Otto Cycle

11 Key excerpts on "Otto Cycle"

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  Thermodynamic Cycles

  Computer-Aided Design and Optimization

  • Chih Wu(Author)
  • 2003(Publication Date)
  • CRC Press

    (Publisher)

  3

  Gas Closed-System Cycles

  3.1   Otto Cycle

  A four-stroke internal combustion engine was built by a German engineer, Nicholas Otto, in 1876. The cycle patterned after his design is called the Otto Cycle. It is the most widely used internal combustion heat engine in automobiles.

  The piston in a four-stroke internal combustion engine executes four complete strokes as the crankshaft completes two revolutions per cycle, as shown in Fig. 3.1 . On the intake stroke, the intake valve is open and the piston moves downward in the cylinder, drawing in a premixed charge of gasoline and air until the piston reaches its lowest point of the stroke called bottom dead center (BDC). During the compression stroke the intake valve closes and the piston moves toward the top of the cylinder, compressing the fuel-air mixture. As the piston approaches the top of the cylinder called top dead center (TDC), the spark plug is energized and the mixture ignites, creating an increase in the temperature and pressure of the gas. During the expansion stroke the piston is forced down by the high-pressure gas, producing a useful work output. The cycle is then completed when the exhaust valve opens and the piston moves toward the top of the cylinder, expelling the products of combustion.

  Figure 3.1

  Otto Cycle.

  The thermodynamic analysis of an actual Otto Cycle is complicated. To simplify the analysis, we consider an ideal Otto Cycle composed entirely of internally reversible processes. In the Otto Cycle analysis, a closed piston–eylinder assembly is used as a control mass system.

  The cycle consists of the following four processes: 1-2 Isentropic compression 2-3 Constant-volume heat addition 3-4 Isentropic expansion 4-1 Constant-volume heat removal

  The p–v and T–s process diagrams for the ideal Otto Cycle are illustrated in Fig. 3.2

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  Biofueled Reciprocating Internal Combustion Engines

  • K.A. Subramanian(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  A spark-ignition engine operates with the thermodynamic cycle of an Otto Cycle, whereas a compression-ignition engine operates with a constant pressure cycle/dual cycle. The principle of operation of a spark-ignition engine and a compression-ignition engine is described next.

  4.4 Otto Cycle

  A spark-ignition engine operates with an Otto Cycle. Air and fuel are inducted during suction stroke (0-1) and compresses the working fluid (1-2) during compression stroke. Ignition energy generated by a spark plug at end of the compression stroke initiates a chemical reaction of the charge and then combustion proceeds. Heat energy is released at a constant volume, resulting in an increase in in-cylinder pressure and temperature. The working fluid at high temperature and pressure pushes down the piston movement through which the heat energy is converted to mechanical work/power (3-4). The burned product (exhaust gas) during exhaust is expelled out to the atmosphere. Thus, a cycle is completed and then next cycle continues. The details of an Otto Cycle process are given in Table 4.1 . The pressure-volume and temperature-entropy diagram of an Otto Cycle are shown in Figure 4.1 .

  Table 4.1 Details of Otto Cycle Processes

  Process	Assumption
  Suction (0-1)	Constant pressure
  Compression (1-2)	Isentropic (adiabatic and reversible), s 1 = s 2
  Heat addition (2-3)	(i) Adiabatic (ii) At constant volume (iii) Complete combustion
  Expansion (3-4)	Isentropic (adiabatic and reversible), s 3 = s 4
  Exhaust (1-0)	(i) Adiabatic (ii) Valves open and close at top and bottom center (iii) Process (4-1) at constant volume (iv) Process (1-0) at constant pressure

  Figure 4.1 Pressure-volume diagram (a–c) and temperature-entropy diagram (d–f) for an Otto Cycle based spark-ignition engine.

  4.4.1 Derivation for Thermal Efficiency of Otto Cycle Based Spark-Ignition Engines

  4.4.1.1 Process 1-2

  Isentropic Compression : The working fluid (charge: air-fuel mixture) is compressed by a piston, which moves from bottom dead center (BDC) to top dead center (TDC). The pressure, temperature, and volume at BDC and TDC are denoted as P 1 , T 1 , V 1 and P 2 , T 2 , V 2 , respectively. The specific heat ratio (γ) is defined by the ratio of specific heat at constant pressure (C p ) and specific heat at constant volume (C v ). The total volume of the engine’s cylinder is summation of swept volume and clearance volume. V 1 and V 2 represent the engine’s total volume and clearance volume, and the ratio of V 1 at BDC and V 2

