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03182011, 09:38 AM  #1  
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Help me for design calculations
Hello everyone
I'm little trouble doing the design calculations for an 75cc engine which i'l be using for my project. Its specifications are 1.Bore=46mm 2.Stroke length=42mm 3.Piston dia=46mm things to be found, 1.I need to calculate the original cc of the engine 2.Force on the piston 3.Input and output torque Thank you all 

03192011, 09:38 AM  #2  
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Re: Help me for design calculations
Pls help me guys


03192011, 10:10 AM  #3  
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Re: Help me for design calculations
Single cylinder engine? Look here to calculate engine displacement.
http://www.mathsteacher.com.au/year9...r/cylinder.htm As for the others? Force? Torque? These are dependent on may factors in engine design and cannot be calculated from the minimal details given. 

03192011, 12:02 PM  #4  
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Re: Help me for design calculations
This took a little time to remember, but here's what I did:
Total volume is given as 75cc. Compressible volume is the calculated cylinder: PI*(46/2)^2*42=69800 cubic mm=69.8cc Compressed volume would be 7569.8=5.2cc Determine compression ratio: 75/5.2=14.423:1 At intake cycle, assume air pressure in cylinder is 1 atmosphere (atm). Thus, compressed pressure is 14.423 atm. Also assume that the air/fuel mix is compressed adiabatically...the mix doesn't heat up due to compression. Let's also assume for simplicity that gasoline is composed of octane C8H18 and in gaseous form and combustion is complete. The chemical equation for it's combustion is: 2•C8H18 + 25•O2 > 16•CO2 + 18•H2O The ratio change in gas pressure will be 34:27 after combustion. Remember though that only the 21% oxygen in the air is consumed. So, only that partial pressure will be affected. The partial pressure before combustion is: 21% * 14.423 = 3.029 atm and after combustion is: 3.029 * 34/27 = 3.814 atm Assuming adiabatic combustion, total chamber pressure is: 14.423*0.79 + 3.814 = 15.208 atm Maximum force on the piston is just after TDC. Area of piston head is PI*(46/2)^2= 1661 mm^2 Using conversion from atm to pascals (Pa or N/mm^2), force would be: 15.208*(.101325 N/mm^2)*1661= 2559.6 N Now assume that maximum torque occurs half way through the combustion cycle. Chamber volume is: 5.2 + 34.9 = 40.1 cc Assuming adiabatic expansion, use gas law to determine new pressure: P1•V1 = P2•V2 15.208•5.2 = P2•40.1 P2 = 1.972 atm Torque arm length is equal to the crank throw or 1/2 the stroke length, 21mm or 0.021 m. Torque is calculated to be: 1.972*.101325*1661*0.021= 6.97 N•m Calculating the torque at just past TDC would require some force vector calculation, but this is a good start I think. Hope this helps! Man, what a way to celebrate my 2,000th post!
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03192011, 05:51 PM  #5  
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Re: Help me for design calculations
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03192011, 06:58 PM  #6  
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Re: Help me for design calculations
Good post, jd,
What I was trying to get at was the engine configuration. A 14.4:1 compression ratio is virtually unusable in a 4 stroke configuration, which tells me the engine design is actually a 2stroke. One can apply the same math if one knows the height of the ports in the cylinder wall. The engine would only develop compression after the ports have been closed by the piston moving upwards. For example, if the top of the port is halfway up the cylinder wall, the effective compression ratio would be cut in half, or 7.2:1. Furthermore, the total compressible volume of the engine would also be cut in half, reducing the total fuel/air mixture available for combustion. 

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03202011, 11:29 AM  #7  
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Re: Help me for design calculations
Yes, the high compression ratio isn't normal for a 4stroke and not for a small engine that was given. But I just rolled with what I was given. At the least, I laid out the major steps of the thought process. Thanks for the nod.
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03252011, 06:40 AM  #8  
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Re: Help me for design calculations
Thank u ppl for helping me, its a 2 stroke engine only. These calculations were of gr8 help. Million of thanks


03252011, 11:05 AM  #9  
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Re: Help me for design calculations
Your very welcome and good luck on your project.
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03262011, 12:59 AM  #10  
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Re: Help me for design calculations
Dear jdmccright, in this eqn PI*(46/2)^2*42=69800 cubic mm=69.8cc, what is PI and what value hav u substituted for it.
Thanks 

03292011, 01:19 PM  #11  
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Re: Help me for design calculations
PI is pi. 3.14159265.... If you have to ask that question, then I would say that you have more than a little trouble on this project's calculations. Good luck.
[EDIT] I will also say that you can adjust the cylinder pressure for the change in gas temperatures as it is compressed and combusted using the same Ideal Gas law: PV=nRT, but you'd have to estimate the temperatures.
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