206
Alternative Fuels for Transportation
TABLe 7.1
LPG Composition (% by Volume) as
Automotive Fuel
Country
Propane
Butane
Austria
50
50
France
35
65
Italy
25
75
Spain
30
70
Sweden
95
5
United Kingdom
100
—
Germany
90
10
Australia
70
30
Source: Saleh, H. E., Fuel, 87, 3031–39,
2008. Reprinted with permission
from Elsevier Publications.
composition, the temperature, and whether it is stored in a liquid or a vapor
state.
LPG and petrol have many similar properties and the good practices appropriate for work on petrol vehicles apply equally to LPG vehicles. The viscosity of liquid LPG (0.15 centi Poise at 15.6ºC) is much lower than water (1 centi
Poise). The value for LPG vapor at atmospheric pressure of 15.5ºC is about
0.08 centi Poise. The flash point of propane is very low, in line with its very
low freezing and boiling points. The autoignition temperature of propane is
nearly twice as high as that of gasoline providing more safety when exposed
to hot surfaces. The stoichiometric air–fuel ratio for propane is only slightly
higher than that of gasoline. As liquid, propane is about 68% of the weight
of gasoline. Its boiling point is very low, which is typical of lower molecular
weight hydrocarbons. Its vapor pressure is much higher than that of gasoline
at 100ºF. The net calorific value of LPG on mass basis is higher but its net
calorific value on volume basis lower than that of gasoline and diesel. The
main difference is that LPG vaporizes more rapidly than petrol, so, as LPG
is stored in the fuel tank and associated fuel lines at elevated pressure (up
to 10 bar g), any leakage will immediately vaporize and disperse. LPG vapor
is highly flammable and mixtures in air of between 2 and 10% will readily
ignite and explode. The rapid expansion resulting from the conversion of liquid propane into a vapor causes severe cooling of the gas. Anyone exposed
to a vapor cloud or in contact with metal surfaces may receive cold burns. LP
gases are 1.5–2 times heavier than air. Leaking LPG will sink to ground level
rather than rise to the atmosphere whereas natural gas or hydrogen goes up.
Table 7.2 shows the physical and chemical properties of LPG as compared to
gasoline and diesel fuels.
The octane rating of propane is higher than that of gasoline. Hence, higher
compression ratios engines can be used to take advantage of the higher
octane number. Though the LPG specifications are specific to the particular
Alternative Fuels for Transportation
TABLe 7.1
LPG Composition (% by Volume) as
Automotive Fuel
Country
Propane
Butane
Austria
50
50
France
35
65
Italy
25
75
Spain
30
70
Sweden
95
5
United Kingdom
100
—
Germany
90
10
Australia
70
30
Source: Saleh, H. E., Fuel, 87, 3031–39,
2008. Reprinted with permission
from Elsevier Publications.
composition, the temperature, and whether it is stored in a liquid or a vapor
state.
LPG and petrol have many similar properties and the good practices appropriate for work on petrol vehicles apply equally to LPG vehicles. The viscosity of liquid LPG (0.15 centi Poise at 15.6ºC) is much lower than water (1 centi
Poise). The value for LPG vapor at atmospheric pressure of 15.5ºC is about
0.08 centi Poise. The flash point of propane is very low, in line with its very
low freezing and boiling points. The autoignition temperature of propane is
nearly twice as high as that of gasoline providing more safety when exposed
to hot surfaces. The stoichiometric air–fuel ratio for propane is only slightly
higher than that of gasoline. As liquid, propane is about 68% of the weight
of gasoline. Its boiling point is very low, which is typical of lower molecular
weight hydrocarbons. Its vapor pressure is much higher than that of gasoline
at 100ºF. The net calorific value of LPG on mass basis is higher but its net
calorific value on volume basis lower than that of gasoline and diesel. The
main difference is that LPG vaporizes more rapidly than petrol, so, as LPG
is stored in the fuel tank and associated fuel lines at elevated pressure (up
to 10 bar g), any leakage will immediately vaporize and disperse. LPG vapor
is highly flammable and mixtures in air of between 2 and 10% will readily
ignite and explode. The rapid expansion resulting from the conversion of liquid propane into a vapor causes severe cooling of the gas. Anyone exposed
to a vapor cloud or in contact with metal surfaces may receive cold burns. LP
gases are 1.5–2 times heavier than air. Leaking LPG will sink to ground level
rather than rise to the atmosphere whereas natural gas or hydrogen goes up.
Table 7.2 shows the physical and chemical properties of LPG as compared to
gasoline and diesel fuels.
The octane rating of propane is higher than that of gasoline. Hence, higher
compression ratios engines can be used to take advantage of the higher
octane number. Though the LPG specifications are specific to the particular