thermodynamics:otto vs diesel cycle

Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles1

Carnot Cycle Otto Cycle Diesela CycleStirling & Ericsson Cycle Brayton Cycle

Contents:

Carnot cycleOtto cycleDiesel cycleStirling cycleEricsson cycle Brayton cycle

Jet Gas Turbine


Carnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle

Gas power cycles Heat engines in which working fluid is gas

Heat sink

Heat source

QH

QL

WnetHeat engineSample

applications

Internal Combustion Engines

Gas Turbines

Introduction


Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton CycleCarnot Cycle

Represents most efficient cycle that operates

between two fixed temperatures TH and TL

Efficiency of Carnot heat engine:

Not practical for real-life applications

H

LCarnotth T

T−=1,η

Acts as reference against which actual cycles can be compared.

Carnot Cyle

Carnot


Otto Cycle Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle

Carnot Cyle

Carnot Cycle

Processes in a Carnot cycle:

1 - 2 Isothermal heat addition

2 - 3 Isentropic expansion

3 - 4 Isothermal heat rejection

4 - 1 Isentropic compression

Enclosed area in T-s & P-v diagrams

=> net work done by the cycle

Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles

5Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle

Reciprocating engines

Carnot Cycle Otto Cycle

Examples of gas power cycle applications that involve piston-cylinder units

Types of reciprocating engine

Combustion initiated by a spark

Ideal process described by Otto cycle

Spark-ignition engine

Compression-ignition engine

Combustion initiated by compression

Ideal process described by Diesel cycle


Diesel Cycle Stirling & Ericsson Cycle Brayton Cycle



TDC : Top dead centre

BDC : Bottom dead centre

Stroke : Distance between TDC and BDC

Bore : Diameter of the piston

Clearance volume : Minimumvolume when piston at TDC

TDC

BDC

VV

VV

r ==min

max

r : Compression ratio





MEP : Mean Effective Pressure : Fictitious pressure that if it

acted on piston during entirepower stroke would producesame amount of net workproduced during actual cycle

minmax VVw

MEP net

−=



Otto cycle


Represents ideal cycle for spark-ignition (SI) engines

Processes in 4-stroke engine cycle:

Air-fuel mixture is

compressed

Spark plug ignite and

combustion starts

High pressure gas

drives piston down

Exhaust gas driven out by piston

Fresh air-fuel mixture

drawn in

Otto: stroke by stroke



Otto cycle


Differences between Otto and actual 4-stroke engines



4-stroke engine

Carnot CycleOtto Cycle

1 - 2 1. Piston moves upward from BDC to TDC 2. Air-fuel mixture is compressed isentropically.

Isentropic compression (Compression stroke)

2 - 3 1. Spark plug fires and combustion takes place 2. Piston moves downward from TDC to BDC,

converting heat energy to work

Constant-volume heat addition (Power or expansion stroke)

3 - 4 1. Piston moves upward from BDC to TDC 2. Exhaust valve open and exhaust gas is removed

Isentropic expansion (Exhaust stroke)

4 - 1 1. Piston moves downward from TDC to BDC 2. Intake valve open and air-fuel mixture drawn in

Constant-volume heat rejection (Intake stroke)

Actual Cycle

Otto Cycle



2-stroke cycle


All four processes in take place in 2 strokes



2-stroke cycle


Compression stroke: Air-fuel mixture drawn in,squeezed in combustion chamber

Power stroke: Combustion takes place, burned gas removed



2-stroke cycle


2-stroke engines generally less efficient than 4-stroke due to:

incomplete expulsion of exhaust gases

partial expulsion of fresh air-fuel mixture

Advantages of 2-stroke engines:

simple and inexpensive

high power-to-weight and power-to-volume ratios

=> suitable for small size and light applications



1st. Law Analysis:


v

P

1

2

3

4

Qin

Qout

s

T

1

2

3

4

Qin

Qout

uwq ∆=−For closed system:

outinnet

out

in

qqwuuquuq

−=−=−=

14

23

in

netOttoth q

w=,η



1st. Law Analysis:


If specific heat is considered constant (i.e. approximate method):

)()(

14,14

23,23TTcuuqTTcuuq

avvout

avvin−=−=−=−=

1,11−

−= kOttoth rη Attention:

*Use suitable method(exact or approximate)

consistently*

v

p

CC

k

r

=

= ration compressio



Some notes:


Efficiency of Otto cycle increases with compression ratio and specific heat ratio 1,

11−

−= kOttoth rη

At high compression ratio (above 8):

further increase in efficiency is

insignificant

premature ignition occurs =>

engine knock. Reduced by anti-

knock agent, e.g. tetraethyl lead

Typical efficiency of SI engines: 25 - 30%


Stirling & Ericsson Cycle Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle

Represents ideal compression-ignition (CI) engine

Diesel Cycle:

Consists of 4 processes

=> Almost similar to Otto cycle

Air compressed to pressure & temperature above self-ignition temperature of fuel

Combustion starts on contact as fuel is injected to hot air



1st. Law analysis:

Exact method: variable specific heat

Approximate method: constant specific heat

)(,)( 1423 TTCqTTCq voutpin −=−=

ratio cutoff2

3

2

31, ,

)1(111 ===

−−

−= − vv

VVr

rkr

r cc

kc

kDieselthη

), 1423 UUqhhq outin −=−=

in

netDieselth q

w=,η



Some notes:

At same compression ratio, Otto has greater efficiency than Diesel engines

Advantages of Diesel engines:☺ able to operate at much higher

compression ratio (12 to 24)i.e higher efficiency (35 - 40%)

☺ able to use cheaper fuel, becauseless constraint on premature ignition problem

Gas Power Cycle Advanced Thermo-fluidsGas Power Cycles

20Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle

Stirling & Ericsson cycle:

Stirling & Ericsson Cycle

Stirling: Two constant-volume regeneration

Ericsson: Two constant-pressure regeneration

Robert Stirling


Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle

Stirling Engine


Brayton CycleCarnot Cycle Otto Cycle Diesel Cycle Stirling & Ericsson Cycle

Advantages:

☺ Ideal Stirling and Ericsson cycles

have Carnot cycle efficiency

☺ Combustion can be done externally

=> more choices of fuel types

Disadvantages:

Difficult to achieve in practice:

- involve heat transfer through

small temperature difference.

- require very large heat transfer

area and very long time.



Brayton cycle:

Actual gas turbine operate on open cycle

Assumptions:Combustion process => const-pressure heat additionExhaust process => const-pressure heat rejection

Represents ideal gas-turbine engine cycle



Made up of 4 processes:

1 - 2 Isentropic compression (compressor)

2 - 3 Const Pressure heat addition (heat exchanger)

3 - 4 Isentropic expansion (turbine)

4 - 1 Const Pressure heat removal (heat exchanger)



1st law analysis:

1423 , hhqhhq outin −=−=

1

2/)1(, ,11

PPr

r pkkp

Braytonth =−=−

η

in

netoutBraytonth q

w ,, =η

If specific heats are assumed constant (approximate method)

4312 , hhwhhw outin −=−=

inoutnetout www −=,

outinnetout qqw −=,



Deviation from ideal Brayton cycle, due to:

pressure drops during heat addition and rejection

irreversibilities in compressor and turbine

a

s

a

scompressor hh

hhww

21

21

−−

≅=η

s

a

s

aturbine hh

hhww

43

43

−−

≅=η



Efficiency of gas-turbine power plant can be increased significantly by combining with steam power cycle=> combined cycle gas turbine (CCGT)

Main applications of Brayton cycle:electricity generation => gas-turbine power plantsaircrafts => jet propulsion enginesmarine => propeller prime mover



Jet propulsion cycle:1 - 2 Air pressure increased slightly in diffuser2 - 3 Air is compressed in compressor3 - 4 Heat addition (combustion) process in burner at constant pressure4 - 5 Partial expansion of exhaust gas in turbine, producing just

enough power to drive compressor and other auxiliaries5 - 6 Gas expansion in the nozzle to ambient pressure at high velocity6 - 1 Heat rejection to surrounding at constant pressure



Turbofan engine

Turboprop engine



SummaryGas power cycles => Heat engineswith gas as working fluidOtto cycle => spark ignitioninternal combustion engine

v

P

1

2

3

4

Qin

Qout

s

T

12

3

4

Qin

QoutDiesel cycle => compression ignition internal combustion engine

Brayton cycle => open cycle gas turbine

thermodynamics:otto vs diesel cycle

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