Compression Ratio Of IC Engine in Simple Words Compression Ratio Of IC Engine in Simple Words. This is the main topic of today's post. Here, you will learn about the compression ratio of different combustion engines. I will cover important points like, what is compression ratio in automobiles ? Why do petrol and diesel have different compression ratios? All such types of questions are simplified in this article. So, let's dive in. What is Compression Ratio in Automobile? It is the ratio between the volume of the cylinder when the piston is at BDC to the volume of the cylinder when the piston is at TDC. This is known as the compression ratio in automobiles. TDC and BDC are the top and bottom positions of cylinders where the piston moves up and down . The full form of TDC is Top Dead Center and BDC is Bottom Dead Center . Here, the cylinder is referred to as the combustion chamber of the petrol or diesel engine. Have a look at the image - there you will easily understan...