# Internal Combustion Engines Nomenclature, Performance Parameters and Power Ratings

Know
about all the engine performance parameters like IHP, BHP Volumetric Efficiency
and also about MCR, astern output etc. These are the basic terms that you need
to know when you are studying the Internal Combustion Engine.

## Internal Combustion Engines Nomenclature

Cylinder
Bore (d)
– The nominal
inner diameter of the working cylinder is called as cylinder bore. It is
denoted by the letter‘d’ and is usually expressed in mm.
Piston
Area (A)
– The area of
circle of diameter equal to the cylinder bore is called as piston area. It is
denoted by the letter ‘A’ and is expressed in cm2.
Stroke
(L)
– The nominal
distance through which piston moves between successive reversals of its
direction of motion is called the stroke. It is denoted by the letter ‘L’ and
is usually expressed in mm.
Stroke
to Bore Ratio (L/d)

This ratio is an important parameter in classifying the size of engine.
If
d < L it is called under-square engine
If
d = L it is called square engine
If
d  > L it is called as over square engine.
Centre
– The position of
working piston and the moving parts which are connected to it, so when the
piston motion is reversed at the either end of the stroke is called the dead
centre. There are two dead centres in an engine. They are Top Dead Centre and
Top
– It
is the dead centre when the piston is farthest from the crank shaft. It is
denoted as TDC for vertical engines and Inner Dead Centre (IDC) for horizontal
engines.
Bottom
– It
is a dead centre when piston is nearest to the crankshaft. It is denoted as BDC
for vertical engines and Outer Dead Centre (ODC) for horizontal engines.
Displacement
or Swept Volume (Vs)

The nominal volume swept by the working piston when travelling from one dead
centre to other is called the displacement volume. It is denoted by Vs and expressed
in cubic centimeter (cc). Vs = A x L
Cubic
capacity or engine capacity

– The displacement volume of a cylinder multiplied by number of cylinders in an
engine will give cubic capacity or engine capacity.
Clearance
Volume (Vc)
– The
nominal volume of combustion chamber above the piston when it is at top dead
centre is known as clearance volume. It is denoted by Vc and expressed in cubic
centimeter (cc).
Compression
Ratio (r) – It is ratio of total cylinder volume when piston is at bottom dead
centre to the clearance volume. It is denoted by letter ‘r’(Total cylinder
volume = Swept volume + clearance volume)          Hence,   r = Vc + Vs
/ Vc = 1 + Vs  / Vc
As
compression ratio increases the thermal efficiency increases but it also leads
higher peak pressure and temperature. The upper limit of compression ratio is
therefore fixed by strength of cylinder, bearing and other parts whose stresses
are determined by peak values of mechanical and thermal loading. Large marine
engines use compression ratio of 12-14 whereas medium speed engines use
compression ratio of 16. Life boat diesel engine which needs good starting
ability even in cold climate have compression ratio  of 20.

## Internal Combustion Engine Performance Parameters

Indicated
Thermal Efficiency

Indicated thermal efficiency is the ratio of energy in indicated power ( i.e.
output generated in the cylinder)  to
input fuel energy in appropriate units. Indicated Power is measured by taking
indicator diagram from engine cylinder.
Brake
Thermal Efficiency