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  Introduction to Modern Vehicle Design

  • Julian Happian-Smith(Author)
  • 2001(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  This is a major consideration when the thermal design of components such as the piston and the engine cooling system are considered. A thermodynamic cycle of a spark ignition engine can be represented in Pressure–Volume coordinates as shown in Figure 12.1. This ideal cycle, known as the Otto Cycle after its inventor Gustav Otto, consists of an isentropic compression process 1–2, a constant volume heat addition process 2–3, an isentropic expansion 3–4, and a constant volume heat rejection process 4–1. The amount of heat received per unit mass of the working fluid per cycle is as shown in equation 12.1. Figure 12.1 An ideal Otto Cycle q + = c v T 3 − T 2 (12.1) Likewise the heat rejected from the cycle will be q = c v T 4 − T 1 (12.2) The network produced by the cycle and the thermal efficiency of the cycle will then be: W net = M c v T 3 − T 2 − T 4 + T 1 (12.3) η th = 1 − q − q + = 1 − T 4 − T 1 T 3 − T 2 (12.4) Using the geometric. relationships in the cycle the expression for the thermal efficiency then becomes as shown in equation 12.5. η th = 1 − 1 r n − 1 (12.5) The efficiency of the cycle is seen to depend upon the compression ratio r and the nature of the working fluid through the polytropic exponent n. The influence of compression ratio on the efficiency is shown in Figure 12.2 for different values of the exponent n. A value of n of 1.4 corresponds to that of an ideal gas with constant specific heats whilst the value of 1.3 is typical of that for products of combustion of petrol–air mixtures. For given values of r and n the efficiency is a constant quantity that is independent of the combustion process and therefore is also independent of the load. Figure 12.2 The influence of compression ratio r on the ideal cycle thermal efficiency for different values of the polytropic exponent The mean effective pressure is often used as a measure of an engine’s performance. It is obtained by dividing the work produced per cycle by the swept volume

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  Energy, Entropy and Engines

  An Introduction to Thermodynamics

  • Sanjeev Chandra(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Assuming that both the expansion and compression strokes are reversible and adiabatic, we can model them as being isentropic processes. Heat addition (due to ignition) and heat rejection (due to the exhaust valve opening) are very rapid processes, during which the piston moves through a very small distance and we can quite reasonably suppose that both are constant volume processes. With these assumptions, a spark-ignition engine can be approximated by the Otto Cycle, which is shown in Figure 10.4 on both P-v (Figure 10.4 a) and T-s (Figure 10.4 b) diagrams. Figure 10.4 The Otto Cycle on (a) a P-v diagram and (b) a T-s diagram. The Otto Cycle consists of four processes: 1→2 Isentropic compression 2→3 Constant volume heat addition 3→4 Isentropic expansion 4→1 Constant volume heat rejection. There is no work done by the engine during heat addition or rejection since both are constant volume processes. The heat added to the engine per unit mass of air is, assuming constant specific heat, (10.2) and the heat rejected is (10.3) The thermal efficiency of an Otto Cycle, (10.4) Substituting Equations (10.2) and (10.3) into Equation (10.4) gives (10.5) The processes 2→3 and 4→1 are both constant volume, so V 1 = V 4 = V max and V 2 = V 3 = V min. Since processes 1→2 and 3→4 are both isentropic, we can write (10.6) where the specific heat ratio γ = c p / c v. Rearranging Equation (10.6) gives (10.7) Substituting Equation (10.7) into Equation (10.5), (10.8) Combining with Equation (10.1), (10.9) Figure 10.5 shows the variation of Otto engine efficiency (η th,Otto) with compression ratio (r), assuming that the specific heat ratio for air γ =1.4. The efficiency increases continuously with r, suggesting that the compression ratio should be made as high as possible. In practice there is a limit to how high the engine compression ratio can be in a spark-ignition engine because the more the gas is compressed the higher its temperature (T 2) becomes at the end of the compression stroke

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  Thermodynamics and Energy Systems Analysis

  • Lucien Borel, Daniel Favrat, Dinh Lan Nguyen, Magdi Batato(Authors)
  • 2012(Publication Date)
  • EPFL PRESS

    (Publisher)

