230
Alternative Fuels for Transportation
range of 10:1–13:1. At higher compression ratios, higher thermal efficiencies
can be achieved. Supercharged NG engines provide higher performance
and emission characteristics. In order to measure the knock resistance of
NG blends, a methane number (MN) scale is used. For this purpose, reference fuel mixtures of methane and hydrogen are used. Pure methane has a
MN of 100 and hydrogen has a MN of zero. The relationship between MN
and Motor Octane Number (MON) was reported by Kubesh, King, and Liss
(1992) and CONCAWE (1995).
Motor Octane Number MON = 0.679 × MN + 72.32
Methane Number MN = 1.445 × MON – 103.42
with R 2 in each case greater than 0.95.
Due to its low-cetane number, in general, for satisfactory use in a diesel cycle
engine it must be used in conjunction with a high-cetane fuel to provide ignition in dual fuel mode operation. In dual fuel mode, diesel is injected into
a premixed charge of air and NG. Natural gas is used in stationary engine
applications in dual fuel mode.
8.2.2 Flammability
Natural gas has a very high-autoignition temperature than that of gasoline/
diesel. CNG is likely to be safer than gasoline or diesel, since its low density,
high-ignition temperature (540ºC) and high-flammability limits give the gas
a high-dispersal rate and make it less likely to ignite. The minimum spark
energy required for methane ignition is much higher than that required for
other petroleum products. Hence, NG engines require high-performance
ignition systems that can generate more than three times the energy required
by gasoline engines (30–40 mJ).
It also has a wider flammability range that allows the engine to operate
lean mixtures also. However, there is a marginal reduction in engine performance as the gas enters the engine through the intake manifold—it reduces
the engine volumetric efficiency. Moreover, methane combustion is very slow
in nature and hence it causes deterioration in performance due to increase in
heat losses to the combustion chamber walls. However, this can be overcome
by creating turbulence.
8.2.3 Wobbe index
Wobbe index is actually the correct representation of the heating value of
NG arriving, from the gas line, at the orifice where a burner is located. It is
the bulk property that accounts for many gas components. Wobbe index or
Wobbe number of a gaseous fuel is determined by its composition. It is the
ratio of higher calorific value of gas divided by the square root of specific
gravity. It can be calculated as
Alternative Fuels for Transportation
range of 10:1–13:1. At higher compression ratios, higher thermal efficiencies
can be achieved. Supercharged NG engines provide higher performance
and emission characteristics. In order to measure the knock resistance of
NG blends, a methane number (MN) scale is used. For this purpose, reference fuel mixtures of methane and hydrogen are used. Pure methane has a
MN of 100 and hydrogen has a MN of zero. The relationship between MN
and Motor Octane Number (MON) was reported by Kubesh, King, and Liss
(1992) and CONCAWE (1995).
Motor Octane Number MON = 0.679 × MN + 72.32
Methane Number MN = 1.445 × MON – 103.42
with R 2 in each case greater than 0.95.
Due to its low-cetane number, in general, for satisfactory use in a diesel cycle
engine it must be used in conjunction with a high-cetane fuel to provide ignition in dual fuel mode operation. In dual fuel mode, diesel is injected into
a premixed charge of air and NG. Natural gas is used in stationary engine
applications in dual fuel mode.
8.2.2 Flammability
Natural gas has a very high-autoignition temperature than that of gasoline/
diesel. CNG is likely to be safer than gasoline or diesel, since its low density,
high-ignition temperature (540ºC) and high-flammability limits give the gas
a high-dispersal rate and make it less likely to ignite. The minimum spark
energy required for methane ignition is much higher than that required for
other petroleum products. Hence, NG engines require high-performance
ignition systems that can generate more than three times the energy required
by gasoline engines (30–40 mJ).
It also has a wider flammability range that allows the engine to operate
lean mixtures also. However, there is a marginal reduction in engine performance as the gas enters the engine through the intake manifold—it reduces
the engine volumetric efficiency. Moreover, methane combustion is very slow
in nature and hence it causes deterioration in performance due to increase in
heat losses to the combustion chamber walls. However, this can be overcome
by creating turbulence.
8.2.3 Wobbe index
Wobbe index is actually the correct representation of the heating value of
NG arriving, from the gas line, at the orifice where a burner is located. It is
the bulk property that accounts for many gas components. Wobbe index or
Wobbe number of a gaseous fuel is determined by its composition. It is the
ratio of higher calorific value of gas divided by the square root of specific
gravity. It can be calculated as