Materials selection of internal combustion engine
Summary
The purpose of this project is to carry out an investigation of the microstructure and metallurgical characteristics of a range of components in a simple internal combustion engine, such as lawnmower, chain saw or strimmer, which is employed in garden machinery.
The principles of operation of simple internal combustion engine are also discussed. The combustion engine consist of various parts that function together to ensure that it works well. These material properties and their operation are well discussed. Grind, polish and etch samples that reveal the microstructure and measure relevant properties such as hardness and micro hardness, quantities of phases and their distribution in the microstructure are also discussed. The correlation between the microstructure and the phase diagrams such as the eutectic phase diagram is also highlighted through optical micrographs. Through these discussions, the material and the functioning of parts of internal combustion engine are well understood.
Contents
- Introduction and objectives
- Literature review
- Experimental procedure
- Result
- Conclusion
- Reference
Introduction and objectives
Engineering is a branch of since that has resulted to advancement of many different technologies and machines that have impacted positively on the life of people. Engineering advances has led to invention of internal combustion engines that are used in various applications since the late 19th century. Internal combustion engines are one of the widely engineering products. These power plant machines are cheap and convenient. They are very essential in facilitating performance of tasks which otherwise could have taken many people to complete. Example of these combustions engines are the lawnmowers, for trimming grass, chain saws and trimmers. These engine burns fuel with the aid of an oxidizer in their combustion chamber, which enables them to perform various tasks. As the combustion happens, high pressures and temperatures are generated which apply direct force to the pistons allowing the machines to carry out their functions. The moves applied move the piston over a distance, transforming chemical energy into a useful mechanical energy. This process is repeated as the pistons do cyclically movement.
The major objective is to carryout an investigation of the microstructure and metallurgical characteristics of a range of components in a simple internal combustion engine (such as is employed in garden machinery such as lawnmower, chain saw or strimmer). This will foster understanding of the basic principles of operation of a simple internal combustion engine. The project will also identify the suitable materials for a simple internal combustion. This will be achieved through reading and doing experiments to understand the working principles in figuring out how the materials are used in every component of the machines.
Literature review
List of Components and working principle
The above diagram is an example of a combustion engine that is found in various garden machines such as lawnmowers that is used to perform various functions. The various parts and their functions are explained and how they work to perform various functions.
Spark plug is one of the parts also called a sparking plug. This device is used to deliver electric current form an ignition system to the combustion chamber of the spark ignition engine to start/ignite the compressed fuel /air mixture by an electrical park.
Other important parts of the combustion engine includes camshaft, cam, mixture in, intake valve, combustion chamber, cylinder block, connecting rod, crankshaft, valve spring, exhaust valve, cylinder head, cooling water, piston and crankcase. All these parts are important to the functioning of the combustion engine as they work in unison.
Suggested alloys and material property of some components
- The piston
Piston is one of the most important parts that allow the functioning of the engine. The material property of piston is as follows; it is light in weight, low inertial forces, it is rustproof and has high thermal conductivity. Suggested materials: aluminum alloys (e.g. LM13, LM28, and LM29)
- Piston rings
Suggested materials: cast iron containing fine flake graphite/ sintered iron (small engines)
- The connecting rod
Suggested materials: Al-3.5Cu-8.5Si alloy (low-duty)/150M36(motor car)/817M40(higher performance, tensile strength of 1000MPa)
- The cylinder block
Material property: Fluidity, small solidification range, strength, rigidity, good thermal conductivity, low density, good resistance to abrasion, wear and corrosion and low thermal expansion
Suggested materials: grey cast iron (satisfactory of all except density)/flake graphite cast iron (no enough power-weight ratio)/aluminum alloys (low resistance to abrasion and wear)
- The cylinder head
Suggested materials: cast iron, aluminum alloy (e.g. LM25) or magnesium alloy
- The inlet manifold
Material property: Low weight, less machining, good surface-finish and automated production
Suggested materials: aluminum alloy (e.g. LM20)/thermoplastic materials (a nylon 66 compound containing 33% glass fiber reinforcement)
- The crankcase
Material property: Strength and rigidity
Suggested materials: cast iron/ aluminum alloy (e.g. LM25)
- The crankshaft
Material property: High integrity, depends on the duty required
Suggested materials: forged steel (diesel engine)/spheroid graphite cast iron (motor car engines)
- The camshaft
Material property: Lightly stressed and wear resistance
Suggested materials: low-carbon steel
- Inlet and exhaust valves
Material property: Temperature and corrosion
Suggested materials: 0.5C-8Cr-3Si alloy (inlet)/ 0.8C-20Cr-1.3Ni-2Si/21-4N (outlet)
- Turbochargers
Suggested materials: high chromium steels and super alloys (e.g. inconel738)
- The compressor
Material property:
Discs- high proof stress, some ductility at blade fixings and resistance to low-cycle fatigue
Blades- strength, stiffness, resistance to fatigue, erosion, and impact damage
Suggested materials: steels, titanium alloys/nickel-base alloys (high-power compressors)
- Combustion chambers and flame tubes
Material property: temperature resistance, readily formable and wieldable.
