Liquefied petroleum gas, LPG
Nov. 04, 2024
Liquefied petroleum gas, LPG
LPG is a mature, but quite niche alternative fuel that can be used in special spark ignition engines or as an auxiliary fuel in dual fuel compression ignition engines together with diesel oil. LPG is a mixture of propane and butane and it is a by-product of gas and oil industries. The use of LPG in transportation is concentrated in few countries (Korea, Turkey, Russia, and Poland) and it is mainly used in bi-fuel light duty vehicles.
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Regulated emissions are dependent on the vehicle type (OEM or retrofit, bi-fuel or dedicated, injection type, age etc.), but with proper designing, somewhat better emission performance can be achieved in modern vehicles compared to gasoline. Compared to diesel, lower NOx and particulate matter emissions can be obtained, whereas CO and HC emissions are typically higher with LPG (similar to gasoline). Compared to natural gas, emission performance is worse, but distribution and storage are easier.
General
Liquefied petroleum gas (LPG) also known as autogas is a widely used alternative fuel. LPG is a mixture of propane and butane, and it is being produced as by-product of natural gas and oil refining industry. Around 60% of total amount of LPG produced is recovered directly from oil and gas fields (WLPGA) in which case no actual refining is needed. The remaining 40% is formed as a by-product in crude oil processing either in distillation phase or after-treatment (cracking) processes.
In , LPG was used to power more than 17 million vehicles around the world. Little more than 9% of the global consumption of LPG is used in transportation. (WLPGA). Rest of the LPG is used, for example, in space and water heating, cooking, electricity production and in many industrial processes. The use of autogas is concentrated in a small number of markets: Korea, Turkey, Russia, Poland and Italy accounted for half of world consumption in and the top ten countries for 75 %. In Korea and Japan, most of the LPG is used in taxis and other light-duty fleet vehicles due to incentives and regulations. In Europe LPG is mostly used in the private sector, in cars which typically have been retrofitted with LPG equipment as opposed to Korea where LPG vehicles are original equipment manufactured (OEM). LPG is not used much in heavy-duty vehicles. (WLPGA). The increasing trend in autogas use can be seen from Figure 1.
Figure 1. LPG gas use in transportation (WLPGA).
Standards and typical properties
LPG consists mostly of propane (C3H8) and butane (C4H10) which can be easily liquefied at moderate pressure. Table 1 lists the basic fuel properties of LPG (as propane/butane). The chemical composition of LPG varies depending on the location and time of the year. For example, LPG sold in the Netherlands contains on average 60% propane and 40% butane, but in northern regions such as Canada, USA or Sweden, LPG consists mostly of propane. At low temperatures, vapor pressure of butane is so low that it will not come out of the tank. LPG used in transportation should contain as little olefins (such as propene) as possible. Olefins have a low octane number and they are known to cause carbon deposits in engines.
ISO has two standards for Petroleum products LPG, but these are mainly meant for international trade and not specifically for vehicle use (ISO -3 and ISO ). The ASTM standard for Liquefied Petroleum Gases covers four basic types of LPG for use in applications such as domestic and industrial heating and as engine fuels. The CEN standard EN 589 "Automotive Fuels LPG Requirements and test methods" covers the use of LPG as vehicle fuel. There are also other standards for automotive LPG. (Rehnlund ). The latest version is EN 589: + A1:. The selected properties and requirements for LPG are listed in Table 1.
Table 1. Examples of properties and requirements for LPG. Complete requirements and standards are available from the respective organizations.
LPG examplea
Standard example
Formula
x%, C3H8
x%, C4H10
Molecular weight, g/mol
44 58
Carbon/hydrogen/oxygen, wt-%
82 83/17 18/0
Applicable compression ratios
11 13
Density, liquid at 20 °C, kg/dm3
0.5 0.58
Boiling point, °C
-42 - -0.5
Research octane number (RON)
94 112
Motor octane number (MON)
89 98
89.0
Blending vapor pressure at 20 °C, kPa*
210 810
LHV heating value, MJ/kg
44 46
LHV heating value, MJ/I
23 26
HHV, MJ/kg
48 50
Heat of vaporization at 20 °C, kJ/kg
358 372
Self-ignition temperature, °C
365 470
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Ignition limits, fuel in air, vol-%
Stoichiometric air to fuel ratio
15.4
Total dienes, %(m/m)
0.5
1,3-Butadiene, %(m/m)
0.09
Propane content, %(m/m)
>20
Hydrogen sulphide
negative
Total sulfur content, mg/kg
30
Copper strip corrosion (1 h at 40 °C)
Class 1
Evaporation residue, mg/kg
60
Vapor pressure gauge at 40 °C b
Water content
pass
Odor
Unpleasant and distinctive at 20% LFL
a Varying butane/propane ratio, e.g. 70% propane & 30% butane to 100% propane [IEA ].
