{"id":51,"date":"2021-05-30T01:20:33","date_gmt":"2021-05-30T01:20:33","guid":{"rendered":"https:\/\/libraryresources.nse.org.ng\/thermo1\/chapter\/state-process-and-cycle\/"},"modified":"2026-03-16T14:00:17","modified_gmt":"2026-03-16T14:00:17","slug":"state-process-and-cycle","status":"publish","type":"chapter","link":"https:\/\/libraryresources.nse.org.ng\/thermo1\/chapter\/state-process-and-cycle\/","title":{"raw":"1.4 State, process, and cycle","rendered":"1.4 State, process, and cycle"},"content":{"raw":"<div class=\"-state,-process-and-cycle\">\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">If a system is isolated from its surroundings or is free from any unbalanced potentials, such as forced flows of mass or energy, the system will eventually reach a uniform condition called [pb_glossary id=\"257\"]equilibrium[\/pb_glossary]. A system in equilibrium has uniform properties throughout the system. The following equilibrium conditions are commonly considered in thermodynamics.<\/p>\n\n<ul>\n \t<li>\n<p class=\"no-indent\" style=\"text-align: left\">A system that features spatially-uniform temperature is in\u00a0[pb_glossary id=\"258\"]thermal equilibrium[\/pb_glossary].<\/p>\n<\/li>\n \t<li style=\"text-align: left\">\n<p class=\"no-indent\" style=\"text-align: justify\">A system free from chemical reactions is in [pb_glossary id=\"259\"]chemical equilibrium[\/pb_glossary].<\/p>\n<\/li>\n \t<li style=\"text-align: left\">\n<p class=\"no-indent\" style=\"text-align: justify\">If there is no tendency for a system to change its pressure over time, the system is in [pb_glossary id=\"260\"]mechanical equilibrium[\/pb_glossary].<\/p>\n<\/li>\n \t<li>\n<p class=\"no-indent\" style=\"text-align: justify\">For a system consisting of a mixture of multiple phases, such as liquid water and water vapour, if the composition of the mixture remains constant over time, the system is in [pb_glossary id=\"261\"]phase equilibrium[\/pb_glossary].<\/p>\n<\/li>\n<\/ul>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">[pb_glossary id=\"262\"]State[\/pb_glossary] refers to the condition of a system, which may be described by a unique set of properties, such as pressure, temperature, and specific volume. The state of a system in equilibrium is called [pb_glossary id=\"263\"]equilibrium state[\/pb_glossary]. A system may change from one state to another state through a [pb_glossary id=\"264\"]process[\/pb_glossary]. Let us consider a container of water initially at 10<sup>o<\/sup>C and 101 kPa, as an example. We set the water in the container as the system. The water is heated until its temperature reaches 50<sup>o<\/sup>C, while its pressure is kept constant 101 kPa. We may say that the water undergoes a constant-pressure, heating process with an initial state of 10<sup>o<\/sup>C and 101 kPa and a final state of 50<sup>o<\/sup>C and 101 kPa.<\/p>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">Typically, there are many possible paths that a system may take between two states; therefore, <em><strong>the exact path of a process is extremely important and must be clearly specified in order to describe a process<\/strong><\/em>! Here are the definitions of some common processes.<\/p>\n\n<ul>\n \t<li class=\"import-Normal\">\n<p class=\"no-indent\" style=\"text-align: left\">[pb_glossary id=\"267\"]Isobaric process[\/pb_glossary]: \u00a0the pressure remains constant in a process.<\/p>\n<\/li>\n \t<li class=\"import-Normal\" style=\"text-align: left\">\n<p class=\"no-indent\">[pb_glossary id=\"268\"]Isochoric process[\/pb_glossary]: the specific volume remains constant in a process.<\/p>\n<\/li>\n \t<li class=\"import-Normal\" style=\"text-align: left\">\n<p class=\"no-indent\">[pb_glossary id=\"269\"]Isothermal process[\/pb_glossary]: the temperature remains constant in a process.<\/p>\n<\/li>\n \t<li class=\"import-Normal\" style=\"text-align: left\">\n<p class=\"no-indent\">[pb_glossary id=\"270\"]Adiabatic process[\/pb_glossary]: no heat transfer occurs between a system and its surroundings in a process.<\/p>\n<\/li>\n \t<li class=\"import-Normal\">\n<p class=\"no-indent\" style=\"text-align: left\">[pb_glossary id=\"271\"]Isentropic process[\/pb_glossary]: the entropy remains constant in a process.<\/p>\n<\/li>\n<\/ul>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\"><a href=\"#process\">Figure 1.4.1<\/a> shows a compression process as the piston moves from the right to the left. States 1 and 2 represent the initial and final states. Each point along the process path represents an equilibrium state. If all states in a process are equilibrium states, the process is called <strong>[pb_glossary id=\"265\"]quasi-equilibrium process[\/pb_glossary]<\/strong>. In this book, we will deal with systems in equilibrium; therefore, all states thereafter refer to equilibrium states, and all processes refer to quasi-equilibrium <a id=\"process\" href=\"\"><\/a>processes.