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West Virginia University's
Alternative Fuel Vehicle Training Program
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INTRODUCTION
GENERAL INFORMATION
LPG (Liquefied Petroleum Gas) is a petroleum derived, colorless gass, typically comprised of primarily either propane, butane, or a combination of the two. LPG has been and continues to be the most widely used alternative motor fuel to gasoline and diesel on a worldwide basis. The acceptance it has enjoyed over the years ensures the place of LPG in clean air scenarios worldwide. Currently, (1992) there are over 500,000 vehicles using propane gas in the United States, most are spark-ignition engines adopted to use either propane or gasoline, and over three million worldwide.
The term propane will be used to refer to LPG.
LPG IN U.S.A
Propane fuel for vehicles is actually a mixture of various hydrocarbons which are gasses at atmospheric pressure and temperature but which liquify at higher pressures. It is one of several mixtures referred to as liquifed petroleum gases (LPG) and is named for the major constituent, propane.
Virtually all U.S, LPG fuel vehicles use propane, thus LPG generally refers to propane gas whose chemical composition is C3H8. LPG is a natural derivative of both natural gas and crude oil. In the United States, approximately 30 % of the LPG is generated during oil refining and 70 % is from natural gas processing and reserves. Domestic production accounts for over 85 % of the LPG supply [10].
In the United States there are more than 10,000 retail propane refueling stations, while in Canada there are about 5,000 stations. Ferrelgass, the second largest propane retailer in the US, operates a fleet of 2,400 vehicles. Of those 2,300 are dedicated propane vehicles, consisting of medium-duty trucks and light-duty pickups. Schimidt reports that Runzheimer International conducted a survey of 118 fleet managers with an average of 449 vehicles in each fleet, propane was by far the most frequently named alternative fuel as shown by the results given in Table 1.
Table 1. Alternative fuels selected by fleet managers ( Percent )
Propane CNG Electric Methanol
Business 86 14 29 0
Utility 71 36 36 14
Government 77 22 33 11
In the United States, the propane industry has attempted to adopt an automotive propane standard known as HD5. [1] Fuel for spark ignition engines must meet certain requirements as set out in the HD5 specification. The standard is not universally observed. Because, the concentration of propane as high as virtually 100 %, to as low as 50 % in certain locations. Much of the remainder of the gas is butane and some other hydrocarbons, both saturated and unsaturated. The utilization of LPG as an automotive fuel varied very widely from one country to another, depending on the cost and availability of the fuel in relation to alternative fuels, notably gasoline and diesel, Table 2.
Table 2. LPG Composition ( % by volume ) as Automotive Fuel in Europe
Country Propane Butane
Austria 50 50
Belgium 50 50
Denmark 50 50
France 35 65
Greece 20 80
Ireland 100 _
Italy 25 75
Netherlands 50 50
Spain 30 70
Sweden 95 5
United Kingdom 100 _
Germany 90 10
Source: Urban 1982
HD5 requires minimum propane content of 90 % and propylene content of less than 5 % (volume basis). The remainder is normally n-butane,with isobutane and butanes also present. The limitation on propylene and other propane unsaturated results its low octane number that means low knock resistance, see Table 3. A second concern with propylene is its photochemical reactivity, which is higher than that of propane. This could be an important factor in formation of smog. Propylene does not occur in LPG obtained from natural gas processing plants but it is found in the LPG resulting from petroleum refinery operations. The minimum propane requirement arises from the need to have sufficient vapor pressure, even at very low temperatures, to deliver fuel to the engine. Vapor pressure of butane is considerably less than that of propane at any given temperature and will not provide adequate pressure for proper equipment operation below about 18-19 C (a minimum of about 0.2 Mpa absolute pressure is required for satisfactory operation of delivery system ).
Table 3. Octane numbers of LPG Components.
Component Research Motor Est. max. ratio comp
propane 111.5 100 11:1
n-butane 95 92 8:1
isobutane 100.4 99 9:1
propylene 100.2 85 7.5:1
n-butane-1 100 80 6.5:1
n-butane-2 101 83 7:1
regular gasoline 92.95 83-86 9:1.
COMPARISON OF PROPANE TO GASOLINE
Performance and drive-ability of propane vehicles is essentially the same as for gasoline vehicles. For propane, the gas displacement effect is 4%, it means that the displacement of air by propane causes reduction in power of 4 % (volumetric efficiency decrease) from an equivalent gasoline counterpart. Gasoline on the other hand, provides evaporotive cooling of the intake air which increases the intake air density and increases the power. Test results show 6 % less power with propane than with gasoline[11]. Propane has a research octane rating of 110 to 120, thus, it resists engine knock better than gasoline (gasoline 87-94 ) allowing a higher compression ratio for the engine, see Figure 1. Propane contains about 5 % more energy per unit mass however the density is nearly 32 % less. The net result is that a liter propane contains 28 % less energy than a litre of gasoline, Table 4. Assuming that an engine is operated on propane and gasoline with equal efficiency, more litres of propane will be consumed to provide equivalent performance. Fortunately, engines generally operate on propane with greater efficiency than on gasoline so that the increase in fuel volume is not as great as the energy comparison suggests. Propane fueled vehicles can achieve the same driving range as a gasoline vehicle by installing a slightly larger tank. Propane use consumes approximately 5 % more fuel for equivalent performance but it costs 15 % less than gasoline. Projections for the next decade, anticipate LPG prices increasing far more more slowly than gasoline[11].
