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Old 08-22-2005, 11:46 PM   #1
fluenceINc.
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Default compresion ratio

Hey im 17 years old and am trying to learn. Can someone give me a good overview of how compresion ratios work? Also i would like to know why a turbo charged car needs a lower compresion ratio than a normaly aspriated car.
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Old 08-23-2005, 01:57 AM   #2
hippy
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Check this article out<click>. Compression ratio is the ratio of space in the cylinder when the piston is all the way up to when it's all the way down. Ie- If there were 8 feet of space in a cylinder when the piston was all the way up, then 1 food of space in the cylinder when the piston was all the way down, the compression ratio would be 8 to 1 since it's taking a given amount of area an puting it into 1/8th of the area.

Higher compression ratios let an engine make more power out of a given amount of air and fuel. Kinda like in the article where it shows the potato shooter. If ya have fuel and air in the back of the potato shooter in a given area, a potato will get pushed out of the barrel with a given fource when the air/fuel is lit and expands. If ya have another shooter with the same amount of air and fuel in it but in a smaller area, the potato will get pushed out of the barrel with more force when the a/f is lit(increasing compression from there being a smaller area). The air/fuel will light quicker and expand faster when it's pushed closer together.

Fuel can only take a given amount of stress. Ie-Heat and pressure can make fuel burn spontaniously, and that's not good. By lowering compression when using turbochargers, we make it so we put the same amount of stress on the fuel, but get more air into the cylinder(by pushing air into the engine at a given pressure). This makes it so we can get the same kinda fource(or close to the same force) out of the air/fuel at a lower compression ratio, but with more air and fuel which will make bigger combustion. Course this is just what I've learned, and I'm no expert.

peace

Last edited by hippy; 08-23-2005 at 02:22 AM.
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Old 08-23-2005, 02:29 PM   #3
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thanks
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Old 08-24-2005, 06:51 PM   #4
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Quote:
Originally Posted by hippy
Compression ratio is the ratio of space in the cylinder when the piston is all the way up to when it's all the way down. Ie- If there were 8 feet of space in a cylinder when the piston was all the way up, then 1 food of space in the cylinder when the piston was all the way down, the compression ratio would be 8 to 1
You got that exactly backwards. 8:1 is 8/1 which is the space when the piston is down over the space when it is up. There is no way you could have 8cu ft with the piston up (TDC) and only 1 ft when it was down (BDC). But other than that, what you said is correct.
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Old 08-25-2005, 09:34 AM   #5
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up in my example ment with most space in cylinder and down ment with the least. I thought that would have been easy to figure out since the higher number would be the most spade and the lower number would be the least. Sorry if anyone was confused because I didn't explain in relation to.......

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Old 08-25-2005, 09:38 AM   #6
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Forced Induction Perforamance Tuning by A. Graham Bell is a pretty good read. His other book, Four-Stroke Performance Tuning is also a good read.
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