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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 help.
Exxon-Mobil
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
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
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™
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.
Converter Temperature
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.
Be sure to fuel your propane vehicle with HD-5 rather than commercial
propane whenever possible. Use an automatic propane converter vapour temperature control
valve in conjunction with an electronic fuel mixture control unit.
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