Group 7 - Lawn Mower Engine
Contents |
Tecumseh 3 H.P, 4-Stroke Lawn Mower Engine: Model Number TNT100
Request for Proposal
Page Contents
- Initial Product Assessment
- Management Proposal
- Work Proposal
- Evaluation
- Dissection Tools
Preliminary Design Review
- Product Disassembly
- Causes for Corrective Action
Product Analysis
Causes For Corrective Action
During this phase of the group project, we have encountered some of the most serious and challenging issues to date. Firstly, due to an increase in club meetings, work, exams, and outside the classroom projects in other classes, several group meetings have been cancelled and others shortened. This problem was overlooked as an increase in e-mail, texting, and other methods of communication were implemented to help alleviate some of the missed meeting time. The second major issue that arose was the loss of group member Dharan Shah. Due to a family emergency, Dharan, who had already been struggling to meet during group time due to his strenuous family job, resigned the course on November 13, 2009, leaving his portion of this gate to even be started. A quick meeting the following Monday determined that each group member would attempt to complete one of the four major sections of the gate, meaning some work would be difficult to divide evenly due to the fact that the engine parts could not be with multiple group members at once over the break. Modeling lead, Christopher Germain was to take over the 3-D modeling process but due to continually increasing difficulties with the Inventor program on his computer, and finally a total loss of work over Thanksgiving break during the Windows 7 upgrade, the group has struggled to find a solution to the problem.
Component Summary
Different Materials were used in the production of different components for several reasons. For the majority of the components, the materials used were chosen based on physical properties. In areas that are subject to high heat, rapid movement, or extensive use, the materials used are usually Iron and steel, sometimes hardened. The connecting rod experiences high heat, but is composed of cast aluminum which may not be as resilient to heat as steel or iron, but cast aluminum is still the best choice because of the incredibly rapid motion and light weight. Other reasons for choosing certain materials are strength, ease of manufacturing, cost, and availability of materials.
In the case of the gas tank, plastic is the most appropriate material. If steel had been used, the gas tank would have a high probability of rusting, and also the large amount of steel needed would prolong the manufacturing process and increase cost while decreasing output on the assembly line. The gas tank is not a complex component, but as far as the injection molding goes, the shapes are too complex to use steel for. Injection molding steel to fit these shapes would also decrease assembly line output and increase cost.
The bolts on the gas tank experience shear. This is due to the movement of the liquid inside the tank when the engine is actually in use.
The general material choices of components have some effect on manufacturing processes. For example, injection molding, which is used for the gas take, is possible with steel, but would be exceptionally difficult if done with any other material than plastic. The melting point of steel is quite high so the mold itself would have to have an even higher melting point to withstand the heat applied to the mold. This would also drive up costs and inhibit manufacturing.
All components are functional, not cosmetic, as engine does not usually show when installed in lawnmower.
| Part # | Component Name | Quantity | Material(s) | Manufacturing Process | Function | Image |
|---|---|---|---|---|---|---|
| 1 | Gas Tank | 1 | Plastic | Several pieces are Injection Molded and fused together with heat. | Acts as a reservoir for the fuel that the motor will consume during operation. |  |
| 2 | Gas Cap | 1 | Aluminum | Rolled, Stamped | Covers the fill hole on the gas tank to keep any fuel from spilling. | |
| 3 | Engine Shroud | 1 | Steel | Sheet metal Forming | Protects users from rotating fan blades and other hazardous parts, also for cosmetic looks. |  |
| 4 | Dip Stick | 1 | Aluminum, Plastic | Injection Molded and Rolled Sheet Metal. | Used to measure the amount of oil in the system and keep oil from leaking. |  |
| 5 | Fill Spout | 1 | Plastic | Injection Molding | Provides a channel for the oil to be poured into the system. | |
| 6 | Carburetor Assembly | 13 Pieces, each found only once per engine. | Various | Various | Controls the flow of fuel and air into the combustion chamber for ideal engine performance. |  |
| 6a | Carburetor Housing | 1 | Iron | Die Cast Iron | Individual components of the carburetor are mounted on this, which is in turn mounted to the engine block | See above image |
