Lecture-5
Prepared under
QIP-CD Cell Project
Internal Combustion Engines
Ujjwal K Saha, Ph.D.
Department of Mechanical Engineering
Indian Institute of Technology Guwahati
1
Chem.Energy
Thermal Energy
Mech. 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.
2
Transmission Loss
(from piston to crankshaft
via the connecting rod)
Friction loss
Pumping loss
fp
∴ ip− fp= bp
‰ The brake power is always less than the
indicated power because of frictional losses.
‰ Indicated power (ip), is the power actually
developed in the cylinder.
‰ Brake power (bp), is the output power
measured at the crankshaft.
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
4
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 =
m f
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. Accordingly,
bsfc =
m f
bp
& isfc =
m f
ip
sfc is a measure of how efficiently the fuel supplied
to the engine is used to produce power. Clearly, a
low value of sfc is desirable since for a given power
level less fuel is consumed.
6
Brake Specific Fuel Consumption vs Engine Size
bsfc
generally
decreases
with
engine size due to
reduced heat losses
from gas to cylinder
wall.
¾
• Note cylinder surface to volume ratio increases with bore diameter.
cylinder surface area 2πrL 1
= 2 ∝
cylinder volume
πr L r
7
Brake Specific Fuel Consumption vs Engine Speed
• There
is
a
minimum in the bsfc
versus engine speed
curve
• At
high speeds the
bsfc increases due
to increased friction
losses.
• 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
8
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.
‰
‰
Engine efficiency can be calculated two ways viz.,
¾
¾
Thermal efficiency and
Mechanical efficiency.
9
Combustion Efficiency
As time available for combustion is very short, a
small fraction of fuel does not react and exits with
the exhaust flow.
‰
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.
‰
Qin = m f Q f ηc
where mf = mass of fuel
Q f = calorific value of fuel
η c = combustion efficiency
∴ Q in = m f Q f ηc
10
Thermal Efficiency: It is the ratio of power produced
to the energy in the fuel burned to produce this
power, and can be expressed as
P
ηth =
f Qf
m
Depending upon whether it is brake power or indicated
power, the terms brake thermal efficiency or indicated
thermal efficiency is used. Accordingly, following two
expressions can be used.
η bth
bp
=
;
fQf
m
η ith
ip
=
fQ f
m
where m f = fuel mass flow rate
Q f = calorific value of fuel
11
Mechanical efficiency:
bp
ηm =
ip
Mechanical efficiency usually lies between 80 to
90 %. It can also be defined as the ratio of brake
thermal efficiency to indicated thermal efficiency.
It also follows that
isfc ηbth
ηm =
=
bsfc ηith
12
Air-Fuel Ratio:
A ma
=
F
mf
A m a
=
F m f
SI engines may have A/F ratio in the range of 12 to 18
based on the operating conditions such as starting,
accelerating, cruising etc.
CI engines, on the other hand, may have A/F ratio in
the range of 18 to 70.
13
Chemically Correct or Stoicheometric F/A: The
mixture that contains optimum proportion of fuel
air ratio.
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 :0.056 ≤ F ≤ 0.083
CI engines :18 ≤ A ≤ 70
CI engines :0.014 ≤ F ≤ 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.
This is often referred to as the breathing capacity
of the engine, and is expressed quantitatively as
volumetric efficiency.
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
nm a
ηv =
ρ a vd N
where, ma = mass of air into the engine in one cycle (kg)
m a = 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).
The volumetric efficiency for a normally aspirated
engine is about 80 %, and this value can be
increased by supercharging or turbocharging
methods.
16
Improving Volumetric Efficiency:
Modifying the intake passages that make it easier
for the mixture to flow through as shown in Figure.
Other changes include reshaping ports to smooth
bends, reshaping the back of the valve heads, or
polishing the inside of the ports.
17
Engine weight w
=
‰ Specific Weight =
Engine power bp
Indicates the relative economy
with which materials are used.
Engine volume Vd
=
‰ Specific Volume =
Engine power bp
Indicates the relative effectiveness
with which engine space is utilized.
‰
Specific Power =
Engine power
Piston face area ( all pistons )
=
bp
Ap
Measures the effectiveness with which piston
area is used regardless of cylinder size.
18
Two-stroke Engines:
For
same
power
generation, air input in a
2-stroke engine is greater
than a 4-stroke engine.
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.
19
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 . 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 . Re siduals ) = m tc
mt
Trapping Efficiency:
λte =
mi
mt
Scavenging Efficiency: λ se =
m tc
Relative Charge:
m tc λ ce
λ rc =
=
mc
λ se
21
Charging Efficiency
= Delivery Ratio x Trapping Efficiency
Charging Efficiency
= Relative Charge x Scavenging Efficiency
Typical values
0.65 ⟨ λ dr ⟨ 0.95
0.50 ⟨ λ ce ⟨ 0.75
0.65 ⟨ λ te ⟨ 0.80
0.75 ⟨ λ se ⟨ 0.90
0.60 ⟨ λ rc ⟨ 0.90
22
Road-Load Power
•A
part-load power level useful for testing
car engines is the power required to drive
a vehicle on a level road at a steady
speed.
