Heat Engines for Basic Mechanical Engineering

HEAT ENGINESPRESENTATION

04/08/2023 JIGAR MEVADA 2

INTRODUCTION What is heat engine?

An engine may be defined as a device which converts one form of energy into mechanical energy. Mechanical energy can be further easily converted into electrical energy which is the most required form of energy.

So, Heat Engine is a device which transforms heat energy into mechanical energy. In every heat engine, some form of fuel is used to convert the chemical(heat) energy into useful mechanical energy (or to produce mechanical work).

The Heat Engines are mainly classified into two types:-1. External combustion Engines, and2. Internal Combustion Engines


TYPES OF HEAT ENGINES External Combustion Engines:-

In this engine, the products of combustion are air and fuel transfer heat to a second fluid which is working fluid of the cycle, as in the case of a steam engine or steam turbine plant.Ex:- The heat of combustion is employed to generate steam which is used in a piston engine or a turbine.

Internal Combustion Engine:- In internal combustion engine, the products of combustion are directly the motive fluid. Petrol, gas and diesel engines, open cycle gas turbines are the examples of I. C. Engine.


CLASSIFICATION OF HEAT ENGINESCLASSIFICATION NAME OF ENGINE RECIPROCATING/ROTARY

(A) Internal combustion Engines

Petrol Engine(SI) Reciprocating

Gas Engine(SI) Reciprocating

Diesel Engine(CI) Reciprocating

Wankle Engine(SI, CI) Rotary

Open Cycle Gas Turbine Rotary

Jet Engine Rotary

Rocket No Mechanism

(B)External combustion engines

Steam Engine Reciprocating

Steam Turbine Rotary

Sterling or Hot Air Engine Reciprocating

Closed Cycle Turbine Rotary


ADVANTAGES OF HEAT ENGINES Advantages of Internal Combustion Engines are:-

1. Greater mechanical simplicity.2. Lower weight and bulk to o/p ratio.3. Lower first cost.4. Higher overall efficiency.5. Lesser requirements of water for dissipation of energy

through cooling system.

Advantages of External Combustion Engines are:-1. Use of cheaper fuels.2. High starting torque.3. Higher weight and bulk to o/p ratio.


COMPARISION BETWEEN INTERNAL & EXTERNAL COMBUSTION ENGINES

Parameter I.C. Engine External Combustion Engine1) Sutablity Small size upto 1 MW. Large sizes

2)Size(weight and bulk) Compact Bulky

3)Fuel Refined Fuel Cheaper Fuels

4)Pollution level Higher Lower

5)Efficiency Lower Higher

6) Complixity of components Less More

7)Expansion Incomplete Complete

8)Heat Rejection At higher temperature At atm. temperature


ESSENTIAL ELEMENTS OF A HEAT ENGINE1. Working Substance:- It is a medium for receiving and rejection heat.2. Heat Source:- It is reservoir from which the working substance receives

heat.3. Heat Sink:- It is the reservoir where the working substance rejects heat

after developing work. It is at a lower temperature than the source temperature.

4. Expander:- It is an enclosure where the working substance does work. It may be either a cylinder or casing.

5. Pump:- it is device used to raise the pressure of the working substance. It utilizes the work give out by the expander.The essential elements of a direct cycle heat engine are shown in fig.

QS = Heat suppiledQr = Heat RejectedW = Work O/P of cycleWp = Pump work


ESSENTIAL ELEMENTS OF HEAT CYCLE

HEAT SINK

HEAT SOURCE

PUMP EXPANDER W

Qr

Qs

CONTROL SURFACE


CARNOT CYCLE Every thermodynamic system exists in a particular thermodynamic state.

When a system is taken through a series of different states and finally returned to its initial state, a thermodynamic cycle is said to have occurred. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine. A system undergoing a Carnot cycle is called a Carnot heat engine, although such a 'perfect' engine is only theoretical and cannot be built in practice.

The working of carnot cycle is shown in fig. The working substance is enclosed in a cylinder having frictionless piston. The walls of the cylinder and piston are taken as perfect insulators of heat. The cylinder head is assumed diathermic which permits the flow of heat. The heat source is at temperature T1 and heat sink at temperature t2. there is also a work reservoir.

The carnot consists of the following four reversible processes:-1. Isothermal expansion2. Adiabatic expansion3. Isothermal compression4. Adiabatic compression


WORKING OF A CARNOT CYCLE The carnot consists of the following four reversible processes:-1. Isothermal expansion2. Adiabatic expansion3. Isothermal compression4. Adiabatic compression


CARNOT CYCLE ON P-V DIAGRAM


CARNOT HEAT ENGINE PROCESS 1-2:- In the first stage, the piston moves downward while the

engine absorbs heat from a source (T1)and gas begins to expand. The portion of the graphic from point 1 to point 2 represents this behavior. Because the temperature of the gas does not change, this kind of expansion is called exothermic. (Heat absorption = Q1,T1=C)

PROCESS 2-3:- In the second stage, the heat source is removed; the piston continues to move downward and the gas is still expanding while lowering in temperature from T2 to T3. It is presented by the graphic from point 2 to point 3. This stage is called a adiabatic expansion.

PROCESS 3-4:- The piston begins to move upward and the cool gas is recompressed in the third stage. The heat goes to sink. Point 3 point 4 represents the decrease in volume and increase in pressure. The engine gives energy to the environment. This stage is called isothermal compression. (Heat rejection =Q2, T2 = C)

PROCESS 4-1:- In the final stage, the piston to move upward and the cool gas is secluded and compressed. Its temperature rises to its original state. Point 3 to point 4 illustrate this behavior; a continuing increase in pressure and decrease in volume to their initial position. So it is an adiabatic compression.


EFFICIENCY OF CARNOT CYCLE


RANKINE CYCLE Process 3-4:- The water

from the hot well at low pressure p1 pumped isentropically into the boiler at high pressure p2.

Process 4-5 :- This is the sensible heating of water along the hot water is heated up to the saturation temperature T1. The area a-3-4-5-b-a represents the heat supplied (hf5 – hf4) during this process.


RANKINE CYCLE Process 5-1 :- The saturated

water at pressure p1 and temperature T1 is completely vaporized into steam. The heat added (h1 – hf5) is represented by area b-5-1-c-b, i.e., the latent heat of vaporization. The state point 1’ shows wet steam, 1 dry and saturated and 1’’ superheated condition of steam.

Process 1-2 :- It is the isentropic expansion of steam in the turbine from p1 to p2.

Process 2-3 :- The exhaust steam is condensed in the condenser at pressure p2 and temperature T2 giving latent heat to water.

RANKINE CYCLE ON p-v

DIAGRAM


EFFICIENCY OF RANKINE CYCLEConsider 1 kg of the working fluid, we have (considering pump

work).


EFFICIENCY NEGLECTING PUMP WORK For this process assuming

polytropic expansion, work done per kg of steam.

= (p1 v1 – p2 v2)

p-v diagram for neglecting pump work

1

23

4

V

P

Adiabatic Process

P1

P2


WORKING OF OTTO CYCLE

Otto cycle on p-v diagram

Otto cycle on T-s diagram

Process 1-2:- Adiabatic compression(Q=0), Process 2-3:- Heat addition at v=c,Process 3-4:- Adiabatic expansion (Q=0), Process 4-1:- Heat Rajection at v=c

This cycle is composed of four internally reversible processes, two adiabatic and two constant volume processes. The p-v and T-s diagrams as shown in fig.


EFFICIENCY OF OTTO CYCLEConsider 1 kg of air flowing through the cycle. Since the air in the cylinder acts as a closed system.So, from first law of thermodynamics for isen tropic compression and expansion, we have q-w = ΔuSo, for v=c heat supplied and rejection processes, since ω = 0 then, q=Δu=cv

ΔT


EFFICIENCY OF OTTO CYCLE


EFFICIENCY OF OTTO CYCLE

The air standard efficiency of otto cycle depends on compression ratio only and increases as compression ratio increases. In actual engine the compression ratio varies from 5 to 8. This is used for S.I. engines working on petrol engine.


M.E.P. OF OTTO CYCLEM.E.P. = Mean Effective Pressure


DIESEL CYCLEThis cycle is used for compresssion ignition internal combustion engines working on diesel oil. The p-v and T-s diagrams are shown in fig. it consists of four internally reversible processes two adiabatic, one p=c and one v=c.


WORKING OF DIESEL CYCLEProcess 1-2:- Adiabatic compression of air, Process 2-3:- Heat

addition at p=cProcess 3-4:- Adiabatic expansion of air, Process 4-1:- Heat rejection

at v=c.


EFFIECIENCY OF DIESEL CYCLE

The efficiency of diesel cycle increases as compression ratio increases but decreases as cut-off ratio increases. The thermal efficiency of diesel cycle is less then that of otto cycle. The compression ratio for diesel cycle varies from 14 to 18.


M.E.P. OF DIESEL CYCLE


THANK YOUPREPERED BY:- JIGAR MEVADA

Heat Engines for Basic Mechanical Engineering

Documents

Transcript of Heat Engines for Basic Mechanical Engineering