<\/p>\n&nbsp;\n\n<\/div>\n<div class=\"-state,-process-and-cycle\">\n\n[caption id=\"attachment_680\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-10.jpg\" target=\"_blank\" rel=\"noopener\"><img class=\"wp-image-45 size-medium\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2021\/05\/Fig.-1-10-300x250.jpg\" alt=\"Piston cylinder device demonstrating the process path in a compression process.\" width=\"300\" height=\"250\"><\/a> <strong>Figure 1.4.1<\/strong>\u00a0<em>Schematic of a process. States 1 and 2 represent the initial and final states; each point along the process path represents an equilibrium state.<\/em>[\/caption]\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">If a system undergoes a series of processes and finally returns to its initial state, we say that the system completes a [pb_glossary id=\"266\"]<strong>cycle<\/strong>[\/pb_glossary]. Thermodynamic cycles are the basis for the operation of thermal equipment. For example, the vapour-compression refrigeration cycle is often used in conventional refrigerators and air conditioners, as shown in <a href=\"#refrigerator\">Figure 1.4.2<\/a>. The cycle consists of four main devices: compressor, condenser, expansion valve, and evaporator. A working fluid called refrigerant circulates through these devices connected by tubes. The refrigerant in the cycle experiences phase changes between vapour and liquid, as shown in <a href=\"#vapour_comp_cycle\">Figure 1.4.3<\/a>. Phase diagrams (see details in Chapter 2) are commonly used to analyze a process or a cycle. <a href=\"#T-v_refrigeration_cycle\">Figure 1.4.4<\/a> illustrates the temperature-specific entropy, [latex]T-s[\/latex], diagram for the vapour-compression refrigeration cycle, where the numbered dots represent different states and the lines with arrows represent different processes in this cycle. For example, the number \"1\" in <a href=\"#vapour_comp_cycle\">Figure 1.4.3<\/a> and <a href=\"#T-v_refrigeration_cycle\">Figure 1.4.4<\/a> refers to the state of the refrigerant at the inlet of the compressor or the exit of the evaporator. The line 1-2 in <a href=\"#T-v_refrigeration_cycle\">Figure 1.4.4<\/a> refers to the compression process in the <a id=\"refrigerator\" href=\"\"><\/a>compressor.<\/p>\n&nbsp;\n\n&nbsp;\n\n[caption id=\"attachment_688\" align=\"aligncenter\" width=\"350\"]<a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-11_Refrigerator-working-principal.jpg\" target=\"_blank\" rel=\"noopener\"><img class=\"wp-image-46\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal.jpg\" alt=\"Refrigerator utilizes the vapour compression cycle. The main components of a refrigerator are compressor, evaporator, expansion valve and condenser.\" width=\"350\" height=\"417\"><\/a> <a id=\"vapour_comp_cycle\" href=\"\"><\/a><strong><em>Figure 1.4.2<\/em><\/strong>\u00a0<em>Refrigerator working on the vapour compression cycle<\/em>[\/caption]\n\n[caption id=\"attachment_697\" align=\"aligncenter\" width=\"500\"]<a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-11_states-labled.png\" target=\"_blank\" rel=\"noopener\"><img class=\"wp-image-47\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-300x246.png\" alt=\"Vapor compression cycle consisting of a compressor, condenser, expansion device and evaporator\" width=\"500\" height=\"410\"><\/a> <a id=\"T-v_refrigeration_cycle\" href=\"\"><\/a><strong><em>Figure 1.4.3<\/em><\/strong>\u00a0 <em>Vapour compression cycle<\/em>[\/caption]\n\n<\/div>\n\n[caption id=\"attachment_690\" align=\"aligncenter\" width=\"387\"]<a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-12_RefrigerationTS.png\" target=\"_blank\" rel=\"noopener\"><img class=\"wp-image-48 size-full\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS.png\" alt=\"Temperature-specific entropy diagram of a vapour compression cycle\" width=\"387\" height=\"319\"><\/a> <em><strong>Figure 1.4.4<\/strong><\/em>\u00a0<em>Temperature-specific entropy (T-s) diagram of a vapour compression cycle<\/em>[\/caption]\n\n<div class=\"-state,-process-and-cycle\">\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">Otto cycle is another thermodynamic cycle. It is an ideal cycle that modells the operation of internal combustion engines. <a href=\"#4stroke_combustion\">Figure 1.4.5<\/a> shows the cycle consisting of four strokes. The pressure-volume diagram, <a href=\"#PV_otto\">Figure 1.4.6<\/a>, illustrates different processes in this cycle.