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 to the position of the crankshaft when the piston is in its topmost position i.e., the position closest to the cylinder head ( position 2 Fig)

2. Bottom dead center (B.D.C.) 

This refers to the position of the crankshaft when the piston is in its lowest position, i.e., the position farthest from the cylinder head ( position 3. Fig)

3. Bore.

Diameter of the engine cylinder is referred  to as the bore. 

4. Stroke

Distance travelled by the piston in moving from T.D.C to the B.D.C (Fig) is called "stroke". 

5. Clearance volume

The volume of cylinder (including the combustion chamber) above the piston when its is in the T.D.C position is referred to as clearance volume (Vc)

6. Piston displacement

This is the volume swept by the piston in moving from T.D.C. to B.D.C. This is also called 'swept volume'. If 'd' is the cylinder bore and 'S' the stroke, the piston displacement, Vs is given by,

                                      π

                             Vs= — d².S 

                                     4

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

                                   Vd=Vs.n 

8. Compression ratio

This indicates the extent to volume above   the piston at T.D.C If 'γ' is the compression, then compression ratio, then  

                                        Vs+Vc

                                  γ=――――

                                           Vc

For petrol engine, compression ratios are about 8 to 9.5:1, whereas for diesel engines, these vary form 16 to 22

9. Power 

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

10. Horse Power (HP) 

This is the amount of energy requied to do 4500 Kgm. of work in one minute. 

11. Indicated horse power (I.H.P.)

The power developed within the engine cylinders is called indicated horse power. This is calculated from the area of the engine indicator diagram.

12. Brake horse power (B.H.P)

This is the actual power delivered at the cranksshaft. It is obtained by deducting various power losses in the engine from the indicated horse power.

13. Engine torque.

It is the force of rotation action about the crankshaft axis at any given instant of time. It is expressed in newton-metres 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