| 6b | Float Bowl | 1 | Steel | Stamped Steel | Where the fuel is held before being mixed | See above Image |
| 6c | Drain Screw | 1 | Steel | Turned on a Lathe then Died | Holds the float bowl on the carburetor assembly and also drains the float bowl if needed. | See above image |
| 6d | Float | 1 | Plastic | Injection Molded, fused with heat | Monitors how much fuel is in the float bowl | See above image |
| 6e | O-Ring | 1 | Rubber | Extruded Rubber | Seals the joint between the float bowl and carburetor housing | See above image |
| 6f | Float Pin, Needle and Spring Clip | 1 each | Steel | Steel is rolled into stands and cut for all 3, the spring is then heated and bent, while the needle is stamped. | Holds Float in place and spring forces the float to hold in the right position in the fuel. | See above image |
| 6g | Primer Bulb | 1 | Rubber | Extruded Rubber | Using suction of air, forces additional fuel and air into the carburetor to aid in starting the engine | See above image |
| 6h | Throttle Lever, Washer, Shutter, Shutter screw | 1 each | Steel | The lever is a combination of a cut steel rod welded to a punched head, the shutter is a stamped plate, the washer is also a stamped, and the screw is turned on a lathe and died. | Opens and closes to control air and fuel flow into the carburetor | See above image |
| 7 | Manifold and Throttle Mount Assembly | Various | Various | Various | Provides a channel for the exhaust of the engine to be directed out and provides a place for the throttle controls to be held in place on the engine block |  |
| 7a | Manifold | 1 | Iron | Die Casting | Provides a channel for the exhaust of the engine to be directed out | See above image |
| 7b | Manifold Gasket | 1 | Composite | Composite is rolled into sheets and the gasket is stamped out. | Seals the manifold to the engine block | See above image |
| 7c | Throttle Mount Brackets | 1 each, 2 total | Steel | Rolled steel is stamped and formed | Holds the throttle controls onto the engine block | See above image |
| 7d | Nuts, Bolts, Screw | 2 Nuts, 2 Bolts, 1 Screw | Hardened Steel | The bolts and screw are Extruded and then died while the bolts are extruded and tapped | Hold the manifold and throttle mounts at various points | See above image |
| 8 | Starter Assembly | Various | Various | Various | The pull cord gives the engine the initial spin and power required to start it while the rest of the assembly holds and contains the pull cord |  |
| 8a | Starter Mounts | 1 each, 2 total | Steel | Stamped Steel and machined holes. | Holds the Starter Assembly onto the engine block | See above image |
| 8b | Starter Handle | 1 | Plastic | Injection Molding then drilled | Provides a comfortable grip on the pull cord to start the engine | See above image |
| 8c | Starter Pulley | 1 | Plastic | Injection Molded | Keeps the pull cord in a neat and unknotted manner | See above image |
| 8d | Gear and gear clip | 1 each | Plastic, Steel | The gear is injection molded and the clip is formed from cut, rolled steel. | Engages the starter to the Crank Shaft | See above image |
| 8e | Pull Cord | 1 | Nylon | Machine wound strands | Operator pulls the cord to spin the starter and engage the gear to start the engine | See above image |
| 8f | Washers | 2 | Aluminum | Stamped | Space the components of the assembly to reduce rub and wear | See above image |
| 8g | Housing Cover | 1 | Steel | Cast, then machined | Covers the tension spring in the Starter Assembly | See above image |
| 8h | Screws | 2 | Steel | Extruded then Died | Holds the cover in place | See above image |
| 8i | Tension Spring | 1 | Steel | A cut steel sheet is heated and wound into the coil. | Sends the starter assembly back to its initial position to allow for repeated use. |  |
| 9 | Cylinder Head | 1 | Iron | Die Cast then machined, drilled | Covers the piston assembly in the engine block |  |
| 9a | Bolts | 8 | Hardened Steel | Extruded Steel is died | Mounts Cylinder Head onto Engine Block | See above image |
| 9b | Gasket | 1 | Aluminum | Stamped, drilled | Seals the Cylinder head to the engine block |  |
| 10 | Blade Mount, Pulley, Key | 1 each | Steel | Cast steel, machined, the key is cut from solid stock | Drives the mower blade and holds the mount and on the base of the crank shaft |  |
| 11 | Crank Case Cover | 1 | Iron | Die Cast, machined | Conceals internal components of the engine block |  |
| 12 | Governor Assembly | Various | Various | Various | Monitors the speed of the engine. |  |
| 12a | Gear | 1 | Plastic, aluminum | Injection Molding, machine assembled with rivets holding counter weights in place | Monitors the speed of the engine. | See above image |
| 12b | Mount, washers, clips | 1 mount, 2 washers, 2 clips | Steel | Machined, Turned on Lathe, stamped | Holds the governor gear | See above image |
