Victa 2 Stroke Engine - 85% PDF

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VICTA TWO-STROKE ENGINE

Manufacturing Engineering 48621

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Contents Introduction.........................................................................................................................................3 How does it work?...........................................................................................................................3 Advantages of a Two-Stroke Engine:.........................................................................................3 Disadvantages of a Two-Stroke Engine:.....................................................................................3 Analysis of Components:.................................................................................................................4 Justification of required accuracy and surface integrity..........................................................4 Diagrams:.........................................................................................................................................5 Material Selection:...............................................................................................................................6 Manufacturing Processes....................................................................................................................7 Flow Chart.......................................................................................................................................7 Manufacturing operations..............................................................................................................7 Alternative Manufacturing Processes/Operations....................................................................8 Required Manufacturing Equipment.............................................................................................9 Critical & non-critical parts of the engine block...........................................................................9 Photos of the Engine Block.......................................................................................................10 Quality Assurance and Inspection................................................................................................11 Estimation of Manufacturing Cost...................................................................................................12 Time:...............................................................................................................................................12 Labour:...........................................................................................................................................12 Equipment:.....................................................................................................................................13 References..........................................................................................................................................13

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Introduction The Victa 2 Stroke Engine is a type of two stroke internal combustion engine that completes a power cycle with two strokes of the piston during only one rotation of the crankshaft. Compared to a four-stroke engine which is also an internal combustion engine and four separate strokes of the piston are completed while turning the crankshaft, a two-stroke engine has a high power to weight ratio with a smaller amount of moving parts. This 2 Stroke Engine is mainly used when “mechanical simplicity, light weight, and high power to weight ratio are design priorities” (Two-Stroke Engine, 2020) for example, it is used in lawn mowers and chainsaws. This report will explore and examine the Victa two-stroke engine and investigate the manufacturing processes and how to fabricate the engine block. How does it work? A cycle of a two-stroke engine consists of the compression stroke and the power stroke. During a compression stroke, the inlet port opens, allowing air-fuel mixture to move into the chamber and become compressed when the piston moves upwards. The compressed air-fuel mixture is then ignited by a spark plug which commences the power stroke. During the power stroke, the piston is put under high pressure exerted by the heated gas, shifting the piston downwards while exhausting waste heat. Advantages of a Two-Stroke Engine: A comparison of a two-stroke engine to a four-stroke engine shows that two stroke engines have a simpler construction comprising only of ports and no valves, lowering its overall weight and cost to manufacture. This makes it easier and more affordable to manufacture than a four-stroke engine. Two stroke engines “fires once every revolution while a four stroke engine fires once every alternate revolution”, giving two stroke engines a significant power boost with the potential for about twice the power in the same size since there are twice as many power strokes per revolution (Reibel, 2020). As mentioned previously, the two-stroke engine has a higher power to weight ratio compared to the four-stroke engine due to the twostroke engine firing twice as often (Reibel, 2020). Another advantage of a two-stroke engine is that it can operate in any position since it is lubricated with oil, which flows throughout the engine. Disadvantages of a Two-Stroke Engine: Although a two-stroke engine have many useful advantages, it also comes with its disadvantages when compared to a four-stroke engine. As mentioned beforehand, a twostroke engine has a simpler design leading to many advantages such as decreasing cost of manufacturing however it also means that there is a lack of a consistent lubrication system meaning that parts wear out faster and subsequently not lasting as long as four-stroke engines (Reibel, 2020). Two-stroke engines require a mixture of oil with the gas to lubricate the crankshaft, cylinder walls, and connecting rod however the oil that a two-stroke engine requires can be quite expensive with a mixing ratio of 50:1, burning around a litre of oil every 1000 kilometres (Reibel, 2020). The fuel usage of the two-stroke engine is inefficient, generating fewer kilometres per litre compared to a four-stroke engine. This occurs since each time a fresh mix of air or fuel fills the combustion chamber, part of it leaks out through the exhaust port. This occurrence is called scavenging. The oil during this process makes the two-stroke engine smoky to a particular degree, consequently creating more pollution contrasted to a four-stroke engine as a result of the combustion of the oil in the gas. (Reibel, 2020) 3

Analysis of Components: A 2-stroke engine is primarily made up of the following components:  

    

Spark Plugs: Spark plugs are a device used to direct electric current from the ignition to the combustion chamber. They ignite the air fuel mixture in the combustion chamber. Piston: The piston is a component of the engine that moves upwards and downwards and is connected to the crank shaft by the connecting rod. This movement rotates the crankshaft, controlling the compression of the air fuel mixture in the chamber. Fuel Intake: The fuel intake is where fuel is allowed to enter the combustion chamber once the piston is at the lowest point. Combustion Chamber: The combustion chamber is where the combustion caused by the spark plugs occur after the piston lowers to allow the fuel air mixture to enter. Exhaust Outlet: The exhaust outlet is a passage that directs the gases after combustion has ensued to exit out of the combustion chamber. Crankshaft: A crankshaft is a rotating shaft that converts the up and down motion of the piston into rotational motion. Connecting Rod: The connecting rod functions as a lever arm that transfers motion from the piston to the crankshaft. (University of Windsor, 2020)

