Mô phỏng động cơ xăng 4 kỳ
Considering the energy crises and pollution problems
today, investigations have concentrated on decreasing fuel consumption by using alternative fuels and
on lowering the concentration of toxic components
in combustion products. Hydrogen is considered an
ideal alternative fuel. The use of hydrogen as an automotive fuel, as a primary or supplementary fuel,
appears to promise a significant improvement in the
performance of a spark ignition engine. Besides being the cleanest burning chemical fuel, hydrogen can
be produced from water (using non-fossil energy)
and, conversely, on combustion forms water again
by closed cycle (Veziroglu et al. 1989; Veziroglu
and Barbir, 1991, 1992). A small amount of hydrogen mixed with gasoline and air produces a combustible mixture, which can be burned in a conventional spark ignition engine at an equivalence ratio
below the lean flammability limit of a gasoline/air
mixture. The resulting ultra-lean combustion produces a low flame temperature and leads directly to
lower heat transfer to the walls, higher engine effi-
ciency and lower exhaust of CO and NOx (Sher and
Hacohen, 1987; Al-Baghdadi, 2000, 2002).
Ethanol is a likely alternative automotive fuel in
that it has properties that would allow its use in
present engines with minor modifications. Alcohol
fuels can be made from renewable resources like locally grown crops and even waste products such as
331AL-BAGHDADI
waste paper or grass and tree trimmings (Morris,
1992). As a fuel for spark-ignition engines, ethanol
has some advantages over gasoline, such as better
anti-knock characteristics and reduction of CO and
UHC emissions. Ethanol fuel has a high heat of
vaporization; therefore, it reduces the peak temperature inside the cylinder and hence reduces the
NOx emissions and increases the engine power (WeiDong et al., 2002; Al-Hasan, 2003; Bang-Quan et al.,
2003).
One of the major areas of development in the internal combustion engine is the development of computer simulations of various types of engines. Their
economic value is in the reduction in time and costs
for the development of new engines and their technical value is in the identification of areas that require specific attention as the design study evolves.
Computer simulations of internal combustion engine
cycles are desirable because of the aid they provide
in design studies, in predicting trends, in serving as
diagnostic tools, in giving more data than are normally obtainable from experiments, and in helping
one to understand the complex processes that occur
in the combustion chamber. In the present work,
a quasi-dimensional model was developed to simulate a 4-stroke cycle of a spark ignition engine fueled
with various types of fuels, i.e. gasoline, hydrogen,
ethanol, and their mixture.
Modeling of The Spark Ignition Engine
(Power Cycle)
The combustion chamber was generally divided into
burned and unburned zones separated by a flame
front (Figure 1). The first law of thermodynamics,
equation of state and conservation of mass and volume were applied to the burned and unburned zones.
The pressure was assumed to be uniform throughout the cylinder charge. A system of first-order ordinary differential equations was obtained for the pressure, mass, volume, temperature of the burned and
unburned zones, heat transfer from burned and unburned zone, and mass flow into and out of crevices.
Burned
zone
R
Heat transfer
(from burned zone)
Heat transfer
(from unburned zone)
Flow into crevices
(from unburned zone)
Flow into crevices
(from burned zone)
Work done
Figure 1. Two-zone thermodynamic model of combustion.
The mass burning rate was modeled by the following equation (Heywood, 1989):
dMb
dt
= Af l
.ρ.ST (1)
The turbulent flame front speed (ST) was modeled by the following equation (Heywood, 1989):
ST = SL.f.
(ρu/ρb)
[(ρu/ρb) − 1]Xmb + 1
(2)
where f is a turbulent flame factor, defined with the
following formula:
f = 1 + 0.0018 × rpm (3)
The laminar flame front speed for mixtures of hydrocarbon and/or alcohol with hydrogen, air, and
residual gas was modeled by the following equation
(Yu et al., 1986):
SL = SLo.
where YH2 is an indication of the relative amount of
hydrogen addition, which was defined by the following formula:
