Energy Conversion Systems
(ME341)
Date: 24th January 2024
Lecture-3
Gas Power Cycles
IIT (BHU), Varanasi
Dr. Akhilendra Pratap Singh
Department of Mechanical Engineering
IIT (BHU), Varanasi, India
Email: akhilendra.mec@itbhu.ac.in
Introduction of Power Cycle
A cycle during which a net amount of work is produced is called a power cycle, and a
power cycle during which the working fluid remains a gas throughout the cycle, is called a
gas power cycle.
The most efficient cycle operating between a heat source at temperature TH and a sink at
temperature TL is the Carnot cycle, and its thermal efficiency is given by,
The actual gas cycles are rather complex. The approximations used to simplify the analysis
are known as the air-standard assumptions.
Under these assumptions, all the processes are assumed to be internally reversible; the
working fluid is assumed to be air, which behaves as an ideal gas; and the combustion
and exhaust processes are replaced by heat-addition and heat-rejection processes,
respectively.
Cold-air-standard assumptions???
Energy Conversion Systems (ME341)
Otto Cycle: The Ideal Cycle for Spark-ignition Engines
The actual cycle does not have the sharp transitions between the different processes that
the ideal cycle has
Energy Conversion Systems (ME341)
Analysis of Otto Cycle
In SI engines, the
compression ratio is
limited by auto ignition
or engine knock.
Thermal efficiency of the ideal Otto cycle The thermal efficiency of the Otto cycle increases
as a function of compression ratio (k = 1.4) with the specific heat ratio k of the working fluid
Energy Conversion Systems (ME341)
Factors Affecting Work per Cycle
The net cycle work of an engine can be increased by either:
Increasing the r (1’2) Or (2’3’)
Increase Qin (23”)
3’’
P 3’
3
4’’
Qin 4
Wcycle
4’
2’ 2
r= V1/V2 1
1’
V2 V1
Energy Conversion Systems (ME341)
Diesel Cycle: The Ideal Cycle for CI Engines
In diesel engines, only air is compressed during the compression stroke,
eliminating the possibility of auto-ignition (engine knock).
Diesel engines can be designed to operate at much higher compression
ratios than SI engines, typically between 12 and 24.
In diesel engines, the spark plug is replaced by a fuel injector.
Energy Conversion Systems (ME341)
Working of Compression Ignition Engine (Diesel Cycle)
Energy Conversion Systems (ME341)
Early CI Engine Cycle vs. Diesel Cycle
FUEL Fuel injected
A
at TC
I
R
Fuel/Air
Mixture Combustion
Actual Cycle Products
Intake Compression Power Exhaust
Stroke Stroke Stroke Stroke
Qin Qout
Air
Diesel Cycle TC
BC
Compression Const pressure Expansion Const volume
Process heat addition Process heat rejection
Process Process
Energy Conversion Systems (ME341)
Compression Ignition Engine (Diesel Cycle)
1-2: isentropic compression
2-3: constant-volume heat addition
3-4: isentropic expansion
4-1: constant-volume heat rejection.
Energy Conversion Systems (ME341)
k 1
T2 V1
r orT2 T1 r
k 1 k 1
T1 V2 Cut-off ratio
T3 v3 T3
rc
T2 v2 T2
k 1
T3 V4 T4
(rc ) k
T4 V3 T1 for the same compression ratio
Energy Conversion Systems (ME341)
Analysis of Diesel Cycle
For higher efficiency of diesel engine, cutoff
ratio should be low.
rc is called the cutoff ratio, defined
as V3/V2, and is a measure of the
duration of the heat addition at
constant pressure.
Thermal efficiency of the ideal
Diesel cycle as a function of
compression and cutoff ratios
(k=1.4).
Energy Conversion Systems (ME341)
Main Injection
Dual Cycle
Pilot
Injection
Conventional Diesel Modern Diesel Engines
Engines with Main with Pilot and Main
Dual cycle: A more Injection Injections
realistic ideal cycle
model for modern,
high-speed
compression
ignition engine.
Pilot Main
Injection Injection
Energy Conversion Systems (ME341)
For the same inlet conditions P1, V1 For the same inlet conditions P1, V1
and the same compression ratio P2/P1 and the same peak pressure P3
Otto Dual Diesel Diesel Dual otto
Energy Conversion Systems (ME341)
Energy Conversion Systems (ME341)
