An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine . It is the thermodynamic cycle most commonly found in automobile engines.
51-417: The Otto cycle is a description of what happens to a gas as it is subjected to changes of pressure, temperature, volume, addition of heat, and removal of heat. The gas that is subjected to those changes is called the system. The system, in this case, is defined to be the fluid (gas) within the cylinder. Conversely, by describing the changes that take place within the system it also describes the system's effect on
102-418: A cooling tower or air cooler to reject the waste heat into the atmosphere. In some cases it is possible to use waste heat, for instance in district heating systems. There are many different approaches to transfer thermal energy to electricity, and the technologies to do so have existed for several decades. An established approach is by using a thermoelectric device, where a change in temperature across
153-445: A heat exchanger before heating in homes or power plants . Anthropogenic heat is heat generated by humans and human activity. The American Meteorological Society defines it as "Heat released to the atmosphere as a result of human activities, often involving combustion of fuels. Sources include industrial plants, space heating and cooling, human metabolism, and vehicle exhausts. In cities this source typically contributes 15–50 W/m to
204-412: A by-product. In the majority of applications, energy is required in multiple forms. These energy forms typically include some combination of heating, ventilation, and air conditioning , mechanical energy and electric power . Often, these additional forms of energy are produced by a heat engine running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to
255-508: A circulating fluid, such as coolant. The gas has returned to state 1. The exhaust valve opens at point 1. As the piston moves from "BDC" (point 1) to "TDC" (point 0) with the exhaust valve opened, the gaseous mixture is vented to the atmosphere and the process starts anew. In this process 1–2 the piston does work on the gas and in process 3–4 the gas does work on the piston during those isentropic compression and expansion processes, respectively. Processes 2–3 and 4–1 are isochoric processes; heat
306-417: A complete cycle, the gas returns to its original state of temperature, pressure and volume, hence the net internal energy change of the system (gas) is zero. As a result, the energy (heat or work) added to the system must be offset by energy (heat or work) that leaves the system. In the analysis of thermodynamic systems, the convention is to account energy that enters the system as positive and energy that leaves
357-578: A fundamental result of the laws of thermodynamics . Waste heat has lower utility (or in thermodynamics lexicon a lower exergy or higher entropy ) than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation. Instead of being "wasted" by release into
408-400: A mass containing a mixture of fuel and oxygen is drawn into the cylinder by the descending piston, it is compressed by the piston rising, the mass is ignited by a spark releasing energy in the form of heat, the resulting gas is allowed to expand as it pushes the piston down, and finally the mass is exhausted as the piston rises a second time. As the piston is capable of moving along the cylinder,
459-400: A particular engine with particular dimensions. In the study of thermodynamic systems the extensive quantities such as energy, volume, or entropy (versus intensive quantities of temperature and pressure) are placed on a unit mass basis, and so too are the calculations, making those more general and therefore of more general use. Hence, each term involving an extensive quantity could be divided by
510-483: A semiconductor material creates a voltage through a phenomenon known as the Seebeck effect . A related approach is the use of thermogalvanic cells , where a temperature difference gives rise to an electric current in an electrochemical cell. The organic Rankine cycle , offered by companies such as Ormat , is a very known approach, whereby an organic substance is used as working fluid instead of water. The benefit
561-462: A source of waste heat by releasing waste heat into the outdoor ambient air whilst cooling indoor spaces. This expelling of waste heat from air conditioning can worsen the urban heat island effect. Waste heat from air conditioning can be reduced through the use of passive cooling building design and zero-energy methods like evaporative cooling and passive daytime radiative cooling , the latter of which sends waste heat directly to outer space through
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#1732782738020612-413: Is disposed of by various thermoregulation methods such as sweating and panting . Low temperature heat contains very little capacity to do work ( Exergy ), so the heat is qualified as waste heat and rejected to the environment. Economically most convenient is the rejection of such heat to water from a sea , lake or river . If sufficient cooling water is not available, the plant can be equipped with
663-464: Is done by the system on the piston. The volume ratio V 4 / V 3 {\displaystyle V_{4}/V_{3}} is called the "isentropic expansion ratio". (For the Otto cycle is the same as the compression ratio V 1 / V 2 {\displaystyle V_{1}/V_{2}} ). Mechanically this is the expansion of the hot gaseous mixture in
