96
Alternative Fuels for Transportation
spark ignition engine application. However, it is more difficult to fuel diesel
engines with methanol because of its very low cetane number, high heat of
vaporization, and some other physicochemical properties.
Methanol (CH 3 OH) is a simple compound. It does not contain sulfur or
any complex organic compounds. However, diesel fuel is a complex mixture of a large number of hydrocarbons (such as C 3 –C 25 hydrocarbons).
For this reason, its fuel properties can change depending on the proportion of hydrocarbon types used in the fuel mixture. Methanol contains an
oxygen atom so that it is accepted as a partially oxidized hydrocarbon. It
has lower energy content than diesel fuel. Therefore, more fuel is needed
to obtain the same amount of power with that of a diesel-fueled engine.
Its low stoichiometric air–fuel ratio, high oxygen content, and high H/C
ratio may be beneficial to improve the combustion and to reduce the soot
and smoke.
Methanol has higher latent heat of vaporization than diesel fuel so that it
extracts more heat as it vaporizes. Therefore, it can lead to a cooling effect
on the cylinder charge. Since methanol has very low viscosity compared
to diesel fuel, it can be easily injected, atomized, and mixed with the air
introduced into the cylinder. Methanol has poor ignition behavior due to its
low cetane number, high latent heat of vaporization, and high ignition temperature. Therefore, it can cause some increase in ignition delay. However,
ignition improver, such as diethyl ether, can be added to the blended fuel to
compensate for the cetane number (Murayama et al. 1982).
The autoignition temperature of methanol is higher than that of diesel fuel,
which makes it safer for transportation and storage. On the other hand, methanol has a much lower flash point than that of diesel fuel; this is a safety disadvantage. At the same time, an additive should be added to methanol to improve
its lubrication (Adelman 1979; Kowalecwicz 1993; Ristinen and Kraushaar 2006;
Wagner et al. 1979). Due to the low solubility of methanol in diesel fuel, a solvent such as oleic acid and iso-butanol (Huang et al. 2004a, 2004b), 1-dodecanol
(Bayraktar 2008), iso-propanol (Murayama et al. 1982) are added to methanol–
diesel blends. In some studies, a mixer was used to prevent phase separation
(Canakci, Sayin, and Gumus 2008; Sayin et al. 2009). Another disadvantage of
methanol is its corrosivity that is more than diesel fuel on copper, brass, aluminum, rubber, and many plastics. This puts some restrictions on the designs
and manufacturing of engines to be used with this fuel.
For diesel engines, combustion and emission characteristics are influenced by fuel spray characteristics, nozzle geometry, injection pressure,
and so on. Therefore, Yanfeng, Shenghua, and Yu (2007) investigated the
effects of opening pressure, ambient density, and nozzle diameter on penetration length and cone angle of methanol sprays. The results obtained by
these researchers are shown in Figures 4.3 through 4.6. They found that the
methanol spray penetration length increased with the increase of the opening pressure. The methanol spray penetration and tip velocity decreased
quickly with the increase of ambient density; on the contrary, they increased
Alternative Fuels for Transportation
spark ignition engine application. However, it is more difficult to fuel diesel
engines with methanol because of its very low cetane number, high heat of
vaporization, and some other physicochemical properties.
Methanol (CH 3 OH) is a simple compound. It does not contain sulfur or
any complex organic compounds. However, diesel fuel is a complex mixture of a large number of hydrocarbons (such as C 3 –C 25 hydrocarbons).
For this reason, its fuel properties can change depending on the proportion of hydrocarbon types used in the fuel mixture. Methanol contains an
oxygen atom so that it is accepted as a partially oxidized hydrocarbon. It
has lower energy content than diesel fuel. Therefore, more fuel is needed
to obtain the same amount of power with that of a diesel-fueled engine.
Its low stoichiometric air–fuel ratio, high oxygen content, and high H/C
ratio may be beneficial to improve the combustion and to reduce the soot
and smoke.
Methanol has higher latent heat of vaporization than diesel fuel so that it
extracts more heat as it vaporizes. Therefore, it can lead to a cooling effect
on the cylinder charge. Since methanol has very low viscosity compared
to diesel fuel, it can be easily injected, atomized, and mixed with the air
introduced into the cylinder. Methanol has poor ignition behavior due to its
low cetane number, high latent heat of vaporization, and high ignition temperature. Therefore, it can cause some increase in ignition delay. However,
ignition improver, such as diethyl ether, can be added to the blended fuel to
compensate for the cetane number (Murayama et al. 1982).
The autoignition temperature of methanol is higher than that of diesel fuel,
which makes it safer for transportation and storage. On the other hand, methanol has a much lower flash point than that of diesel fuel; this is a safety disadvantage. At the same time, an additive should be added to methanol to improve
its lubrication (Adelman 1979; Kowalecwicz 1993; Ristinen and Kraushaar 2006;
Wagner et al. 1979). Due to the low solubility of methanol in diesel fuel, a solvent such as oleic acid and iso-butanol (Huang et al. 2004a, 2004b), 1-dodecanol
(Bayraktar 2008), iso-propanol (Murayama et al. 1982) are added to methanol–
diesel blends. In some studies, a mixer was used to prevent phase separation
(Canakci, Sayin, and Gumus 2008; Sayin et al. 2009). Another disadvantage of
methanol is its corrosivity that is more than diesel fuel on copper, brass, aluminum, rubber, and many plastics. This puts some restrictions on the designs
and manufacturing of engines to be used with this fuel.
For diesel engines, combustion and emission characteristics are influenced by fuel spray characteristics, nozzle geometry, injection pressure,
and so on. Therefore, Yanfeng, Shenghua, and Yu (2007) investigated the
effects of opening pressure, ambient density, and nozzle diameter on penetration length and cone angle of methanol sprays. The results obtained by
these researchers are shown in Figures 4.3 through 4.6. They found that the
methanol spray penetration length increased with the increase of the opening pressure. The methanol spray penetration and tip velocity decreased
quickly with the increase of ambient density; on the contrary, they increased