Brake thermal efficiency is the ratio of energy in the brake power ( i.e.
output available at the driving shaft of engine) to input fuel energy in
appropriate units. Brake Power can be measured by using hydraulic dynamometer.
Mechanical
Efficiency
– Mechanical
efficiency is defined as ratio of brake power (delivered power) to the
indicated power ( power provided to the piston ).
Volumetric
Efficiency
– Volumetric
efficiency is ratio of volume of air drawn in the cylinder (at normal
temperature and pressure) to the swept volume. It is one of the important
parameters which decides the performance of four stroke engines. Four stroke
engines have distinct suction stroke and therefore the volumetric efficiency
indicates the breathing ability of the engine. It must be noted that the
availability of air and its utilization determines the power output of the
engine. In a naturally aspirated four stroke engine the volumetric efficiency
is between 85% to 90%.
Mean
Effective Pressure
– It
is that theoretical constant pressure which may be assumed to act on piston
during its power stroke. It is equal to the average pressure inside the
cylinder of an IC engine based on calculated or measured power output.
ip
= pimLAnK / 60 x 1000 then
pim = 60000 x ip / LAnK    where
ip = indicated power (kW)
pim = indicated mean effective pressure
( N/m2)
L = Length of stroke (m)
A
= Area of the piston (m2)
N = Speed in RPM
n = Number of power strokes
N/2 for 4 stroke engine and N
for 2 stroke engine
K = Number of cylinders
Another
way of specifying indicated mean effective pressure pim is from engine
indicator diagram (p–V diagram). In this case pim =  Area of indicator diagram /
length of indicator diagram.
Mean
Piston Speed (sp)
– It
is an important engine parameter and is defined as   sp  =
2LN where    L = stroke and  N = RPM.
It is often a more appropriate parameter than crank shaft speed for
correlating engine behavior as a function of speed.
The
above expression which is a part of
power equation  suggests that
power can simply be increased by increasing m.p.s. However it can be done only
the limitations imposed by following factors:
1) Increase
in m.p.s increases mechanical stresses on bearings, bearing bolts and other
moving parts. It also increases inertia forces and peak turning moments.
2) Increase
in m.p.s also decreases service life of reciprocating and rotating pairs of
engine components. (Though speed is not the only criteria for wear when working
conditions involve   higher temperatures
and pressures with lubrication being in the thin film region speed needs to be
limited for greater reliability and safety. )
3) Beyond
certain speed scavenge efficiency drops sharply and this lowers engine output.
At higher m.p.s. resistance of gases to flow in and out of cylinder increases
and scavenge efficiency drops.
Mean
Piston Speed  Classification based on it:
Low
speed  –
4.5 m/s to 7 m/s
Medium
speed  –
7m/s to 10 m/s
High
speed  –
10 m/s to 15 m/s
Specific
Fuel Consumption  (sfc)
– The fuel consumption characteristics
of an engine are generally expressed in terms of specific fuel consumption  in gms / kWhr. It is an important parameter
that reflects how good the engine performance is. It is inversely proportional
to the thermal efficiency of engine. Fuel consumption is measured by carrying
out a test which subjects engine to a constant rated load for half an hour and
consumption of fuel by engine is measured by suitably located fuel flow meters.
The test is repeated and average value taken as sfc.
Scavenge
Efficiency
– The ratio
of volume of air (at normal temperature and pressure) contained in cylinder at
the start of compression to the volume swept by the piston from top edge of the
ports to the top of its stroke is called as scavenge efficiency.
Air
Charge Ratio
– The ratio
of volume of air (at normal temperature and pressure) contained in cylinder at
the start of compression to the swept volume of piston is called as air charge
ratio. It is also referred to as air mass ratio or air supply ratio.  This term has now more or less replaced
the  terms scavenge efficiency and
volumetric efficiency.
Air
Charge Ratio – In four stroke engines for naturally aspirated engines air
charge ratio is about 0.85 whereas for
highly super charged engine it is 4 or above it. In two stroke engines
the value is about 0.85 for engines have ported scavenge and exhaust and it is
up to 2.5 for supercharged engines.

## Internal Combustion Engine Power Ratings

Maximum
Continuous Rating (MCR)

– The maximum continuous rating signifies the maximum output at which the
engine can run safely and continuously. This output forms the basis of
calculation for the strength of engine.
Normal
or Standard Rating
– The
normal of standard rating signifies the output at service speed. The service
speed is that speed which is regarded as economical for efficiency and
corresponds to thermal and mechanical load best suited from maintenance point
of view.