The processes that occur during diesel combustion are still somewhat mysterious, in spite of extensive attempts at photography and other, more advanced, optical diagnostics. The environment in the diesel combustion chamber appears designed
to resist detailed investigation. It is small, hot, and subject to high pressure and intense vibration. When windows are installed
in the chamber to observe the combustion, soot blocks the passage
of light and quickly deposits on the windows, obscuring further investigation. The combustion involves gas, liquid, and solid
phases as well as complex physical processes and chemical reactions. In spite of this complexity, researchers generally agree about the sequence of processes that occurs in the chamber.
Diesel combustion is the process that occurs when a fuel blend, chosen for its readiness to auto-ignite, is injected into a volume of turbulent air that has been compressed to a high temperature and pressure. The fuel does not ignite immediately.
A time period elapses, called the ignition delay, during which the fuel must vaporize, mix with air, and undergo preflame chemical reactions that produce the chemical species necessary for spontaneous ignition. Because the air temperature is above the thermodynamic critical point of many of the fuel components, vaporization takes place very quickly. In some engines, vapori-zation is complete within a few millimetres of the injection nozzle.After sufficient time has elapsed, ignition will occur spon-taneously in regions of fuel-air mixture that have fuel-air ratios close to the stoichiometric, or chemically correct mixture. Combustion proceeds very rapidly because of the backlog of prepared or nearly prepared fuel-air mixture formed during the ignition delay period. The rapidly rising temperature and pressure in the cylinder accelerate the combustion in an uncontrolled manner until the backlog is depleted. The fuel in the spray core is still too rich to burn, and the fuel in the periphery of the spray is too lean to burn, so combustion slows down and is controlled
by the rate at which the air is entrained and a combustible mixture formed. The first phase of combustion, where prepared fuel burns quickly, is known as the premixed phase and the second phase is known as the diffusion or mixing-controlled phase. The rate of burning during the mixing-controlled phase depends on the air motion and fuel spray momentum. The burning rate starts quite high because there is considerable excess air and the fuel spray entrains air rapidly. After the end of fuel injection, particularly at high loads when there is not as much excess air as with light loads, the burning rate decreases gradually
to zero. Each of the important features of the diesel combustion process will be discussed in this chapter. First, some basic combustion
theory will be presented. Then, the ignition delay and fuel-air mixing processes will be discussed. Finally, combustion system design issues will be presented before moving to a discussion of diesel fuels and their effect on diesel combustion, emissions,
and performance.
Basic combustion theory
Combustion is the chemical reaction that converts the energy
contained in the fuel to the internal energy of product gases.
The internal combustion engine serves as a mechanism to convert
this internal energy into useful work. This section discusses the
basic chemical reactions that relate to diesel combustion and
how the reactions associated with chemical equilibrium and
chemical kinetics influence combustion. A brief discussion of
hydrocarbon combustion is also included.
4.1.1.1 Stoichiometric combustion
Although diesel engines never intentionally run with the chemi-
cally correct, or stoichiometric, amount of air, it is useful to
compare the actual fuel to air ratio to the stoichiometric amount
as a measure of air utilization. Since diesel fuel composition
varies considerably, it is desirable to have a laboratory analysis
of the fuel that gives its composition. Method D5291 from the
American Society for Testing and Materials (ASTM) can give
the percentages of hydrogen and carbon in the fuel. If an average
molecular weight is also available, an equivalent hydrocarbon
molecule can be determined. Universal Oil Products Method
375-86 can be used to estimate the fuel molecular weight using
the fuel viscosity, density, and distillation curve1
.
A typical No. 2 diesel fuel will have a molecular weight of
183, a carbon mass fraction of 86.57%, and a hydrogen mass
fraction of 13.43%. For a hydrocarbon molecule of the form
CxHy, x and y need to be determined to match this measured
data. Because MWcarbon =12.0111 and MWhydrogen = 1.00797,
then
412.0111) + XL00797) = 183 (4.1)
In 1 kg of fuel, there is 0.8657 kg of carbon/12.0111 = 0.0721
kmol of carbon and 0.1343 kg of hydrogen/1.00797 = 0.1332
kmol of hydrogen. Thus,
ylx = 0.1332/0.0721 (4.2)
This system of two equations and two unknowns can be solved
to get x= 13.2 and y = 24.4. The equivalent diesel fuel molecule
1
s
C13.2^24-4.
The stoichiometric reaction for this fuel can be obtained by
atom balances to be:
Ci32H244 + 91.90 (0.21 O2 + 0.79 N2) => 13.2 CO2
+ 12.2 H2O+ 72.60 N2 (4.3)
The molar air-fuel ratio is 91.90 kmol air/kmol fuel, which can
be converted to a mass basis as follows:
91 9Q kmol air x 28.97 kg air ^ kmolfuel
kmol fuel kmol air 183 kg fuel
=
14
-
55m <4
-
4
kg fuel >
The equivalence ratio is defined as the actual fuel-air ratio
divided by the stoichiometric fuel-air ratio. If an engine using
the fuel described above were running with a 30:1 air-fuel
ratio, then its equivalence ratio would be
0=|^= 1/30 =Q485
FIX) stoich 1/14.55
This ratio indicates that the engine is using less than half of the
air supplied for combustion. Diesel engine air utilization is
generally limited to 0 < 0.7. Higher equivalence ratios cause
excessive smoke emissions. This can make it difficult for naturally
aspirated diesel engines to develop as much power per unit of
displacement as spark ignited engines.
المشاركات
ليست هناك تعليقات:
اضافة تعليق