Lecture-9
Prepared under
QIP-CD Cell Project
Internal Combustion Engines
Ujjwal K Saha, Ph.D.
Department of Mechanical Engineering
Indian Institute of Technology Guwahati
1
Actual Cycles
Corrected for
characteristics of
fuel-air mixture
Air Standard
Cycles
Corrected for
Losses
Fuel-Air
Cycles
Actual
Cycles
2
Losses – in Actual cycles
‰ Time Loss
‰ Heat Loss
‰ Blow-down Loss
‰ Blowby Loss
‰ Rubbing Friction Loss
‰ Pumping Loss
3
Time Loss
‰ It is due to the time required for mixing of
air-fuel mixture and complete combustion.
‰ Heat addition is not instantaneous, and
spread over a period (30 to 40 degrees of
crankshaft revolution). Therefore, pmax is not
at TDC, but just after TDC.
‰ Time loss depends upon flame velocity
which, in turn, again depends on type of
fuel used, A/F ratio, and shape of
combustion chamber.
4
This is to have pmax
at TDC. If the spark is
initiated
at
TDC,
peak pressure would
be
low
due
to
expansion of gases.
Further, if the spark is
initiated too early,
additional work is
required to compress
the burning gases –
which is a direct loss.
Therefore,
the
optimum time to start
the combustion is 150
– 300 bTDC.
‰
Spark Advance
Spark at TDC, Advance 00
5
Spark Advance of 350
Optimum Spark Advance
Remark: Best compromise is to go for moderate
spark advance so as to have smaller losses in
both compression and expansion strokes.
6
Cycle Performance at
Various Ignition Timing
7
Heat Loss
‰ This is due to the transfer of heat
through water jackets and cooling fins.
Also, some heat is being transferred
during compression and expansion
processes.
‰ Due to heat loss, temperature (Tmax)
decreases, and specific heat gets
reduced.
This
decreases
the
efficiency.
8
Blow-down Loss
Blowdown loss is due to the early opening of
exhaust valves. This results in drop in pressure, and
a loss of work output during expansion stroke. Too
early opening results in loss of expansion work.
Best compromise is between 400 – 600 bBDC.
‰
9
The effects of exhaust valve early
opening greatly exaggerated
10
Time Loss, Heat Loss and Exhaust Loss
11
Blowby Losses
The blowby loss is due to the leaking of gas flow
through crevices/gaps between the piston, piston
rings and cylinder walls. The gas usually
leaks/flows through them to the crankcase.
‰
12
Compression
Rings
13
Rubbing Friction Losses
‰ Rubbing friction loss is due to friction
between the piston and chamber walls,
friction in various bearings and also includes
the energy spent in operating various
auxiliary equipments such as cooling fans,
water pumps etc.
‰ The piston ring friction increases rapidly
with engine speed. It also increases to a
small extent with increase in mean effective
pressure. The bearing friction and the
auxiliary friction also increase with engine
speed.
14
‰ The
efficiency of an engine is
maximum at full load and decreases
at part load. This is because the % of
direct heat loss, pumping loss and
rubbing friction loss increase at part
loads. The approximate losses for an
SI engine using chemically correct
mixture are shown as % of fuel energy
input.
15
Remark
™ Increasing the load (load may be defined
as the ratio of power developed to normal
rated power at same speed) increases the
maximum pressure in the cylinder which
results in slight increase in friction values. At
the same time, increase in load results in
increase in temperature inside the cylinder
and also the temperature of lubricating oil.
The decrease in the oil viscosity (due to
higher temperature) reduces the friction
slightly.
16
17
Pumping Losses
Pumping work is the difference between the
work done in expelling the gases (during exhaust
stroke) and the work done in inducing the fresh
charge (during suction stroke). The loss is due to
the pumping gases from low inlet pressure to high
exhaust pressure.
‰
18
Summary
1. Real engines operate on an open cycle
with changing composition. Not only does
the inlet gas composition differ from what
exists, but often the mass flow rate is not the
same.
2. Engines which add fuel into the cylinders
after the air induction is complete (CI
engines and some SI engines) change the
amount of mass in the gas composition.
Thus, there is a greater mass exiting the
engine than what entered.
19
Summary
3. During combustion, total mass remains
about the same but the molar quantity
changes. Finally, there is a loss of mass
during the cycle due to crevice flow and
blowby past the pistons. This blowby can
decrease the amount of mass in the
cylinders as much as 1 % during
compression and combustion.
20
Summary
4. Air-standard analysis treats the fluid through the
entire engine as air, and approximates air as an
ideal gas. A more serious error is introduced by
assuming constant specific heats for the analysis.
Specific heats of a gas have a fairly strong
dependency on temperature and can vary as much
as 30 % in the temperature range of the engine.
5. There are heat losses during the cycle of a real
engine. Heat loss during combustion lowers actual
peak temperature and pressure from what is
predicted. The actual power stroke, therefore, starts
at a lower pressure, and the work output during
expansion is decreased.
21
Summary
6. Combustion requires a short but finite time to
occur, and heat addition is not instantaneous at
TDC. By starting combustion bTDC, cylinder pressure
increases late in the compression stroke, requiring
greater negative work in that stroke. Further, as
combustion continues slightly aTDC, some power is
lost at the beginning of expansion stroke.
7. Another loss in the combustion process of an
actual engine occurs because of combustion
efficiency, which is less than 100 %. This happens
due to imperfect mixing, local variations in
temperature and air-fuel due to turbulence, flame
quenching etc. SI engines usually have a
combustion efficiency of about 95 %, while CI
engines are generally 98 % efficient.
22
Summary
8. Blow-down process requires a finite time to occur
and does not occur at constant volume as assumed
in the air-standard cycle analysis. For this reason
exhaust valve open 400 to 600 bBDC, and work
output at the latter end of the expansion is lost.
9. Engine valves require a finite time actuate.
Ideally,
valves
would
open
and
close
instantaneously, but this is not possible when using a
camshaft. Cam profiles must allow for smooth
interaction with the cam follower, and this results in
fast but finite valve actuation. To ensure that inlet
valve is fully open at the beginning of suction stroke,
it must start to open bTDC. Similarly, the exhaust
valve must remain fully open until the end of
exhaust stroke, with final closure occurring aTDC.
23
Valve Mechanisms
rocker
valve
push rod
piston
valve lifter
camshaft
Timing marks
cam
crankshaft
24
Summary
10. Because of these differences, results obtained
from the air-standard analysis will have errors and
will deviate from actual conditions. Interestingly,
however, these errors are not great, and property
values of temperature and pressure are very
representative of actual engine values, depending
on the geometry and operating conditions of the
real engine.
11. By changing operating variables such as inlet
temperature, and/or pressure, compression ratio,
peak temperature etc. in air-standard analysis,
good approximations can be obtained for output
changes that will occur in real engine.
25
Summary
12. Indicated thermal efficiency of a real four stroke
SI engine is always somewhat less than the what airstandard Otto cycle analysis predicts. This is caused
by the heat losses, friction, ignition timing, finite time
of combustion and blow-down, and deviation from
ideal gas behavior of the real engine.
13. It has been found that over a large range of
operating variables, the indicated thermal
efficiency of an actual SI four-stroke cycle engine
can be approximated by:
(ηt)actual ≈ 0.85 (ηt)Otto
This will be correct to within a few % for large
ranges of air-fuel equivalence ratio, ignition timing,
engine speed, compression ratio, inlet pressure,
exhaust pressure and valve timing.
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.
27
Web Resources
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
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