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# Compression Ratio and Clearance Volume

6 Jun

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:

H orsepow er

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.

Brake horsepower refers to the amount of usable power delivered by the engine to the crankshaft.  Indicated horsepower can be as much as 15% higher than brake horsepower. The difference is due to internal engine friction, combustion inefficiencies, and parasitic losses, for example, oil pump, blower, water pump, etc.

The ratio of an engine’s brake horsepower and its indicated horsepower is called the mechanical efficiency of the engine.  The mechanical efficiency of a four-cycle diesel is about 82 to 90 percent.  This is slightly lower than the efficiency of the two-cycle diesel engine. The lower mechanical efficiency is due to the additional friction losses and power needed to drive the piston through the extra 2 strokes.

Engines are rated not only in horsepower but also by the torque they produce.  Torque is  a measure of the engine’s  ability to apply the power it is  generating.

Torque is commonly given in units of lb-ft.

Sum m ary

The important information in this chapter is summarized below.

D iesel E ngines S um m a ry

The compression ratio is the volume of the cylinder with piston at

TDC divided by the volume of the cylinder with piston at BDC.

Bore is the diameter of the cylinder.

Stroke is the distance the piston travels from TDC to BDC, and is determined by the eccentricity of the crankshaft.

The combustion chamber is the volume of space where the fuel air mixture is burned in an engine.  This is in the cylinder of the engine.

The following components were discussed and identified on a drawing.

a.       Piston and rod b.        Cylinder

c.       Blower

d.      Crankshaft

e.       Intake ports or valve(s)

f.       Exhaust ports or valve(s)

g.      Fuel injector

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