During the process of vaporization some heavy ends will collect within the regulator over time. This can be reduced by mounting the regulator with the outlet pointed down and if there are high coolant temperatures a "Thermstat" can be installed in the line coming out of the regulator which keeps the coolant at about 160° F in the regulator chamber The "Thermstats" are available from Gann Products in CA. Their number is (562) 862-2337. It is an inline thermostat that can be purchased with 5/8 " hose barbs on both ends or 3/8 female NPT on both ends. The regulators produced by IMPCO have a working temperature from -40° to 250° F. If you are working with vehicles that are feedback controlled from the OEM factory on gasoline, you need to use the recommended thermostat because the engine will not go into closed-loop until a certain water temperature is reached. Therefore, from what I have found about heavy ends, I think that it is more likely that the heavy ends are always present in propane fuel and will always accumulate in the converter if the converter is oriented in such a way that doesn’t allow them to drain into the mixer. Since the converter temperature also has an effect on the fuel density supplied to the mixer and thus fuel consumption, I think that one solution would be to install a control valve in the line supplying water to the converter. In my opinion, the converter should ideally be supplied with water at a constant temperature. The control valve could be either a globe valve with a hand wheel or a Thermstat. With the hand valve, I would try running the converter with the valve wide open and establish a baseline fuel consumption. After determining the baseline fuel consumption, I would then close the valve by a half turn and measure the new fuel consumption. I would keep repeating this test until I saw that the fuel consumption began to decrease and then I would restore the valve opening to the point where fuel consumption were the lowest. I have no experience with the Thermstat valve but have asked Gann Products for more information. |
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