| 13 | Camshaft | 1 | Iron and Steel | Die cast Shaft is then Assembled with a steel wound spring and a steel stamped mount | Opens and closes the valves at the proper time in conjunction with the speed of the motor |  |
| 13a | Balance Shaft and Sleeve | 1 each | Steel and Plastic | Injection Molded sleeve, The Shaft is turned from solid stock | Balances Camshaft while governor and camshaft are engaged | See above image |
| 14 | Flywheel | 1 | Iron | Die-Cast and machined to specifications | Generates current in electromagnet to spark the spark plug, air also hits fins to keep engine cool |  |
| 15 | Electromagnet and wire | 1 each | Various, Steel Aluminum, Plastic, Iron, Rubber, Copper | The magnet is factory assembled and cannot be dismantled without breaking the component, the wire is copper strands woven together with a rubber jacket over them. | Generates current in to spark the spark plug |  |
| 16 | Piston Assembly | Various | Various | Various | Powered by combustion, it forces the crankshaft to rotate and spin the blade |  |
| 16a | Piston Head | 1 | Aluminum Alloy | Cast and machined | Powered by combustion, it forces the crankshaft to rotate and spin the blade | See above image |
| 16b | Compression Rings | 2 | Steel | Punched | Seal the combustion chamber and supports heat transfer from the piston to the cylinder wall | See above image |
| 16c | Oil Rings | 2 | Steel | Punched and machined, cut steel from a sheet | Seal the combustion chamber and supports heat transfer from the piston to the cylinder wall | See above image |
| 16d | Wrist Pin and clips | 1 pin, 2 clips | Steel | Pin is machined from solid stock, Clip is shaped and heated to hold form. | Holds Piston head to Connecting Rod | See above image |
| 16e | Connecting Rod | 1 | Aluminum | Cast Aluminum | Connects Piston to Crank Shaft | See above image |
| 16f | Connecting Rod cap, bolts | 1 cap, 2 bolts | Aluminum cap, hardened steel bolts. | Cast Aluminum, Extruded steel is turned and died. | Connects Piston to Crank Shaft | See above image |
| 17 | Crank Shaft | 1 | Iron | Die-Cast Iron is machined, turned, and brought to specifications | Drives the engine |  |
| 18 | Air Filter Assembly | 1 each | Various | Various | Keeps dirt and contaminate out of engine and combustion system |  |
| 18a | Air Filter Cover | 1 | Plastic and Rubber | Injection Molded Plastic is brought together with extruded rubber | Keeps dirt and contaminate out of engine and combustion system | See above image |
| 18b | Air hose | 1 | Plastic and aluminum | Injection Molded Plastic is brought together with stamped aluminum cover | Keeps dirt and contaminate out of engine and combustion system | See above image |
| 18c | Air Filter | 1 | Composite | Composite is shredded and cut to shape | Keeps dirt and contaminate out of engine and combustion system | See above image |
| 19 | Valve Cover | 1 | Steel | Punched steel | Allows access to the valve springs |  |
| 20 | Valve Assembly | 2 each | Steel | Various | Controls compression and combustion of the engine |  |
| 20a | Valve and Lifter | 2 each | Steel | Extruded steel is machined, turned and welded | Controls compression and combustion of the engine | See above image for valve, lower image for lifters |
| 20b | Valve Spring | 2 | Steel | Rolled Steel is cut and heat wound into desired form | Forces Valves to move up and down | See above image |
| 20c | Valve Spring clips | 2 top, 2 bottom | Steel | Stamped Steel | Holds springs in place | See above image |
| 21 | Various Other Parts | 7 bolts, 2 keys, 4 screws | Steel | Extruded steel is then turned and died, the key is cut from solid stock | Hold various components in place |  |
Design Revisions
1. In general, a lawn mower of this size is used as a push mower, and as such it is preferred if the weight of the engine is minimized while maintaining the structural stability needed to provide enough power to cut the grass at a substantial speed. Cost must also be considered, because generally the lighter the metal the more expensive it is to manufacture; in part due to its demand, as well as complexity of the material. However, upon analyzing some of the components it can be noted that some design changes should be able to be made without too much of an increase in product cost:
- Engine Shroud – currently steel, could be made lighter by using aluminum because it is primarily a protective covering and strength is not much of a consideration
- Pulleys – (those for the belt from the engine to the blades) could also be made of aluminum instead of steel, as the primary component of force is from the tension in the belt which acts towards the axis of the pulley. This compression on the pulley should not act much differently on aluminum than steel
- Gear Shaft – perhaps this could also be made of aluminum instead of steel, but it is suggested that much testing be implemented before this change is put into effect.
