The inner diameter of the engine cylinder is known as bore. It can be measured precisely by a vernier calliper or bore gauge. As the engine cylinder wears out with the passage of time, so the bore diameter changes to a larger value, hence the piston becomes lose in the cylinder, and power loss occurs. To correct this problem reboring to the next standard size is done and a new piston is placed. Bore is denoted by the letter ‘D’. It is usually measured in mm (S.I. units) or inches (metric units). It is used to calculated the engine capacity (cylinder volume).
The distance traveled by the piston from its topmost positions (also called as Top dead centre TDC), to its bottom most position (or bottom dead centre BDC) is called stroke it will be two times the crank radius. It is denoted by letter h. Units mm or inches (S.L, Metric). Now we can calculate the swept volume as follows: (L = 2r)
If D is in cm and L is also in cm than the units of V will be cm3 which is usually written as cubic centimeter or c.c.
The volume above the T.D.C is called as clearance volume, this is provided so as to accommodate engine valves etc. this is referred as (VC).Then total volume of the engine cylinder.
=VS + VC
It is calculated as follows
Mean Effective Pressure (Pm or Pmef)
Mean effective pressure is that hypothetical constant pressure which is assumed to be acting on the piston during its expansion stroke producing the same work output as that from the actual cycle.
The constant depends on the mechanism used to get the indicator diagram and has the units bar/m.
Indicated Mean Effective Pressure (Pim) Indicated power of an engine is given by
Break Mean Effective Pressure (Pbm) Similarly, the brake mean effective pressure is given by
where; ip = indicated power (kW)
bp = Break Powder (kW)
Pim = indicated mean effective pressure (N/m2)
Pbm = Break mean effective Pressure (N/m2)
L = length of the stroke
A = area of the piston (m2)
N = number of power strokes
= rpm for 2-stroke engines = rpm/2 for 4-stroke
K = no. of cylinder.