THERMAL ENGINEERING(ME 2301 )

M.R.SWAMINATHANAssistant Professor

Department of Mechanical EngineeringAnna 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-air-standard 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.

TDC

BDC

min

max

VV

VV

r minmax

net

VVW

MEP

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 CYCLEThe Carnot cycle is composed of totally four reversible processes:

•isothermal heat addition,

•isentropic expansion,

•isothermal heat rejection

•isentropic compression

H

LCarnot,th T

T1

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 air-standard 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:12 Isentropic compression23 Constant volume heat addition34 Isentropic expansion41 Constant volume heat rejection

Thermal Efficiency of an Otto CycleThe Otto cycle is executed in a closed system, and disregarding the changes in kinetic and potential energies, we have

1k

2

1

232

141

23

14

in

out

in

netOttoth,

14v14out

23v23in

outinoutin

r11

TT1

1/TTT1/TTT1

TTTT1

qq1

qwη

TTCuuqTTCuuqΔuwwqq

2

1

2

1

min

max

3

41k

4

31k

1

2

2

1

υυ

VV

VVr and;

TT

υυ

υυ

TT Where,

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 constant-

pressure heat-addition process.

This is the ONLY process where the

Otto and the Diesel cycles differ.

Ideal Cycle for CI Engines

111111 1

23

14

1414

232323

c

kc

kin

out

in

netDiesel,th

vout

pinout,bin

rkr

rTTkTT

qq

qw

TTCuuq

TTChhquuwq

2

3

2

1

cr

r

and

Where,

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 TURBINEThe open gas-turbine cycle can be modeled as a closed cycle, as shown in the figure below, by utilizing the air-standard assumptions

12 Isentropic compression (in a compressor)

23 Constant pressure heat addition

34 Isentropic expansion (in a turbine)

41 Constant pressure heat rejection

BRAYTON CYCLE - PROCESSES

/k1kp

232

141

23p

14p

in

out

in

netBraytonth,

r11

1/TTT1/TTT1

TTCTTC

1

qq1

qwη

ratio. pressure the is PPr and ,

TT

PP

PP

TT where

1

2p

4

3/k1k

4

3/k1k

1

2

1

2

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.