714-479: Is energy removed from the system from 3–4–1. In terms of work and heat added to the system Equation 1b: Each term of the equation can be expressed in terms of the internal energy of the gas at each point in the process: The energy balance Equation 1b becomes To illustrate the example we choose some values to the points in the illustration: These values are arbitrarily but rationally selected. The work and heat terms can then be calculated. The energy added to
765-404: Is lost to the environment may instead be used to advantage. Industrial processes, such as oil refining , steel making or glass making are major sources of waste heat. Although small in terms of power, the disposal of waste heat from microchips and other electronic components, represents a significant engineering challenge. This necessitates the use of fans, heatsinks , etc. to dispose of
816-432: Is not normally calculated in state-of-the-art global climate simulations. Equilibrium climate experiments show statistically significant continental-scale surface warming (0.4–0.9 °C) produced by one 2100 AHF scenario, but not by current or 2040 estimates. Simple global-scale estimates with different growth rates of anthropogenic heat that have been actualized recently show noticeable contributions to global warming, in
867-416: Is one contributor to urban heat islands . Other human-caused effects (such as changes to albedo , or loss of evaporative cooling) that might contribute to urban heat islands are not considered to be anthropogenic heat by this definition. Anthropogenic heat is a much smaller contributor to global warming than greenhouse gases are. In 2005, anthropogenic waste heat flux globally accounted for only 1% of
918-549: Is performed on the system during the lower isentropic compression process. Heat flows into the Otto cycle through the left pressurizing process and some of it flows back out through the right depressurizing process. The summation of the work added to the system plus the heat added minus the heat removed yields the net mechanical work generated by the system. The processes are described by: The Otto cycle consists of isentropic compression, heat addition at constant volume, isentropic expansion, and rejection of heat at constant volume. In
969-418: Is reversible. The compression process requires that mechanical work be added to the working gas. Generally the compression ratio is around 9–10:1 ( V 1 : V 2 ) for a typical engine. The piston is momentarily at rest at TDC . During this instant, which is known as the ignition phase, the air/fuel mixture remains in a small volume at the top of the compression stroke. Heat is added to the working fluid by
1020-485: Is that this process can reject heat at lower temperatures for the production of electricity than the regular water steam cycle. An example of use of the steam Rankine cycle is the Cyclone Waste Heat Engine . Waste of the by-product heat is reduced if a cogeneration system is used, also known as a Combined Heat and Power (CHP) system. Limitations to the use of by-product heat arise primarily from
1071-580: Is the Drake Landing Solar Community in Alberta , Canada, which, by using a cluster of boreholes in bedrock for interseasonal heat storage, obtains 97 percent of its year-round heat from solar thermal collectors on the garage roofs. Another STES application is storing winter cold underground, for summer air conditioning. On a biological scale, all organisms reject waste heat as part of their metabolic processes , and will die if
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#17327827380201122-413: Is transferred into the system from 2—3 and out of the system from 4–1 but no work is done on the system or extracted from the system during those processes. No work is done during an isochoric (constant volume) process because addition or removal of work from a system requires the movement of the boundaries of the system; hence, as the cylinder volume does not change, no shaft work is added to or removed from
1173-466: The energy flux created by anthropogenic greenhouse gases. The heat flux is not evenly distributed, with some regions higher than others, and significantly higher in certain urban areas. For example, global forcing from waste heat in 2005 was 0.028 W/m , but was +0.39 and +0.68 W/m for the continental United States and western Europe, respectively. Although waste heat has been shown to have influence on regional climates, climate forcing from waste heat
1224-452: The infrared window . The electrical efficiency of thermal power plants is defined as the ratio between the input and output energy. It is typically only 33% when disregarding usefulness of the heat output for building heat. The images show cooling towers , which allow power stations to maintain the low side of the temperature difference essential for conversion of heat differences to other forms of energy. Discarded or "waste" heat that
1275-639: The second law of thermodynamics , therefore a heat engine will always produce a surplus of low-temperature heat. This is commonly referred to as waste heat or "secondary heat", or "low-grade heat". This heat is useful for the majority of heating applications, however, it is sometimes not practical to transport heat energy over long distances, unlike electricity or fuel energy. The largest proportions of total waste heat are from power stations and vehicle engines. The largest single sources are power stations and industrial plants such as oil refineries and steelmaking plants. Conventional air conditioning systems are
