# Internal Combusion Engines: Frequently Used Terms

You may have been using your vehicle for a number of years or maybe you are new to them; you will frequently come across some terms related to internal combustion (IC) engines. For instance when you are purchasing a new vehicle you will look for the engine’s displacement volume (cc), its horse power, etc.

Here are some frequently used terms related to the engine of your vehicle:

1) Cylinder bore (d): The internal diameter of the cylinder in which the piston reciprocates is called the cylinder bore.

2) Piston area (A): The external diameter of the piston is almost same as the cylinder bore. The area of this external diameter of the piston is the piston area.

3) Stroke or stroke length (L): The piston moves inside the cylinder between its ends with some clearance at both the ends. The distance traveled by the piston inside the cylinder is called its stroke.

4) Stroke to bore ratio (L/d): This is an important parameter used to classify the size of the engine. It is the ratio of the stroke of the piston to the cylinder bore. The engines in which bore is greater than the stroke length are called over-square engines. Since their bore is larger than the stroke length, such engines can operate at high speeds.

5) Dead center: The two extreme positions between which the piston operates inside the cylinder is called the dead center. The Top Dead Center (TDC) is the position where the piston is farthest away from the crankshaft. The Bottom Dead Center (BDC) is the position where the piston is nearest to the crankshaft.

6) Displacement or Swept Volume (Vs) of engine: The volume swept by the piston as it moves inside the cylinder between top dead center and bottom dead center is called the displacement of the engine.

7) Cubic Capacity or Engine Capacity: The total cubic capacity of the engine is the displacement volume of one cylinder multiplied by the total number of cylinders in the engine.

8) Clearance volume: The area in the top portion of the cylinder above the top dead center position of the piston is called clearance volume. In this area combustion of fuel occurs.

9) Compression ratio: The ratio of total volume of the cylinder to the clearance volume of the cylinder is called the compression ratio.

10) Horse power (hp): It is the net power generated by the engine. It depends on the size of cylinder bore and the total number of cylinders in the engine.