Table 4. Energy Density Comparison
HD5-propane Gasoline
Liquid Density * 499 732
kg / m3
Lower Heating Value 46.3 43.9
Mj / kg
Energy Density 23.1 32.2
Density at 20 C & corresponding sat. pressure
The flammability range for propane is from 2.4 to 9.6 % in air. This compares to a flammability range for gasoline of 1 to 7.6 % in air. The ignition temperature, a identification of anti-knock-property, of propane ( 457 C ) is at the higher end of ignition temperature range for gasoline ( 227 to 471 C ).
Gasoline, being a normal liquid, exhibits very little change over the normal temperature or pressure range. Propane, however, is gas at normal temperatures and pressures. Its physicsal properties depend strongly on the temperature and pressure at which they are being stored. The vapor pressure and liquid density of propane are shown in Fig 2 and Fig 3. The first figure defines the pressure that will exist in a propane fuel tank as the ambient temperature changes. The second figure shows why propane tanks can not be filled completely. Some ullage space must be left in the tank because the liquid volume expands significantly if the tank encounters increasing ambient temperatures. Between 27 and 99 F, for example, the liquid volume expands by 13 %. Due to this, and its lower density, propane requires a 35 % greater storage volume than gasoline. Propane systems have some kind of safety fill stop device to limit tank fills to no more than 80 % to 85 % of tank volume. This allows room for liquid expansion if the temperature rises after the tank is filled. Due to the low viscosity of propane and its storage under pressure, it may leak through small cracks, pumps, seals and gaskets more readily than gasoline.
Propane fuel systems, being totally enclosed and pressure tight, have no of refueling, evaporative, running losses and emissions from the fuel storage system.
It is not sufficient to merely consider mass of exhaust emissions. One must also consider how hydrocarbon emissions and nitrogen oxides combine in the atmosphere to form smog.Smog is a ground level photo chemical ozone phenomenon that is a consequence of emissions and sunshine in a relatively stagnant air basin. The less the rate of reactive organic gas emitted, the les ozone formed in time. No ozone is formed without oxides of nitrogen being present and there is a certain ratio of reactive organic gas to NOx that maximizes the ozone formed per unit mass of each of the reactive organic gases. This ratio is often near this optimum in many urban environments. Reactive organic gas emission controls are important in reducing the mass and virulence of the compounds. NOx controls are important in minimizing the rate at which ozone is formed from any reactive organic gas and instrumental in minimizing the spread and duration ozone episode. Since, smog is not directly emitted, most emission standards and test procedures fail to make a rational connection with the health oriented air quality standard for ozone. Smog forming potential is estimated by calculating the atmospheric reactivities of each of the individual components in a vehicle`s exhaust emission. Such calculations show a clear-cut advantage for propane, Fig 4. Every gallon of gasoline that can be replaced by propane should cut typical exhaust ozone potential by almost one-half. The high ratio of hydrogen to carbon, in propane results in lower production of both toxic carbon monoxide and carbon dioxide, which is the principal greenhouse gas. As a result HC, CO and CO2 emission are lower with propane but NOx emission is higher than with gasoline, Fig 5.
COMPARISON OF PROPANE AND NATURAL GAS
Natural gas vehicle fuel is stored on the vehicle in either the form of compressed natural gas (CNG) stored in cylinders at 2400 to 3600 psi or liquefied natural gas (LNG) stored in tanks at 10 to 30 psi and -163 C
Pipeline quality natural gas is composed of several different gases, of which methane typically accounts for 85 to 95 % . Other hydrocarbons present in natural gas include ethane, propane, some butanes, and trace amounts of other hydrocarbons. Nitrogen, helium, carbon dioxide and trace amounts of hydrogen sulfide, water and odorants are also present. The removal of all CO2, water, hydrogen sulfide and odorants is required for liquefaction, thus LNG does not contain these constituents.
The specific gravity of natural gas relative to air (air = 1.00) is 0.56 to 0.62 depending on gas composition. This indicates that natural gas is lighter than air. In the event of a natural gas leak, the gas will rise and dissipate given open conditions. Propane vapors are heavier than air (specific gravity = 1.5) thus propane will stay low, against the ground and may collect in sewers and other low areas before ultimately dispersing into the air with the aid of wind or ventilation systems.
