Liquefied Petrol Gas (LPG) is a widely used alternative fuel. It has substantial reserves due to its dual origins from natural gas processing and crude oil refining. Liquefied Petrol Gas (LPG) powered passenger cars have about 10% lower tailpipe CO2 emission than comparable gasoline powered cars. When compared to a diesel car, there is no significant CO2 emission reduction per km driven; however, LPG powered vehicles do have substantially lower NOx emissions than diesel powered vehicles.
Liquefied Petroleum Gas (LPG) (also called Autogas) is a liquefied mixture of propane and butane. It is an inevitable by-product of the crude oil refining process and of natural gas processing. In natural gas processing, the natural gas is cleaned off heavy hydrocarbons such as propane and butane before distribution. About 60% of global LP Gas supply comes from natural gas processing (WLPGA, 2011). In crude oil refining, LPG is a by-product of the refining process. LPG is gaseous at room temperature and changes to a liquid when compressed at moderate pressure or chilled. The chemical composition of LPG can vary, but is usually made up of butane and propane with a 30-99% propane mix.
LPG can be used as an alternative fuel in vehicles, and may lead to lower vehicle maintenance costs, lower emissions, and fuel costs savings compared to conventional gasoline and diesel.
An important difference between LPG and conventional vehicles is the method of fuel storage. LPG is gaseous at room temperature and a pressurized storage tank is required at the fuelling station as well as in the vehicle. In the vehicle, liquid LPG is converted to vapour in the vehicle’s engine. The vapour is then mixed with filtered air before it enters the combustion chamber where it is burned to produce the energy to propel the vehicle. There are also liquid propane injection engines, which do not depend on vaporizing the LPG, but instead burn the liquid fuel (US DOE, 2010).
LPG has a very high energy content per tonne compared to most other oil products and burns easily in the presence of air. This has made LPG a popular fuel for domestic heating and cooking, commercial use, agricultural and industrial processes, and increasingly as an alternative transport fuel (WLPGA, 2011).
LPG can be used in dedicated LPG vehicles or in vehicles converted from gasoline use. The availability of dedicated LPG models is limited, and most LPG powered passenger vehicles have a modified combustion engine. Such converted vehicles normally operate in bi-fuel mode, using either LPG or regular gasoline. Older bi-fuel passenger cars often had poor performance because the engine was optimized for gasoline and not for LPG, resulting in relatively high NOx emissions and no tailpipe CO2 emission reduction. However, modern bi-fuel vehicles use electronically controlled gas injection systems lowering the NOx and CO2 emissions substantially. However, bi-fuel cars rarely achieve the full emission benefits because a compromise in engine tuning is required for the two fuels. The advantage of a bi-fuel vehicle is that the car owner is less dependent on a LPG refuelling infrastructure with sufficient coverage. In areas, where LPG is not available, regular gasoline can be used. A drawback of the bi-fuel vehicle is that two fuelling tanks need to be available, lowering the available space in the vehicle. Figure 1 shows an LPG tank in the position where normally the spare tire is located, saving space in the trunk of the vehicle.
LPG also has a lower energy density than gasoline, and therefore requires more storage volume for an equivalent drive range. [media:image:1]
The safety risk of LPG is higher than of other (alternative) fuels, although LPG liquefies at moderate pressures (just over 10 bar). In Europe, the number of accidents related to LPG vehicles is very low, however, the extent of an accident can be serious. Pressurized storage of liquid can give rise to a so called Boiling Liquid Expanding Vapour Explosion (BLEVE). These type of explosions occur when the storage vessels rupture. Moreover, LPG has a higher density than air which means that in case of a leak there can be a build-up of the gas around the fuelling station (Roeterdink et al, 2010). In the past, most accidents have happened during the transport of LPG. These safety problems make it generally necessary to locate an LPG fuelling station well outside an urban location.
The use of LPG as a transport fuel is a well developed technology, and LPG is a widely used alternative fuel. According to an industry organization, the European LPG Association (AEGPL) (AEGPL, 2009), there are more than 7 million LPG powered vehicles in Europe, and LPG accounts for about 2% of the fuel mix of passenger cars in Europe. AEGPL (2009) also estimates that LPG could account for 10% of Europe’s passenger car fuel mix by 2020.
Worldwide there are more than 17.4 million Autogas vehicles and over 57,000 refuelling sites. Figure 2 below shows a summary of the largest markets across the globe for LPG vehicles. LPG also has substantial reserves because of its duel origins of natural gas processing and crude oil refining. Demand between 2000-2010 increased by 60%, but remains concentrated in a small number of markets with the top 5 countries accounting for 53% of world consumption in 2010. [media:image:2]
The primary reason why governments in many countries actively encourage Autogas use is the environment. Autogas is shown in many studies to perform better environmentally than its gasoline and diesel counterparts (WLPGA, 2011).
