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Engine: Idealized cycles and processes
Dr. Md. Zahurul Haq
ProfessorDepartment of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh
[email protected]://teacher.buet.ac.bd/zahurul/
ME 417: Internal Combustion Engines
http://teacher.buet.ac.bd/zahurul/ME417/
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 1 / 25
IC Engine Classifications
T533
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 2 / 25
T555
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 3 / 25
Spark or Compression Ignition Engines (SIE vs CIE)
T503� Spark initiates the flame in SIE; high compression in CIE
generates high temperature for ignition in CIE.
� Part-load efficiency of CIE is better as load is regulated by fuel
injection adjustment; in SIE throttling is used to reduce load
which increases pumping work.
� SIEs are high speed engines with higher temperature exhaust.
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 4 / 25
Four Stroke Reciprocating IC Engine
T523
Four stroke cycle eventsc
Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 5 / 25
Two Stroke Reciprocating IC Engine
T524
Two stroke cycle events
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 6 / 25
T924
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 7 / 25
4s vs. 2s IC Engines
T515 T516
� Basic difference: 2s engine accomplishes the four processes (intake,
compressing, expansion & exhaust) in a single revolution.
� Advantages of 4s engines: better fuel economy, better lubrication
and easier control.
� Advantages of 2s engines: fewer moving parts, lighter weight and
smoother operation.
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 8 / 25
Wankel Four Stroke Rotary SI Engines
T906
◮ smooth rotary motion, ◮ sealing is critical.c
Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 9 / 25
Gas Turbine
T371
A comparison between the working cycle of a turbo-jet engine and a
piston engine.
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 10 / 25
Engine Ideal Cycles
Four Stroke Reciprocating SI Engine: Otto Cycle
T498
P-v diagram of Otto cycle
T500
Cycle with gas-exchange processes
� 1-2: Isentropic compression from BDC to TDC
� 2-3: Constant-volume heat addition at TDC
� 3-4: Isentropic expansion from TDC to BDC
� 4-1: Constant-volume heat rejection at BDC
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 11 / 25
Engine Ideal Cycles
� thermal efficiency, ηth ,otto = 1 −1
r k−1c
→ compression ratio, rc ≡ VBDC/VTDC
→ ratio of specific heat capacities, k = cp/cv
T501 T502
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 12 / 25
Engine Ideal Cycles
Actual 4s SI engine cycle vs. Otto cycle
T499
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 13 / 25
Engine Ideal Cycles
T528
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 14 / 25
Engine Ideal Cycles
Air Standard Diesel Cycle
T504
� 1-2: Isentropic compression from BDC to TDC
� 2-3: Constant-pressure heat addition at TDC
� 3-4: Isentropic expansion to BDC
� 4-1: Constant-volume heat rejection at BDC
T505
◮thermal efficiency, ηth ,diesel = 1 −1
r k−1c
[
βk−1k (β−1)
]
; cut-off ratio, β ≡ V3V2
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 15 / 25
Engine Ideal Cycles
Air Standard Dual Cycle
T506
� 1-2: Isentropic compression from BDC to TDC
� 2-3: Constant-volume heat addition at TDC
� 3-4: Constant-pressure heat addition at TDC
� 4-5: Isentropic expansion to BDC
� 5-1: Constant-volume heat rejection at BDC
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 16 / 25
Engine Ideal Cycles
Comparison: Otto, Diesel & Dual Cycle
T603T509
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 17 / 25
Engine Ideal Cycles
Miller Cycle
T891
� compression ratio, rc ≡ V1/V2
� expansion ratio, re ≡ V4/V3
T892
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 18 / 25
Ideal Cycle: Effect of Engine Parameters
Ideal Four Stroke Processes : Inlet & Exhaust Flow
T893
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 19 / 25
Ideal Cycle: Effect of Engine Parameters
Ideal Four Stroke Processes: Exhaust Stroke
T894
� Te = T4
(
PeP4
)k−1k
� Residual gas fraction, fr
� fr =1rc
(
PeP4
)k−1k
� 0.03
Ideal Cycle: Effect of Engine Parameters
Otto Cycle: Effect of rc and Heat Input
� ηotto = 1 −(
1rc
)k−1
�
imepP1
=(
QinP1V1
)(
rcrc−1
)
ηotto
T889
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 21 / 25
Ideal Cycle: Effect of Engine Parameters
Otto Cycle: Effect of rc, fr and φ
T609 T883
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 22 / 25
Ideal Cycle: Effect of Engine Parameters
Otto Cycle: Effect of Fuel Type
T884
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 23 / 25
Ideal Cycle: Effect of Engine Parameters
Otto Cycle: Effect of Pi/Pe
T885 T886
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 24 / 25
Ideal Cycle: Effect of Engine Parameters
Diesel Cycle: Effect of rc and Heat Input
� β is not an independent
variable.
� β = 1 + kk−1
(
QinP1V1
)
1r k−1c
⇒ Qin = 0 → β = 1 → Otto cycle
T890
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Dr. Md. Zahurul Haq (BUET) Engine: Idealized cycles and processes ME 417 (2018) 25 / 25
Engine Ideal CyclesIdeal Cycle: Effect of Engine Parameters
fd@rm@0: fd@rm@1: fd@rm@2: