Environmental Engineering Reference
In-Depth Information
taBle 25.7
combustion Parameters
dynamic
Injection timing
(°ca btdc)
Ignition delay (°ca)
Peak hrr
(J/ °ca)
location of Peak
hrr (°ca)
Peak Pressure
(bar)
location of Peak
Pressure (°ca)
Biodiesel
From-to
duration
ROME
14
346-353
7
63
356
63
370
LOME
14
346-353
7
66
357
65
370
KOME
13
347-353
6
67
358
66
371
NOME
13
347-353
6
71
359
67
371
MOME
12
348-354
6
73
360
68
371
timing will increase NO
x
emissions (Szybist et al. 2005a). For a given engine at the same operating
conditions, the dynamic injection may not be the same for different fuels; that is, dynamic injection
timing can greatly be affected by fuel properties.
The fuel injection timing can be influenced by the
bulk modulus, i.e., the compressibility of the fuel. The bulk modulus or compressibility is a measure
of how easily a unit of fluid volume can be decreased when increasing the pressure working on it.
A higher bulk modulus indicates that the fluid is relatively incompressible.
The most common type of diesel injector is the pump-line-nozzle injector. In this configuration,
the pressure is applied to the fuel upstream of the injector nozzle. The pressure increases until it
reaches the nozzle opening pressure whereupon injection occurs. If a fuel is less compressible, the
pressure will build more quickly and the fuel will be injected into the combustion chamber earlier
in the compression cycle. Fuels with higher compressibility require a longer time to reach the nozzle
opening pressure, which results in late injection. The presence of oxygen and double bonds in bio-
diesel fuels can increase the bulk modulus, thereby advancing the injection timing (McCormick
et al. 2005). The actual start of injection (SOI) was not experimentally determined in this investiga-
tion. It was calculated based on the dip in the heat release rate diagram.
The bulk modulus of unsaturated biodiesel is higher than that of saturated biodiesel because of
the introduction of double bond carbons. A carbon-carbon double bond introduces a bend in the
structure and thereby distorts the linearity of a carbon-carbon single bond. This bend configuration
may foster intra- or intermolecular interactions in the fuel that reduce compressibility, leading to
earlier injection. It was reported (Szybist et al. 2005b) that the bulk modulus of diesel was 2% lower
than B20 soybean, yielding a shift in the fuel injection timing by 0.1-0.3 crank angles.
When biodiesel is injected, the pressure rise produced by the pump is quicker as a consequence
of its lower compressibility (higher bulk modulus). It also propagates quicker toward the injectors
as a consequence of its higher sound velocity. In addition, higher viscosity reduces leakages in the
pump, leading to an increase in the injection line pressure. Therefore, a quicker and earlier needle
opening is realized with respect to the case of more unsaturated biodiesel fuel. It can therefore be
stated that higher density fuels can have a higher value of bulk modulus because of an increase in
unsaturation.
From the above statements, it can be concluded that the injection timing is advanced for higher
unsaturated and hence higher density fuels. It can be observed from Table 25.7 that high unsaturated
content biodiesel (ROME), which has a higher density, advances the injection timing by approxi-
mately 2° crank angle (CA) compared with relatively low unsaturated biodiesel (MOME). A correla-
tion analysis was done to find out the relationship between dynamic injection timing, density, and
percentage of unsaturation using equation (25.2). The correlation coefficients are shown in Table 25.8.
It can be observed from the table that the dynamic injection timing is highly positively correlated
with percentage of unsaturation and the density of biodiesel fuels. Figure 25.9 illustrates the varia-
tion of dynamic injection timing with percentage of unsaturation. A good r2 value of 0.879 can be
observed from Figure 25.10 between the percentage of unsaturation and dynamic injection timing.