<\/p>\n\n<ol>\n \t<li style=\"list-style-type: none\">\n<ol>\n \t<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Intake stroke, line 0-1 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During the intake stroke, the inlet valve opens and the outlet valve remains closed. Air is drawn into the cylinder as the piston moves to the bottom dead center (BDC).<\/li>\n \t<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Compression stroke, line 1-2 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During the compression stroke, both valves remain closed. The air is compressed as the piston moves from BDC to the top dead center (TDC).<\/li>\n \t<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Ignition and power stroke, line 2-3-4 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During this stroke, both valves remain closed. The piston is at TDC momentarily while the fuel-air mixture is ignited by the spark. The burning of the fuel-air mixture generates a large force, pushing the piston from TDC to BDC.<\/li>\n \t<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Exhaust stroke, line 4-1-0 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During the exhaust stroke, the outlet valve opens and the inlet valve remains closed. The piston remains at BDC momentarily, allowing a certain amount of heat to release to the surroundings. <a id=\"4stroke_combustion\" href=\"\"><\/a> Then the piston moves from BDC towards TDC to reject the exhaust and more heat to the surroundings.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n&nbsp;\n\n<\/div>\n<div class=\"-state,-process-and-cycle\">\n\n[caption id=\"attachment_692\" align=\"aligncenter\" width=\"400\"]<a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-13_FourStroke_PSF.png\" target=\"_blank\" rel=\"noopener\"><img class=\"wp-image-692\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13_FourStroke_PSF-scaled.png\" alt=\"Four-stroke combustion engine consisting of intake, compression, ignition and exhaust strokes\" width=\"400\" height=\"228\"><\/a> <em><strong>Figure 1.4.5<\/strong><\/em>\u00a0<a id=\"PV_otto\" href=\"\"><\/a> <em>Four-stroke combustion engine<\/em>[\/caption]\n\n[caption id=\"attachment_691\" align=\"aligncenter\" width=\"400\"]<a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-13.png\" target=\"_blank\" rel=\"noopener\"><img class=\"wp-image-50\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13.png\" alt=\"Pressure-volume diagram of an Otto cycle\" width=\"400\" height=\"382\"><\/a> <em><strong>Figure 1.4.6<\/strong><\/em>\u00a0<em>Pressure-volume diagram of an Otto cycle<\/em>[\/caption]\n\n<\/div>\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\n<p class=\"textbox__title\">Practice Problems<\/p>\n\n<\/header>\n<div class=\"textbox__content\">\n\nThe original version of this chapter contained H5P content. You may want to remove or replace this element.\n\n<\/div>\n<\/div>\n&nbsp;","rendered":"<div class=\"-state,-process-and-cycle\">\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">If a system is isolated from its surroundings or is free from any unbalanced potentials, such as forced flows of mass or energy, the system will eventually reach a uniform condition called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_257\">equilibrium<\/a>. A system in equilibrium has uniform properties throughout the system. The following equilibrium conditions are commonly considered in thermodynamics.<\/p>\n<ul>\n<li>\n<p class=\"no-indent\" style=\"text-align: left\">A system that features spatially-uniform temperature is in\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_258\">thermal equilibrium<\/a>.<\/p>\n<\/li>\n<li style=\"text-align: left\">\n<p class=\"no-indent\" style=\"text-align: justify\">A system free from chemical reactions is in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_259\">chemical equilibrium<\/a>.<\/p>\n<\/li>\n<li style=\"text-align: left\">\n<p class=\"no-indent\" style=\"text-align: justify\">If there is no tendency for a system to change its pressure over time, the system is in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_260\">mechanical equilibrium<\/a>.<\/p>\n<\/li>\n<li>\n<p class=\"no-indent\" style=\"text-align: justify\">For a system consisting of a mixture of multiple phases, such as liquid water and water vapour, if the composition of the mixture remains constant over time, the system is in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_261\">phase equilibrium<\/a>.