Definitions Used in Engine Terminology

1. Top dead center (T.D.C.)

In a reciprocating engine, the piston moves to and fro motion in the cylinder. When the piston moves upper direction in the cylinder, a point at which the piston comes to rest or change its direction known as the top dead center. It is situated at the top end of the cylinder.

2. Bottom dead center (B.D.C.)

When the piston moves in the downward direction, a point at which the piston comes to rest or change its direction known as the bottom dead center. It is situated in the bottom side of the cylinder.

3. Stroke (L)

The maximum distance travel by the piston in a single direction is known as a stroke. It is the distance between the top dead center and bottom dead center.

 

4. Bore (b)

The inner diameter of cylinder known as the bore of the cylinder.

 

5. The maximum or total volume of a cylinder (Vtotal)

It is the volume of the cylinder when the piston is at bottom dead center. Generally, it is measured in centimeter cube (c.c.).

 

6. Minimum or clearance volume of a cylinder (V clearance)

It is the volume of the cylinder when the piston is at top dead center.

7. Swept or displace volume (Vswept)

It is the volume which swept by the piston. The difference between total volume and clearance volume is known as swept volume.

Swept volume = Total volume – Clearance volume

8. Compression ratio

The ratio of maximum volume to the minimum volume of a cylinder is known as the compression ratio. It is 8 to 12 for spark ignition engine and 12 to 24 for compression ignition engine.

Compression ratio = Total volume / Clearance volume

9. Ignition delay

It is the time interval between the ignition start (spark plug start in S.I. engine and inject fuel in C.I. engine) and the actual combustion starts.

 

10. Stroke bore ratio

Stroke bore ratio is the ratio of the bore (diameter of cylinder) to the length of the stroke. It is generally equal to one for small engine and less than one for the large engine.

Stroke bore ratio = inner diameter of cylinder/length of stroke

11. Mean effective pressure

The average pressure acting upon the piston is known as mean effective pressure. It is given by the ratio of the work done by the engine to the total volume of the engine.

Mean effective pressure = Work done by engine / Total volume of the cylinder

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