13-11-2012, 04:54 PM
THERMAL ANALYSIS OF A SI-ENGINE USING SIMPLIFIED FINITE ELEMENT MODEL
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Abstract
Simplified finite element model of spark ignition (SI) engine to analyze combustion heat transfer is presented. The 2D model is made up with main features of engine including combustion chamber, valves, manifold, cylinder body, piston head and cooling jacket, all projected at the cross section of the cylinder. The model was discretized with 2D thermal elements of global length 0.001. The fuel type is gasoline. Internal nodal temperature of cylinder body is defined as 2100C to represent occurrence of gasoline combustion. The presence of cooling is modeled by assigning convection coefficient on cooling jacket. Material information and isotropic material properties are taken from published report. The transient heat transfer analysis is done for the instant of combustion. The model is validated by comparing the computed maximum temperature at the piston surface with the published result. The computed temperature gradient at the crucial parts are plotted and discussed. It has been found that the critical component likely suffered from thermal fatigue was the exhaust port near the cylinder head and the materials used to construct the engine parts strongly influenced the temperature distribution in the engine. The model is capable to analyze heat transfer in the engine reasonably and efficiently.
INTRODUCTION
HE automobile engines are major parts that contribute to means of transportation. Researchers in automotive field have emphasized on improvement of engine design since fuel economy and environmental impact from transportation become a global concern. Research focus, to name a few, include introducing new engine material, bio-fuel cell, spark-free operating, hybrid – engine, and electric engine. Methodology in these researches relies on huge amount of experimentations. Although the engine designs have been considerably improved, the fuel economy and environmental impact are still under the subject of research. One reason of being inefficient, high fuel consumption and pollution is that quarter of energy is wasted as heat. In this regard, knowledge of temperature distribution in the engine components is important to tackle the problem. [1 – 3].
RELATED LITERATURE
Since environmental impact from transport sector which mainly utilizes energy from combustion of fossil fuel awakened many people around the world, widespread global initiatives have taken place in the light of this awareness. The development of hybrid electric vehicles and solar cars is one example. The use of alternative fuels such as biofuel, hydrogen fuel cells, and nano energy are among others. However, investment in electric vehicles received failing mark. It is much more expensive than gasoline fueled peers. All are under the subject of research to make them commercially viable. There are still much more to be done to resolve cost and performance issues with these initiatives [1]. Because the world economy is so far dependent on oil in a way that no other energy source can claim, improving SI engine performance still needs to pay attention.
METHODOLOGY
Finite element model of the gasoline SI engine was developed in general-purpose FE code [5]. The model was simplified into 2D geometry with its computational domain comprising one cylinder and its major components including combustion chamber, water jacket, piston head, cylinder head with inlet/outlet manifolds, and intake/exhaust valves. The dimensions and materials of all parts were based on the actual engine of a passenger car. Table 1 shows typical materials used for the engine parts [6, 20]. The properties of these materials were available inside the FE package [5] used.
RESULTS AND DISCUSSION
Simplified finite element model of SI engine provides
promising results. The computed results are comparable with
published reports. Figure 3 depicts the computed overall
temperature distribution across the engine components under
consideration. The computed result was validated by
comparing with the published result of [9] in which the
piston surface temperature due to combustion was reported.
The computed maximum temperature at the piston surface
was about 220C while that in the published data was
223C. The percentage of error is only 1.37%. Therefore the
computed results are acceptable.
Figure 4 shows the temperature gradient plotted for the
node picked up at the exhaust valve surface (Refer to the
contour plot shown together). After combustion had taken
place, heat transfer from combustion to the exhaust valve
surface causes its surface temperature reaches the maximum
of 430C.
CONCLUSION
Finite element model of gasoline spark ignition engine has been successfully developed and simulated to analyze heat transfer during combustion process. The computational analysis had been carried out in order to obtain temperature distribution across the major engine components. The results of finite element analysis have been found to be in good agreement with the published report.
The finite element prediction has indicated that thermal effect in the combustion chamber is influenced by major parameters such as combustion flame temperature, convection of cooling system, and thermal properties of engine component materials. Apparently, the choice of material for part component in the combustion chamber is one of the solutions in order to improve engine performance and efficiency. In addition, the geometry and dimension of the engine parts also can be considered in order to improve the engine performance. The proposed model is simple, yet efficient to analyze thermal condition of the engine component during engine operation and even performance of engine, choice of suitable material improvement of component and design etc.