<\/p>\n<\/li>\n<\/ul>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_262\">State<\/a> refers to the condition of a system, which may be described by a unique set of properties, such as pressure, temperature, and specific volume. The state of a system in equilibrium is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_263\">equilibrium state<\/a>. A system may change from one state to another state through a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_264\">process<\/a>. Let us consider a container of water initially at 10<sup>o<\/sup>C and 101 kPa, as an example. We set the water in the container as the system. The water is heated until its temperature reaches 50<sup>o<\/sup>C, while its pressure is kept constant 101 kPa. We may say that the water undergoes a constant-pressure, heating process with an initial state of 10<sup>o<\/sup>C and 101 kPa and a final state of 50<sup>o<\/sup>C and 101 kPa.<\/p>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">Typically, there are many possible paths that a system may take between two states; therefore, <em><strong>the exact path of a process is extremely important and must be clearly specified in order to describe a process<\/strong><\/em>! Here are the definitions of some common processes.<\/p>\n<ul>\n<li class=\"import-Normal\">\n<p class=\"no-indent\" style=\"text-align: left\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_267\">Isobaric process<\/a>: \u00a0the pressure remains constant in a process.<\/p>\n<\/li>\n<li class=\"import-Normal\" style=\"text-align: left\">\n<p class=\"no-indent\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_268\">Isochoric process<\/a>: the specific volume remains constant in a process.<\/p>\n<\/li>\n<li class=\"import-Normal\" style=\"text-align: left\">\n<p class=\"no-indent\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_269\">Isothermal process<\/a>: the temperature remains constant in a process.<\/p>\n<\/li>\n<li class=\"import-Normal\" style=\"text-align: left\">\n<p class=\"no-indent\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_270\">Adiabatic process<\/a>: no heat transfer occurs between a system and its surroundings in a process.<\/p>\n<\/li>\n<li class=\"import-Normal\">\n<p class=\"no-indent\" style=\"text-align: left\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_271\">Isentropic process<\/a>: the entropy remains constant in a process.<\/p>\n<\/li>\n<\/ul>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\"><a href=\"#process\">Figure 1.4.1<\/a> shows a compression process as the piston moves from the right to the left. States 1 and 2 represent the initial and final states. Each point along the process path represents an equilibrium state. If all states in a process are equilibrium states, the process is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_265\">quasi-equilibrium process<\/a><\/strong>. In this book, we will deal with systems in equilibrium; therefore, all states thereafter refer to equilibrium states, and all processes refer to quasi-equilibrium <a id=\"process\" href=\"\"><\/a>processes.<\/p>\n<p>&nbsp;<\/p>\n<\/div>\n<div class=\"-state,-process-and-cycle\">\n<figure id=\"attachment_680\" aria-describedby=\"caption-attachment-680\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-10.jpg\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" class=\"wp-image-45 size-medium\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2021\/05\/Fig.-1-10-300x250.jpg\" alt=\"Piston cylinder device demonstrating the process path in a compression process.\" width=\"300\" height=\"250\" srcset=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2021\/05\/Fig.-1-10-300x250.jpg 300w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2021\/05\/Fig.-1-10-65x54.jpg 65w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2021\/05\/Fig.-1-10-225x188.jpg 225w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2021\/05\/Fig.-1-10.jpg 315w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-680\" class=\"wp-caption-text\"><strong>Figure 1.4.1<\/strong>\u00a0<em>Schematic of a process. States 1 and 2 represent the initial and final states; each point along the process path represents an equilibrium state.