Heat Engine Cycles

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A Proposal on Heat Engines, a topic in Chemical Engineering Thermodynamics.
This work aim at studying the process involved in the conversion of heat energy to mechanical work and in effect the principles which engine operate.
Heat engines are systems that convert heat or thermal energy to mechanical energy which can then be used to do mechanical work. This is done basically by bringing a working substance from a higher state temperature to a lower state temperature. The working substance is brought to a high temperature by a heat source which generates thermal energy. This energy is converted to work by exploiting the proportion of the working substance during which the heat is transferred to the colder destination until it reaches a lower temperature state.
The conversion of this heat to mechanical work follow certain routes which ends at the start point and hence are called cycles. This work will in essence focus on these cycles. Otto cycle, Atkinson cycle and brayton cycle are some of the cycle that represent models for heat engine operations. The condition to which the working fluid is subjected in the process, is what distinguishes one cycle from the other.

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Heat Engine Cycles

  1. 1. GROUP 4 ’S PROPOSAL NAME MATRICULATION NUMBER BRIGHT CYRIL OMOREGIE ENG1002112 CHIAJUYA ANTHONY CHUKWUKA ENG1002113 EBIKADE EBAKOLE MICHEAL ENG1002115 EBIRERI OGHENERUKEVWE GLORY ENG1002116 EDAFE COLLINS EFEJIRO ENG1002118 EFEROBOR EBENEZER ENG1002119 EKHATOR PROSPER ENG1002121 CHUKWUDI EMEKA DESTINY ENG1004285 CONTENT  EXECUTIVE SUMMARY  INTRODUCTION  AIM AND OBJECTIVES  JUSTIFICATION  METHODOLOGY  CONCLUSION  REFERENCES
  2. 2. EXECUTIVE SUMMARY This work aim at studying the process involved in the conversion of heat energy to mechanical work and in effect the principles which engine operate. Heat engines are systems that convert heat or thermal energy to mechanical energy which can then be used to do mechanical work. This is done basically by bringing a working substance from a higher state temperature to a lower state temperature. The working substance is brought to a high temperature by a heat source which generates thermal energy. This energy is converted to work by exploiting the proportion of the working substance during which the heat is transferred to the colder destination until it reaches a lower temperature state. The conversion of this heat to mechanical work follow certain routes which ends at the start point and hence are called cycles. This work will in essence focus on these cycles. Otto cycle, Atkinson cycle and brayton cycle are some of the cycle that represent models for heat engine operations. The condition to which the working fluid is subjected in the process, is what distinguishes one cycle from the other. INTRODUCTION Heat is a form of energy that is transferred from one body to another as a result of a difference in the temperatures of the bodies. The effect of the transfer of energy is usually (but not always) an increase in the temperature of the colder body and a decrease in the temperature of the hotter body (the exception being during phase change). An engine is a machine designed to convert energy into useful mechanical motion. The energy is obtained usually from the burning or consumption of fuel and the work is performed by exerting a torque or linear force to drive machinery that generates electricity, pumps water or compress gas. In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical energy which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat “source” generates thermal energy that brings the working substance to the high temperature state. The working substance generates work in the “working body” of the engines while transferring heat to the colder destination until it reaches a low temperature state. During this process, some of the thermal energy is
  3. 3. converted into work by exploiting the properties of the working substance. The working substance is usually a gas or liquid Heat engines distinguish themselves from other types of engines (e.g. electric motor, physically powered motor, etc.) by the fact that their efficiency is fundamentally limited by Carnot’s theorem, which specifies the limits on the maximum efficiency any heat engine can obtain which solely depends on the difference between the hot and cold temperature reservoir. Thermodynamics is basically the study of the relationships between heat and work. The first and second laws of thermodynamics constrain the operation of a heat engine. The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow. Heat engines such as automobile engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle. Fig. 1 Heat engines in thermodynamics are often modelled using standard engineering models which are referred to as the “heat engine cycles” such as the Otto cycle, Atkinson cycle, Brayton cycle, etc. In reality, very few actual implementations of heat engines exactly match their underlying thermodynamic cycle and as a result one could say that a thermodynamic cycle is an ideal case of a mechanical engine. In any case, fully understanding an engine and its efficiency requires gaining a good understanding of the (possibly simplified or idealized) theoretical model, the practical nuances of an actual mechanical engine, and the discrepancies between the two.
