# Basic Definitions Used in Engine Terminology

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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 top dead center. It is situated at top end of cylinder.

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

3. Stroke (L)
The maximum distance travel by the piston in single direction is known as stroke. It is the distance between top dead center and bottom dead center.

4. Bore (b)
The inner diameter of cylinder known as bore of cylinder.

5. Maximum or total volume of cylinder (Vtotal)

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

6. Minimum or clearance volume of cylinder (Vclearance)
It is the volume of 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 minimum volume of 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 bore (diameter of cylinder) to length of stroke. It is generally equal to one for small engine and less than one for 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 engine.

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