Compression ratio?

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Can someone explane to me about Compression ratio? I dont understand it. Here is a example. The McLaren F1 compression ratio is 10.5:1 The Jaguar XJ220 Compression ratio is 8.3:1. What is the diff? Dose it give it more HP,RPM,torque?
 
I found a website not so long ago that explained it perfectly, but, I can't find it. But this should be in the Automotive Forum anyways. :)
 
Originally posted by Cobraboy
I found a website not so long ago that explained it perfectly, but, I can't find it. But this should be in the Automotive Forum anyways. :)
Click to expand...

And so it shall.

www.howstuffworks.com is always worth a look on this sort of stuff.

From memory I think it's the ratio of air to fuel inside the cylinder at the time the spark ignites the mixture.
 
erm, isn't that the air to fuel mixture ratio?

the compression ratio, iirc, is the volume of the cylinder when TDC to when it is bottomed out. i think. that's a very simple expaination. (sp?)
 
explanation ;).
generally, i believe the higher that redline in relation to engine size (i.e. 7000rpm 7.0 = 4000rpm 4.0) means a generates compression ratio.
 
No, compression ratio refers to how much the air/fuel mixture is squeezed (compressed) in the cylinder. Let's say that we have a cylinder that has a maximum capacity of 100 CCs. That's at BDC (bottom dead center), when the piston is at the lowest point in its travel.

Now, the piston travels to TDC (top dead center) or the highest point in its travel. For the sake of argument (and easy calculations :D) we'll say that the cylinder has a capacity of 10 CCs at this point. So, the A/F mix has been compressed into an area one-tenth its original size - or, it has been compressed by a factor of 10:1. There you have it!

I hope that made some sense. :)
 
It's a ratio of the uncompressed volume of air taken into a combustion chamber to the compressed volume. Let's say a piston is at BDC (chamber volume is as big as it gets) and you have 600cc in there. When the piston is at TDC (minimum volume) let's say you've got 60cc. Your compression ratio is 600:60 or more simply put, 10:1. Or something...
 
Hey, and to explain even more.

Everything Risingson said is right.

A high compression needs higher octane fuel, so it will all burn fast and at the same time. Whereas low octane fuel might ignite from the high pressure before the rest of the ignited fuel gets to it. (If you understand what I just said, give yourself a pat on the back!)

Two things cause ignition of the Air/Fuel mix: A spark, or pressure. A deisel can have compression around 15:1 because diesels dont have spark plugs. They just pressurize all of the air and then inject the diesel which automatically ignites from the immense pressure.

And the reason the Mclaren F1 has a high ratio as opposed the the Jaguar XJ220 is because the Jaguar has a turbo, so it already has alot of extra air crammed in the cylinder, and if it compressed as much as the Mclaren the Air/Fuel mix would ignite before the spark plug went off. It would also be very hard on the engine to put that much stress on it.


I hope that explains it completely to you.
 
Originally posted by j2s
explanation ;).
generally, i believe the higher that redline in relation to engine size (i.e. 7000rpm 7.0 = 4000rpm 4.0) means a generates compression ratio.
Click to expand...

Compression ratio is the same at all rpms, and it doesnt account for turbo or supercharged air.

Moreover, it is simply a measure of how big the chamber was at the largest point and at the smallest point, which was explained already, so I wont ramble.
 
Actually, higher octane fuels have a much higher ignition temperature (resistance to burning). This is to keep the A/F mix from combusting before it's supposed to. Why is that bad? Well, if the mix starts to burn before the piston reaches TDC, the resulting expansion starts pushing the piston back down before its time. This leads to very rough running and eventually (or not so eventually) engine damage.

Oh, and thanks for the quotable. :smilewink
 
Originally posted by risingson77
Well, if the mix starts to burn before the piston reaches TDC, the resulting expansion starts pushing the piston back down before its time. This leads to very rough running and eventually (or not so eventually) engine damage.
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This is also referred to as detonation, or more commonly as pinging or knocking.
 
Originally posted by CrackHoor
It's a ratio of the uncompressed volume of air taken into a combustion chamber to the compressed volume. Let's say a piston is at BDC (chamber volume is as big as it gets) and you have 600cc in there. When the piston is at TDC (minimum volume) let's say you've got 60cc. Your compression ratio is 600:60 or more simply put, 10:1. Or something...
Click to expand...

basically what i said, in fewer words.
 

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