What is LPG?
"Liquefied Petroleum Gas (LPG) is often incorrectly identified as
propane. In fact, LPG is a mixture of petroleum and natural gases that
exist in a liquid state at ambient temperatures when under moderate
pressures (less than 1.5 MPa or 200 psi). The common interchanging of
the two terms is explained by the fact that in the U.S. and Canada LPG
consists primarily of propane (see Table 3). In many European
countries, however, the propane content in LPG can be as low as 50% or
"LPG is synthesised by refining petroleum or natural gas; it was first
produced in 1910 by Dr. Walter Snelling, and the first commercial
products appeared in 1912. It currently provides about 3% of the
energy consumed in the United States."
Technical Aspects of LPG
"LPG (Liquefied Petroleum Gas) is a petroleum derived, colorless gas,
typically comprised of mainly propane, butane, or a combination of
these two constituents. LPG fuel for vehicles is actually a mixture of
various hydrocarbons which are gases at atmospheric pressure and
temperature but which liquefy at higher pressures like less than 200
psi. LPG is a natural derivative of both natural gas and crude oil.
Commercially three different grades of LPG are available, Table 1.
Standard HD5 requires minimum propane content of 90 % and propylene
content of less than 5 % (volume basis)."
Chemical Composition of LPG (click link for diagrams)
"There are two LP gases that can be stored in liquid form with only
moderate pressurization -- propane and butane. Isobutane, which has
the same simple chemical formula as butane but has a different
chemical structure, is also used. Usually, butane and isobutane are
mixed with propane in various proportions, depending on the intended
use of the fuel."
"Propane is particularly useful as a portable fuel because its boiling
point is -44 F (-42 C). That means that even at very low temperatures,
it will vaporize as soon as it is released from its pressurized
container. This results in a clean burning fuel that doesn't require a
lot of equipment to vaporize it and mix it with air. A simple nozzle
Butane's boiling point is approximately 31 F (-0.6 C), which means it
will not vaporize in very cold temperatures. This is why butane has
more limited uses and is mixed with propane instead of being used by
A single pound of propane can generate 21,548 BTU (British Thermal
Units) of energy, while butane can produce 21,221 BTU per pound [ref].
For comparison, here is how LP Gases stack up to other fuels in terms
* Propane: 21,500 BTU per pound
* Butane: 21,200 BTU per pound
* Gasoline: 17,500 BTU per pound
* Coal: 10,000 BTU per pound
* Wood: 7,000 BTU per pound"
"The Source of LP Gas
LP gas is a fossil fuel, like oil and natural gas. It can be refined
from oil and natural gas the same way gasoline is refined from crude
oil. While most energy companies are not focused on LP gases, they
produce them nonetheless because they are a by-product of the refining
process for other fuels."
"When an energy company draws natural gas from the ground, about 90
percent of it is methane. The rest is in the form of various LP gases,
which the company separates from the methane before the methane is fed
into pipelines for use in our homes. The amount of LP gas that comes
from natural gas varies, but it is usually from 1 to 3 percent [ref].
LP gases are separated from crude oil, as well. The refining process
produces about a 3 percent yield of LP gases, although if refineries
were retooled to focus on LP-gas extraction, that number could be as
high as 40 percent"
"The major sources of commercial LPG are natural gas processing and
petroleum refining. Raw natural gas often contains excess propane and
butanes which must be removed to prevent their condensation in
high-pressure pipelines. In petroleum refining, LPG is collected
during distillation, from lighter compounds dissolved in the crude
oil, as well as generated in the "cracking" of heavy hydrocarbons.
Therefore, LPG can be considered a by-product and its exact
composition and properties will vary greatly with the source."
"LPG is widely used as a "green" fuel for internal combustion engines
as it decreases exhaust emissions. It has an octane rating (RON) that
is between 90 and 110 and an energy content (higher heating value?HHV)
that is between 25.5 megajoules per liter (for pure propane) and 28.7
MJ/L (for pure butane.) Toyota made a number of LPG engines in their
1970s M, R, and Y engine families.
