Chapter 4 Principles of Engine Operation 83 Copyright Goodheart-Willcox Co., Inc. Engine displacement is the volume that is displaced by the upward strokes of the engine’s pistons. To calculate engine displacement, multiply the displacement of a single cylinder by the number of cylinders in the engine. Note: There are 61.0237 cubic inches in one liter. To convert cubic inches of displace- ment to liters, simply divide by 61.0327. Compression Ratio The amount of compression developed in a cylinder is expressed by the compression ratio. This ratio compares the cylinder volume when the piston is in the bottom dead center position to the cylinder volume when the piston is in its top dead center position, Figure 4-44. The volume of air inside the cylinder that is compressed is known as the clearance volume. The clearance volume and cylinder displacement can be used to calculate the compression ratio as follows: compression ratio = cylinder displacement + clearance volume clearance volume While cylinder displacement does not change without altering either the bore or stroke, clearance volume is controlled by cylinder head and piston design. Different part options might be available to achieve var- ious compression ratios on a given engine. Care must be taken during engine rebuild to not unknowingly alter the compression ratio. Pressure Pressure (P) is defined as force per unit area. Gas and liquid pressures are normally stated in pounds per square inch (psi). The metric unit of pressure is the pascal (Pa). One psi equals 6,895 pascals, or 6.895 kilopascals (kPa). Atmospheric Pressure The weight of the atmosphere, which is called atmospheric pressure plays a key role in the operation of naturally aspirated diesel engines. It is constantly exerting force on all objects. At sea level, this force creates a pres- sure equal to 14.7 psi. At higher elevations, the pressure is less and may cause engine performance problems. Volumetric Efficiency The amount of air that is actually pulled into a cyl- inder compared to the total volume that could theoretically fill the cylinder is known as the volumetric efficiency (VE). Volumetric efficiency can be calculated using the fol- lowing formula: VE = volume of air taken into cylinder maximum possible cylinder volume Most naturally aspirated engines have a VE between 80% and 90%. This means the amount of air drawn into the cylinder is slightly less than the cylinder’s full capacity. In a turbocharged or supercharged engine, air is forced into the cylinder at pressures greater than atmo- spheric pressure. Consequently, the amount of air forced into the cylinder is not directly related to the piston displacement, and volumetric efficiency does not apply. In the case of a supercharged engine, the term scavenge efficiency is used. Scavenge efficiency is an indication of how efficiently the burned gases are removed from the cylinder and replaced with a charge of fresh air. Stroke length Diameter of cylinder Top of piston Combustion chamber Figure 4-43. Engine bore and stroke measurements. Clearance volume Top Dead Center (TDC) Bottom Dead Center (BDC) Figure 4-44. Compression ratio is the relationship between the cylinder volume when piston is at bottom dead center and the cylinder volume when the piston is at top dead center.