when a piston on the power stroke is coming up, with all valves closed, its compressing a measured amount of air and fuel.

 

wow! captain chrome asking a motor question? a sure sign of the apocalypse


The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber; from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.
In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.
Picture a cylinder with the piston at the bottom of its stroke containing 1000 cc of air. When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.
Higher compression ratios will however make gasoline engines subject to engine knocking, also known as detonation and this can reduce an engine's efficiency or even physically damage it.




:crash

 

http://en.wikipedia.org/wiki/Compression_ratio :thumbup

I didn't want rocalotopus to get credit for something that can easily be found on the internet! :crash

 

I did the "compression for dummies" post

 

LOL

:crash

except you didn't explain anything. like what the number is. :lol


(aron is the fastet man alive and he has the ban stick i fear him greatly!!)

 

it's the ratio of a cylinder's volume with the piston at bottom dead center to the volume with the piston at top dead center.

generally, if you hold all other things constant, a higher number results in more power.

too high a CR results in detonation (ignition before the plug fires) because the fuel / air mixture's temperature rises proportionally to how much it is compressed.