Project 1 Engine Block for studoc PDF

Preview

Summary

Download Project 1 Engine Block for studoc PDF

Description

Victa 2 Stroke Engine Manufacturing Report Minor Project

Introduction The Victa 2 Stroke Engine has been manufactured for the past 65 years. The Australian based lawn mower company has mass produced these engines for their Victa Lawn mowers, selling over 8 million lawn mowers worldwide also including their 4 stroke engine lawn mowers. The Victa 2 Stroke engine is what drives the grass trimmer, which consists of a blade that spins to cut the grass it is drawn over. The Victa 2 Stroke engine lawn mower is a common domestic lawn mower typically used for mowing garden grass areas as well as commercial mowing of lawns at parks and reserves.

Analysis of Components The engine block of early engine designs comprised of the components of the cylinder, the cylinder head, transfer port, intake and exhaust ports and the crankcase being bolted together to form the block. However a more efficient method of single piece manufacturing of the engine block including all the components in one makes for a more efficient design. Components of the Victa 2 Stroke Engine Block: Cylinder The Cylinder houses the piston and conrod, allowing for a guided passage of movement of the piston, while also hosting the internal combustion of fuel and air mixtures that causes these movements of the piston. This transverse movement of the piston, along with the conrod, within the cylinder, ultimately drives the crankshaft that then drives the blades of the lawnmower. Cylinder Head The Cylinder Head houses the spark plug and along with the cylinder, also helps to contain and host the internal combustion of the fuel and air mixtures. The Cylinder Head is the casing for the cylinder, closing in the cylinder for the combustion process to take place. The sparkplug is housed through the cylinder head and into the cylinder where it periodically ignites the fuel air mixtures in the combustion chamber throughout the strokes. Transfer Port The Transfer Port transfers fresh fuel and air mixture from the crankcase to the area of the cylinder currently above the piston, where the internal combustion process takes place. When the piston moves downwards it pushes the fresh fuel and air mixture, which has been transferred to within the crankcase through the intake port, up through the transfer port and into the combustion chamber of the cylinder.

Intake Port The Intake Port is the port that deposits the fuel and air mixture into the crankcase for it to then be transferred via the transfer port to the cylinder for combustion. It usually consists of a valve that allows for periodic opening and closing of the port throughout the strokes. Exhaust Port The Exhaust Port is the port that deposits the burnt and post-combustion gases out of the cylinder and out through the exhaust. It consists of a run off pipe leading to the exhaust, and the gasses escape through the pipe after the combustion has occurred. Crankcase The Crankcase protects the crankshaft and contains the fuel air mixture after it is deposited from the intake port. It is also where the counter weight is protected along with the crank pins and journal.

Justification of Required Accuracy and Surface Integrity The overall rendering of the engine block is unique to the various components and features. The externals of the engine block do not require such precision as they are mainly static or have minimal movement against surrounding parts, as the engine block is stationary on the lawn mower, with only vibrations being the dynamics it undertakes. However there are places where accuracy is paramount. This is because moving parts usually require certain finishes of low friction depending on application and also certain finishes for the sealing of components. These places include the connection plate with the cylinder, where it bolts on, the internal cylinder wall where the piston moves and the diameter of the crank shaft housing. Machining methods of achieving the required accuracy and surface finishes include honing, lapping and super finishing. Although high precision and accuracy is typically needed for moving parts, joining of parts usually requires precision as both parts need to be able to fit well. For example, the joining of the cylinder head to the engine block must be accurate as the 1cm join screws, as seen in Figures 1, undergo tensile stress as combustion within the cylinder adds forces pushing against the walls of the combustion chamber, as well as vibrations from the rpms. It is also important to have the walls and edges of the cylinder alignments between the cylinder head component and the engine block, as movement of the piston can be disrupted if misaligned. Also the joining between the crankcase housing and external engine block mount as seen in Figures 1 also require alignment accuracy for the parts to join, where surface finishes between joints should be smooth for fitment accuracy. It is also important to note that attention to the thickness of joining screws and threads should allow for the shear stresses acting on the screws from component vibration when engine is running.

