EDUC6048 Research Methodology For the Theoretical Calculations

Answer:

Introduction

The paper is aimed to design, analysis and optimization of the exhaust valve of an engine. The design has performed the theoretical calculations and also automated calculations, through the design and analysis done, in the ANSYS Workbench, by performing the finite element analysis and method. The method makes use of the AutoCAD designs.

Engine Exhaust Valves

Exhaust valves are designed with the objective to allow escaping of the exhaust gases, into the exhaust manifold, in internal combustion engines.

The valve stands as the quite significant part of the engine valve mechanism and determines the overall service life and structural strength, when considered working with higher temperatures. Hence, it influences the overall engine performance. Since, it works under the environment of higher pressure and temperature, frequently, it has to bear not only various mechanical loads, like cylinder gas pressure, valve spring elastic and friction forces and reciprocating inertia forces, but also by the thermal load, when worked in increased temperature (Deng, et al. 2014).

Design Analysis

Since, the practical design and analysis in physical terms demands more time, efforts and money, all of them can be saved to a great extent, when the engine exhaust valve is designed with software, preferably, AutoCAD and Ansys Workbench.

An analysis of design against its performance and failure are carried out with various analysis and the analysis is done over the gasoline engine valve. It has been observed and shown that the material hardness and microstructure, through scanning, by the electron microscope and performing thermal deformation, in terms of coordinate measuring machine. The tests have shown the failure of valve, because of the mechanical bending force, because of the valve shift misalignment and because of thermal deformation. Other tests conducted have shown the results of analysis of failure as initiation of the cracks, from the valve shaft outer surface and propagating towards inner part of the surface. Other fractographic study conducted on the valve shows the decomposition of the matrix and formation of the lamellar structure that has ?Cr23 C6 that influence and decrease the harness, toughness, valve plate  material gas corrosion resistance, all resulting towards exhaust valve failure. Overheating of valve occurred because of the extensive surface oxidation, galling or fretting of the valve have resulted in significant loss of hardness. Valve also gets affected from the stress that is increased due to closing acceleration, valve train dynamics and combustion temperature and pressure (Jeff, et al. 2014). Temperature gradient, such as near to seat face, thermal stress etc., would increase the stresses and fails the valve, in its head area and result in traversal cracks.

Numerical Method 

Valve drawing is done through CATIA model and then converted to .igs file and then the file is imported to the ANSYS Workbench. Then meshing is done for the model. The physical and mechanical properties of the material used for the valve are to be entered in the engineering data. The mesh has to represent the component geometry, accurately, especially, in the critical areas and here stress is a vital factor (Sagar et al., 2015).

Option loads are used for applying the force and the support is fixed, through the option displacement. Once the supports and forces are applied selection of the von misses to be done, through the option, stresses and solution is obtained, based on the parameters given in the engineering data, after clicking on the solve button. 

Finite Element Method

ANSYS along with the Finite Element Method and Application help to obtain the knowledge of both theoretical and practical, along with the necessary skills for engineering problems analysis. The analysis can better be done with the adopton of the APDL (ANSYS Parametric Design Language) and GUI (Graphical User Interface). Using the FEM, practical modeling of the machines and components, such as engine exhaust valve can be done with practical considerations, of various parameters of the components. For example, the problems related to the engine exhaust valve moisture diffusion, heat transfer and nonlinear structural problems can be explored. Additionally, sub-structuring, sub-modeling, capability of interaction with the external files are offered.

Exhaust valves can be designed virtually, analyzed and optimized for better performance. The design of the values consider various factors, like material strength, fatigue life, temperature and manufacturing processed, so that they can be operated without premature failure. Studies have explored the better material used for the valves for better material strength and can provide stronger valves with lesser cost and lesser weight, such as Nimonic 105 A and Magnesium Alloy. Deisgn of the valve through modifying the exhaust valve, through varying the shape, position and certain considerations, like structural and thermal considerations, can increase the heat transfer rate from the exhaust valve seat portion, so that possibility of knocking can be reduced. Finite element analysis can be utilized for the design and optimization of the exhaust valves giving no affect of structural strength and thermal strength.