<\/em><\/figcaption><\/figure>\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">If a system undergoes a series of processes and finally returns to its initial state, we say that the system completes a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_51_266\"><strong>cycle<\/strong><\/a>. Thermodynamic cycles are the basis for the operation of thermal equipment. For example, the vapour-compression refrigeration cycle is often used in conventional refrigerators and air conditioners, as shown in <a href=\"#refrigerator\">Figure 1.4.2<\/a>. The cycle consists of four main devices: compressor, condenser, expansion valve, and evaporator. A working fluid called refrigerant circulates through these devices connected by tubes. The refrigerant in the cycle experiences phase changes between vapour and liquid, as shown in <a href=\"#vapour_comp_cycle\">Figure 1.4.3<\/a>. Phase diagrams (see details in Chapter 2) are commonly used to analyze a process or a cycle. <a href=\"#T-v_refrigeration_cycle\">Figure 1.4.4<\/a> illustrates the temperature-specific entropy, [latex]T-s[\/latex], diagram for the vapour-compression refrigeration cycle, where the numbered dots represent different states and the lines with arrows represent different processes in this cycle. For example, the number &#8220;1&#8221; in <a href=\"#vapour_comp_cycle\">Figure 1.4.3<\/a> and <a href=\"#T-v_refrigeration_cycle\">Figure 1.4.4<\/a> refers to the state of the refrigerant at the inlet of the compressor or the exit of the evaporator. The line 1-2 in <a href=\"#T-v_refrigeration_cycle\">Figure 1.4.4<\/a> refers to the compression process in the <a id=\"refrigerator\" href=\"\"><\/a>compressor.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_688\" aria-describedby=\"caption-attachment-688\" style=\"width: 350px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-11_Refrigerator-working-principal.jpg\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" class=\"wp-image-46\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal.jpg\" alt=\"Refrigerator utilizes the vapour compression cycle. The main components of a refrigerator are compressor, evaporator, expansion valve and condenser.\" width=\"350\" height=\"417\" srcset=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal.jpg 494w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal-252x300.jpg 252w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal-65x78.jpg 65w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal-225x268.jpg 225w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_Refrigerator-working-principal-350x417.jpg 350w\" sizes=\"(max-width: 350px) 100vw, 350px\" \/><\/a><figcaption id=\"caption-attachment-688\" class=\"wp-caption-text\"><a id=\"vapour_comp_cycle\" href=\"\"><\/a><strong><em>Figure 1.4.2<\/em><\/strong>\u00a0<em>Refrigerator working on the vapour compression cycle<\/em><\/figcaption><\/figure>\n<figure id=\"attachment_697\" aria-describedby=\"caption-attachment-697\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-11_states-labled.png\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" class=\"wp-image-47\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-300x246.png\" alt=\"Vapor compression cycle consisting of a compressor, condenser, expansion device and evaporator\" width=\"500\" height=\"410\" srcset=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-300x246.png 300w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-1024x839.png 1024w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-768x630.png 768w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-65x53.png 65w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-225x184.png 225w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled-350x287.png 350w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-11_states-labled.png 1220w\" sizes=\"(max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-697\" class=\"wp-caption-text\"><a id=\"T-v_refrigeration_cycle\" href=\"\"><\/a><strong><em>Figure 1.4.3<\/em><\/strong>\u00a0 <em>Vapour compression cycle<\/em><\/figcaption><\/figure>\n<\/div>\n<figure id=\"attachment_690\" aria-describedby=\"caption-attachment-690\" style=\"width: 387px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-12_RefrigerationTS.png\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" class=\"wp-image-48 size-full\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS.png\" alt=\"Temperature-specific entropy diagram of a vapour compression cycle\" width=\"387\" height=\"319\" srcset=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS.png 387w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS-300x247.png 300w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS-65x54.png 65w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS-225x185.png 225w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-12_RefrigerationTS-350x289.png 350w\" sizes=\"(max-width: 387px) 100vw, 387px\" \/><\/a><figcaption id=\"caption-attachment-690\" class=\"wp-caption-text\"><em><strong>Figure 1.4.4<\/strong><\/em>\u00a0<em>Temperature-specific entropy (T-s) diagram of a vapour compression cycle<\/em><\/figcaption><\/figure>\n<div class=\"-state,-process-and-cycle\">\n<p class=\"import-Normal no-indent\" style=\"text-align: justify\">Otto cycle is another thermodynamic cycle. It is an ideal cycle that modells the operation of internal combustion engines. <a href=\"#4stroke_combustion\">Figure 1.4.5<\/a> shows the cycle consisting of four strokes. The pressure-volume diagram, <a href=\"#PV_otto\">Figure 1.4.6<\/a>, illustrates different processes in this cycle.