2. The Fly Wheel Head showed significant wear in that a major crack formed about where the shaft is bolted on. This implies that this component should actually be made of a heavier or more durable material than what it currently is. Another possible improvement would be to thicken the metal in the area where the cracking occurred to decrease the likelihood of this happening during prolonged use.
3. The Cylinder Head also showed some broken fins on its exterior. This could be due to the fact that it was dropped or not well maintained, but it could mean that the fins should be thickened slightly to increase their durability. On the other hand, this section seems to be designed to dissipate heat quickly, and it could be that the metal used was poorly chosen, and becomes brittle at high temperatures. If this is the case research should be done to find a durable metal that behaves more favorably to heating.
4. Another option would be to increase the size of the Piston Head, as this would allow for potentially more power to be generated by the engine. This is not necessarily recommended because this smaller-sized engine does not really need that much power, and the efficiency of the engine would likely decrease with having to move a heavier piston.
5. Using only one type of bolt or screw would decrease the production costs and make working on the engine that much easier and more efficient. Some components were even attached using different types of screws, and that just doesn’t make sense. Not only would there be fewer screws to make but also the holes bored for them would also all be the same dimensions. The only possible constraint would be if there was a significant amount of force present in one area or the other.
Another direction that the design revisions could make is moving towards modern materials. Carbon fiber is generally very strong and durable, but it is also much more expensive to manufacture. Another question is how well the carbon fiber stands up to high temperatures for extended periods of time. One safe use of carbon fiber could again be the engine shroud, or maybe even the gas tank, if price was not an issue. New composites being developed every day are also potential alternatives, such as this aluminum composite that is potentially stronger and cheaper than carbon fiber, and is apparently immune to metal fatigue!
"Revolutionary Aluminum composite:" http://www.tgdaily.com/trendwatch-features/34052-revolutionary-aluminum-composite-stronger-and-lighter-than-carbon-fiber
Solid Modeled Assembly
Engineering Analysis
A key component for a lawn mower engine is the spark plug, as this provides the spark that ignites the fuel-air mixture above the piston head which allows for combustion to occur. An important aspect in the design of a spark plug is the minimum voltage required to create an electric arc across the gap of the spark plug. The designer must consider how a spark plug works, and then how much voltage is lost in the process of transferring energy from the wire to where the spark occurs. Therefore a problem statement set forth for engineering analysis might be “what is the voltage needed to ignite a spark plug of these specifications?”
The next step would be to enumerate the assumptions made for the system, as well as take measurements of the desired dimensions. For instance, the internal resistance of the wiring may be taken to be negligible. One major simplification of this problem would be to consider the system as a basic transformer, where the current generated initially by the pull start and later by the perpetuation of the engine transfers a given voltage to the spark plug, which must be large enough to cause an electrical arc in the engine. A diagram of the system would help to show the similarities as well as illustrate the differences between a spark plug and a simple transformer. If dimensions such as the length and diameter of the wiring being used are known, these can be used in the calculation of the minimum voltage required. Otherwise educated assumptions can be made for these dimensions, in order to determine if the resulting answer is reasonable. Other variables can be researched, such as the resistivity of the material being used as the wire, the gauge of the wire, or the number of turns used for each coil of the transformer. Another important measurement would be the diameter of the coil itself, in order to determine the approximate length of the wire in each part of the system.
Because this spark plug analysis has been simplified to a simple transformer, the governing equations are also rather simple to evaluate, with the most elaborate analysis involving a dynamic system which would require a first order differential equation to be modeled properly. Other than this, fundamental algebraic equations should be able to be used for this system. These include voltage equals current times resistance (V=I*R) and the definition of resistance in terms of its physical dimensions and resistivity (R=ρ*L/A). With these equations, one can compare the predicted input voltage to a potential output voltage. This output voltage needs to be large enough to allow for the electrons to ‘jump’ from one side of the gap to the other. This voltage of course is determined by the metal involved and the size of the gap to be crossed.
After a probable output voltage has been established, it must be determined if this solution is reasonable. For instance, can the necessary input voltage be supplied from the original pull-start of the engine? This may in fact prompt another analysis question – as to the magnitude of the voltage that is capable to be supplied to the spark plug. These two solutions should validate each other if the assumptions that have been made are correct. Finally, discussion would be made as to whether this answer as well as the primary assumptions and equations used are reasonable and hence could be used as a reference point for the manufacturing of this spark plug. If the solutions found do not make sense, but the assumptions seem to be correct, it could be that the design of the plug needs to be reevaluated, in that the gap is too wide or the number of coils is too few, etcetera. This process would be continued until an acceptable version of the spark plug is developed.