• The road-load power (Pr) is the engine
power needed to overcome rolling
resistance and the aerodynamic drag of
the vehicle.
2
1
Pr = (C R M v g +
ρ
C D Av Sv ) ⋅ Sv
a
2
23
Road-Load Power
2
1
Pr = (C R M v g +
ρ
C D Av Sv ) ⋅ Sv
a
2
where CR = coefficient of rolling resistance (0.012 - 0.015)
Mv = mass of vehicle
g = gravitational acceleration
ra = ambient air density
CD = drag coefficient (for cars: 0.3 - 0.5)
Av = frontal area of the vehicle
Sv = vehicle speed
™ Modern midsize aerodynamic cars only
need 5-6 kW (7-8 HP) power to cruise at 90
km/hr, hence the attraction of hybrid cars!
24
Summary
‰ Specific volume, specific weight and
specific power are the important
parameters for engines used in
transportation vehicles such as boats,
automobiles, airplanes, where keeping
weight to a minimum is necessary. For
land-based stationary engines, weight
is insignificant.
‰ Modern
automobile engines usually
have brake power per displacement in
the range of 40 to 80 kW/L.
25
Summary
‰ The Honda eight-valve/cylinder V4
motorcycle engine generates about
130 kW/L, an extra example of a highperformance racing car engine.
‰ One
main reason for continued
development in two-stroke engines is
that they produce 40 % greater power
output per unit weight.
26
References
Crouse WH, and Anglin DL,
DL (1985), Automotive Engines, Tata McGraw Hill.
2. Eastop TD, and McConkey A, (1993), Applied Thermodynamics for Engg.
Technologists, Addison Wisley.
3. Fergusan CR, and Kirkpatrick AT, (2001), Internal Combustion Engines, John
Wiley & Sons.
4. Ganesan V, (2003), Internal Combustion Engines, Tata McGraw Hill.
5. Gill PW, Smith JH, and Ziurys EJ, (1959), Fundamentals of I. C. Engines, Oxford
and IBH Pub Ltd.
6. Heisler H, (1999), Vehicle and Engine Technology, Arnold Publishers.
7. Heywood JB, (1989), Internal Combustion Engine Fundamentals, McGraw Hill.
8. Heywood JB, and Sher E, (1999), The Two-Stroke Cycle Engine, Taylor & Francis.
9. Joel R, (1996), Basic Engineering Thermodynamics, Addison-Wesley.
10. Mathur ML, and Sharma RP, (1994), A Course in Internal Combustion Engines,
Dhanpat Rai & Sons, New Delhi.
11. Pulkrabek WW, (1997), Engineering Fundamentals of the I. C. Engine, Prentice Hall.
12. Rogers GFC, and Mayhew YR,
YR (1992), Engineering Thermodynamics, Addison
1.
Wisley.
13. Srinivasan S, (2001), Automotive Engines, Tata McGraw Hill.
14. Stone R, (1992), Internal Combustion Engines, The Macmillan Press Limited, London.
15. Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice, Vol. 1 & 2,
The MIT Press, Cambridge, Massachusetts.
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Web Resources
1.
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http://www.mne.psu.edu/simpson/courses
http://me.queensu.ca/courses
http://www.eng.fsu.edu
http://www.personal.utulsa.edu
http://www.glenroseffa.org/
http://www.howstuffworks.com
http://www.me.psu.edu
http://www.uic.edu/classes/me/ me429/lecture-air-cyc-web%5B1%5D.ppt
http://www.osti.gov/fcvt/HETE2004/Stable.pdf
http://www.rmi.org/sitepages/pid457.php
http://www.tpub.com/content/engine/14081/css
http://webpages.csus.edu
http://www.nebo.edu/misc/learning_resources/ ppt/6-12
http://netlogo.modelingcomplexity.org/Small_engines.ppt
http://www.ku.edu/~kunrotc/academics/180/Lesson%2008%20Diesel.ppt
http://navsci.berkeley.edu/NS10/PPT/
http://www.career-center.org/ secondary/powerpoint/sge-parts.ppt
http://mcdetflw.tecom.usmc.mil
http://ferl.becta.org.uk/display.cfm
http://www.eng.fsu.edu/ME_senior_design/2002/folder14/ccd/Combustion
http://www.me.udel.edu
http://online.physics.uiuc.edu/courses/phys140
http://widget.ecn.purdue.edu/~yanchen/ME200/ME200-8.ppt 28