Currently, a number of automobile manufacturers?CitroŽn, Daewoo, Fiat,
Ford, Hyundai, Opel/Vauxhall, Peugeot, Renault, Saab and Volvo?have
OEM bi-fuel models that will run equally well on both LPG and
gasoline.Vialli have OEM LPG powered scooters and LPG powered mopeds
that run equally well on LPG."
"According to the World Liquefied Petroleum Gas Association (WLPGA),
more than 9 million vehicles in 38 countries currently operate on LP
gas. It's not a new idea: Propane-powered vehicles have been around
for decades. The benefits include reduced emissions, quoted by WLPGA
as "50% less carbon monoxide, 40% less hydrocarbons, 35% less nitrogen
oxides (NOx) and 50% less ozone forming potential compared to
gasoline" [ref]. With government incentives and tax breaks figured in,
LP gas used in cars (known as autogas) can be much cheaper than
gasoline. Even without the incentives, it is usually much cheaper.
Autogas is a high-octane fuel, offering performance comparable to
gasoline and diesel, and many owners claim that autogas runs more
smoothly, resulting in less wear and tear on engine components."
"Rarely (though more often in Europe), cars are built solely with an
LP-gas fuel system. The engine is tuned and adjusted specifically to
run at maximum efficiency on propane, which results in reduced engine
wear and better mileage in the long run. However, many consumers are
worried that a propane filling station could be hard to find. For that
reason, most LP gas cars have dual fuel systems -- one for gasoline
and one for propane. The systems are set to switch automatically
between the two fuels as appropriate. This allows owners to use clean,
cheap propane when it's available, but they still have the option of a
quick fill-up at the local gas station if they need it. "
How the system works
LPG Engine Specs
"Mechanical conversions systems for LPG and CNG are basically
identical with some exceptions. Storage tanks are different with LPG
system. Vaporizer is necessary in LPG conversion system. In LPG
conversion system, high pressure regulator is not necessary.
Electronic conversion systems are also available for LPG.
Early propane mixers operated as a conventional venturi-controlled
devices in a manner quite similar to gasoline carburetors. Vaporized
propane is drawn through a fixed orifice in response to engine air
flow. The basic design principles have remained unchanged over 30
years. As intake air enters the engine, a venturi effect is created
through the mixer air-valve. This slight pressure drop acts on a
spring-loaded diaphragm is proportionally with air flow,. This may be
best described as a highly accurate flow meter which controls engine
fuel flow as a function of air flow.
Vaporizer converts the liquid propane to a gas. The primary heat
source for this vaporization is engine-jacket water which flows
through specially designed water jackets cast into the vaporizer body.
It is necessary that propane fuel systems draw from the bottom of the
tank rather than the top. If engine feed were drawn from the gas
phase, the heavier, higher boiling components in LPG would gradually
become concentrated in the liquid phase creating a liquid mass with a
for vapor pressure and a high freezing point. This liquid would create
various problems in the feel feed system .Therefore, L.P.G systems
draw from the bottom of the tank and send the liquid through a
vaporizer that is heated by engine coolant.
The function of the regulator is to provide precise fuel pressure
regulation to the mixer. As demand on the regulator increases with
engine load, regulator allows higher flow; demand on system decreases,
regulator restricts flow to maintain flow pressure. The high pressure
regulator is unnecessary due to low pressure in LPG storage tank."
"Propane fuel tank is installed, along with a refueling port,fuel
lines, and pressure safety valves. A filter" fuelock" removes
particles that may be present in the propane. Propane tanks are
constructed of heavy gauge steel, in compliance with the Boiler and
Pressure Vessel Code of the American Society of Mechanical Engineers (
ASME ) to whitstand a pressure of 1000 psi. Normal working pressures
of the tanks vary depending upon ambient temperatures and the quantity
of fuel in the tank. Propane systems normally limit the liquid level
to 80% of tank total tank volume by a stop fill valve. Common
operating pressures are in the range of 130-170 psi. Tanks are
equipped with pressure relief valves that will release propane vapors
to the atmosphere to prevent tank explosion under abnormally high
Do-It-Yourself Automotive LPG Conversion
Calor: LPG Conversion
More info on LPG Engines
LPG Automotive Options
"LPG cars produce 90% fewer particulate emissions and 90% less
Nitrogen Oxides than diesel engines. LPG engines produce 75% less
Carbon Monoxide than petrol and have 87% less Ozone forming potential.