<\/p>\n<ol>\n<li style=\"list-style-type: none\">\n<ol>\n<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Intake stroke, line 0-1 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During the intake stroke, the inlet valve opens and the outlet valve remains closed. Air is drawn into the cylinder as the piston moves to the bottom dead center (BDC).<\/li>\n<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Compression stroke, line 1-2 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During the compression stroke, both valves remain closed. The air is compressed as the piston moves from BDC to the top dead center (TDC).<\/li>\n<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Ignition and power stroke, line 2-3-4 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During this stroke, both valves remain closed. The piston is at TDC momentarily while the fuel-air mixture is ignited by the spark. The burning of the fuel-air mixture generates a large force, pushing the piston from TDC to BDC.<\/li>\n<li class=\"import-Normal no-indent\" style=\"text-align: justify\">Exhaust stroke, line 4-1-0 in <a href=\"#PV_otto\">Figure 1.4.6<\/a>. During the exhaust stroke, the outlet valve opens and the inlet valve remains closed. The piston remains at BDC momentarily, allowing a certain amount of heat to release to the surroundings. <a id=\"4stroke_combustion\" href=\"\"><\/a> Then the piston moves from BDC towards TDC to reject the exhaust and more heat to the surroundings.<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<\/div>\n<div class=\"-state,-process-and-cycle\">\n<figure id=\"attachment_692\" aria-describedby=\"caption-attachment-692\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-13_FourStroke_PSF.png\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" class=\"wp-image-692\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13_FourStroke_PSF-scaled.png\" alt=\"Four-stroke combustion engine consisting of intake, compression, ignition and exhaust strokes\" width=\"400\" height=\"228\" \/><\/a><figcaption id=\"caption-attachment-692\" class=\"wp-caption-text\"><em><strong>Figure 1.4.5<\/strong><\/em>\u00a0<a id=\"PV_otto\" href=\"\"><\/a> <em>Four-stroke combustion engine<\/em><\/figcaption><\/figure>\n<figure id=\"attachment_691\" aria-describedby=\"caption-attachment-691\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/499\/2021\/06\/Fig.-1-13.png\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" class=\"wp-image-50\" src=\"https:\/\/libraryresources.nse.org.ng\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13.png\" alt=\"Pressure-volume diagram of an Otto cycle\" width=\"400\" height=\"382\" srcset=\"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13.png 468w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13-300x287.png 300w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13-65x62.png 65w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13-225x215.png 225w, https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-content\/uploads\/sites\/10\/2026\/03\/Fig.-1-13-350x334.png 350w\" sizes=\"(max-width: 400px) 100vw, 400px\" \/><\/a><figcaption id=\"caption-attachment-691\" class=\"wp-caption-text\"><em><strong>Figure 1.4.6<\/strong><\/em>\u00a0<em>Pressure-volume diagram of an Otto cycle<\/em><\/figcaption><\/figure>\n<\/div>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\">Practice Problems<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>The original version of this chapter contained H5P content. You may want to remove or replace this element.<\/p>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_51_257\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_257\"><div tabindex=\"-1\"><p>Equilibrium refers to a uniform condition throughout a system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_258\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_258\"><div tabindex=\"-1\"><p>Thermal equilibrium is an equilibrium condition. A system in thermal equilibrium has a uniform temperature everywhere.