Ignition and Combustion Properties
Any alternative fuel source must be reasonably consistent so that the combustion equipment can be designed to suit its particular combustion characteristics. With the exception of Hydrogen, most alternative fuels have similar ignition and combustion characteristics to existing fuels. For a fuel to be effectively used as an alternative to the conventional fuels it should be comparable to the ignition and combustion properties of conventional fuels. The properties are defined below.
Gross Calorific Value
The calorific value is the measure of the direct energy content of the fuel. The gross calorific value assumes that any condensables, such as water, actually condense. High pressure may be required for reasonable storage volume, but a low pressure is better for distribution as potential leakage is reduced.
Octane Number
The Octane number is the measure of the knock rating of gasoline. It is measured by the percentage by volume of isooctane in a measure of isooctane and heptane. The concept of octane number is based on isooctane having an octane number of 100 and heptane having an octane number of 0. Blends of heptane and isooctane are deemed to have an octane number corresponding to the volume percentage of isooctane in the blend. If the octane number is too low, gas will spontaneously ignite causing knocking. On the other hand, if the flame reaches the end gas zone before buildup of peroxides, the combustion mixture will be without knock.
Flash Point
Flash point is the liquid temperature at which the vapor pressure is sufficient to form a flammable concentration with air above the liquid. This can predict possible fire hazard during transportation, handling, and storage of fuel. Liquids with high flash points are likely to explode if ignited. Liquids below their flash points do not form flammable vapors unless sprayed out under pressure.
Cetane Number
The Cetane number refers to the volume of n-cetane. Fuels with a higher Cetane number have a lower ignition delay period than fuels with a lower Cetane number. In diesel engines, a low cetane number will result in poor performance and fouling of the engine.
Volatility (Vapor Pressure)
Volatility is a liquids tendency to change to the vapor or gaseous form. It is a necessary characteristic of liquid gasoline fuels. The volatility of gasoline affects the performance of the engine in ways such as ease of starting, rate of warm up, vapor lock, carburetor icing, and crankcase dilution. The fuel must be sufficiently volatile to give easy starting, rapid warm up, and adequate vaporization for proper distribution between cylinders. It can’t be too volatile that vapor losses from the gasoline tank are excessive or vapor is formed in the gas line impeding the flow of gasoline. Volatility is directly related to a substances vapor pressure. At a given temperature a substance with a higher vapor pressure vaporizes more readily than a substance with lower vapor pressure.
Auto Ignition Temperature
This is the temperature at which vapor is ignited spontaneously in a confined space. Within a large space, auto ignition temperature becomes lower which causes safety concerns with large storage scale of inflammable fuels.
Table 1 compares the ignition and combustion properties of the four alternative fuel sources that are discussed and conventional gasoline.
Gross Calorific Value
The calorific value is the measure of the direct energy content of the fuel. The gross calorific value assumes that any condensables, such as water, actually condense. High pressure may be required for reasonable storage volume, but a low pressure is better for distribution as potential leakage is reduced.
Octane Number
The Octane number is the measure of the knock rating of gasoline. It is measured by the percentage by volume of isooctane in a measure of isooctane and heptane. The concept of octane number is based on isooctane having an octane number of 100 and heptane having an octane number of 0. Blends of heptane and isooctane are deemed to have an octane number corresponding to the volume percentage of isooctane in the blend. If the octane number is too low, gas will spontaneously ignite causing knocking. On the other hand, if the flame reaches the end gas zone before buildup of peroxides, the combustion mixture will be without knock.
Flash Point
Flash point is the liquid temperature at which the vapor pressure is sufficient to form a flammable concentration with air above the liquid. This can predict possible fire hazard during transportation, handling, and storage of fuel. Liquids with high flash points are likely to explode if ignited. Liquids below their flash points do not form flammable vapors unless sprayed out under pressure.
Cetane Number
The Cetane number refers to the volume of n-cetane. Fuels with a higher Cetane number have a lower ignition delay period than fuels with a lower Cetane number. In diesel engines, a low cetane number will result in poor performance and fouling of the engine.
Volatility (Vapor Pressure)
Volatility is a liquids tendency to change to the vapor or gaseous form. It is a necessary characteristic of liquid gasoline fuels. The volatility of gasoline affects the performance of the engine in ways such as ease of starting, rate of warm up, vapor lock, carburetor icing, and crankcase dilution. The fuel must be sufficiently volatile to give easy starting, rapid warm up, and adequate vaporization for proper distribution between cylinders. It can’t be too volatile that vapor losses from the gasoline tank are excessive or vapor is formed in the gas line impeding the flow of gasoline. Volatility is directly related to a substances vapor pressure. At a given temperature a substance with a higher vapor pressure vaporizes more readily than a substance with lower vapor pressure.
Auto Ignition Temperature
This is the temperature at which vapor is ignited spontaneously in a confined space. Within a large space, auto ignition temperature becomes lower which causes safety concerns with large storage scale of inflammable fuels.
Table 1 compares the ignition and combustion properties of the four alternative fuel sources that are discussed and conventional gasoline.
Table 1
*(at 273 K and standard atmospheric pressure)
** at 100 degrees F
· Biodiesel has the same ignition and combustion properties as conventional diesel fuel, but its higher flash point reduces explosion hazard.
· Ethanol presents an explosion risk in automotive fuel tanks because the vapor concentration is within the flammable range for long periods of time when the temperature is above the flash point.
· Hydrogen is different in its properties than existing fuels and presents fire and explosion hazards. These properties are different than those exhibited by conventional fuel and the alternative fuels discussed.
*(at 273 K and standard atmospheric pressure)
** at 100 degrees F
· Biodiesel has the same ignition and combustion properties as conventional diesel fuel, but its higher flash point reduces explosion hazard.
· Ethanol presents an explosion risk in automotive fuel tanks because the vapor concentration is within the flammable range for long periods of time when the temperature is above the flash point.
· Hydrogen is different in its properties than existing fuels and presents fire and explosion hazards. These properties are different than those exhibited by conventional fuel and the alternative fuels discussed.