In internal combustion engine, the combustion takes place internally.

**10.2.1 Bore**

The inner diameter of the cylinder is called bore.

**10.2.2 Top Dead Centre (T.D.C.)**

The extreme position of the piston at the top of the cylinder (head side) is known as top dead centre (TDC). In case of horizontal engines this is known as the outer dead centre (ODC). At the dead centres the piston reverses the direction of its motion and its velocity therefore equals to zero at these dead centres.

**10.2.3 Bottom Dead Centre (B.D.C.)**

** **The extreme position of the piston at the bottom of the cylinder is known as bottom dead centre (B.D.C.)

**10.2.4 Stroke**

The distance between the extreme positions of the piston i.e., the distance between two dead centres is called the stroke length. Stroke length is equal to double the crank radius.

**10.2.5 Swept Volume**

The volume swept through by the piston in moving between top dead centre and bottom dead centre is called the swept volume and is denoted by V _{s}

V s = π / 4 d^{2} x1

Where d = cylinder bore and 1 = stroke length

**10.2.6 Clearance Volume**

The space above the piston TDC is known as combustion chamber space and its volume is called the clearance volume and is denoted by v c-

**10.2.7 Volume of the Cylinder**

It is the sum of the clearance volume and swept volume.

Cylinder volume= V=V c + V s

**10.2.8 Compression Ratio**

The ratio of the total volume of the cylinder to that of the clearance volume is called the compression ratio (r).

r = V /V c = V c + V s / V c = 1 + V s / V c

compression ratio varies from 5 to 10.5 in 5.1. (spark ignition) engine and 12 to 25 for C.I. (compression ignition) engine.

**10.2.9 Mean Effective Pressure (Pro)**

The mean effective pressure represents the theoretical constant pressure for which the Work done during volume change V5 is the same as net work.

W net = Pm x V s

In Fig. 10.1, area 1 – 2-3-4-1 = area a – b – c-d -a.

Thus Pm = W net / V s = Net work done per cycle / swept volume

So in other words the mean effective pressure is the theoretical constant pressure which if it acted on the piston during power stroke, would produce the same net work as actually developed in one cycle.