Manufacturing Different Kind Of Components

Published Date: 02 Nov 2017

Introduction

Material selection is one the main and most important steps to be taken into a consideration while manufacturing different kind of components that will serve under different applications.

Moreover, manufacturing a component of a right material selected will help in boosting up the level of operation and help into widely boosting the life span of the material at the same time if it is compared to a similar design but with less effective material.

The other section of this assignment will discuss the different kind of forces that is acting up on each of the piston and the exhaust valve as well as the different conditions that is found in the surrounding of those two components.

As well as how each of those conditions will affect the choice of the right material to manufacture those two components , and how each of those two materials are chosen relaying on different kind of information , in which some of them are taken from the aid of a program and some of them are more human judgement .

As it always proven that relaying 100% on programs maybe give you close solutions but a human factor is always required to make the final decision towards the end .

Function

Piston :

Piston is considered to be one of the most important part of the internal combustion engine which helps into converting the chemical energy obtained from the combustion into useful work .It has a cylindrical shape that moves up and down inside the cylinder block. Good sealing between the piston and the cylinder wall provided by the piston rings which fitted into the grooves on the piston circumference groves (Schafer, 2004).

Piston function is basically summarized by (KHURMI & GUPTA, 2005) :

Transferring the force produced from burning air-fuel mixture inside the combustion chamber to the crankshaft via the wristpin and the connecting rod.

Reciprocate in the cylinder as a gas tight plug causing suction, compression, expansion and exhaust strokes.

To form a guide and bearing to the small end of the connecting rod and to take the side thrust due to the obliquity of the rod.

To seal the combustion chamber against gases escaping and the lubrication oil by the piston rings.

There are three major areas in the piston which are head, skirt and boss as it shown below:

Figure Piston major areas (KHURMI & GUPTA, 2005).

1.2 Exhaust valve :

The exhaust valve is precision engine components used to open to permit the burned gases to escape out from the cylinder. Therefore , the exhaust valve is exposed to thermal loads and chemical corrosion. This type of valve manufactured using different types of materials to satisfy the limit condition during the engine operation at a very high temperature (KHURMI & GUPTA, 2005).

There are three major parts in the exhaust valve which are the head, face , stem and tip as it shown in the figure below (KHURMI & GUPTA, 2005) :

Figure Exhaust valve major areas (KHURMI & GUPTA, 2005).

Operational conditions

Figure operation of Exhaust valve and piston

2.1 Piston :

Mechanical loading (Schafer, 2004):

Piston head/ Gasoline engine: ignition pressure 50 to 90 bar.

Diesel engine : Ignition pressure 80 to 180 bar.

Piston skirt/ Side force: approximate 6% to 8% of the maximum ignition pressure

Piston boss : allowable surface pressure , temperature dependent

High temperature in combustion chamber (Schafer, 2004):

The mean gas temperature inside the combustion chamber is approximately 1000 Câ—¦ , the distribution of temperature in different areas of the piston is (Schafer, 2004):

Piston head/edge of recess: 200-400 â—¦C for ferrous materials: approx. 350 to 500 â—¦C .

Wristpin boss : the temperature between 150 to 260 â—¦C (Michael Ashby, 2007) .

Piston skirt : temperature between 120 to 180 â—¦C .

Acceleration (Schafer, 2004) :

The acceleration of the piston at high speed in some cases far above 25 000 m/s2 , therefore, the piston material should be light to reduce the inertia forces and moment of inertia .

Friction (Schafer, 2004):

Pistons exposed to friction at the skirt, in the wristpin bearings and also there is a sliding friction in the ring grooves , also, poor lubrication will increase friction forces on the piston which will reduce the service life .

Thermal expansion (Schafer, 2004):

The piston-to-cylinder clearance should be kept minimum by choosing a material with low thermal expansion .

Therefore, Piston material should have the following requirements:

Good castability.

Good resistance to surface abrasion, to reduced skirt & ring groove wears.

Good thermal conductivity to keep piston temperatures low.

High hot strength.

High strength-to-mass ratio.

Low thermal expansion to maintain lower clearance between piston and cylinder walls.

2.2 Exhaust valve :

Mechanical loads (KHURMI & GUPTA, 2005):

Longitudinal cyclic stresses caused by returning spring load and the inertia response of the assembling valve.