If you spill LPG, it evaporates rather than soaking into and polluting
the ground. LPG engines run up to 50% more quietly than diesel
But what will helping to reduce pollution mean for you and your car?
Reduced vibration in LPG car engines, means longer engine life. As LPG
is a gas in the engine, it doesn?t wash away oil from the cylinder
walls. Wear and tear on LPG cars cylinder bores is lessened. This
means that you don?t have to replace lubricating oils as frequently as
on conventional engines. LPG cars are much cheaper than the equivalent
petrol or diesel and there is often favourable tax or licensing
concessions, which means running on LPG can save you money too. Crash
and fire tests show that LPG tanks are safer than petrol."
"LPG provides about 8% more energy per unit weight (LHV = 19,757
BTU/lbm) than gasoline. Theoretically, vehicle operation with LPG
should be more efficient than with gasoline, i.e., the vehicle should
attain better specific fuel consumption and improved mileage. However,
this will only happen if the engine design is optimized for LPG fuel.
If a gasoline engine is converted to operate on LPG this increased
efficiency will not be realized due to the lower density of LPG
compared to gasoline and also its slightly higher oxygen demand (LPG
stoichiometric A/F = 15.8). The lighter density fuel displaces air in
the intake manifold, and thus, less air per cycle is induced to the
cylinders. This translates to a decreased volumetric efficiency and a
loss of power compared to the original gasoline rating of the engine."
"The major harmful emissions from LPG engines are similar to those
from other internal combustion engines:
* Carbon monoxide (CO)
* Hydrocarbons (HC)
* Nitrogen oxides (NOx)
Unlike diesel engines, there are practically no particulate emissions
from LPG engines."
"Carbon monoxide is generated in the exhaust as the result of
incomplete combustion of fuel. CO is a very toxic, colorless and
odorless gas. LPG emissions may contain considerable amounts of CO.
When engines operate in enclosed spaces, such as warehouses, buildings
under construction, or tunnels, carbon monoxide can accumulate quickly
and reach concentrations which are dangerous for humans. It causes
headaches, dizziness, lethargy, and death. CO is usually the major
concern whenever LPG engines are used indoors."
IOWA STATE UNIVERSITY of Science and Technology† † † AEN-206
Agricultural and Biosystems Engineering Extension
CARBON MONOXIDE POISONING
"Is carbon monoxide a problem with diesel engines? Usually not,
although any engine, including diesel, produces CO when combustion is
incomplete. Diesel (compression ignition) engines run with an excess
of air and often produce less than 1200 ppm CO. When diesel fuel is
burned incompletely or when overloaded and over-fueled (rich mixture),
diesel engines will produce high concentrations of CO. Diesels usually
pollute the air with particulates and nitrogen oxides, not CO.
Is carbon monoxide a problem with LPG engines? Yes, and the same
precautions against running a gasoline engine in an enclosed space
should be observed with an LPG engine.† Industry sources report a
properly tuned LPG engine will produce from 200 to 20,000 ppm,
depending on load.† A difference in CO production from an engine
operating on LPG and one operating on gasoline usually results from
more complete combustion of the LPG because it is already a vapor. †
Unfortunately, most LPG engines have simple fuel delivery systems
which can easily be adjusted too rich, allowing extra fuel into the
engine and the subsequent high production of carbon monoxide.† On one
new engine, adjustment of the idle mixture reduced CO concentrations
from 44,500 ppm to 600 ppm."
"Hydrocarbons are also a product of incomplete combustion of fuel. LPG
emissions, because of the composition of fuel, contain only short
chain hydrocarbons. They are not likely to contain toxic components
which are found in gasoline HC emissions. Also the environmental
impact of LPG hydrocarbon emissions (ozone reactivity contributing to
smog) is much smaller than that of gasoline. However, hydrocarbon
derivatives are responsible for the characteristic smell which is
often a nuisance when LPG engines operate indoors."
"Nitrogen oxides are generated from nitrogen and oxygen under the high
temperature and pressure conditions in the engine cylinder. NOx
consists mostly of nitric oxide (NO) and some nitrogen dioxide (NO2).
