144
Alternative Fuels for Transportation
TABLe 5.3
E85 Emission Reduction in comparison with Gasoline
CO
VOC
PM
NO x
Sulfate
40
15
20
10
80
irritation of the eyes, skin, and the respiratory tract when exposed to acetaldehyde vapors. Ethylene emission amounts were higher for E10 fuel than for
the base fuel at different cruising stages. Ethanol in fuel blends favors combustion of methane, which was mainly produced by the decomposition of
other long-chain hydrocarbons. Moreover, the amounts of butane, pentane,
and hexane emission at each driving mode were decreased to some extent
when E10 fuel was used.
A U.S. EPA420-F-00-035 report on clean alternative fuels, ethanol highlights the emission characteristics of E85. It was reported that E85 produced
fewer total toxic emissions, higher ethanol and aldehydes emissions and lowreactivity hydrocarbon emissions. The comparison emission characteristics
of E85 with conventional gasoline are depicted in Table 5.3.
5.5 Ethanol–Diesel Engine Tests
Combustion in a CI engine is fundamentally different from that in a SI
engine. Unlike a SI engine that has a spark plug to ignite the air-fuel mixture, the CI engine relies on high compression pressure to raise the temperature in the combustion chamber above self ignition temperature (SIT)
of the fuel. However, the fuel does not ignite immediately upon reaching
its SIT. Instead, there is an ignition delay period that starts upon injection of fuel into the combustion chamber and ends after certain preflame
reactions are completed. Figure 5.11 is a conceptual illustration of combustion in a CI engine. All of the fuel that vaporizes and mixes with air during
the ignition delay period combusts very quickly upon ignition, resulting in
the triangular peak of fast energy release known as premixed combustion.
The subsequent, slower combustion rate is limited by the rate at which
fuel vapor and air can diffuse into each other and is known as diffusion
combustion.
The cetane rating of CI engine fuels is closely related to the ignition delay;
that is, a fuel with a high cetane rating has a short ignition delay and vice
versa. Lowering the cetane rating of the fuel lengthens the ignition delay
and thus shifts more combustion into the premixed mode. However, lowering the cetane rating too much can result in ignition being delayed until
after the piston has moved far enough into the power stroke to lower the
chamber temperature enough to prevent ignition entirely. Ethanol has a very
Alternative Fuels for Transportation
TABLe 5.3
E85 Emission Reduction in comparison with Gasoline
CO
VOC
PM
NO x
Sulfate
40
15
20
10
80
irritation of the eyes, skin, and the respiratory tract when exposed to acetaldehyde vapors. Ethylene emission amounts were higher for E10 fuel than for
the base fuel at different cruising stages. Ethanol in fuel blends favors combustion of methane, which was mainly produced by the decomposition of
other long-chain hydrocarbons. Moreover, the amounts of butane, pentane,
and hexane emission at each driving mode were decreased to some extent
when E10 fuel was used.
A U.S. EPA420-F-00-035 report on clean alternative fuels, ethanol highlights the emission characteristics of E85. It was reported that E85 produced
fewer total toxic emissions, higher ethanol and aldehydes emissions and lowreactivity hydrocarbon emissions. The comparison emission characteristics
of E85 with conventional gasoline are depicted in Table 5.3.
5.5 Ethanol–Diesel Engine Tests
Combustion in a CI engine is fundamentally different from that in a SI
engine. Unlike a SI engine that has a spark plug to ignite the air-fuel mixture, the CI engine relies on high compression pressure to raise the temperature in the combustion chamber above self ignition temperature (SIT)
of the fuel. However, the fuel does not ignite immediately upon reaching
its SIT. Instead, there is an ignition delay period that starts upon injection of fuel into the combustion chamber and ends after certain preflame
reactions are completed. Figure 5.11 is a conceptual illustration of combustion in a CI engine. All of the fuel that vaporizes and mixes with air during
the ignition delay period combusts very quickly upon ignition, resulting in
the triangular peak of fast energy release known as premixed combustion.
The subsequent, slower combustion rate is limited by the rate at which
fuel vapor and air can diffuse into each other and is known as diffusion
combustion.
The cetane rating of CI engine fuels is closely related to the ignition delay;
that is, a fuel with a high cetane rating has a short ignition delay and vice
versa. Lowering the cetane rating of the fuel lengthens the ignition delay
and thus shifts more combustion into the premixed mode. However, lowering the cetane rating too much can result in ignition being delayed until
after the piston has moved far enough into the power stroke to lower the
chamber temperature enough to prevent ignition entirely. Ethanol has a very