Thermal loads (Schafer, 2004):

The temperature of the exhaust valve reaches 600 to 800 ͦC , while the temperature distribution across the valve are shown in the figure below :

Figure Temperature distribution in exhaust valve (Schafer, 2004)

So , there is thermal stress in the circumferential and perpendicular directions because of the large temperature gradient from the center of the head to its periphery and from the crown to the stem (KHURMI & GUPTA, 2005).

Creep conditions due to working at high temperature conditions, especially in the case of the valve head (KHURMI & GUPTA, 2005).

Corrosion and wear conditions (KHURMI & GUPTA, 2005).

Therefore , the exhaust valve material should have the following requirement :

High hot strength and hardness to resist tensile loads and stem wear.

High fatigue and creep resistance .

Low thermal expansion coefficient to avoid increasing thermal stresses in the head of the valve .

High thermal conductivity for better heat dissipation .

Suitable corrosion resistance .

CES Materials

Piston material

By using CES Edupack 2012 program , hot strength material can be chosen by drawing a graph between strength σ (MPa) and the maximum service temperature (◦C) as it's shown below :

Figure Material graph Yield strength Vs Maximum service temperature (CES EduPack 2012)

By using the limitation of maximum service temperature between 200-400 â—¦C (Schafer, 2004) , fracture toughness of metals in engineering almost have values of K1c above 15 MPa.m1/2 (Michael Ashby, 2007) the material in the above graph will reduced as it shown below :

Figure Passed Materials after applying limits(CES EduPack 2012)

The thermal conductivity K (W/m.K) should be high whereas thermal expansion coefficient α (m /°C) low for the piston in another word, K/α should be high . Also , the piston should be light and stiff , to satisfy that ,density ρ kg/m3 should be low and the Young Modulus E high , in another word E/ρ high .A graph drawn between the E/ρ and the K /α in CES as it is shown below and box selection used in order to choose the highest values in the high right corner of the graph .

Figure Final 3 candicates chosen(CES EduPack 2012)

Making another graph between the fracture toughness and the price will show :

Figure Final materials fracture toughness vs price graph(CES EduPack 2012)

As it appear from the above graph, Aluminum/Silicon carbide composite Al-SlC not the cheapest material but it will be the best choice for the piston material because it have the highest maximum service temperature 227-367°C, while, Age-hardening wrought Al-alloy and Cast Al-alloy have a maximum service temperature of 120-200°C and 130-220°C respectively (Ashby, 2011) also it has a less thermal expansion and more resist against wear (Schafer, 2004).

Exhaust valve materials

The thermal conductivity K (W/m.K) should be high whereas thermal expansion coefficient α (m /°C) low for the exhaust valve in another word, K/α should be high . Also , the exhaust valve should be light and resist fatigue stresses , to satisfy that ,density ρ kg/m3 should be low and the fatigue stress σe high , in another word σe/ρ high .A graph drawn between the σe/ρ and the K /α in CES as it is shown below:

Figure thermal properties Vs. Fatigue strength

By using the limitation of maximum service temperature between 600-800 â—¦C (Schafer, 2004) , fracture toughness of metals in engineering almost has values of K1c above 15 MPa.m1/2 (Michael Ashby, 2007) the material in the above graph will reduced as it shown below :

Figure Passed Materials after applying limits(CES EduPack 2012)

It is better to convert to level 3 in CES , the material will be as it shown below:

Figure Stainless Steel Alloys in level 3 (CES EduPack 2012)

Graph has been drown between specific strength σy / ρ (MPa.m3/Kg) and the price as it's shown below :

Figure Yield strength Vs. Price (CES EduPack 2012)

Low price and high strength materials have been choosen by using box selection as it shown below :

Figure Box selection (CES EduPack 2012)

Finally, the specific type of material for the exhaust valve is Stainless steel, martensitic, AISI, wrought, hard temper

Reason:

High resistance to general corrosion

Mechanical strength is very high

Physical properties that offer design advantages

High resistance to erosion corrosion and corrosion fatigue

Low coefficient of thermal expansion

Surface considerations

Piston

The piston is subjected to high wear and a lot of thermal loads , in order to increase protection against wear a surface coating helps in improving the resistance for such applications.

Furthermore ,the coating surface of the piston applying phosphate graphite, partial coating surface of the piston skirt with different materials such as iron or at least hard anodized finishing at the piston head to resist the effect of temperature alternation and to prevent fissuring (Schafer, 2004).

Figure Hard anodized piston head to resist high temperature (Schafer, 2004)

Exhaust valve