Introduction:

Compression ratio is a crucial parameter that plays a vital role in the performance of internal combustion engines. A higher compression ratio typically leads to better engine efficiency and more power output. In this article, we will discuss how to calculate the compression ratio and understand its significance in engine performance.

What is Compression Ratio?

Compression ratio refers to the ratio of the maximum cylinder volume when the piston is at its lowest point (bottom dead center or BDC) to the minimum cylinder volume when the piston is at its highest point (top dead center or TDC). In simpler terms, it’s the relationship between cylinder volume before and after compression.

Calculating Compression Ratio:

To calculate the compression ratio, you’ll need to know two key measurements: the total volume of a cylinder in an engine (V1) and the clearance volume left after compression (V2). The formula for calculating compression ratio can be defined as follows:

Compression Ratio (CR) = V1 / V2

where,

V1 = Volume of the cylinder when piston is at BDC

V2 = Volume of the cylinder when piston is at TDC

Step-by-Step Guide to Calculating Compression Ratio:

1. Determine V1:

To get V1, you need to know the total volume of a cylinder in an engine. This can be calculated using the following formula:

V1 = π/4 × B² × S

where,

π (pi) = 3.14159265359 (approximately)

B = Bore Diameter

S = Stroke Length

2. Determine V2:

To find V2, you’ll need to measure clearance volume – which is the total volume of combustion chamber with piston at TDC.

The simplest way to obtain V2 is by using a graduated burette filled with liquid – such as paraffin oil or alcohol. Place it inside a spark plug hole and then move the piston to TDC. The amount of liquid that gets displaced into the burette represents V2.

3. Calculate Compression Ratio:

Now, use the formula mentioned above to calculate compression ratio:

Compression Ratio (CR) = V1 / V2

Example:

Consider an engine with a bore diameter (B) of 87 mm, a stroke length (S) of 84 mm, and a clearance volume (V2) of 55 cm³. Here’s how you can calculate the compression ratio:

1. Calculate V1:

V1 = π/4 × B² × S

V1 = π/4 × (8.7²) × 8.4

V1 = 4907.78 cm³ (approximately)

2. Calculate Compression Ratio:

CR = V1 / V2

CR = 4907.78 / 55

CR = 89.23

In this example, the compression ratio is approximately 89.23:1.

Conclusion:

Calculating compression ratio is an essential step for understanding how an internal combustion
engine works and how its performance can be improved. By accurately calculating the compression ratio, you’ll be able to optimize your engine and achieve better efficiency and power output.