Air cavity can be created at insider the stem of the valve, as it acts in the form of insulating medium and eventually, heat flow is prevented. So, need of insulation coating can be minimized. The objective of the air cavity creation is the engine weight reduction and thermal coating cost as well. The air cavity would further be optimized in the valve, so that the temperatures and thermal stresses can be lightly decreased at every nodal point.  

Finite Element Simulation with Ansys workbench

Towards the analysis of thermal and structural capacity, an exhaust valve is selected with a single-cylinder engine.

Specifications

Specifications of the Engine are as the following.

The valve is designed and analyzed with the Aluminium Alloy EN52 material, having certain mechanical properties (Ram, 2011).

The design, analysis and optimization are done with the following assumptions.

1. During normal operation, stresses get arise from the seating and are moderate, when it is seated at the cam ramp properly. the stress become very high, when the valve train gets engineered improperly, resulting in valve bounce or when lash is set improperly or engine is spread overly. The analysis stresses occurred from valve seating are considered.
2. Arise of valve misalignment with the set result in distortion stresses and valve head has to deflect, for seat accommodation and result in bending stresses.
3. The analysis is done for the medium range engines, assuming that the cooled with air.
4. Water chamber exhausts the generated heat in chamber, around cylinder head and liner.
5. Valve pops up and down and stationary valve analysis is done with the assumption that the valve fatigue life is much more and resulting stress is neglected. 

Heat Flux and Structural Stress Calculated Theoretically  

Mean effective pressure

Diameter of cylinder bore = 50 mm

Length of stroke = 50 mm

Power = 7.2 BHP = 7.2 * 0.746 = 5.37 KW

Rotations Per Minute = 8000

Cylinder Area (A) = π/4 * D2

= 0.7857 * 50 * 50 = 1963.5 mm².

BHP = (L * N * A * Pm * K ) / 60000

So, Pm (Mean Effective Pressure) = BHP * 60000 / (L * N * A  * K )

Pm = 0.41 MPa

But Heat Flux, q = - kAl

Here, is valve temperature (5880 K) / length * direction (7.03 cm)

And k = thermal activity = 0.021 W/mm K

From the CAD model, the stem valve weight = 0.0159 kg

Valve legth = 65.3 + 5 = 70.3 mm

So, main valve main area of valve, Al = weight / (Density * L1)

= 28.811 mm² 

Therefore,

q = - (1/70) * (-10 *0.021 * 28.81)

so, heat flux = -0.08606 W 

Stress of exhaust valve

σ = P/A

here, P = pressure or load on head of valve = 0.41 MPa = 128.81 N 

Total change or deflection

is,

Δl = α x Δt x L1,

Δt = difference of temperature between the regions of cold and hot = 588 – 578 = 10 K

E = Modulus of Elasticity = 210 KN / mm²

α = thermal expansion co-efficient = 11.635 * 10 ^-6 

Δl = 0.00817 mm 

So, length change = 0.00787 mm

σ (Thermal stress ) = ε x E = 24.43 MPa

ε, Thermal Strain = Δl / L1 = 1.16 * 10^-4 

Finite Element Analysis

The solution is obtained with finite element analysis, which is a numerical procedure for heat transfer, stress analysis, electromagnetism and fluid flow and other classes of engineering systems. Experimental analysis helps understand the stresses arising in the device and components, so the prevention can be worked upon. ANSYS finite element program is one of the FEM software, used in the design, analysis and optimization of the exhaust valve.

Conditions of Thermal boundary considered are,

Conclusion 

The numerical analysis results obtained suggest that the design of the valve can possibly optimized, so that the weight would be reduced, with no affect of deformation values and permissible stress values. Stress reduction can be observed, because of the air cavity and improves the strength of the valve, further. The valve weight is finally 17% reduced that can be reduced further, by repeating the same procedure. Great amount of material is saved, by considering the mass production and so helps in reducing the cost of manufacture, to a great extent.

References