What is a compression ratio anyway? Is it the same as A/F ratio? What is the difference between a 10.1:1 and 13.9:1? What does it all mean?

 

Compression ratios are calculated with fairly complex measurements and mathmatic equations.

But basically, you know a piston goes up and down, right? You know that the battery and starter motors turn them to start, and then they get going on thier own, with aid of fuel and air, and compression, and spark.

You have a 4 stroke engine. First, its at TDC (Top Dead Center, meaning the piston is as far up in the combustion chamber as it can be). Then it intake valves open, the piston goes down, sucking air in (fuel comes with the air, the ratio of Air to Fuel is the Air/Fuel, or A/F Ratio). That is the "Intake Stroke". At the bottom (BDC or Bottom Dead Center), the combustion chamber will be full of air and fuel, The volume of this air in fuel can loosely be defined as the "displacement" of that cynlindar. Then, the valves close, and the piston comes back up, compressing the air and fuel. That's the "Compression Stroke". When the piston is as far up as it can go, at TDC again, the volume of the compressed air can also be recorded. The ratio of the volume at BDC, to volume at TDC is the Compression Ratio. The more compressed the air and fuel is, the easier it is going to be to ignite, and the more power it will produce.

When to air and fuel is compressed as far as it's going to be the spark plug ignites the fuel and air. That explodes the piston back down the chamber, pushing the crank around, producing power (That's called the "Power Stroke"). When it's gone exploding, and is at BDC again, the Exhaust valves open up, and the piston drives back up the chamber, pushing all the gases out (Called the "Exhaust Stroke"). At the Top, The exhaust valves close, the Intake valves open, and it starts all over again.


Of course, some of this stuff is easily changed, and tuned, but for the sake of simplicity, that's the idea.

So a 10.1:1 compression ratio would mean the engine compresses 10.1 "units" into 1 "unit". So a 13.9:1 ratio is compressing the air/fuel MUCH more.

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This is the only part I needed. The rest I knew. Thanx.

 

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