Tech topic #1

[Blue Mule]

Hi m8s

Question;

Whats the difference between compression ratio and compression? ve heard people often mix those two terms...

 

[Blue Mule]

Is there any difference?

 

[telford_mike]

No difference:

e.g. a ratio of 10 to 1 = 20 to 2

compression is compression, when expressed relative to the original volume of the cylinder (or whatever) its a compression ratio.

 

[BenR]

lost me a bit.

CR is a quantative value and compression is a process.

In what context are we talking of compression?

 

[Blue Mule]

Ive heard that people mix those two terms in this way:

compression ratio of my car is 9:1
compression of my car is 9:1

Im told that those two terms are not the same...

 

[Infinicar]

Compression ratio is the relationship between the cylinder at zero compression (BDC) and volume of the cylinder at maximum compression (TDC)

So in simple terms, if the cylinder capacity is 100cc in total, and 10cc when compressed, thats a compression ratio of 10:1 - The fact the two terms of are often used to mean the same thing, whilst technically wrong, is just an idiosyncrosy of tech talk about cars, which is often overlooked.

Compression is the process of the cylinder moving up the bore from BDC, "squashing" the air and fuel into a volume x times smaller (x = the comp ratio ) than the cylinder size.

I hope ive explained this clearly enough - trying to relate the two terms in their correct contexts.

Paul

 

[Blue Mule]

OK Mitch, thanks a lot

 

[Nick Read]

Although compression can be meant as the capability of the engine to maintain a good seal in each cylinder. It really depends what youre talking about, the context of compression. If you have problems with your piston rings or your head gasket, you will lose compression because the pressure which should be building in the cylinder as the piston moves upwards, is being lost through a blowing head gasket or piston ring.