Natural gas has a research octane rating of about 130 (research octane rating of propane is between 110-120) making it more resistant to engine knock. The anti-knock property is a result of the high ignition temperature, resistance to autoignition, and the relatively low flame speed of natural gas. Methane can be used at higher compression ratios (therefore higher efficiency) than gasoline, propane falls between the two.
The volumetric air- fuel ratio for CNG is 9.6. One cubic meter of fuel is required for every 9.6 cubic meters of air charged. Because the fuel gas must displace air, CNG results in volumetric reduction of about 9.3 % with a corresponding drop in potential power (for propane 4 %).
With respect to almost all defined fuel characteristics, values for propane lie between those for methane and gasoline, Table 5.
The volumetric heating value of the fuels, the volumetric air/fuel ratios, and the volumetric heating values of the stoichometric air/fuel mixtures for different gaseous fuels are shown in Table 6 and Fig 6.
Table 5. Comparative Engine Use Characteristics
Units Methane Propane Gasoline
Autoignition point F 1,000-1,350 874 365
Autoignition point C 538-732 468-494 185
Flammability Limits vol percent 5-15 2.1-9.5 1.4-7.6
Stoichometric A/F kg / kg 17.3 15.3 14.7
Stoichometric A/F m3/m3 9.7 24.6
Research octane number 130 112-125 91-95
Motor octane number 105 97-111 82-88
Relative CO2/Btu 0.76 0.92 1.0
Table 6 illustrates the fact that the power obtainable from an engine is more a function of the amount of air that can be charged than of the heating value of the particular fuel being used.
Table 6 Volumetric Heating Value (LVH) of fuel gases and energy content of
stoichometric mixtures with air
Fuel gas Calorific Value Air required m3/ m3 Energy content Mj/m3
Propane 93.2 24.65 3.63
Octane 233.3 59.50 3.92
Natural Gas 31.7 8.53 3.32
Methane 35.9 9.67 3.36
If compared according to emissions data, both methane and propane engines may emit more NOx. NOX is primarily a function of peak combustion temperature. Gasoline enters the combustion chamber at least partially as liquid. The energy used to vaporize the gasoline results in a lower peak combustion temperature. Methane is also considered to contribute to greenhouse gases because methane is a highly persistent and highly absorbtion gas that collects in the upper atmosphere.Propane is oxidized more quickly and generally does not reach upper atmosphere levels. The non-toxic methane has near zero reactivity in the production of photochemical smog, but propane represents a more reactive exhaust hydrocarbon component than with methane.
COMPARISON OF PROPANE AND METHANE TO GASOLINE
The first line in the Table 7 gives the ratio of the energy of the fluids under typical fuel tank conditions, to that of gasoline. In computng this number, the liquid density of saturated propane at 60 F was used. For methane calculations a tank pressure of 3,660 psi was assumed. From line 2 in Table 7, methane requires 3.85 times as much storage volume as gasoline, propane requires 35 % greater storage volume than gasoline.
Table 7.Comparision of Energy Storage Efficiency
Units Methane Propane Gasoline
Energy density ratio to gasoline 0.26 0.74 1.0
Tank volume for 20 GGE Gal 76.9 27.0 20
Tank weight for 20 GGE Lb 530 89 25
Ullage and heel limits percent 4 15 0
Corrected tank volume for 20 GGE Gal 80 31.8 20
Effective energy density raio 0.25 0.63 1.0
Effective tank volume ratio 4.0 1.59 1.0
Weight of fuel lb 107 115 124
Corrected tank weight, 20 GGE lb 552 * 99 25
Full tank weight lb 659 214 149
* 4 tanks, each 20 gallons, Al / Fiberglass
GGE = Gallons of gasoline equilvalent
Natural gas and propane are generally considered to reduce engine maintenance and wear in spark-ignited engines. The most commonly cited benefits are extended oil change intervals, increased spark plug life, nd extended engine life. Natural gas and propane both exhibit reduced soot formation over gasoline. Reduced soot concentration in the engine oil is believed to reduce abrasiveness and chemical degradation of the oil.Gasoline fueled engines ( particularly carburated engines ) require very rich operation during cold starting and warm up. Some of the excess fuel collects on the cylinder walls, " washing " lubricating oil off wals and contributing to accelerated wear during engine warm up [11]. Gaseous fuels do not interfere with cylinder lubrication.
Gaseous fueled engines are generally considered easier to start than gasoline engines in cold weather. Because they are vaporized before injection to into engine. However, under extremely cold temperatures, there is cold-start difficulty for both propane and narural gas.This is probably due to ignition failure because very difficult ionization conditions, sluggishness of mechanical components. Hot starting can present difficulties for gaseous fueled vehicles, especially in warm weathers. After an engine is shut down, the engine coolant continues to absorb heat from the engine, raising its temperature. If the vehicle is restarted within a critical period after shutdown, ( long enough for the coolant temperature to rise, but before the entire system cools ), the elevated coolant temperature will heat the gas more than normal, lowering its volumetric heating value and density. This would result in mixture enleanment.
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