A common complaint among LPG vehicle owners is the appearance of heavy end residuals or 'heavy ends' in their converters. This is a dark, smelly, waxy substance that can plug up propane converters with enough accumulation. But where does it come from?
A common explanation of this problem is that coolant flow of too high a temperature through the converter causes the propane fuel to 'crack' (as in petrochemical refining) into these residuals. While researching converter temperatures, I came across two postings on Eng-Tips Forum: Propane Mixing Temperature (qid=47012) and Propane Converter Temperature (qid=53767). One of the interesting points I read in the first posting is the following 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.
Further internet research about converter temperatures and about the formation of hydrocarbon heavy ends failed turn up any collaborating information so I turned to the petrochemical companies, Impco, and Dual Curve™ for advice.
Exxon-Mobil's fuel department had the following answer:
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/converter 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.
Subsequently, Exxon-Mobil followed up with the following message:
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.
PetroCanada replied with the following answer:
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.
PetroCanada then followed up with this answer:
Most propane as it is produced at a gas plant (the majority of product in Western Canada) or refineries is very clean. However, during distribution it can pick up contaminants such as traces of gasoline or diesel fuel (if pipelined through a common products pipe line, or in storage caverns) or extract some plasticizers from hoses and gaskets. Some of these contaminants, particularly diesel fuel and lube oil range materials, have low volatility - so as propane is evaporated in a converter (changing from a liquid to a gas), the contaminants remain behind at a low point in the system - which can be the bottom of the converter, or a low-lying loop in a fuel pipe delivering propane vapours to the carburetor. So there is no 'conversion' or 'breakdown' of propane into oily residues in a converter - the residues are contaminants left behind when the propane evaporates. Unfortunately, the current propane specification allows rather a lot of oily residues - up to 500 ppm. I've seen instances of 6 - 12 ppm oily contaminants (6 - 12 litres of oil from a million litres of propane used in a high volume heating situation) being enough to cause problems with build-up of the oil in the bottom of large converters.
While instances of contaminants in propane have been on-going for decades, and appear in different forms (oily materials, 'grease-like', 'black shoe-polish', and waxy deposits), they are usually sporadic, even seasonal, and we (the industry) have not been successful in finding the sources of all the contaminants. It is clear that potential future uses of propane, such as fuel cells, will require very clean product, and current contaminants will be totally unacceptable.
Impco's technical department replied with the following answer about whether the effect of water temperature reaching the converter makes a significant difference to the operation of a propane system:
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.
(ED. - I have tried unsuccessfully to contact Gann Products several times about their Thermstat product. It does not appear to be currently available.)
Dual Curve™'s technical department has this to say about temperature stabilization:
The long-term performance of both the Vaporizer/Regulator and Mixer is greatly enhanced by our proprietary temperature stabilization technology. This technology stabilizes both the vapor temperature and the internal temperature of the Vaporizer/Regulator. By regulating the temperature of LPG vapors as they leave the Vaporizer/Regulator to a narrow window close to 70°F, several very beneficial effects are obtained:
- One effect, validated by extensive in-field testing with commercially available propane, is that the buildup of heavy ends, both in the Vaporizer/Regulator and the mixer is virtually eliminated. This result was verified using old-style non-coated aluminum vaporizer/regulators and mixers. When this effect of temperature stabilization is combined with Safe Controls coating and low surface activity materials, the results are extraordinary.
- A second effect of temperature stabilization enhances the superior air/fuel ratio tracking ability of the Safe Controls Mixer. All mixer flow is calibrated at 70°, but is subject to a number of factors that prevent most available products from maintaining true linearity under actual engine operation. One reason for erratic performance is the varying expansion rates of metal components. As noted, the Safe Control mixer utilizes extremely temperature-stable materials. However, another factor that affects mixer stability is the varying temperature of the propane gas entering the mixer. Under heavy load, most vaporizer/regulators, in order to satisfy demand, produce very low-temperature, higher density vapor. This variation in density makes it very difficult to track air/fuel ratios consistently over a range of engine operating conditions. Temperature stabilization greatly enhances the already outstanding linear performance of the Safe Controls Mixer
A Fuels Consultant
Finally, an individual actively involved with Canadian (CGSB) and American (ASTM) fuel standards and test methods provided some some further insight into propane motor fuel:
“HD-5” stands for ‘heavy duty (propane), 5% propylene (also called propene) maximum. This grade was developed about 30+ years ago to establish a grade of propane, based on composition, that would be suitable for automotive uses. In other words, if the composition of propane met the HD-5 requirement [maximum 5% propylene and max. 2.5% butanes and heavier (also shown as C4+)], then the fuel would be suitable for automotive engines both for stability and for octane quality, without actually testing the batch for octane or stability.
The issue around propylene is that propylene, with a double bond, is a lot less stable than propane and can polymerize (under hot conditions, as occur in an engine compartment) to form gums and varnishes – which interfere with engine performance. (Think of polypropylene plastic!) Experience showed that limiting the propylene concentration to 5 % gave an acceptable product.
Some states, I think, California, have allowed an HD-10 – meaning 10% propylene – as a means of extending supply and /or reducing production costs, but there has not been any move within the industry to follow this lead, and I don’t know if any product is being sold to this specification.
While HD-5 is the common grade of propane (LPG) available in Canada and the US, some marketers sell ‘commercial propane’, which can have a wide range of propylene (generally 10 – 50+ %). This product is presumably satisfactory for heating applications (but might give problems if used in vehicles).
Further correspondence with this individual about whether hot converter temperatures (200°F+) could cause the polymerization of the propylene to heavy-end residuals that are a concern with propane-fuelled engine revealed:
Directionally, yes. Remember that the temperature of an automotive converter is the temperature of the coolant fluid – generally below 200ºF. However, the higher the temperature, and the more propylene present, the greater the risk of polymerization leading to residues in the fuel system.
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 did have a detrimental effect on my fuel consumption. I found that reducing the coolant flow through converter had the effect of cooling the propane vapor and making it denser resulting in the enrichment of the fuel mixture.
From what I have found about heavy ends, it is likely that the heavy end contaminants are always present in propane fuel to some extent 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. High temperatures can potentially cause the polymerization of the propylene fraction in the fuel.
For best operation, the converter should ideally be supplied with water at a constant temperature. The manual method of controlling the coolant flow through the converter with a hand valve is unreliable at best. At worst, manually limiting converter water flow could result in reduced engine power output.
At this time, only Dual Curve™ and Technocarb make automatic LPG converter vapour temperature control valves. Both companies recommend that their control valves be used in conjunction with their closed loop electronic fuel mixture control units. If used on an open loop system, the cooler fuel vapour would cause the fuel mixture to become too rich.