  4. 4. In general terms, the larger the difference in temperature between the hot source and cold sink, the larger the potential thermal efficiency. Also, the heat which cannot be used to do work is exhausted. Examples of heat engines are Refrigerator, Carnot cycle, dual pump, diesel engines. Examples of everyday heat engines include the steam engine (for example in trains), the diesel engine, and the gasoline (petrol) engine in an automobile. Heat engine processes. Cycle Process 1-2 (Compression) Process 2- 3 (Heat Addition) Process 3-4 (Expansion) Process 4- 1 (Heat Rejection) Notes Power cycles normally with external combustion - or heat pump cycles: Bell Coleman adiabatic isobaric adiabatic isobaric A reversed Brayton cycle Carnot isentropic isothermal isentropic isothermal Carnot heat engine Ericsson isothermal isobaric isothermal isobaric the second Ericsson cycle from 1853 Rankine adiabatic isobaric adiabatic isobaric Steam engine Hygroscopic adiabatic isobaric adiabatic isobaric Hygroscopic cycle Scuderi adiabatic variable pressure adiabatic isochoric
  5. 5. and volume Stirling isothermal isochoric isothermal isochoric Stirling engine Stoddard adiabatic isobaric adiabatic isobaric Power cycles normally with internal combustion: Brayton adiabatic isobaric adiabatic isobaric Jet engines the external combustion version of this cycle is known as first Ericsson cycle from 1833 Diesel adiabatic isobaric adiabatic isochoric Diesel engine Lenoir Isobaric isochoric adiabatic Pulse jets (Note: Process 1-2 accomplishes both the heat rejection and the compression) Otto adiabatic isochoric adiabatic isochoric Gasoline / petrol engines Each process is one of the following:  isothermal (at constant temperature, maintained with heat added or removed from a heat source or sink)  isobaric (at constant pressure)  isometric/isochoric (at constant volume), also referred to as iso-volumetric
  6. 6.  adiabatic (no heat is added or removed from the system during adiabatic process)  isentropic (reversible adiabatic process, no heat is added or removed during isentropic process) AIMS AND OBJECTIVES The aim of this work is to study in details the different processes (cycles) involved in the conversion of heat energy to mechanical work. Objectives  To review the concepts of heat and heat engines.  To study heat engine cycles.  To compare the different heat engine cycles as well as their applications. JUSTIFICATION Doing this work will give a knowledge of the sequence of steps involved in the conversion of heat to mechanical work as well as the limitation of each step and how they differ from one another. Although the term heat engine, is a common one, this work seeks to explain the actual operations that take place during the conversion of heat to mechanical work. Also gives us a knowledge about how the wrong timing of the engine cycle (fuel being blown up at an inappropriate time either early or late) which could be caused by the piston slapping next to the side of the engine cylinder walls could lead to catastrophic engine failure. Improved engine design and more precise ignition timing steadily improves engine efficiency. The study of the idealistic cycle gives an analysis and a picture of the specific process in order to get a particular output.
  7. 7. METHODOLOGY In carrying out this work, a theoretical review of heat engine and heat engine cycles is to be done. In order to make the subject (processes involved in) clearer, motion pictures will be employed to provide a vivid illustration of these operations. If possible, we also hope to show live displays of some components in heat engine for easier comprehension and assimilation. CONCLUSION Even with the aforementioned limitations. Heat engines have an advantage over other types of engines in that most forms of energy can be easily converted to heat by processes like exothermic reactions (e.g. combustion). Since the heat source that supplies thermal energy to the engine can be powered by virtually any kind of energy, heat energy are very versatile to have a wide range of applicability. Heat engines are often confused with the cycles they attempt to mimic. Typically engines refer to physical device. REFERENCES  J.M. Smith, H.C. Van Ness & M.M. Abbott 2011, “production of power from heat”, 6th Edition, McGraw Hill, New York.  http://en.wikipedia.org/wiki/Thermal_energy  http://en.wikipedia.org/wiki/Heat_engine  Yunus A. Cengel, Michael A. Boles 2001, “Thermodynamics: An Engineering Approach", Mcgraw-Hill College; 4th edition (June 2001)

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