The cylinder, crankcase, bushing housings and bearing housing seen in Figures 2, also require accuracy to allow for efficiency. The cylinder needs to be of correct diameter as well as height to allow for the correct movement of the piston along its cylindrical wall. Also surface integrity is important as the cylinder needs to be smoothly finished for friction reduction as well as provided an accurate fit for sealing the combustion chamber, as the piston propagates when the lawn mower is engaged. The crankcase also needs to be of correct diameter as it needs to cater for the crankpin and counter weights as they move through the crank chamber. The surface integrity also needs to be relatively smooth as the fuel air mixture moves through the chamber and therefore requires low friction walls of the chamber for fluid efficiency. For the bushing and bearing housings for the crankshaft, correct alignment as well as smooth surface integrity is needed for the bushings and bearing to be sealed and fitted.

Surface Integrity (SI) Standards There is a lack of set definitive quantitative relationships between SI and part performance, although there has been numerous research papers published to justify these correlations, including between surface finishes and fatigue life. Except for some cases there is no specificity on set physical, mechanical, metallurgical and chemical properties for surface finishes through machining metals and strengthening metal processes for manufacturing. Therefore there is more flexibility for machining, strengthening as well as other process to meet needs, like dimensions, surface finishes and stress requirements in manufacturing to help better satisfy target manufacturing costs. However there are various ISO (international) standards of texture and surface parameters. The parameters presented, with brief definitions and comments, in Sections 2.2.1 and 2.2.2 are mostly included in ISO 13565-2: 1997 standard, which is based on the “M” (mean line system). (Davim, 2010) ISO Standards on Surface Finish: • ISO 1302 – 2001 Indication of Surface Texture • ISO 3274 – 1996 Nominal Characteristics of Contact (Stylus) Instruments • ISO 4287 – 1997 Terms, Definition and Surface Texture Parameters • ISO 4288 – 1996 Rules and Procedures for Assessment of Surface Texture • ISO 5436-1 – 2000 Calibration, Measurement Standards • ISO 5436-2 – 2000 Calibration, Soft Gages 2 Surface Texture Characterization and Evaluation Related to Machining 49 • ISO 8785 – 1999 Surface Imperfections – Terms, Definitions and Parameters • ISO 11562 – 1996 Metrological Characteristics of Phase Correct Filters • ISO 12179 – 2000 Calibration of Contact (Stylus) Instruments • ISO 12085 – 1996 Surface Roughness and Waviness – Motif Method • ISO 13565-2 – 1996; Geometrical Product Specifications (GPS) − Surface texture: Profile method; Surfaces having stratified functional properties − Part 2: Height Characterization using the linear material ratio curve.

Material As the engine block is required to withstand internal and external forces, a high strength, resistant to thermal expansion and wear resistant metal is required. As the 2- Stroke Victa Lawn Mower is a mass produced product the engine block for it must also be cost effective to save manufacturing costs, therefore the choice of metal needs to be inexpensive but still strong and durable. Therefore the requirements of the engine block material are:      

Manufacturability Inexpensive Resistance to thermal expansion Good thermal conductor Strong Wear resistant

Some possible candidates for material selection include:  Cast Iron  Aluminium  Magnesium Alloy  Mild Steel  Titanium

Selection Criteria for Engine Block Material 1. Manufacturability The main determinants of manufacturability are are: 







Abundance/Availability- As it is mass produced the abundance of the material needs to be high to allow for more units of production. Cast Iron is relatively abundant compared to Aluminium and Magnesium. Castability- As the engine block is cast as part of the manufacturing process, a metal that is easy to cast is needed. Cast Iron is easy to cast into any shapes whereas Aluminium is difficult and tends to oxidize in casting processes. Cast Iron can be sand casted which is an efficient and easy way to cast. Economy- Related to abundance, the cost of Cast Iron is cheaper compared to other metals, averaging at 1.43 USD/kg, compared to Aluminium; 1.50 USD/kg, Titanium 4.20 USD/kg and Magnesium 1.99 USD/kg (2017 Measurements, www.iron-foundry.com) Machinablility, Surface Finish & Lubrication- Cast Iron is easy to machine, with aluminium posing even easier with the added ability to get higher quality finishes.

