Internal Combustion Engines Nomenclature, Performance Parameters and Power Ratings

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 cm .

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.

Dead 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 Bottom Dead Centre.

Top Dead Centre (TDC) – 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 Dead Centre (BDC) – 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 1214 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 -10m/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.

Overload Rating – The overload rating signifies the extent of overload that can be

safely exerted on the engine for a short period.

Astern Output – It signifies the maximum output while going astern.

So these are the terminology that we use for the Internal Combustion Engine.