Nitrogen dioxide is a reactive gas, very toxic for humans.
Accumulation of NOx in a warehouse atmosphere may be also detrimental
for the stored goods. For example, only a few ppm of NOx in the
ambient air can change the color of paper stock from white to
yellowish. NOx emissions are also a serious environmental concern
because of their ozone reactivity and important role in smog
"LPG was introduced to the market as a "clean-burning" fuel. Is it
still clean today, many years down the road? LPG definitely had the
potential to become a clean fuel. The reasons for the superior
emissions performance were the following:
* Reduced emissions of carbon monoxide compared to gasoline engines
(but not as low as in diesel engines).
* No heavy hydrocarbon emissions. HC which are emitted, are of short
carbon chain and low ozone-forming reactivity.
* Low emission of toxic air contaminants such as benzene and 1,3-butadiene.
* Low cold-start emissions.
* Likely better emissions durability than that of gasoline engines.
LPG emissions should not increase as dramatically with the engine wear
and deposit build-up.
* Zero evaporative and running losses due to the sealed fuel system."
Not a main alternative fuel option
"For a variety of reasons LPG is not considered the alternative fuel
of the future any more. Its place has been taken by natural gas
competing with diesel and biodiesel. Consequently, there has been
little development in dedicated LPG engine technology. On the other
hand, gasoline engines and their emissions improved tremendously over
the last decade. As a result of that development, some of the
used-to-be advantages of LPG fuel, especially the low CO emissions,
are now less pronounced. "
Gasoline engine conversion limits the environmental benefits of LPG
"Essentially all LPG engines are gasoline engine conversions. As such,
they are not engineered to take advantage of the low emission
potential of LPG. Their engine/fuel control system is not optimally
calibrated for the new fuel, often sacrificing performance, fuel
economy, and emissions. The performance and emissions vary between
different engines and conversion kits. Electronic LPG conversion kits
are available now which should provide the lowest emissions and best
fuel economy, but little data exists so far to verify that statement.
Many of the mechanical conversions produce engines not even remotely
resembling the ideal, low-emission LPG picture. It is wise to request
emission data from the vendor when buying an LPG vehicle for indoor
application. Unfortunately, brand new LPG conversions emitting CO
levels of 2 to 4% are not uncommon. As a generous guideline, an
acceptable LPG engine should have exhaust CO concentration of less
than 1% under any steady-state condition."
Indoor LPG Vehicles can be dangerous
"Emissions from LPG vehicles used indoors pose a health hazard to
workers. As such, they are indirectly regulated by all occupational
health and safety air quality standards. There are many regulatory
authorities in different jurisdictions. Occupational Safety and Health
Administration (OSHA) within the Department of Labor sets air quality
standards at the federal level in the U.S."
"Most problems with indoor operation of LPG engines are related to
carbon monoxide emissions. Sometimes also NOx emissions may be an
issue. TLVs for carbon monoxide vary in different jurisdictions
between 25 and 50 ppm. It is recommended that ambient concentrations
of carbon monoxide be monitored on a regular basis wherever LPG
engines are used."
Methods of limiting LPG emissions
(chemical equations included at linked site)
"Oxidation catalyst systems are commonly used today for emission
control of LPG forklifts and off-road vehicles. The oxidation
catalyst, sometimes also called 2-way catalyst, is effective in
reducing two major exhaust pollutants, i.e., carbon monoxide and
hydrocarbons. Both are removed from the exhaust through oxidation to
carbon dioxide and water vapor "
"A newer catalyst technology is known as the 3-way catalyst. Three
pollutants are simultaneously removed from the exhaust gas in the
3-way catalyst. They are carbon monoxide, hydrocarbons, and nitrogen
[PDF] LIQUEFIED PETROLEUM GAS (LPG) Fact sheet
Global Autogas Industry Network
World LP Gas Association: About LP Gas
NPGA.org: Snelling Discovery
BTU Content Comparisons of Common Fuels
-"Liquified Petroleum Gas"
-"Liquified Petroleum Gas" and emissions
-"Liquified Petroleum Gas" and "chemical properties"
-"Liquified Petroleum Gas"
How Stuff Works
-"Liquified Petroleum Gas"
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