Conclusion: Cast Iron is abundant, easy to cast and relatively cheap. Aluminium is not as abundant, requires technical casting processes like hot/cold chamber die casting systems, but is only slightly more expensive than Cast Iron and Magnesium alloy is not abundant, lacks Castability and is expensive, Titanium is extremely expensive.

2. Thermal Properties 



Thermal Expansion- With temperatures reaching over 700oC, the engine block will undergo thermal expansion and the block will experience shape deformation. Therefore the selected material should resist thermal expansion as much as possible to hold structurally sound. In Table 0.1, Cast iron shows relatively low thermal expansion, with Titanium showing even lower. However Aluminium’s thermal expansion rate is not ideal with Magnesium with an even higher thermal expansion rate. Thermal Conductivity- To prevent any focused thermal concentration; thermal conductivity of the material should be high. Aluminium shows a high thermal conductivity however its high thermal expansion counteracts its performance. Cast Iron shows an average mark of thermal conductivity.

Conclusion: Cast Iron thermal properties are appealing to the requirements. Aluminium shows a high thermal conductivity however its high thermal expansion counteracts its performance. Titanium’s low thermal expansion rate is appealing however its thermal conductivity is poor.

3. Wear & Corrosion Resistance The wear resistance of the material must be high as the engine block undergoes stresses and wear from environmental conditions as well as internal conditions. Surface hardness is important for wear resistance as it allows for the material to withstand wear. Anti Wear Additives provide for high surface hardness of a material, with additives like phosphorous being commonly added to gray cast iron. Zinc is another element additive that helps provide hardness to resist high loads and provide durability.

4. Weight The weight of the engine block material can affect the efficiency of the lawn mower as a heavy engine block will reduce the ergonomics of the lawn mower. However it is not a requirement for the engine to be super lightweight like race car engines, but reasonable. Cast Iron is heavier than Aluminium, but for the size of the engine block of the Victa 2 stroke it is within functional reason.

Table 0.1

Conclusion Grey Cast Iron satisfies the criteria performance the highest out of the other materials, and provides for a sufficient fit for the Victa 2 Stroke engine block’s material. Grey cast iron contains 2.5 – 4 % of carbon, 1 -3 % of silicon, 0.2 - 1% manganese, 0.02 - 0.25 % of sulfur, and 0.02 - 1 % of phosphorus.

Manufacturing The proposed initial (casting) manufacturing process of the engine block is Sand Casting. Sand casting is a relatively simple casting method that is easy to cast with Grey Cast Iron. It is also relatively cheap among other advantages, making it roughly the most common casting technique for metal casting accounting for 70% of all metal casting, being sand casted. The process of sand casting, known as the Cope and Drag, is as follows: 1.

2.

3. 4. 5. 6.

Place a pattern in sand to create a mould - a skilled pattern maker creates a pattern (model) of the design out of simple materials like wood, metals and polystyrene. The metal as it is casted, will contract, therefore the pattern should be made a little larger than the final dimensions. The patterns require core prints that register into sand cores. This is usually done for components that can’t otherwise be easily moulded. Incorporate the pattern and sand in a gating system- The patterned sand moulds are then configured into a gating system, where there a channels for the molten metal to be poured into to cast the engine block. The gating system creates the sand mould cast, that will eventually cast the Grey Cast Iron. Pattern is usually pre coated in talcum powder to aid in removal. The sand is then poured over the pattern and compounded, usually by mechanically vibrating it, to pack down the sand. The pattern is then turned on the other side and repeated however dowel is placed joining the cope and drag to aid in removal. Remove the pattern- The dowels are then carefully pulled from the sand and the cope and drag are separated carefully removing the pattern. Fill the mould cavity with molten metal- The molten metal is poured through the sprue into the cavity and back up the riser. Allow the metal to cool Break away the sand mould and remove the casting