1326-436: The Otto cycle uses isentropic processes during the compression (process 1 to 2) and expansion (process 3 to 4) the isentropic equations of ideal gases and the constant pressure/volume relations can be used to yield Equations 3 & 4. Waste heat Waste heat is heat that is produced by a machine , or other process that uses energy , as a byproduct of doing work . All such processes give off some waste heat as
1377-448: The Otto cycle, there is no heat transfer during the process 1–2 and 3–4 as they are isentropic processes. Heat is supplied only during the constant volume processes 2–3 and heat is rejected only during the constant volume processes 4–1. The above values are absolute values that might, for instance , have units of joules (assuming the MKS system of units are to be used) and would be of use for
1428-624: The above it appears as if the system gained one unit of heat. This matches the energy produced by the system as work out of the system. Thermal efficiency is the quotient of the net work from the system, to the heat added to system. Equation 2: Alternatively, thermal efficiency can be derived by strictly heat added and heat rejected. Supplying the fictitious values η = 1 + 1 − 4 9 − 5 = 1 + − 3 4 = 0.25 {\displaystyle \eta =1+{\frac {1-4}{9-5}}=1+{\frac {-3}{4}}=0.25} In
1479-453: The ambient environment, sometimes waste heat (or cold) can be used by another process (such as using hot engine coolant to heat a vehicle), or a portion of heat that would otherwise be wasted can be reused in the same process if make-up heat is added to the system (as with heat recovery ventilation in a building). Thermal energy storage , which includes technologies both for short- and long-term retention of heat or cold, can create or improve
1530-506: The ambient temperature is too high to allow this. Anthropogenic waste heat can contribute to the urban heat island effect. The biggest point sources of waste heat originate from machines (such as electrical generators or industrial processes, such as steel or glass production) and heat loss through building envelopes. The burning of transport fuels is a major contribution to waste heat. Machines converting energy contained in fuels to mechanical work or electric energy produce heat as
1581-402: The case of a four-stroke Otto cycle, technically there are two additional processes: one for the exhaust of waste heat and combustion products at constant pressure (isobaric), and one for the intake of cool oxygen-rich air also at constant pressure; however, these are often omitted in a simplified analysis. Even though those two processes are critical to the functioning of a real engine, wherein
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1632-462: The combustion of the injected fuel, with the volume essentially being held constant. The pressure rises and the ratio ( P 3 / P 2 ) {\displaystyle (P_{3}/P_{2})} is called the "explosion ratio". The increased high pressure exerts a force on the piston and pushes it towards the BDC . Expansion of working fluid takes place isentropically and work
1683-406: The cylinder known as expansion (power) stroke. The piston is momentarily at rest at BDC . The working gas pressure drops instantaneously from point 4 to point 1 during a constant volume process as heat is removed to an idealized external sink that is brought into contact with the cylinder head. In modern internal combustion engines, the heat-sink may be surrounding air (for low powered engines), or
1734-413: The details of heat transfer and combustion chemistry are relevant, for the simplified analysis of the thermodynamic cycle, it is more convenient to assume that all of the waste-heat is removed during a single volume change. The four-stroke engine was first patented by Alphonse Beau de Rochas in 1861. Before, in about 1854–57, two Italians ( Eugenio Barsanti and Felice Matteucci ) invented an engine that
1785-463: The engineering cost/efficiency challenges in effectively exploiting small temperature differences to generate other forms of energy. Applications utilizing waste heat include swimming pool heating and paper mills . In some cases, cooling can also be produced by the use of absorption refrigerators for example, in this case it is called trigeneration or CCHP (combined cooling, heat and power). Waste heat can be used in district heating . Depending on
1836-493: The environment. The purpose of the Otto cycle is to study the production of net work from the system that can propel a vehicle and its occupants in the environment. The Otto cycle is constructed from: The isentropic process of compression or expansion implies that there will be no inefficiency (loss of mechanical energy), and there be no transfer of heat into or out of the system during that process. The cylinder and piston are assumed to be impermeable to heat during that time. Work
1887-408: The gas is exhausted to the environment. Mechanical work is produced during the expansion process and some of that used to compress the air mass of the next cycle. The mechanical work produced minus that used for the compression process is the net work gained and that can be used for propulsion or for driving other machines. Alternatively the net work gained is the difference between the heat produced and
1938-399: The heat removed. A mass of air (working fluid) is drawn into the cylinder, from 0 to 1, at atmospheric pressure (constant pressure) through the open intake valve, while the exhaust valve is closed during this process. The intake valve closes at point 1. Piston moves from crank end (BDC, bottom dead centre and maximum volume) to cylinder head end ( TDC , top dead centre and minimum volume) as