  Chapter 13 Thermodynamic cycles 13.A Beau-de-Rochas or Otto Cycle Description The real cycle of a spark ignition internal combustion engine is complex. To study it, a first approach is to represent it by the theoretical cycle of a hot air engine, shown in the P -v diagram of Figure 13.1 (Beau-de-Rochas or Otto Cycle). Figure 13.1 The air flowing through the cycle undergoes the following processes: 1-2: Adiabatic compression without dissipation, 2-3: Isochoric heating, 3-4: Adiabatic expansion without dissipation, 4-1: Isochoric cooling. Let us study how to improve the cycle efficiency by increasing either the volu- metric compression ratio χ = (V 0 + V c )/V 0 , or the pressure P 1 . 380 Beau-de-Rochas or Otto Cycle Hypotheses • The changes of kinetic and potential energies are negligible. • Air can be considered as a perfect gas. Data • Temperature of air at point 1: ˆ T 1 = 20 ◦ C • Molar mass of air: ˜ m = 28.85 kg/kmol • Calorific ratio of air: γ = 1.389 • Specific heat energy received by air: 3 2 q + = 2 400 kJ/kg Questions • Draw the pressure at point 3 and the power effectiveness of the cycle as a function of the pressure at point 1 (P 1 = 1 to 2 bar), with the volumetric ratio of compression as a parameter (χ = 2, 4, 6 and 8). • Determine the thermodynamic state (P, T ) of the points of the cycle for the cases corresponding to the maximum values of the pressure in point 3 and of the power effectiveness, in the domain considered in the first question. Solution Pressure in point 3 and power effectiveness The calorific factor is, according to (2.36): Γ ≡ γ - 1 γ = 0.28 The isochoric specific heat is, according to (2.74) and (5.66): c v = 1 γ - 1 ˜ r ˜ m = 741.035 J/(K kg) The process 2-3 being isochoric, the pressure in point 3 is, according to the state equation (5.8): P 3 = P 2 T 3 T 2 Relation (2.2) shows that the process 1-2 is isentropic.

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  Thermodynamics

  A Smart Approach

  • Ibrahim Dinçer(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  In addition, the exergy efficiency can be calculated as η ex = w net q in 1 -T 0 T s = 300 8 kJ kg 634 4 kJ kg 1 -300 K 2000 K = 55 78 % 6.6 Dual Cycle The difference between the Otto Cycle and the Diesel cycle is mainly in the step of heat addi-tion, where, as explained above, the Otto Cycle heat addition step occurs at a constant vol-ume (isochoric) process while in the Diesel cycle the heat addition process occurs at a constant pressure (isobaric) process. In modern engines some are equipped with high-speed compression ignition systems. In high-speed compression ignition (turbo) engines, the fuel is injected into the chamber earlier than in Diesel engines, where the fuel starts to ignite at later stages of the compression process, where this part of the combustion can be approxi-mated as heat addition at a constant volume. However, in these engines the combustion does not stop at the top dead center, it continues; this results in the pressure inside the pis-ton – cylinder device remaining high during the expansion process, which can be approxi-mated as constant pressure heat addition in the simplified cycle. The simplified or ideal cycle of the earlier mention mode of combustion is referred to as the dual cycle. This is also recognized as another heat engine cycle, which is technically a combination of the heat addition processes of both the Otto and Diesel cycles; it is believed to have been first dis-covered by a Russian – German engineer Gustav Trinkler. Some still call it the Trinkler cycle. It is, however, commonly named the Dual cycle. Figure 6.10 shows a schematic of the Dual cycle, which can be considered a combination of the Otto Cycle and the Diesel cycle.

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  Fundamentals of Heat Engines

  Reciprocating and Gas Turbine Internal Combustion Engines

  • Jamil Ghojel(Author)
  • 2020(Publication Date)
  • Wiley-ASME Press Series

    (Publisher)

