Engine Nomenclature and classification

Cylinder Bore (d) :

The working cylinder's nominal inner diameter is called the cylinder bore.
It usually expressed in millimetre (mm).

Piston Area (A) :

Piston area is the area of a circle of diameter equal to the cylinder bore is called piston area.
It usually expressed in square centimetre (cm2).

Engine Nomenclature
Engine Nomenclature

Dead Centre :

The engine has two dead centres:
1. Top Dead Centre (TDC)
2. Bottom Dead Centre (BDC)

Stroke :

The distance travelled by the piston from TDC to BDC is called a stroke. 
In other words, the maximum distance travelled by the piston in the cylinder is also known as a stroke. It is equal to twice the radius of a crank.

Stroke to Bore Ratio :

Stroke to bore ratio is L/d is an important parameter in classifying the size of the engine.

Stroke to bore ratio = Inner diameter of circle / Stroke length 
  • If d<L is called under-square engine
  • If d=L is called square engine
  • If d>L is called over-square engine
An over-square engine can operate at higher speeds because of large bore and shorter stroke.

Displacement or Swept Volume (Vs) :

The nominal volume swept by the working piston when travelling from one dead centre to another dead centre is called displacement volume.
It is expressed in terms of a cubic centimetre (cc).

Vs = A × L

Cubic Capacity or Engine Capacity : 

It is known as the total volume of a cylinder.
The displacement volume of a cylinder multiplied by a number of the cylinder in an engine will give the cubic capacity.

Cubic Capacity = Vs × K

Where k = Cylinders in an engine

Clearance Volume (Vc) :

The nominal volume of the combustion chamber above the piston, when it is at the TDC, is the clearance volume.
It expressed in a cubic centimetre (cc).

Swept or displacement volume (Vswept) :

The difference between total volume and clearance volume is called swept volume.

Swept volume = Total volume - Clearance volume 

Compression Ratio (r) :

The ratio of the total cylinder volume when the piston is at the BDC to clearance volume is called compression ratio.

r = Vt/Vc = Vc+Vs/Vc = 1+Vs/Vc

Compression ratio = Total volume / Clearance volume

Mean effective pressure : 

The mean effective pressure is known as the average pressure acting on the piston.

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

Mean piston speed :

The distance per unit time travelled by the piston.

Mean piston speed = 2ln / 60
Where l = stroke length (mm)
N = rpm