Suggested materials: Nimonic86/Haynes188
Working process
- The piston
For these pistons to work efficiently solid aluminum alloys are needed to manufacture the pistons. The rods are cut into small pieces that are called slugs. Punch press is pre-heat and dyed at the same temperature with the slugs in the oven. Slugs are placed into the punch and pressure is applied unto them forging them into their basic shapes of pistons.
The forgings are cooled and then taken to an oven for another two times heating. The first time heating requires high temperatures than the second to strengthen and stabilize the metal. Excess material of the forgings is cut to make them from the basic shape to their final shape. Tiny holes are drilled in the sides, making them possible to be lubricated when they are in use. Then piston rings are impressed into the top of the pistons to help them glide, and form airtight seals. A large hole is drilled through both sides of the piston (for the piston pin) (Charles, 1997). Some metals are shaven up of each side of the piston where the large holes were drilled, where the three rings were impressed and the top (to reduce the overall weight of the piston and to allow the piston to expand when temperature rises. Every sharp edge is smoothened out of the piston generated during production and engraves production information. Finally, the pistons are cleaned by hot, deionized water.
- Piston rings
A short tube of oval cross section is made to fit the shape of the piston rings. The piston rings are then cut and machined from the tube. The piston rings should be made to have tension properties probably by using one of these two methods below.
Thermally Tensioned Ring
It is one of the methods with lowest cost to cause the tension of piston rings but only of small engines. The piston ring is machined from circular tube in this method. A gap is cut on the final form of the ring and a small piece of metal is inserted in the gap, which expands the ring and induces a tension in the ring. After expanding, the ring and the distance piece should be heated to reduce the stress caused during the process. The ring may lose its tension when it is heated.
Oval Pot Cam Turning Method
The oval pot cam turning method is relatively expensive, but with this method, the rings can keep their tension under the heat of the engine. The rings are machined in a cam-turning lathe. By changing the pressure distribution around the ring, the tension is induced.
- The connecting rod
Powder forging method guarantees less material waste, energy cost and engine weight. These rods are prepared through the following steps;
- a) Mix the powder of proper material
- b) Press and sinter the mixture in a sealed container
- c) Forge the shape of connecting rod under high temperature
- d) Cool down the forging.
- The cylinder block
Sand casting is mainly used in manufacturing process of engine blocks (sand will not wear out due to the high temperature of the molten metal like dies)
a) Make patterns by metal with high melt point.
- b) Pour the mixture of silica sand, clay, and water in to the one-half of the aluminum block pattern and then apply pressure to the mould.
- c) Repeat the process above for the other half of the mold. Then remove both moulds from the pattern.
- Create a resin model by computer aided design (part 1 of Fig. 1).
- A layer of fine polystyrene powder is laid down with an arbitrary thickness on a platform (Part 2 of Fig. 1).
- A laser beam is launched through the surface of the resin powder laminate as set by CAD for both resin model forming and casting, then the resin powder is sintered (Part 3 of Fig. 1).
- When the first layer has been entirely sintered, another thin layer is generated, and laser beam scanning is again used to sinter this layer, thus forming layer on top of the first sintered layer. Repeating this layer-forming procedure several times, the model is created (Part 4 of Fig. 1).
