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Lecture-5

Prepared under
QIP-CD Cell Project

Internal Combustion Engines

Ujjwal K Saha, Ph
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Department of Mechanical Engineering

Indian Institute of Technology Guwahati
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Chem
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Work

Losses
Energy in Fuel

Heat

Not wholly convertible
to drive the piston
Loss
to
coolant,
radiation and exhaust

Remainder is converted to Power (to drive
the piston), and this is the indicated power
...

Indicated power (ip), is the power actually
developed in the cylinder
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3

Indicated power (ip) can be expressed as

( imep ) LAnK
ip =
60×1000
where, ip = indicated power (kW)
imep = indicated mean effective pressure (kN/m2)
L = length of stroke (m)
A = cross-sectional area of piston (m2)
n = number of power strokes
n=N/2 for four strokes, and n=N for two-strokes
N = crankshaft speed (revolutions per minute)
and
K = number of cylinders

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Brake power (bp) can be expressed as

(bmep ) LAnK
bp =
60×1000
Brake power (bp) obtained at the output shaft can
also be related as

2π NT
bp =
60×1000
where bp = brake power (kW)
N = crankshaft speed (revolutions per minute)
and
T = engine torque (N-m)
5

Specific Fuel Consumption: It is defined as the
fuel flow rate per unit power output, and can be
expressed as

sfc =

mf
P

Depending upon whether it is brake power or
indicated power, the terms brake specific fuel
consumption (bsfc), or indicated specific fuel
consumption (isfc) is used
...
Clearly, a
low value of sfc is desirable since for a given power
level less fuel is consumed
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• Note cylinder surface to volume ratio increases with bore diameter
...

• At lower speeds, the bsfc increases due to
increased time for heat losses from the gas to the
cylinder and piston wall, and thus a smaller ip
• As compression ratio is increased, fuel consumption
decreases due to greater thermal efficiency

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Engine Efficiency
Efficiency is the relation between the power
delivered and the power that could be obtained if
the engine operates without loss of power
...
,
Thermal efficiency and
Mechanical efficiency
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A Combustion Efficiency is defined to account for
the fraction of fuel burnt, and typically has values in
the range of 95 % to 98 % when an engine is
operating properly
...
Accordingly, following two
expressions can be used
...
It can also be defined as the ratio of brake
thermal efficiency to indicated thermal efficiency
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CI engines, on the other hand, may have A/F ratio in
the range of 18 to 70
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φ = Equivalence Ratio
=

Actual F

A

Ratio

Stoicheometric F

A

Ratio

φ = 1; Chemically Correct
φ 〈 1; Lean M ixture
φ 〉 1 ; Rich M ixture
SI engines :12 ≤ A ≤ 18

SI engines :0
...
083

CI engines :18 ≤ A ≤ 70

CI engines :0
...
056

F

F

A

A

14

Volumetric Efficiency: The power output of an
engine depends directly on the amount of charge
that can be inducted in the cylinder
...

It can be defined as the ratio of the volume of air
induced to the swept volume of the cylinder, and
can be expressed as

ma
ηv =
ρ a vd
nma
ηv =
ρ a vd N

where, ma = mass of air into the engine in one cycle (kg)
ma = mass flow rate of air into the engine (kg/s)
ρa = air density at atmospheric conditions (kg/ m3)
Vd = displaced volume (m3)
N = engine speed (revolutions per minute)
and
n = number of revolutions per cycle
15

Volumetric Efficiency:

Actual mass of ch arg e inducted
ηv =
Theoretical mass of ch arg e inducted
Actual mass is always less than theoretical mass
because of pressure losses in the ducting system
and due to heat transfer (process is not adiabatic)
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16

Improving Volumetric Efficiency:

Modifying the intake passages that make it easier
for the mixture to flow through as shown in Figure
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17

Engine weight w
=
Specific Weight =
Engine power bp
Indicates the relative economy
with which materials are used
...


Specific Power =

Engine power
Piston face area ( all pistons )

=

bp
Ap

Measures the effectiveness with which piston
area is used regardless of cylinder size
...

As there is a loss in the
scavenging period, the
term volumetric efficiency
(as applied to a 4-stroke
engine) is replaced by the
terms delivery ratio and
charging efficiency
...
Re siduals ) = m tc
Delivery Ratio:

λ dr

Charging Efficiency:

∴ λ dr 〉 λ ce

mi
=
mc
mt
λ ce =
mc
Because some mixture is
lost out of exhaust port
before it is closed
20

Two-stroke Engines:
Cylinder Volume = Swept Volume = V d
Cylinder M ass = ρ aV d = m c
M ass of Fresh Ch arg e Delivered / Ingested = m i

M ass of Fresh Ch arg e Re tained / Trapped = m t
M ass of Ch arg e Lost ( Short − circuiting ) = m i − m t
M ass of Ch arg e Trapped ( including Exh
...
65 〈 λ dr 〈 0
...
50 〈 λ ce 〈 0
...
65 〈 λ te 〈 0
...
75 〈 λ se 〈 0
...
60 〈 λ rc 〈 0
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• The road-load power (Pr) is the engine
power needed to overcome rolling
resistance and the aerodynamic drag of
the vehicle
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012 - 0
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3 - 0
...
For
land-based stationary engines, weight
is insignificant
...

25

Summary
The Honda eight-valve/cylinder V4
motorcycle engine generates about
130 kW/L, an extra example of a highperformance racing car engine
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26

References
Crouse WH, and Anglin DL, (1985), Automotive Engines, Tata McGraw Hill
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Eastop TD, and McConkey A, (1993), Applied Thermodynamics for Engg
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Wisley
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Srinivasan S, (2001), Automotive Engines, Tata McGraw Hill
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Stone R, (1992), Internal Combustion Engines, The Macmillan Press Limited, London
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Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice, Vol
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Web Resources
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mne
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edu/simpson/courses
http://me
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ca/courses
http://www
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fsu
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personal
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edu
http://www
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org/
http://www
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com
http://www
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psu
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uic
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ppt
http://www
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gov/fcvt/HETE2004/Stable
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rmi
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php
http://www
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com/content/engine/14081/css
http://webpages
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edu
http://www
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edu/misc/learning_resources/ ppt/6-12
http://netlogo
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org/Small_engines
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ku
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ppt
http://navsci
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edu/NS10/PPT/
http://www
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org/ secondary/powerpoint/sge-parts
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tecom
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mil
http://ferl
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org
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cfm
http://www
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fsu
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me
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edu
http://online
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uiuc
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ecn
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edu/~yanchen/ME200/ME200-8

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