Diesel Engine Fundamentals
Compression Ratio and Clearance Volume
Clearance volume is the volume remaining in the cylinder when the piston is at TDC.
Because of the irregular shape of the combustion chamber (volume in the head) the
clearance volume is calculated empirically by filling the chamber with a measured amount
of fluid while the piston is at TDC. This volume is then added to the displacement
volume in the cylinder to obtain the cylinders total volume.
An engine's compression ratio is determined by taking the volume of the cylinder with
piston at TDC (highest point of travel) and dividing the volume of the cylinder when the
piston is at BDC (lowest point of travel), as shown in Figure 15. This can be calculated
by using the following formula:
displacement volume clearance volume
Figure 15 Compression Ratio
Power is the amount of work done per unit time or the rate of doing work. For a diesel
engine, power is rated in units of horsepower. Indicated horsepower is the power
transmitted to the pistons by the gas in the cylinders and is mathematically calculated.