15-07-2011, 11:15 AM
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ABSTRACT
In this paper an attempt has been made to increase the reliability of engine using Al-SiCp composites as an alternative material for the engine valve guides. Aluminum matrix composites are finding increased application in automotive, aircraft and aerospace industries and hold the greatest promise for the future growth. The finite element analysis of the Al-SiCp composite engine valve guide was done using Ansys software. The temperature, principal stress and principal strain distribution over the entire surface of the engine valve guide were obtained. The stresses were found to be well below the allowable stress for the Al-SiC composites.
INTRODUCTION
In response to increasing global competition and growing concern for environment the automobile manufacturers have been encouraged to meet the conflicting demands of increased power and performance, lower fuel consumption, lower pollution emission and decreased noise and vibration. In order to fulfill these newer and emerging demands the automotive industry has recognized the need for material substitution. Metal matrix composites are offering outstanding properties in a number of automotive components such as piston, cylinder liner, engine valves, brake discs, brake drums, clutch discs, connecting rods etc. Several work has been reported on the substitution of presently used material by the aluminum matrix composites for different automotive components viz. piston, cylinder liner, engine valves, valve seat inserts etc. [1–6]. Al-SiCp composite engine valve guides have been fabricated through casting and powder metallurgy processes. The radial crushing strength, hardness, and wear resistance of the Al-SiCp composite and cast iron engine valve guides were measured and compared [3, 4]. Al-SiCp composites with 5 to 30 wt. % of SiC were found to have higher Rockwell hardness and radial crushing strength than the cast iron engine valve guides as shown in Table 1. Al-SiCp composite engine valve guides with 20 and 30 wt. % of SiC were found to have higher wear resistance than the cast iron engine valve guides as shown in Table 1 [4]. Present work incorporates the finite element technique to envisage the prospects of Al-SiCp composites as a possible alternative material for the engine valve guides. The finite element analysis of the engine valve guides was done using Ansys software. Temperature, pressure and displacement boundary conditions were applied and the temperature, stress and strain distribution over the entire surface of the engine valve guides was obtained.
At present the engine valve guides are made of iron-based materials, which cause a number of problems in an automotive engine:
• During cold start condition the viscosity of oil is high and also sufficient lubricant is not available therefore high wear of the valve stem / valve guide takes place. In the adverse conditions the valve may jam in the guide.
• During running condition of the engine the temperature of the valve stem and valve guide increases to about 500°C. Therefore at high temperature the clearance between valve stem and valve guide decreases due to thermal expansion, which results in high wear of the valve stem and the guide.
• The superimposed rocking motion in addition to the sliding of the engine valve causes high wear at the ends of the valve guide called ‘bell-mouthing’, generally more pronounced in the rocker arm actuation mechanism.
These problems call for a high wear resistant material with low coefficient of thermal expansion and engine valve guides based on the Al-SiCp composites are expected to provide a better solution.
The finite element analysis is the most widely accepted computational tool in engineering analysis. The finite element analysis of the engine valve guide has been carried out using Ansys software. Table 1 shows the properties of different materials used for the finite element analysis. To make the analysis simple following assumptions were made:
1. The Al-SiCp composites are homogeneous and isotropic materials.
2. The effect of other neighboring components was neglected.
