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Alternative Fuels for Transportation
30% at very lean conditions. There are many reports about the effects of the
LPG on the engine performance and emission characteristics. However, it is
apparent that very little information is available about the cyclic variability
when using LPG as a fuel. There are some studies about the lean operating
performance of LPG-used SI engines. The studies showed that ignition timing strongly affected the overall engine lean misfire limit. The experimental
results show that as the fuel/air equivalence ratio leaned, the MBT ignition
timing needed to be advanced, where lean mixtures required advancing in
ignition timing to provide more time for reaction due to the excess presence
of oxygen in the fuel/air mixture. The engine lean misfire limit increases as
the engine speed increases, with the same effect of preheating inlet air temperature, and as the engine load increases, the lean misfire limit increases. It
has been also shown that the higher laminar flame speed of LPG and good
mixing of gaseous fuels with air causes a decrease in cyclic variations, and
higher H/C ratio of LPG decreases the engine emissions.
LPG and other gaseous fuels have common properties that provide them
some advantages and disadvantages relative to gasoline. Before discussing its
usability for engines, its properties are compared with those of other engine
fuels in Table 7.1. If the properties of propane are compared with those of
gasoline and alcohols, the following benefits and/or shortcomings can be
expected when it is used as the SI engine fuel.
Propane has lower density and stoichiometric fuel–air ratio than gasoline,
and thus, it could reduce the specific fuel consumption and exhaust emissions. If a propane fueled SI engine operates at the same equivalence ratio as
a similar gasoline fueled engine, higher effective power could be expected
due to the higher calorific value of propane. However, as will be explained
below, this advantage may be balanced by decreasing volumetric efficiency.
On the other hand, propane can be used at higher compression ratios due to
its higher octane number, and as a consequence of this property, engine performance; that is, engine power and thermal efficiency, would be improved.
The above mentioned properties of propane make it an attractive alternative fuel for SI engines. The most important drawback of this fuel is that it
reduces the engine volumetric efficiency and consequently the fresh charge
mass, which is mainly due to its rising inlet temperature and its entering the
intake system in the gaseous state. This problem can be removed by cooling;
that is, offsetting the heat in the inlet manifold.
It has been found that in the case of using LPG in SI engines, the burning
rate of fuel is increased, and thus, the combustion duration is decreased. As
a consequence of this, the cylinder pressures and temperatures predicted
for LPG are higher than those obtained for gasoline. The maximum cylinder pressures and temperatures predicted for LPG are higher. This may
cause some damages on engine structural elements. LPG reduces the engine
volumetric efficiency and, thus, engine effective power. Furthermore, the
decrease in volumetric efficiency also reduces the engine effective efficiency
and consequently increases specific fuel consumption. It has been also found
Alternative Fuels for Transportation
30% at very lean conditions. There are many reports about the effects of the
LPG on the engine performance and emission characteristics. However, it is
apparent that very little information is available about the cyclic variability
when using LPG as a fuel. There are some studies about the lean operating
performance of LPG-used SI engines. The studies showed that ignition timing strongly affected the overall engine lean misfire limit. The experimental
results show that as the fuel/air equivalence ratio leaned, the MBT ignition
timing needed to be advanced, where lean mixtures required advancing in
ignition timing to provide more time for reaction due to the excess presence
of oxygen in the fuel/air mixture. The engine lean misfire limit increases as
the engine speed increases, with the same effect of preheating inlet air temperature, and as the engine load increases, the lean misfire limit increases. It
has been also shown that the higher laminar flame speed of LPG and good
mixing of gaseous fuels with air causes a decrease in cyclic variations, and
higher H/C ratio of LPG decreases the engine emissions.
LPG and other gaseous fuels have common properties that provide them
some advantages and disadvantages relative to gasoline. Before discussing its
usability for engines, its properties are compared with those of other engine
fuels in Table 7.1. If the properties of propane are compared with those of
gasoline and alcohols, the following benefits and/or shortcomings can be
expected when it is used as the SI engine fuel.
Propane has lower density and stoichiometric fuel–air ratio than gasoline,
and thus, it could reduce the specific fuel consumption and exhaust emissions. If a propane fueled SI engine operates at the same equivalence ratio as
a similar gasoline fueled engine, higher effective power could be expected
due to the higher calorific value of propane. However, as will be explained
below, this advantage may be balanced by decreasing volumetric efficiency.
On the other hand, propane can be used at higher compression ratios due to
its higher octane number, and as a consequence of this property, engine performance; that is, engine power and thermal efficiency, would be improved.
The above mentioned properties of propane make it an attractive alternative fuel for SI engines. The most important drawback of this fuel is that it
reduces the engine volumetric efficiency and consequently the fresh charge
mass, which is mainly due to its rising inlet temperature and its entering the
intake system in the gaseous state. This problem can be removed by cooling;
that is, offsetting the heat in the inlet manifold.
It has been found that in the case of using LPG in SI engines, the burning
rate of fuel is increased, and thus, the combustion duration is decreased. As
a consequence of this, the cylinder pressures and temperatures predicted
for LPG are higher than those obtained for gasoline. The maximum cylinder pressures and temperatures predicted for LPG are higher. This may
cause some damages on engine structural elements. LPG reduces the engine
volumetric efficiency and, thus, engine effective power. Furthermore, the
decrease in volumetric efficiency also reduces the engine effective efficiency
and consequently increases specific fuel consumption. It has been also found