For some parameters only separate data for 100% propane and 100% butane have been found.
b In addition, min. 150 kPa for several grades at different temperatures.
Compatibility
Engines
Similar to natural gas, LPG forms easily a homogenous mixture with air. This combined with the relatively simple chemical structure of LPG, it burns cleanly and is well-suited for spark-ignition engines. For compression ignition (diesel) engines, LPG is not suitable as the sole fuel.
LPG vehicles are available as OEM vehicles and as retrofit vehicles. Generally OEM vehicles perform better than retrofit vehicles. LPG is used mostly in bi-fuel vehicles, which start on gasoline. Spark ignition engines using gasoline can be converted to LPG or bi-fuel engines quite easily by changing the fuel system or adding a parallel fuel system for LPG. Liquid or gaseous LPG is sequentially injected in the inlet ports of an engine. The LPG-kit can be implemented in nearly all petrol cars. The advanced LPG vehicles have lambda control, which enables good catalyst performance. (Verbeek et al. ).
In spark ignition engines, similar compression ratios are typically used with LPG as with gasoline, even though the octane number of LPG (112 for propane, 94 for butane) is higher than that of gasoline. This is due to the fact that the combustion temperature is higher when LPG is used and this lowers the knock limit especially at high engine loads. Exceptions to this are the engines in which LPG is injected in liquid form. In bi-fuel cars, the upper limit for compression ratio is restricted by gasoline. Efficiency of LPG engines is similar to gasoline engines.
When diesel engines, typically used in buses and trucks, are converted to LPG use, spark-ignition must be added. In addition, compression ratio must be reduced, combustion chamber must be reshaped and, of course, the whole fuel system must be replaced. It is, however, also possible to use LPG in diesel engines as an auxiliary fuel similar to methane. In so called gas-diesels, diesel is needed as ignition fuel and gas can be the main fuel. Gas-diesel engines work on the diesel process and energy efficiency is good. Dual fuel gas-diesel is more complicated and more difficult to control on transient operation than spark ignited gas engines.
Infrastructure
A major difference between conventional fuels and LPG is the storage, as LPG is gaseous at room temperatures and atmospheric pressure. Thus a pressurized storage tanks are needed both in the fuelling stations and in vehicles.
Compared to natural gas, distribution of LPG is simpler and fuelling stations are significantly cheaper due to the fact that LPG is liquid already at moderate pressures. For fuelling stations, LPG is usually transported by tank trucks, which have a pressure less than 25 bar. In vehicles, fixed pressure tanks with pressure levels typically in the range of 515 bar are used (with the safety valve set to 25 bar). Due to pressure-proof structure, LPG tanks are somewhat more expensive, heavier and require more space than gasoline or diesel tanks.
The needed pressure is, however, only around one tenth of that of needed for compressed natural gas. The volumetric energy content of LPG is lower than that of gasoline or diesel (around 70% of that of diesel). In addition, the diesel process is also more efficient than the otto cycle. Therefore, the volume of LPG tanks in vehicles has to be about twice as big as those of diesel vehicles for covering the same travel distance.
Exhaust emissions
Certification and emission requirements of LPG vehicles vary (Verbeek ). Fuel consumption and CO2 emissions are typically the same or little bit lower with the LPG fuel than with gasoline. Compared to diesel engines, LPG engine is 1015% less efficient when it is operating at its optimal range. In practice the share of partial load is dominating, therefore "the real world" difference compared to diesel can be higher.
In a study by Tasic et al. () emissions with gasoline and LPG were compared using a modern Opel Zafira with four cylinder cc Ecotec engine as the test vehicle. It had been converted with a Landi Renzo kit to run also with LPG. The results showed that the emissions were clearly lower with LPG than with gasoline. According to TNO's (Dutch Organization for Applied Scientific Research) measurements, regulated emissions of OEM equipped LPG vehicles are generally equivalent or lower than those of gasoline fuelled vehicles (Figure 2, Table 2, Verbeek et al. ). Diesel vehicles emitted lower CO, HC, NH3 and CO2 emissions compared to LPG, whereas other emissions from LPG vehicles were lower than for diesel. Particulate matter emissions of diesel vehicles were high when compared with LPG vehicles. Verbeek et al. () studied also unregulated emissions with LPG vehicles (polyaromatic hydrocarbons, aldehydes and individual hydrocarbons). Overall, the human health effects were very low for LPG with a hot engine. LPG bi-fuel vehicles typically start with gasoline. Therefore during the cold start and warming up emission behavior of LPG vehicles resemble that of gasoline vehicles.(Hendriksen , Verbeek ).
