THERMAL ENGINEERING
(ME 2301 )
M.R.SWAMINATHAN
Assistant Professor
Department of Mechanical Engineering
Anna University Chennai
Chennai-25
SYLLABUS
• Unit-I
– Air Standard Cycles, Valve Timing
• Unit-II
– IC Engines
• Unit-III - Steam Turbines & Nozzles
• Unit-IV - Air Compressors
• Unit-V - Refrigeration & Air-Conditioning
AIR STANDARD CYCLES
•
•
•
•
The working fluid is air, which continuously
circulates in a loop and always behaves as an
ideal gas.
All the processes that make up the cycle are
internally reversible.
The combustion process is replaced by a
heat-addition process from an external
source.
The exhaust process is replaced by a heat
rejection process that restores the working
fluid to its initial state.
• Another assumption utilised to simplify
the analysis even more is that the air has
constant specific heats whose values are
determined at room temperature (25°C, or
77°F).
• Assumptions are called the cold-airstandard assumptions.
• A cycle for which the air-standard
assumptions are applicable is frequently
referred to as an air-standard cycle
COMPRESSION RATIO
The
ratio
of
the
maximum
volume
formed in the cylinder to
the minimum (clearance)
volume is called the
compression ratio of the
engine.
Vmax VBDC
r

Vmin VTDC
Wnet
MEP 
Vmax  Vmin
The compression ratio is a volume ratio
and should not be confused with the
pressure ratio.
Mean effective pressure (MEP) is a
fictitious pressure that, if it acted on the
piston during the entire power stroke,
would produce the same amount of net
work as that produced during the actual
cycle.
CARNOT CYCLE
The Carnot cycle is composed
of totally four reversible
processes:
•isothermal heat addition,
•isentropic expansion,
•isothermal heat rejection
•isentropic compression
TL
th ,Carnot 1 
TH
CARNOT CYCLE
The Carnot cycle can be executed in a
closed
system
(a
piston-cylinder
device)and either a gas or vapor can be
used as the working fluid.
Otto Cycle: The ideal Cycle for Spark-Ignition Engines
Figures below show the actual and ideal cycles in spark-ignition (SI)
engines and their P- diagrams.
Ideal Otto Cycle
The
thermodynamic
analysis of the actual
four-stroke or two-stroke
cycles can be simplified
significantly if the airstandard assumptions are
utilized. The T-s diagram
of the Otto cycle is given
in the figure at left.
The ideal Otto cycle consists of four
internally reversible processes:
12
Isentropic compression
23
Constant volume heat addition
34
Isentropic expansion
41
Constant volume heat rejection
Thermal Efficiency of an Otto Cycle
The Otto cycle is executed in a closed system,
and disregarding the changes in kinetic and
potential energies, we have
qin  qout   w in  w out   Δu
 qin  u3  u2  C v T3  T2 
 qout  u4  u1  C v T4  T1 
ηth,Otto
w net
qout
T4  T1

 1
 1
qin
qin
T3  T2
T1 T4 /T1  1
T1
1
 1
 1
 1  k 1
T2 T3 /T2  1
T2
r
T1  υ2 
Where,
  
T2  υ1 
k 1
 υ3 
  
 υ4 
k 1
Vmax V1 υ1
T4
 ; and r 


T3
Vmin V2 υ2
Engine Knock and Thermal Efficiency
The thermal efficiency of the ideal Otto cycle
increases with both the compression ratio
and the specific heat ratio.
 When
high compression ratios are used, the
temperature of the air-fuel mixture rises
above
the
auto-ignition
temperature
produces an audible noise, which is called
engine knock. (antiknock, tetraethyl lead? 
unleaded gas)
 For
a given compression ratio, an ideal Otto
cycle using a monatomic gas (such as argon
or helium, γ = 1.667) as the working fluid will
have the highest thermal efficiency.
DIESEL CYCLE
The diesel cycle is the ideal cycle for CI
(Compression-Ignition) reciprocating engines.
The CI engine first proposed by Rudolph Diesel
in the 1890s, is very similar to the SI engine,
differing mainly in the method of initiating
combustion.
In diesel engines, ONLY air is compressed
during the compression stroke, eliminating
the possibility of auto-ignition.
Diesel engines can be designed to operate at
much higher compression ratios, typically
between 12 and 24.
The fuel injection process in diesel engines
starts when the piston approaches TDC and
continues during the first part of the power
stroke.
Therefore, the combustion process in these
engines takes place over a longer interval.
Because of this longer duration, the
combustion process in the ideal Diesel
cycle is approximated as a constantpressure heat-addition process.
This is the ONLY process where the
Otto and the Diesel cycles differ.
Ideal Cycle for CI Engines
qin  wb ,out  u3  u 2  qin  h3  h2  C p T3  T2 
qout  u 4  u1  Cv T4  T1 
th ,Diesel
wnet
qout
T4  T1
1  rck  1 

1
1
 1  k 1 

qin
qin
k T3  T2 
r
 k rc  1 
Where,
1
r
2
and
rc 
3
2
Thermal efficiency of Ideal Diesel Cycle
Under the cold-air-standard
assumptions, the efficiency
of a Diesel cycle differs from
the efficiency of Otto cycle by
the quantity in the brackets.
The quantity in the brackets is always
greater than 1. Therefore, th,Otto > th, Diesel
when both cycles operate on the same
compression ratio.
Also the cuttoff ratio, rc decreases, the
efficiency of the Diesel cycle increases.
BRAYTON CYCLE – GAS TURBINE
The open gas-turbine cycle can be modeled as
a closed cycle, as shown in the figure below, by
utilizing the air-standard assumptions
BRAYTON CYCLE - PROCESSES
12 Isentropic
compression (in a
compressor)
23 Constant pressure heat
addition
34 Isentropic expansion (in
a turbine)
41 Constant pressure heat
rejection
w net
qout
ηth,Brayton 
 1
qin
qin
Cp T4  T1 
T1 T4 /T1  1
 1
 1
Cp T3  T2 
T2 T3 /T2  1
 1
1
rp
k 1/k
T2  P2 
where
  
T1  P1 
k 1/k
 P3 
  
 P4 
k 1/k

T3
P
, and rp  2 is the pressure ratio.
T4
P1
The highest temperature in the cycle occurs at
the end of the combustion process, and it is
limited by the maximum temperature that the
turbine blades can withstand.