- Add other components to the model, and then apply wax is then to the model to improve the adhension of the resin model and the gypsum slurry. Following this, the resin model is setup within a depressurized molding flask, and gypsum slurry is then poured into the molding flask and allowed to harden (Part 5 of Fig. 1).
- Next, the gypsum mold is removed from the molding flask, and by placing this into an oven, the resin model can be melted and removed, and the gypsum mold can be calcined (Part 6 of Fig. 1).
- The mold is then cooled down to the pouring temperature, and molten aluminum is poured into the mold (Part 7 in Fig. 1).
- After cooling and mold removing, the casting is subjected to solution treatment to modify the metal structure of the combustion chamber walls, and then artificial hardening treatment (T6) is carried out, thus completing the cylinder-head material (Part 8 of Fig. 1).
- The cylinder head
- The inlet manifold
This is the pipe, which acts like the heart of a machine. It allows the engine to bring. It is a series of tubes connected to cylinders to allow right amount air to mix with gas. Once the intake valves closes after sucking the air in the cylinder, the process of compression, combustion and exhaustion takes place allowing the machine to operate. These inlet manifolds are made form fluid rails of ¾ square tubing with 0.5 to 0.65 walls. This shape is selected because it allows easier jigging and drilling. ¼ holes are drilled at the port spacing interval. In their assembly, the pieces are cut and deburred whereby head flange are clamped to the scrap head to reduce warpage. They are also required to be straight to ensure that they function well once assembled.
- The crankcase
This is the part of the engine that holds all the other parts of internal combustion engine together. It is always strong and light and is normally made form different materials such as aluminum.
- The crankshaft
These are formed through forging process by heating a billet of suitable size to appropriate forging temperature. The temperature usually ranges form 1950-2250 degrees F. After the heating, they are pressurized or pounded into desired shapes through squeezing the billets through pairs of dies. For complex shapes, more deformations are required and are achieved by applying more than one dies to accomplish the shaping.
- The camshaft
They are used to operate poppet valves in the combustion engines. It has a cylindrical rod that runs the length of a cylinder bank with oblong lobes that protrude form it. They force the valves to open by pressing the valves as they rotate. They are made of various materials such as chilled iron castings and billet steel
- Inlet and exhaust valves
Inlet and exhaust valves are also important parts of the combustion engine. They help in the regulating of the air that enters sin the cylinder of the combustion engine.
- Combustion chambers and flame tubes
Combustion chambers and flame tunes play important role in the performance of the combustion engines. The flame tubes are holes that allow the burnt air out of the engine helping it to refresh and eliminate the wastes. Combustion chambers when being produced must met the following; they must ensure that fuel is completed burnt, minimize pressure drop, ensure god temperature stability at the turbine inlet and must ensure that here is proper cooling of the walls.
The binary eutectic phase diagram
Phase diagram of Ai-Si
Consider an alloy having this composition that is cooled from a temperature within the liquid phase region down the vertical line yy’ in f9.11. as the temperature is lowered, no changes occur until we reach the eutectic temperature, 183 .c. upon crossing the eutectic isotherm, the liquid transforms to the two x and b phases.
During this transformation, there must necessarily be a redistribution of the lead and tin components, inasmuch as the x and b phases have different compositions neither of which is the same as that of the liquid. This redistribution is accomplished by atomic diffusion. The microstructure of the solid that results from this transformation consist of alternating layers (sometimes called lamellae) of the x and b phases that form simultaneously during the transformation. This misconstrue, represented schematically in f 9.11. Point I, is called a eutectic structure, and is characteristic of this reaction. A photomicrograph of this structure for the lead- tin eutectic is shown in f 9.12. Subsequent cooling of the alloy from just below the eutectic to room temperature will result in only minor microstructure alterations (Callister, 1940).
The microstructure change that accompanies this eutectic transformation is represented schematically in f9.13. Here is shown the x-b layered eutectic growing into and replacing the liquid phase. The process of the redistribution of lead and tin occurs by diffusion in the liquid just ahead of the eutectic- liquid interface. The arrows indicate the directions of diffusion of lead and tin atoms.
