What is Compression Ratio and How Do I Calculate It?

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If you're not familiar with combustion engines, you probably don't know the term "static compression ratio," let alone know how to calculate one. If you are learning about combustion engines, how to calculate static compression ratios is valuable knowledge to have under your belt.
The first thing you need to know is that there are two kinds of compression ratios–static compression ratios and dynamic compression ratios.
The static compression ratio represents the ratio between the volume of the combustion chamber and the cylinder when the piston is at the bottom and top of its stroke, whereas the dynamic compression ratio is a more comprehensive calculation that also takes the gasses flowing in and out of the cylinder (or cylinder pressure) into account.
Here's what you need to know about static compression.

What is compression ratio?

The first part of understanding compression ratio is knowing the definition of a "ratio." A ratio indicates how many times one number goes into another number. So, a ratio is essentially a comparison of two quantities.
The two quantities compared in a static compression ratio are the volume of the combustion chamber and cylinder volume when the piston is at the top of its stroke and when it is at the bottom of its stroke.
These ratios are used to determine how well the air fuel mixture is being utilized by the engine.
When it comes to a static compression ratio, a big gap between the comparable numbers is desirable because that represents more energy being generated by the piston cycle.
The ideal static compression ratio varies depending on the combustion engine.
  • Gasoline engines usually range between 8:1 and 12:1.
  • Direct-injection diesel engines usually range between 14:1 and 23:1.
  • Indirect-injection diesel engines usually range between 18:1 and 23:1.
The type of fuel your vehicle uses also influences the static compression ratio. Higher octane fuel, for instance, contributes to a higher compression ratio.
Key Takeaway: The compression ratio is a comparison between the combustion chamber and cylinder volumes at the top and bottom of the piston strokes. Generally, a bigger gap in numbers is better for this ratio, but the ideal range varies depending on the kind of fuel and engine used.

How to calculate static compression ratio

To calculate the compression ratio, you need two values–displacement volume and clearance volume. These values allow you to determine a ratio between the volume of the combustion chamber and cylinder at the top of the piston stroke (pre-compression) and at the bottom (post-compression).
Displacement volume is the amount of air fuel mixture that is displaced as a result of the piston pushing down. The clearance volume is how much mix (or area) remains when the piston is at the bottom position. The formula to calculate compression ratio is: (displacement volume + clearance volume) / clearance volume.
For example, if the displacement volume is 20 and clearance volume is 5, the equation is: (20 + 5)/5 = 5. That means the ratio is 5:1. This is a lower compression ratio, which would indicate that the amount of energy drawn from the piston cycle is poor.
If you calculate your static compression ratio and discover that your piston volume and combustion chamber volume are low, it's a good idea to take your vehicle to a professional auto mechanic to find the root cause of your combustion engine's low compression ratio.
Key Takeaway: You can get the compression ratio by adding the displacement and clearance volumes together, then diving their sum by the clearance volume. If the piston and combustion chamber volumes turn out to be low, you should get your car professionally checked
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