01-06-2013, 01:06 PM
COUPLED FIELD ANALYSIS OF EXHAUST VALVE USING ANSYS
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Abstract
Exhaust valve of an Internal Combustion engine is one of the most critical parts. It is the seat of most problems like pre-ignition, run-on etc. Design of valves depends on many parameters, like fluid dynamics of exhaust gas, fatigue strength of valve material, oxidization characteristics of valve material and exhaust gas, behavior of material at high temperature, the configuration of the cylinder head, the coolant flow, the shape of the exhaust port. The most significant factor underlining the performance of a valve is its operating temperature. The importance of temperature can best be appreciated by its affect on the physical properties of the valve material. Most automotive exhaust valves operate in the temperature range of from 1200o F to 1600o F. It is therefore proper care has to be given in selecting the material for the valve operating at high temperatures.
INTRODUCTION
Exhaust valve of an IC engine is one of the most vital parts.design of valves depends on many parameters, strength of the valve material,behaviour of material at high temperature, the pattern of the cylinder head, the shape of the exhaust port the operating temperature of the valve decides its performance. The importance of temperature can best be appreciated by its affect on the physical properties of the valve material. It is therefore proper care has to be given in selecting the material for the valve operating at high temperatures.
VALVE MATERIAL SELECTION
The key difference between intake and exhaust valves is the temperature at which they operate. The exhaust valve is regularly subjected to highly corrosive exhaust gases at temperatures that may exceed 1400 degrees Fahrenheit. In comparison, the intake valves are cooled by the incoming air/ fuel mixture, and do not reach such high temperatures.
WHY VALVES FAIL
Let‟s talk about burning first. Exhaust valves are the ones most likely to burn because they run hotter than the intakes.the intake valves are cooled by the incoming air and fuel,and consequently operate at about 800F.Exhaust valves,on the other hand receive little such cooling and are blasted by the hot combustion gases as they exit through the exhaust port.exhaust valves run at 1200 to 1350F.on average,which makes them much more vulnerable to erosion and burning than intakes. The higher operating temperature requires a tougher alloy, so exhaust valves are usually made of stainless steel or have stainless steel heads (typically 21-2N or 21-4N alloy with a high chromium and nickel content).for heavy duty and gasoline and diesel applications where heat is even more of a problem, a tough new material may be needed on the exhaust valve face to overcome the failure.
ELEMENT GEOMETRY
The element type PLANE 35 2D 6 Node Triangular solid is taken for the transient analysis of the disc brake rotor. The disc brake rotor is assumed to be axisymmetric problem in 2D as the geometry of the rotor is symmetric about its axis. To reduce the computational costs of the analysis, the problem is treated as 2D axisymmetric. for the analyze of any 2D axisymmetric thermal problem, PLANE 35 2D 6 Node Triangular is considered to be the right choice as each of it nodes degree of freedom represent temperature. As the temperature distribution is our concern in the thermal transient analysis, this element type fulfills it.
CONCLUSION
In this study, the steady state thermal analysis of the exhaust valve for the different materials such as 21 4N, Nimonic 80A and Nimonic105A have been performed. ANSYS software is applied to the steady state thermal analysis problem with outer and inner surface temperature as thermal boundary conditions. To obtain the simulation of thermal behavior appearing in different valve material, the basic governing equation for the heat conduction is solved with the initial boundary conditions with thermal conductivity as the property is solved for the two materials.
The second part of the project i.e., structural analysis for the three materials produces excellent result by treating the problem as coupled field analysis. From the structural analysis results for all the three valve materials, its been concluded that the displacements values for the Nimonic105A is very less than the values of other material steel for the same thermal load and Structural loads. It‟s evident from the analysis, the best material for the valve is Nimonic105A as far as thermal and structural behavior is concerned.