Justification of required accuracy and surface integrity For an internal combustion engine such as the two-stroke engine to function well, all the components need to be made with accuracy and with attention to surface integrity. Surface Integrity refers to the condition of the surface of a part after manufacturing. Surface integrity in addition to accuracy while manufacturing parts is important for the overall assembly since all the parts needs to be able to fit together properly for the engine to run smoothly especially since the engine will undergo vibrations, friction and high temperatures while running. During production, the measurements of all the parts must be accurate and precise since any minor inaccuracies can lead to strength reduction and damages particularly when combustion occurs frequently. To avoid any failures, manufacturers must follow standards, in this case ISO 8062 which defines the system of tolerance grades and machining allowance grades for cast metals and their alloys (ISO 8062-3 para.10). A misalignment or incorrect calculation could cause wear to the engine block which can lead to other parts of the engine such as the piston or crank shaft not being able to operate accurately. Other consequences could include loose parts as a result of incomplete joining from screws that were not aligned correctly or were not fitted properly. Surface integrity is important since the condition on the surface of the part must be of good quality to take advantage of its mechanical and physical properties. Inadequate surface integrity leads to poor performance of the part as the material below the surface is not in good condition. With lower mechanical and physical properties, the part is more susceptible to damages such as cracks and wears, therefore the surface integrity of all the parts of an engine block must remain consistent and of good quality. 4

Diagrams:

Top View

Front and Back Views

Side View

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Isometric View

Material Selection: As seen above in the diagrams, it is evident that an engine block is the main housing of the engine and has many components that have necessary requirements for smooth operation. These requirements include maintenance, wear resistance, being able to last a long time and being able to withstand the pressure created when combustion takes place. Additionally, the engine has to withstand vibration and high temperatures while the engine is running. To meet these requirements, the material used to produce the components of the engine is critical. In order for the two-stroke engine to withstand both internal and external forces while running, it is imperative that the material used for manufacturing the components should contain a high strength, modulus of elasticity, wear resistance, ability to withstand vibrations, high thermal conductivity, low thermal expansion, and corrosion resistance (Reibel, 2020). The material must have high strength as it has to withstand the combustion process that takes place within the engine and will be subjected to the ensuing stresses from the combustion as well as vibrations from moving internal parts such as the crank shaft and pistons (Reibel, 2020). Wear resistance and corrosion resistance of the material must be high as it is also subjected to stresses and moving parts. With combustion continuously occurring within the engine, the engine will be functioning under high temperatures hence the material will have to have high thermal conductivity and low thermal expansion to allow the heat to disperse more rapidly and for the parts of the engine to remain a consistent size. The material should also be lightweight and low in density to increase the efficiency of the engine since for example it was to be used in a lawnmower and the material is heavy, it would be more difficult to use the lawnmower. Based on the elements above, grey cast iron alloys and aluminium alloys are the most commonly sourced material to manufacture the cylinder block. 1. Grey Cast Iron Alloys: These alloys are the most suitable and is the material most commonly used for the manufacturing of engine blocks. Cast iron alloys are suitable since they contain good mechanical properties, meaning that it can be casted to develop complex shapes and designs. The most appealing property of these alloys is that they are cheaper and more affordable than other materials and are also readily available. Cast iron contains 2.54% of carbon, contributing to its high tensile strength and it additionally delivers outstanding damping absorption, good wear, and thermal resistance (Reibel, 2020). Compared to aluminium, cast iron is heavier and cannot conduct heat as well either. 2. Aluminium Alloys: There are two aluminium alloys that are mainly used in the manufacturing of engine blocks and they are 319 and A356. Although the characteristics of aluminium alloys are very similar to cast iron, aluminium alloys have a lighter weight which is more appealing when considering the power to weight ratio and also having the overall weight of the product lower. Aluminium alloys have good machinability properties and a better surface finish compared to cast iron alloys however they are more expensive. The 319-aluminium alloy has good corrosion resistance, casting features, good thermal conductivity, and when it is under the heat treatment of the T5 process, it generates high strength and rigidity for the engine block (Reibel, 2020). Compared to the 319-aluminium alloy, the A356 aluminium alloy is similar however when it is under the heat treatment process T6, it gains a higher strength but has a lower modulus of elasticity (Reibel, 2020).

