Nomenclature

There are various terms which are frequently used in an IC engine and are discussed below.

• Cylinder bore (d): The nominal inner diameter of a cylinder is called cylinder bore which is designated by an English letter ‘d’ and expressed in millimetre (mm).
• Piston area (A): The area of the inner diameter of a cylinder is known as piston area. It is measured in terms of square centimetre (cm²) or square millimetre (mm²).
• Stroke (L): The axial distance for which a piston moves inside a cylinder in one stroke is known as stroke or stroke length (Figure 6.3) which is designated by an English letter ‘L’ and measured in terms of millimetre (mm). Figure 6.3 Stroke Length and Stroke Volume in an IC Engine
• Dead centres: The positions of the piston, at the moments when the direction of the piston motion is reversed are known as dead centres. There are two dead centres—top dead centre (TDC) and bottom dead centre (BDC). The farthest position of the piston head from the crankshaft is known as TDC and nearest position of the piston head from the crankshaft is known as BDC as shown in Figure 6.3.
• Displacement/stroke/swept volume (V_s): The nominal volume swept by the working piston when travelling from one dead centre to the other is called the displacement volume. It is expressed in terms of cubic centimetre (cc) and is given by,  
• Clearance volume (V_c): The nominal volume of the combustion chamber above the piston when it is at the TDC is known as clearance volume (V_c) and is expressed in cc.
• Compression ratio (r_v): It is the ratio of the total cylinder volume when the piston is at BDC to the clearance volume.