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_259\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_259\"><div tabindex=\"-1\"><p>Chemical equilibrium is a state in which the forward and backward reactions proceed at the same rate, causing no net change of the concentrations in either the reactants or the products. A system free from chemical reactions is in chemical equilibrium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_260\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_260\"><div tabindex=\"-1\"><p>Mechanical equilibrium refers to an equilibrium condition, in which the pressure of a system has no tendency to change over time.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_261\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_261\"><div tabindex=\"-1\"><p>Phase equilibrium is an equilibrium condition. For a system consisting of a mixture of multiple phases, if the composition of the mixture remains constant over time, the system is in phase equilibrium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_262\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_262\"><div tabindex=\"-1\"><p>A state refers to a specific condition of a system that is described by a unique set of thermodynamic properties, such as pressure, temperature, specific volume, specific enthalpy, and so on.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_263\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_263\"><div tabindex=\"-1\"><p>An equilibrium state refers to a state of a system in equilibrium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_264\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_264\"><div tabindex=\"-1\"><p>A process refers to the change in a system from one state to another state.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_267\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_267\"><div tabindex=\"-1\"><p>An isobaric process refers to a process whose pressure remains constant.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_268\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_268\"><div tabindex=\"-1\"><p>An isochoric process refers to a process of constant specific volume.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_269\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_269\"><div tabindex=\"-1\"><p>An isothermal process refers to a process whose temperature remains constant.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_270\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_270\"><div tabindex=\"-1\"><p>An adiabatic process is a process, in which heat transfer does NOT occur between a system and its surroundings.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_271\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_271\"><div tabindex=\"-1\"><p>An isentropic process refers to a process that is reversible and adiabatic. The entropy remains constant in an isentropic process.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_265\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_265\"><div tabindex=\"-1\"><p>A quasi-equilibrium process refers to a process, in which all states are equilibrium states.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_51_266\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_51_266\"><div tabindex=\"-1\"><p>A cycle consists of a series of processes. The final state of a cycle is always identical to its initial state.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":1,"menu_order":5,"template":"","meta":{"pb_show_title":"","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-51","chapter","type-chapter","status-publish","hentry","chapter-type-standard"],"part":27,"_links":{"self":[{"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/chapters\/51","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":2,"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/chapters\/51\/revisions"}],"predecessor-version":[{"id":310,"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/chapters\/51\/revisions\/310"}],"part":[{"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/parts\/27"}],"metadata":[{"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/chapters\/51\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/wp\/v2\/media?parent=51"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/pressbooks\/v2\/chapter-type?post=51"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/wp\/v2\/contributor?post=51"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/libraryresources.nse.org.ng\/thermo1\/wp-json\/wp\/v2\/license?post=51"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}