Figure 2. The emissions with LPG vehicle compared to gasoline fuelled vehicle. Gasoline = 100%. (Hendricsen ).
Table 2. Example of emissions with gasoline, diesel and LPG fuelled vehicles.
Retrofitted LPG vehicles gave higher emissions than OEM equipped LPG vehicles, although the performance of retrofit kits has improved already in . (Hendriksen , Verbeek ).
Aakko and Nylund () studied different alternative fuels at normal, +5 and -7 °C temperatures. The LPG car in this study was a prototype. The LPG-powered car produced higher CO, HC and NOx emissions than gasoline powered cars (Figure 3). Compared to the diesel fuelled car, LPG showed lower NOx and particulate matter emissions. Formaldehyde emissions were higher for the LPG car than for the gasoline car, but at the same level as with diesel cars. CO, HC and individual hydrocarbon emissions increased substantially at low temperatures when compared to normal temperature, similar to gasoline cars. In the study by Nylund et al. () The LPG car showed low emissions in all conditions when compared to gasoline and diesel cars at that time.
Figure 3. Regulated emissions with diesel (TDI, IDI), gasoline (MPI, G-DI), E85, CNG, and LPG fuels. (Aakko and Nylund ).
References
Aakko, P. and Nylund, N-O. () Particle emissions at moderate and cold temperatures using different fuels. IEA/AMF Task 32. Project report PRO3/P/03. EN 589:+A1:, JRC () Well-to-Wheels analysis of future automotive fuels and powertrains in the European context, WELL-to-WHEELS Report Version 2c, March .
JRC () Well-to-Wheels analysis of future automotive fuels and powertrains in the European context, TANK-to-WHEELS Report version 3, October .
Hendriksen, P. Vermeulen, R., Rijkeboer, R., Bremmers, D., Smokers, R. and Winkel, R. () Evaluation of the environmental performance of modern passenger cars running on petrol, diesel, automotive LPG and CNG, TNO-report 03.OR.VM.055.1/PHE.
Nylund, N-O., Ikonen, M., Lappi, M., Kytö, M., Westerholm, M. and Laurikko, J. () Performance evaluation of alternative fuel/engine concepts -. Final report including addendum of diesel vehicles. VTT Publications 271. ISBN951-38--5.
Rehnlund, B. () IEA/AMF Outlook on standardization of Alternative vehicle fuels Global, Regional and National level. Task 28 Sub task Report, October .
Tasic, T., Pogorevc, P. & Brajlih, T. () Gasoline and LPG exhaust emissions comparison, Advances in production engineering & management, 6 () 2, 87-94, ISSN -.
Verbeek, R., Smokers, G., Kadijk, A., Hensema, G.L.M., Passier, E., Rabé, B., Kampman, I.J. and Riemersma, I. () Impact of biofuels on air pollutant emissions from road vehicles, TNO report, MONRPT- 033-DTS--, June
WLPGA, World LP Gas Association, website http://www.worldlpgas.com (Accessed 15th November )
History
Early years
In Germany kicked off a programme to increase independence from oil imports. The following year in several alternatively fuelled vehicles (three diesel, one each fuelled with methane, LPG and methanol as well as two steam powered) were presented at the international automobile fair in Berlin. By Autogas systems had becoming very popular due to the scarcity of conventional fuels. The supply of LPG was plentiful: As aviation grade and other liquid fuels were being synthesised from lignite, considerable volumes of LPG were co-synthesised. Unlike in other markets, German Autogas consumption was about 50 times higher than what was delivered to households for cooking.
These first generation systems were developed for spark ignition engines and heavy duty diesel engines (dual fuel). The liquefied fuel stored in cylinders or fixed tanks is fed in liquid phase to a converter which vaporises the fuel and regulates the pressure to a set value. The gas is then fed into a mixer located in front of the throttle valve at the beginning of the intake system. The mixer restricts the diameter of the intake accelerating the flow of air at the narrowest point thus locally reducing the pressure. The mixture composition is regulated through the interaction between the regulated pressure and the amount of air flowing through the venturi. Manual adjustments can be made to ensure satisfying operation across the entire speed and load ranges. The system is entirely mechanically operated and has very few moving parts. Slightly more complex systems, relying on a diaphragm to regulate the gas flow depending on manifold pressure were developed in the US.
The technical solutions elaborated in those years set the reference for decades to come. The simple technology spread to other countries and was used in the south of France and in Italy (with both LPG and natural gas). Its main principles are still used today on carburetted engines, like those found on motor scooters or in generators. Even the advent of electronically controlled carburettors and early fuel injection systems did not change the robust and reliable technology.
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