The energy efficiency of modern LPG powered passenger cars is comparable to the energy efficiency of gasoline powered vehicles (JRC,2007). LPG has a lower carbon content (i.e. higher hydrogen-carbon ratio) than gasoline, leading to about 10-12% lower tailpipe CO2 emission than for gasoline powered cars (RDW,2010). The tailpipe CO2 emission of LPG powered vehicles are higher than from comparable diesel powered cars.
However, replacing diesel powered passenger cars by modern LPG powered passenger cars can greatly reduce NOx, CO2 and particulate matter (PM) emissions (WLPGA, 2011). Figure ? below shows how Autogas performs well with respect to regulated emissions relative to Gasoline and Diesel under the ‘Euro regulations’ (developed by the United Nations Economic Commission for Europe (UNECE) and uniformly applied across Europe). The Euro 5 regulation is currently in force.
A study by Hanschke et al. (2009) compared Well-to-Wheel (WTW) CO2 emissions of LPG vehicles with gasoline and diesel powered vehicles. Well-to-Wheel (WTW) CO2 emissions also consider the energy which is needed to extract and refine the fuels. This study concludes that the corresponding WTW CO2 emissions per MJ of energy content are lower for LPG than for gasoline and diesel (see Table 2). As the energy efficiency of an LPG powered car is comparable to the energy efficiency of a gasoline powered car, the WTW CO2 emissions per km are calculated to be about 11% lower for LPG than for a gasoline powered car. The WTW CO2 emissions per km of an LPG powered vehicle are comparable than for a diesel powered car. This mainly due to by the higher vehicle efficiency of the diesel powered car.
WTW CO2emission [g/MJ]
WTW CO2emission [g/km]
Table 1: Comparison of Well-to-Wheel (WTW) CO2 emissions (Hanschke et al, 2009).
Other benefits of using LPG as an automotive fuel include lower maintenance costs and a longer engine life-time. This is due to LPG’s high octane rating and low carbon and oil contamination, which puts less pressure on the engine.
Using LPG may also increase energy security as it may be available domestically or, if it’s imported, may diversify a country’s fossil fuel sources.
The initial infrastructure costs required to expand the sales of LPG in the transport sector, are mainly determined by the investment costs of the LPG refuelling infrastructure. The costs incurred relate mainly to service-station storage and dispensing facilities. Autogas, however, generally makes use of existing service-station infrastructure for distribution of conventional fuels therefore additional costs for Autogas are relatively low compared to other alternative fuels e.g. the cost of installing a tank, pumo and metering equipment for autogas alongside existing gasoline or diesel facilities is about one third the cost for equivalent CNG (Compressed Natural Gas) capacity (WLPGA, 2011).
The initial infrastructure costs required to expand the sales of LPG in the transport sector, are mainly determined by the investment costs of the LPG refueling infrastructure. In Europe the purchase cost for an LPG vehicle is estimated to be € 1.500 to € 2.500 higher than for a comparable fossil fuel powered car (Roeterdink et al, 2010). However, fuel costs are considerably lower . In 2008 the average price for LPG was estimated to be only about half the price of gasoline. Most of the price difference in Europe is due to lower fuel taxes.
Vehicle-conversion costs vary between countries depending upon equipment and local labour costs. The costs are also dependent on the type of car with older cars being less expensive to convert due to less sophisticated engines to adapt. Costs vary from around $500 in developing countries to $3,500 in the US (WLPGA, 2011).
Depending on the (road) tax regimes for LPG vehicles and on fuel prices, the financial breakeven point for the initial additional investment for an LPG vehicle can be several years.
AEGPL (2003). Automotive LPG, the practical alternative motor fuel
AEGPL (2009). Autogas in Europe, The Sustainable Alternative – An LPG Industry Roadmap. Available at http://www.aegpl.eu/media/16300/autogas%20roadmap.pdf 
Hanschke, C.B., M.A. Uyterlinde, P. Kroon , H. Jeeninga, H.M. Londo (2009). Duurzame innovatie in het wegverkeer. Een evaluatie van vier transitiepaden voor het thema Duurzame mobiliteit. Energy research Centre of the Netherlands, ECN-E—08-076, 2009.
JRC (2007). Well to Wheels analysis of future automotive fuels and power trains in the European context. Tank to Wheels Report Version 2c EUROCAR, CONCAWE Institute for Environment and Sustainability of the Joint Research Centre (JRC), Ispra, Italy, 7 March 2007.
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US DOE (2010). What is a propane vehicle? US Department of Energy, Alternative Fuels and Advanced Vehicles Data Center. Available at http://www.afdc.energy.gov/afdc/vehicles/propane_what_is.html 
World LP Gas Association (WLPGA) (2005). Health Effects and Costs of Vehicle Emissions. Available at http://www.worldlpgas.com/uploads/Modules/Publications/healtheffects_medres.pdf 
World LP Gas Association (WLPGA) (2011). Autogas Incentive Policies: a country-by country analysis of why and how governments promote Autogas & what works. Available at http://www.worldlpgas.com/uploads/Modules/Publications/autogas-incentive-policies-2012-updated-july-2012.pdf