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Old 03-18-2011, 09:38 AM   #1
Dwarakesh
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Smile 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
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Old 03-19-2011, 09:38 AM   #2
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Smile Re: Help me for design calculations

Pls help me guys
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Old 03-19-2011, 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.
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Old 03-19-2011, 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 75-69.8=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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Last edited by jdmccright; 03-19-2011 at 12:19 PM. Reason: Added stuff
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Old 03-19-2011, 05:51 PM   #5
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Re: Help me for design calculations

Quote:
Originally Posted by jdmccright View Post
Man, what a way to celebrate my 2,000th post!

There are considerably less intelligent ways to do it


Now we wait while Curtis checks your math...........
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Old 03-19-2011, 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 2-stroke.

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 half-way 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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Old 03-20-2011, 11:29 AM   #7
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Re: Help me for design calculations

Yes, the high compression ratio isn't normal for a 4-stroke 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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Old 03-25-2011, 06:40 AM   #8
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Thank u ppl for helping me, its a 2 stroke engine only. These calculations were of gr8 help. Million of thanks
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Old 03-25-2011, 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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Old 03-26-2011, 12:59 AM   #10
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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
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Old 03-29-2011, 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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Last edited by jdmccright; 03-30-2011 at 12:11 PM. Reason: Added info
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