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3. Alternative – Graphite Cast Iron: As technology increases and more knowledge is gained, new materials such as graphite cast iron has been uncovered and is found to be lighter and stronger than the grey cast iron alloys. The graphite cast iron when compacted is revealed to have a higher tensile strength and modulus of elasticity when contrasted to grey cast iron (Reibel, 2020). It is similar to grey cast iron with good damping absorption and thermal conduction however it has low machinability which has restricted wide usage (Reibel, 2020). Manufacturing Processes The manufacturing of engine blocks including the Victa 2-stroke engine are predominantly performed using sand casting which is the most widely used metal casting process characterised by using sand as the mould material (Sand Casting, 2020). Sand casting is a type of expendable mould casting meaning that more complex shapes can be created however the mould is destroyed to remove the casting and a new mould is required for each new casting. The manufacturing processes are detailed below. Flow Chart Pattern Making

Sand

Preparation of Sand

Mould Making

Core making (If necessary)

Raw Metal

Melting

Pouring

Solidification and Cooling

Cleaning and Inspection

Removal of Mould

Manufacturing operations

The casting of Victa engine follows sand casting convention, 7

comprised with the three-step approach but with an added degree due to the complexity of the engine geometry. The three steps are the following: 1. Mould making: The process begins by machining the pattern and groves with machinable material such as wood. The sand mould is then patched onto the surface of the 8

finished machined product to replicate its shape. A green sand mould is often implemented for engine casting. It is comprised of a myriad of compounds such as silica, sand, clay, and water. The mould is stiffened by pressure onto the timber frame and vibration are applied to rid of bubbles. Then, the 9

cast can be removed from the pattern and it shall be oversized to account the shrinkage of cast iron. As a further measure, the core is then painted to prevent the formation of gas during the casting. The gas formation contributes to casting defects such as porosity (Ask chemicals 2019, para.9). 10

The process will then be repeated to make the mould for the reciprocal side, the two are then fastened using clamps and chaplets to withstand the pressure and buoyancy of pouring molten during the casting phrase (Custom Part 2018, para. 10). 2. Casting: Once the mould is made, molten alloy is pouring into the 11

mould, which was heated in a furnace. The molten is fills the cavity of the mould and is then left to solidify as it cools. Due to thermal expansion, the metal alloy shrinks as it cools; molten needs to be resupplied as it shrinks to get the correct dimension. Due to the density differential 12

between the alloy and the sand cast, the two mould and cast shall not diffuse into another. 3. Removal: The final step in the sand-casting procedure removes the cast for further processing. A trail of residue metal at the entry of the cast is trimmed and cleaned. Further refining might be 13

needed to get the correct dimension and smooth surfaces. Additional machining processes such as machine boring is also needed. Only then, the product is polished 1. Mould Making: A key tool necessary for sand casting is the mould which is created by a combination of sand, clay, and water with the mixture being moist and containing a binder to maintain its shape (Reibel, 2020). This sand mixture is called green sand. To form the mould, a pattern is required and is normally easily machined by wood or aluminium. The pattern has the shape of the part, which is usually oversized to allow for shrinkage of the metal during solidification and cooling. The sand mixture is then packed around the pattern to the desired shape. Vibrations are applied to free the mixture from air bubbles and once the mould has been hardened and dried, it is ready for the casting process (Reibel, 2020). When the pattern is removed, the remaining cavity of the packed sand has the shape of the cast part. This process will be repeated for the other half of the cast. If necessary, a core can be made and placed within the cavity. For protection, the core can be painted to seal the gas that has formed during the process within the core and aluminium reinforcing rods are used to give more strength to the core (Blogger, n.d.). These rods get melted from the molten metal in the next stage of manufacturing. 2. Clamping: To prepare to pour the molten metal, the surface of the mould cavity must be lubricated to make it easier to remove the casting. The two halves of the mould are subsequently joined together and tightened using clamps to endure the pressure the molten metal creates when being poured (Blogger, n.d.). It is very important to make sure that both halves of the mould are secured tightly. 3. Pouring:

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Once the two moulds are secured tightly together, the metal is heated to the molten temperature sufficient for casting by a heating furnace. Once heated, the molten metal is poured into the mould through a channel called the sprue in the gating system which directs the flow into the mould cavity (Blogger, n.d.). Since shrinkage occurs during the solidifying process as a result of thermal expansion, additional molten metal is poured into the mould through the use of a riser to compensate for the shrinkage. 4. Cooling: After making sure the molten metal has flowed into all the regions of the mould, it is then left to cool and solidify. This is the step where most of the possible defects occur since some of the molten metal may cool faster which can lead cracks or incomplete sections (Blogger, n.d.). 5. Removal: After the cooling time has elapsed and the casting process is over, the sand is removed from the casted engine block through the application of vibrations onto the casting (Blogger, n.d.). At this stage, once the sand is removed, the casting will have some lingering sand on the surface. 6. Cleaning: To further refine the engine block, the block is passed through machines such as computerised milling machines and boring machines to get the correct dimensions and surface finish necessary. Alternative Process – Die Casting An alternative manufacturing process for the Victa 2 Stroke engine is die casting. Die casting is a permanent mould casting process in which molten metal is injected into the mould cavity under high pressure. The pressure is maintained durin...