example: 12.0:1.. 12 of what to one of what?? Jw bc I never understood what it meant

 

im in the same boat... im curous as well, i know what the 1 means it think, i think its the num of stroke. 1 full roatation, as for the 12 i have no clue that is what im curious to find out as well,

 

compression ratio compares cylinder volumes with the piston at extreme TDC. AT bottom dead center BDC, a cylinder has maximum volume. at top dead center it has minimum volume. An engines compression ratio controls how tightly the air/fuel mixture is pressurized or squeezed on the compression stroke.

A compression ratio is expressed in relation to 1. for example an engine my have a compression ratio of 9:1 this means the maximum cylinder volume is nine times greater than the minimum cylinder volume.

When a gasoline engine is at BDc the cylinder volume might be 40 cubic inches. when its a TDC its volume might be 5 cubic inches. if you dived 40 by 5 it givesd you 8...that engine has a compression ratio of 8:1

 

thanks

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by stocker2shocker91 &raquo;</TD></TR><TR><TD CLASS="quote">compression ratio compares cylinder volumes with the piston at extreme TDC. AT bottom dead center BDC, a cylinder has maximum volume. at top dead center it has minimum volume. An engines compression ratio controls how tightly the air/fuel mixture is pressurized or squeezed on the compression stroke.

A compression ratio is expressed in relation to 1. for example an engine my have a compression ratio of 9:1 this means the maximum cylinder volume is nine times greater than the minimum cylinder volume.

When a gasoline engine is at BDc the cylinder volume might be 40 cubic inches. when its a TDC its volume might be 5 cubic inches. if you dived 40 by 5 it givesd you 8...that engine has a compression ratio of 8:1</TD></TR></TABLE>

Very good post...

 

ok so if it has to do with displacment, then how does the dome of a piston change the actual compression ratio?

 

because the dome of the piston makes the volume at TDC smaller

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dave_Dc2 &raquo;</TD></TR><TR><TD CLASS="quote">ok so if it has to do with displacment, then how does the dome of a piston change the actual compression ratio?</TD></TR></TABLE>

Engine displacement is defined as the total volume of air & fuel an engine draws during one complete cycle. This would be the total volume of all four cylinders above the compression ring when the cylinder is at its lowest point, bottom dead center. In a four stroke engine, that would be two rotations of the crankshaft. (intake, compression, combustion, exhaust) Increasing cylinder volume is achieved by increasing bore or stroke.

When you add material to the piston above the compression ring, you are decreasing the cylinder volume when the piston is at top dead center. This in turn increases the cylinder pressure. When compression is increased, the area you are compressing air & fuel into is reduced.

Check this out: http://auto.howstuffworks.com/engine1.htm

Look at the diagram. Imagine adding material to the top of the piston.