1989-452: The heat. For example, data centers use electronic components that consume electricity for computing, storage and networking. The French CNRS explains a data center is like a resistor and most of the energy it consumes is transformed into heat and requires cooling systems. Humans, like all animals, produce heat as a result of metabolism . In warm conditions, this heat exceeds a level required for homeostasis in warm-blooded animals, and
2040-451: The local heat balance, and several hundred W/m in the center of large cities in cold climates and industrial areas." In 2020, the overall anthropogenic annual energy release was 168,000 terawatt-hours; given the 5.1×10 m surface area of Earth, this amounts to a global average anthropogenic heat release rate of 0.04 W/m . Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in dense urban areas. It
2091-448: The mass, giving the terms units of joules/kg ( specific energy ), meters/kg (specific volume), or joules/(kelvin·kg) (specific entropy, heat capacity) etc. and would be represented using lower case letters, u, v, s, etc. Equation 1 can now be related to the specific heat equation for constant volume. The specific heats are particularly useful for thermodynamic calculations involving the ideal gas model. Rearranging yields: Inserting
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2142-432: The specific heat equation into the thermal efficiency equation (Equation 2) yields. Upon rearrangement: Next, noting from the diagrams T 4 / T 1 = T 3 / T 2 {\displaystyle T_{4}/T_{1}=T_{3}/T_{2}} (see isentropic relations for an ideal gas ), thus both of these can be omitted. The equation then reduces to: Equation 2: Since
2193-402: The summation is zero as expected for a complete cycle that returns the system to its original state. From the energy balance the work out of the system is: The net energy out of the system as work is -1, meaning the system has produced one net unit of energy that leaves the system in the form of work. The net heat out of the system is: As energy added to the system as heat is positive. From
2244-408: The system as work during the compression from 1 to 2 is The energy added to the system as heat from point 2 to 3 is The energy removed from the system as work during the expansion from 3 to 4 is The energy removed from the system as heat from point 4 to 1 is The energy balance is Note that energy added to the system is counted as positive and energy leaving the system is counted as negative and
2295-452: The system is accounted as negative. Equation 1a. During a complete cycle, the net change of energy of the system is zero: The above states that the system (the mass of gas) returns to the original thermodynamic state it was in at the start of the cycle. Where E in {\displaystyle E_{\text{in}}} is energy added to the system from 1–2–3 and E out {\displaystyle E_{\text{out}}}
2346-408: The system. Four different equations are used to describe those four processes. A simplification is made by assuming changes of the kinetic and potential energy that take place in the system (mass of gas) can be neglected and then applying the first law of thermodynamics (energy conservation) to the mass of gas as it changes state as characterized by the gas's temperature, pressure, and volume. During
2397-527: The temperature of the waste heat and the district heating system, a heat pump must be used to reach sufficient temperatures. These are an easy and cheap way to use waste heat in cold district heating systems, as these are operated at ambient temperatures and therefore even low-grade waste heat can be used without needing a heat pump at the producer side. Waste heat can be forced to heat incoming fluids and objects before being highly heated. For instance, outgoing water can give its waste heat to incoming water in
2448-506: The utility of waste heat (or cold). One example is waste heat from air conditioning machinery stored in a buffer tank to aid in night time heating. Another is seasonal thermal energy storage (STES) at a foundry in Sweden. The heat is stored in the bedrock surrounding a cluster of heat exchanger equipped boreholes, and is used for space heating in an adjacent factory as needed, even months later. An example of using STES to use natural waste heat
2499-410: The volume of the gas changes with its position in the cylinder. The compression and expansion processes induced on the gas by the movement of the piston are idealized as reversible, i.e., no useful work is lost through turbulence or friction and no heat is transferred to or from the gas during those two processes. After the expansion is completed in the cylinder, the remaining heat is extracted and finally
2550-408: The working gas with initial state 1 is compressed isentropically to state point 2, through compression ratio ( V 1 / V 2 ) . Mechanically this is the isentropic compression of the air/fuel mixture in the cylinder, also known as the compression stroke. This isentropic process assumes that no mechanical energy is lost due to friction and no heat is transferred to or from the gas, hence the process
2601-413: Was rumored to be very similar, but the patent was lost. The first person to build a working four-stroke engine, a stationary engine using a coal gas-air mixture for fuel (a gas engine ), was German engineer Nicolaus Otto . This is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines. The cycle has four parts:
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