Internal Combustion Engine Cycles

Internal Combustion Engine Cycles •Classification of I/C Eng • I/C engine cycles & •Criteria performance of I/C eng

By: Ms. Farm Yan Yan

Combustion engines

Combustion engine is a machine that converts heat energy into mechanical energy.  Internal combustion engine  External combustion engine

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Definition of I.C.E and E.C.E •I.C.E (internal combustion engine) -----is any engine that operates by burning its fuel inside the engine.  Fuel (chemical energy) burning-heat energy-inside of cylinderexpanding gases-force the piston to move (mechanical energy). •E.C.E (external combustion engine) -----is any engine that operates by burning its fuel outside the engine. Fuel (chemical energy) burning-heat energy-outside of boilerwater-steam-force the piston to move (mechanical energy). 3

Simple external and internal combustion engine engine

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Three elements which all ICEs rely on: Fuel: contains potential energy for operating the engine. Air :contains oxygen necessary for combustion. Ignition: it can start the combustion.

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I.C.E versus E.C.E (1) Similarity: ① both are heat engines. ② energy transforms in the same way -reciprocating motion to rotary motion (2) Differences: ①the fuel burns in different place ②the combustion rate 6

Classification of ICE  Type of fuel used a) Petrol b) Diesel c) Gas

 Method of igniting the fuel a) Spark ignition engine (S.I engine) b) Compression ignition engine (C.I engine)

Classification of ICE  No of strokes per cycle a) 4 stroke cycle engine b) 2 stroke cycle engine

 Cycle of operation a) Otto b) Diesel c) Dual combustion cycle

 Method of fuel injection a) Carburetor engine b) Air injection engine c) Airless/ Solid injection engine

Classification of ICE     

No. of cylinder Speeds Arrangement of cylinder Valve mechanism Method of governing

APPLIED THERMODYNAMICS KM 30803

Spark ignition Engine

Petrol Engine

Gas Engine

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• Carnot Cycle • Joule Cycle • Otto Cycle • Dual Combustion Cycle • Diesel Cycle

Assumption for all the cycles • The working medium assume to be perfect gas, & follow pV=mRT • There is no change in the mass of working medium • All the process that constitute the cycle are reversible • Heat supplied from a constant high T source NOT chemical reaction during the cycle • Some heat assume to be rejected to a constant low T sink during the cycle.

• It is assume that there is no heat losses from the system to the surroundings

APPLIED THERMODYNAMICS KM 30803

The Otto Cycle • Constant- volume heat addition cycle  Spark ignition engines •There are four processes: I. Compression stroke II. Ignition Stroke III. Expansion Stroke IV. Exhaust stroke 1

2

Compression stroke

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3

Ignition

4

Expansion

Exhaust

APPLIED THERMODYNAMICS KM 30803

The Otto Cycle

-Air compressed isentropically - work put in and no heat transfer occur

T

p

2

1

2

-Air is heated (Qin) at constant volume - No work is done

T

2

2 1

T

1

s

3

p

3

T

2

1

s

3

p

3

V

2 2

4 1

2

1

V

3

p

3

s

2

4 1

1

s

V -Air compressed isentropically - work output and no heat transfer occur

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3

4

4

4 1

-Air is cooled (Qout)at constant volume, P and T back to original - No work is done.

V

APPLIED THERMODYNAMICS KM 30803

Thermal Efficiency of the Otto cycle:

Wnet Qnet Qin  Qout Q    1  out Qin Qin Qin Qin The thermal efficiency becomes

 th 

Qout Qin mCv (T4  T1 )  1 mCv (T3  T2 )

 th , Otto  1 

Since process 1-2 and 3-4 is isentropic,

Compression ratio

Hence, thermal efficiency of the Otto cycle

Ideal case of air, ɣ=1.4 Prepared by: Ms Farm Yan Yan Not to be produce without permission

Work output- Otto Cycle

 Work done = Heat absorbed – Heat rejected =mCv (T1-T4)-mCv (T2-T3) If replace all the T1, T2, T3& T4: Work done =

𝑝3𝑉3−𝑃4𝑉4 𝛾−1

−

𝑃2𝑉2−𝑃1𝑉1 𝑝2 ; where, 𝛾−1 𝑝1

=

𝑝3 𝑝4

=𝛾

Mean Effective Pressure (Pm)- Otto Cycle The average pressure inside the cylinders of an ICE based on the calculated/ measured power output.  Mean effective pressure, Pi =

𝑁𝑒𝑡 𝑎𝑟𝑒𝑎 𝑑𝑖𝑎𝑔𝑟𝑎𝑚 𝑊𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑑𝑖𝑎𝑔𝑟𝑎𝑚 𝑆𝑤𝑒𝑝𝑡 𝑉𝑜𝑙𝑢𝑚𝑒

l

v

APPLIED THERMODYNAMICS KM 30803

The Diesel Cycle - Used in oil diesel engine

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APPLIED THERMODYNAMICS KM 30803

Diesel Cycle P-V & T-s Diagrams

Process 1- 2 isentropic compression Process 2-3 reversible constant pressure heating Process 3-4 isentropic expansion Process 4-1 reversible constant volume cooling

APPLIED THERMODYNAMICS KM 30803

The Diesel Cycle 3-4 2-3

Constant Pressure Heating Reversible adiabatic expansion

1-2

4-1

Reversible adiabatic compression

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Reversible constant volume heat rejection- cooling

Thermal efficiency of Diesel Cycle

Mean Effective Pressure (Pm)- Diesel Cycle

Dual Combustion Cycle

Comparison of Otto, Diesel & Dual Combustion cycle  Figure shows comparison of Otto, diesel and dual combustion cycles at various compression ratios and with given cut off ratio for diesel and dual cycles. For the given compression ratio.