Basic engine terminology

 A number of basic terms are used to describe and compare engines. A few commonly used terms are explained here.

1. Top dead centre(T.D.C.)- This refers the position of the crankshaft when the piston is in its topmost position i.e.. the position closest to the cylinder head.

2. Bottom dead centre(B.D.C.)- This refers to the position of the crankshaft when the piston is in its lowest position, i.e., the position farthest to the cylinder head.

3. Bore- Diameter of the engine cylinder is referred to as the bore.

4. Stroke- Distance travelled by the piston in moving from its topmost position to the lowest position is called stroke. 

5. Clearance volume- It is the volume of cylinder above the piston when it is in the top dead centre position. (including the combustion chamber). 

6. Piston displacement- This is the volume swept by the piston in moving from top dead centre to bottom dead centre. It is also called "swept volume". If 'd' is the cylinder bore and 'S' is the stroke, the piston displacement, Vs is given by- 

7. Engine capacity- This is a total piston displacement or swept volume of all the cylinders. If 'n' is the number of cylinders and Vs is the piston displacement, then 'engine displacement' or 'engine capacity' Vd, is given by, 

8. Compression ratio- This indicates the extent to which the charge in the engine is compressed. This is calculated as the ratio of the volume above the piston at B.D.C. to the volume above the T.D.C. If r is the compression ratio, then 

For petrol engines, the compression ratios are about 8 to 10, whereas for diesel engines, these vary from 15 to 24

9. Mean effective pressure- This is the average effective pressure throughout the whole power stroke. In fact the cylinder pressure varies considerably during the power stroke. Thus it is more helpful to refer to the mean pressure instead. It is expressed in bars or kilo pascals (1bar= 100kPa).

10. Power- It is the work done in a given period of time. Doing the same amount if work in a lesser time would require more power.

11. Indicated power(I.P.)- The power developed within the engine cylinders is called indicated power. This is calculated from the area of the engine indicator diagram. It is usually expressed in kilowatts(kW).

12. Brake power- This is the actual power delivered at the crankshaft. It is obtained by deducting various power losses in the engine from the indicated power. It is measured with a dynamometer and is expressed in kilowatts(kW). It is always less than the indicated Power, due to frictional and pumping losses in the cylinders and the reciprocating mechanism. 

13. Engine torque- It is the force of rotation acting about the crankshaft axis at any given instant of time. 

    It is given by, 

T=Fr

where, T= The engine torque in N-m

            F= Force applied to the crank

            r= Effective crank radius in 'm'

as the value of 'r' varies during the power stroke, the torque on the power stroke is continually varying. Moreover, there is no torque delivered during the three idle strokes. Therefore the engine manufacturer always quote the average value of torque throughout the engine cycle. 

Engine torque goes through the vehicle transmission system, to the road wheels and is responsible for rotation of the latter and hence for pulling of the vehicle.  

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