Experimental procedure
Making specimen
Grinding and polishing
Grinding clears up the saw marks and extra graphite on the surface. Polishing does the remaining work after grinding. The platen for grinding is bonded with the abrasives while the platen for polishing has no abrasives on it- the abrasive particles are existed in the lubricant and can slide across the platen and specimen.
Grinding
- To grind a specimen, firstly set a reference point on the surface, such as point Q (12 o’ clock) shown in (a).
- Pour the lubricant on the platen.
- Put the specimen on the platen and hold it tightly, do not let it rotate with respect to the platen.
- Keep grinding to clear up the saw marks and extra graphite on the surface until there are only parallel scratches on the surface, as shown in (b).
- Clean the surface by warm tap water. Rotate the reference point Q to the 3 o’clock position, as in (c), and grind the specimen until the new parallel scratches generated, which lie at a 90° angle to the previous ones, as in (d).
- Rotation of the mount by 90° as shown in (e) and repeat previous step.
- Clean the specimen by warm tap water to clean the abrasives on the surface.
In many cases, all the grinding can be accomplished in a single step.
Polishing
After the well grinding and cleaning, polish the specimen on polishing platen without abrasives and loaded with lubricant. Rotate the specimen 90° every time, as in (a–e), and clean it after each polishing step by warm tap water. More polishing step with 0.05 μm γ-Al2O3 suspension is not mandatory. The microstructure after each polishing step is shown in (a–d).
Etching
Result
The piston
Function: Transfers force from expanding gas in the cylinder to the crankshaft via a piston rod or connecting rod
Material property: Low weight, low inertial forces, high thermal conductivity
Suggested materials:
Alloy | Cu% | Mg% | Si% | Ni% | Fe% | Coefficient of thermal expansion per(20-100℃) |
LM13 | 0.7-1.5 | 0.8-1.5 | 10.0-12.0 | 1.5 max | 1.0 max | 19.0*10-6 |
LM28 | 1.3-1.8 | 0.8-1.5 | 17.0-20.0 | 0.8-1.5 | 0.7 max | 17.5*10-6 |
LM29 | 0.8-1.3 | 0.8-1.3 | 22.0-25.0 | 0.8-1.3 | 0.7 max | 16.5*10-6 |
Piston pin
Function: connects the piston and the rod
Predicted material: Ti-6Al-4V, Ti-17 (Ti-5Al-2Sn-4Mo-2Zr-4Cr) HV (300 gf): 420.5
Big end bearing
Function: bearing between the rod and the bent axle
Predicted material: lead copper HV (300 gf): 462.0
Piston pin bushing
Function: fixates the piston pin
Predicted material: steel
HV (300 gf): 789.2
Piston skirt
Function: keeps the piston from rocking excessively in the cylinder
Material: Al-13wt%Si alloy HV (300 gf): 126.7
Piston rings
Function: Seals the combustion/expansion chamber/supporting heat transfer from the piston to cylinder wall/ regulating engine oil consumption
Suggested materials: Cast iron containing fine flake graphite/ sintered iron (small engines) HV (300 gf): 391.4
The connecting rod
Function: Connects the piston to the crank or crankshaft
Suggested materials:
Low-duty engines- LM24 Al-3.5Cu-8.5Si alloy
Motor car- 150M36
Higher performance, tensile strength of 1000MPa- 817M40
HV (300 gf): 100.7
Conclusion
Internal combustion engine goes through various processes in its manufacture. It requires technical abilities to be able to manufacture it and to ensure that it functions well. Various materials are also used in the manufacture of various components of the engine, which ensures that it is able to perform its, functions well. As discussed above, in-depth knowledge has been gained through close study of the microstructure and metallurgical characteristics of a various components of the internal combustion engine.
Reference list
Callister, W 1940, ‘Materials science and engineering: An introduction.’ / 4th ed. New York: Wiley, c1997
Charles, J 1997, ‘Selection and use of engineering materials.’ 3rd ed. / J.A. Charles, F.A.A. Crane, J.A.G. Furness. Oxford: Butterworth-Heinemann.
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