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__I.C Engines all Basic Important Terms, definitions and formulas__:

__I.C Engines all Basic Important Terms, definitions and formulas__:

**Top Dead Centre (T.D.C):-**

When the piston is at its top most position,
i.e. the piston is closet to cylinder head, it is called top dead centre.

**Bop Dead Centre (B.D.C):-**

When the piston is at its lowest position,
i.e. the piston is farthest from the cylinder head, it is called bottom dead
centre.

**Bore:-**

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 re-boring 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 calculate the engine capacity (cylinder volume).**Stroke Length or Stroke:-**

The distance
travelled 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*= 2*r*)*V*

_{S}*= (πD²/4)*

*x*

*L*

If

*D*is in cm and*L*is also in cm than the units of*V*will be cm^{3}which is usually written as cubic centimeter or c.c.**Clearance Volume:-**

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 (*V*)._{C}**Swept Volume or Piston Displacement:-**

The volume
swept by piston while moving from T.D.C to B.D.C is called swept volume. This
is referred as (

*V*)._{S}
Therefore, the
total volume of the engine cylinder

*V*

*=V*

_{S}+ V_{C}**Compression Ratio:-**

It is the
ratio of volume above the piston at B.D.C to the volume above the piston at
T.D.C. It is the ratio of total volume of the cylinder

*(V*to the clearance volume_{S}+ V_{C}),*(V*_{C}).
It is
calculated as follows

*r*= Total volume/Clearance volume

_{k}*r*

_{k}*= (V*

_{S}+ V_{C})/V_{C}
For petrol engine, it ranges from 8 to 12.

For diesel
engine, it ranges from 15 to 24.

**Power:-**

It is the
work-done in a given period of time. More power is required to do the same
amount of work in a lesser time.

**Indicated power (I.P):-**

The power
developed inside the engine cylinder is called the indicated power. It is
expressed is kW. It is given by the area under engine indicator diagram.

Indicated
power of an engine is given by,

*I.P**= P*_{im}L A N K/60,000**Indicator diagram:-**

An indicated
diagram is a graph between pressure and volume. The former being taken on
vertical axis and the latter on the horizontal axis. This is obtained by an
instrument known as indicator. The indicator diagrams are of two types;

(

*a*) Theoretical or hypothetical
(

*b*) Actual.
The
theoretical or hypothetical indicator diagram is always longer in size as
compared to the actual one. Since in the former losses are neglected.

The area of the
indicator diagram represents the magnitude of the net work-done by the system in
one engine cycle.

The area of
the diagram = a

_{d}
The length
of the diagram = l

_{d}
Therefore,
the mean effective pressure (m.e.p) is defined as

*P*= (Area of Indicator diagram/Length of diagram) x constant

_{m}
=

*(a*_{d }/ l_{d})*x**k*
Work-done in
one engine cycle = P

_{m}A L
For 2-stroke
engine, work-done in one min. = P

_{m.}A. L. N
For 4-stroke
engine, work-done in one min. = P

_{m.}A. L. N/2**Diagram Factor:-**

The ratio of
the area of the actual indicator diagram to the theoretical one is called
diagram factor.

**Brake Power (B.P):-**

This is the
actual power available at the crank shaft. The indicated power minus various
power losses in the engine like, friction and pumping losses in the engine,
gives brake power. It is measured by using Dynamometer and expressed in kW.

Brake power
of an engine is given by,

*B.P = T. ω*
Or, B.P = 2 π NT/60,000

**Mean Effective Pressure**

*(P*_{m}or P_{mef }):-
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.

Or,

As piston
performs power stroke, cylinder pressure decreases. Thus it is required to
refer an average effective pressure throughout the whole power stroke. It is
expressed in bars.

Mathematically,

*P*= Work Output/ Swept volume =

_{m}*W*

_{net }/(V₁ - V₂)
It can also
be shown as

*P*= (Area of Indicator diagram/Length of diagram) x constant

_{m}
=

*(a*_{d }/ l_{d})*x**k*
The constant
depends on the mechanism used to get the indicator diagram and has the unit,
bar/m.

**Indicated Mean Effective Pressure (P**

_{im})
Indicated
power of an engine is given by

*I.P*

*= P*

_{im}L A N K/60,000
Therefore, P

_{im}= (60,000*x**I.P)/L A N K***Break Mean Effective Pressure (P**

_{bm})
Similarly,
the brake mean effective pressure is given by

*P*= (60,000

_{bm}*x*

*B.P)/L A N K*

**Engine Torque:-**

It is the force of rotating action about the
crank axis at any given instant of time.

It is given by,

*T = F. r*
Where;

*I.P =*Indicated Power (kW)

*B.P*= Break Powder (kW)

*P*= Indicated mean effective pressure (N/m²)

_{im}*P*= Break mean effective Pressure (N/m²)

_{bm}*L*= Length of the stroke

*A*=

*(πD²/4) =*Area of the piston (m²)

*N*= Number of power strokes

= rpm for 2-stroke engines = rpm/2 for
4-stroke

*K*= Number of cylinder.

*T*= Engine Torque (Nm)

*F*= Force applied to the crank (N)

*r*= Effective crank radius (m).

*ω*= Average velocity of crankshaft (rad/sec)

**Mechanical Efficiency of Engine:**

*η*

_{mech}= B.P/I.P

**Otto cycle Efficiency:**

*η*

_{otto}= 1- (1/r_{k}^{ ɤ - 1})**Diesel**

**cycle Efficiency:**

*[1 - (1/ɣ). (1/r*

_{k}^{ ɤ - 1})]*x*

*[(r*

_{c}^{ɤ}- 1)/r_{c}- 1)]
Where,

*r*= Compression ratio_{k}= v₁/v₂*r*= Expansion ratio

_{e}= v₄/v₃*r*= Cut-off ratio

_{c}= v₃/v₂
Also,

*r*_{k}= r_{e}*x**r*_{c.}