Figure 0.2

The following proposed part of the manufacturing process, is the machining processes. Once the engine block has been casted machining processes help to finish surfaces and improve accuracy of dimensions. After the casting the engine block’s surfaces can be finished by processes like:   

Lathing Honing Lapping

Lathing would be useful for helping to correct dimensions of the crankshaft and bearing housing’s and the cylinder. This would incorporate the use of a Lathe, with various types of lathes with a tracer lathe being an appropriate option for the mass produced Victa 2 stroke engine block. Honing would be used for the smooth finishes of the joining contact plates and the cylinder wall. It involves rubbing or filing down to smoothness of a metal with an abrasive stone. This requires a honing machine. Lapping would be used for smoothing finishes and also accuracy of dimensions. It involves a similar process to honing, but instead uses a tool plate or wheel for improved accuracy and finish quality to honing. This requires a lapping machine.

Alternatives -Die Casting- This provides close size and shape tolerances, high consistency with component dimensions and a reduced need for post casting machining. However this requires high pressures, high energy and is expensive -Investment Casting- This provides greater dimensional accuracy, allows for thin walled parts, can cast both ferrous and nonferrous materials and has a relatively high finish quality. This is a costly process compared to sand casting.

Quality Assurance and Inspection To ensure that the finished sand-casted engine block can be accepted by industry it must be fulfil requirements and meet the standards of:       

Permeability Thermal Stability Sand can be reused, at low energy and cost High enough Strength of Sand Acceptable cooling rates Planning of set risers, so the metal does not cool to quickly Compressibility of the sand

To ensure the finished engine block can be acceptable for function on the market and in intended use, it must meet the machining and finishing standards and requirements of:

   

Smoothness of moving parts Accuracy of dimensions Joining applications must be well fitted Screw joints and holes must be of correct size and applicable

A way to inspect and quality assure the engine block, is to compare it to a control. The control must be of correct dimensions and surface finishes, and acts as a benchmark of quality to compare and ensure the next engine block on the manufacturing line is of quality.

Estimation of Manufacturing Cost 





Time- The time taken to manufacture the engine block adds to costs. With the correct set up of tools and access and availability of materials these costs can be made minimal. Also depending on the scale of production, experience of production can reduce manufacturing times as new manufacturing processes take time to learn. Labour- The amount of work or labour needed to manufacture the engine block needs to be carefully considered. With difficult manufacturing techniques, more experienced workers as well as more technical machinery may be required, which in all will increase costs. Equipment- The more equipment or the more technical the equipment the more costly the manufacturing process is. If the engine block can be made from simple and fewer machines as well as less energy required the less the manufacturing process will cost.

In all maximising efficiency in the manufacturing process is important. Paying close attention to each step of the process while devising and revising new and more efficient methods will reduce Manufacturing Costs.

Appendix

References Richardson, M.V. 2006, Patent Example Victa Lawn Mower, IP Australia, Canberra, viewed 5 September 2017, 2016, Victa, Wikipedia, viewed 5 September 2017 2017,Victa Homepage, Briggs & Stratton, viewed 5 September 2017 < http://www.victa.com.au/> Stein, A. 2017, Function Cylinder Head, It Still Runs, viewed 5 September 2017,

Shourya, S. 2017, What is the use of transfer ports in a two stroke engine, Quora, viewed 5 September 2017, Pateriya, D. 2016, What is the function of exhaust port, Quora, viewed 5 September 2017,

Davim, J.P. 2010, Surface Integrity In Machining, Springer-Verlag, London 2017, Scrap Metal Prices, Scrap Metal Prices, viewed 5 September 2017,

2017, cast-iron-prices-pound-kilogram-ton, Iron foundry, viewed 5 September 2017,

2017, Comparing Casting Processes, Thomasnet, viewed 5 September 2017,

Table 0.1: https://www.scribd.com/doc/98724458/Case-Study-Engine-Block, viewed 6 September 2017 Figure 0.2 https://en.wikipedia.org/wiki/Sand_casting viewed 6 September 2017...