  In this comparison, the Diesel cycle is the most efficient and the Otto Cycle the least efficient. Problems 3.1 Consider the following data for the ideal piston-engine cycle with constant-volume heat addition (Otto Cycle): p 1 = 0.1 MPa , T 1 = 293 K , 𝜀 = 4.5, 𝛼 = 3.5, 𝛾 = 1.4. Assuming constant specific heat, determine Problems 117 (a) Properties at the characteristic points of the cycle (b) Amounts of heat supplied and rejected (c) Thermal efficiency (d) Net cycle work 3.2 The following information provided is for an ideal piston-engine cycle with constant-pressure heat addition (Diesel cycle): p 1 = 0.1 MPa , T 1 = 293 K , 𝜀 = 12.7, 𝛾 = 1.4, c pa = 1.0117 kJ / kg . K . Determine (a) Properties at the characteristic points of the cycle (b) Amounts of heat supplied and rejected (c) Thermal efficiency (d) Net cycle work 3.3 The hypothetical cycle 1 − 2 − 3 − 4 has two constant-pressure and two constant-volume processes. If the specific heats at constant pressure and constant volume are assumed constant, and 𝛼 = p 2 / p 1 , 𝜌 = v 3 / v 2 , show that the thermal efficiency is given by the following equation: 𝜂 t = 1 − 𝛽 ( 𝛼 − 1 + 𝛾 ) − 𝛾 𝛼 − 1 + 𝛾𝛼 ( 𝛽 − 1 ) p V 4 1 3 2 3.4 The initial pressure and temperature in an air-standard cycle with dual combustion process are 90 kPa and 340 K , respectively. The total heat input into the cycle is Q in = 1090 kJ / kg , and the compression ratio 𝜀 = 10. If the maximum cycle pressure is not to exceed 6.5 MPa , calculate the percentage of the total heat that must be provided at constant volume. Assume constant heat capacity throughout. 3.5 A small truck has a four-cylinder, four-litre compression ignition (CI) engine operating on the air-standard dual cycle shown. The heat q in provided externally per cycle to each cylinder is from the combustion of 0.05 g of light diesel fuel with lower heating value of 42 500 kJ/kg . The compression ratio of the engine is 15:1, and the cylinder bore is 100 mm.

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  Principles of Engineering Thermodynamics, SI Edition

  • John Reisel(Author)
  • 2021(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Although better predictions could be gained either by using a specific heat value at an average cycle temperature or by using variable specific heats, predic- tions of the peak temperature and pressure and thermal efficiency will usually still be high. QUESTION FOR THOUGHT/DISCUSSION While the basic four-stroke, spark-ignition engine cycle has changed little over the past 100 years, what are some modifications that have been made to such engines that improve engine performance but also push the actual cycle farther from the pure Otto Cycle? 7.7 DIESEL CYCLE The Diesel cycle, developed by Rudolph Diesel in 1893, models early types of compression- ignition (or diesel) engines. Although modern compression-ignition engines often have a modified thermodynamic cycle that describes their performance more accurately, the basics of the Diesel cycle are useful in understanding how the compression-ignition engine differs from the spark-ignition engine. 7.7.1 The Basic Four-Stroke, Compression-Ignition Engine Figure 7.35 shows the basic processes in a four-stroke, compression-ignition engine. Initially, the air is inducted into the cylinder during the intake stroke. At the end of the intake stroke, the intake valve is closed and the air is compressed in the compression stroke. Near the end of the compression stroke, fuel is injected into the hot, compressed air in the cylinder. Combustion begins and continues into the expansion stroke; fuel will also continue being injected into the expansion stroke if large amounts of power are needed. Near the end of the compression stroke, the exhaust valve opens, allowing for exhaust blowdown to occur. Finally, the exhaust stroke is used to force the combustion products out of the cylinder. Although the compression-ignition engine cycle is fairly similar to the spark-ignition engine cycle, there are some important differences.

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  Thermodynamics and Heat Power, Ninth Edition

  • Irving Granet, Jorge Alvarado, Maurice Bluestein(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  Figure 9.23 . The air-standard efficiency of the ideal Ericsson engine therefore is equal to the Carnot efficiency.

  The use of regeneration in the steam cycle or in the Stirling and Ericsson cycles is seen to cause the efficiencies of these cycles to approach the efficiency of the Carnot cycle. In the case of the Stirling and Ericsson cycles, the reduction of theory to actual working engines presents practical difficulties that cause the actual engine efficiencies to be below that of the Carnot cycle.

  9.10 Review

  In Chapter 8 , we considered vapor cycles in which the working fluid underwent a change in phase. In this chapter, we have concerned ourselves with cycles in which the working fluid is a gas and remains a gas throughout the cycle. Some of the cycles considered in this chapter have internal combustion; others have external combustion. Some have spark ignition; others have compression ignition. In order to simplify the calculations and to permit us to arrive at general conclusions, we utilized the air-standard cycle analysis. The air-standard cycle analysis assumes that air is the working fluid and that it behaves as an ideal gas. Each process of the cycle is assumed to be internally reversible, and the specific heats of the air are assumed to be constant throughout the cycle. In effect, the same air remains in the engine from cycle to cycle in the air-standard analysis.

  Based on the air-standard analysis, the efficiency of the Otto Cycle was determined to be a function only of the compression ratio, r c . The efficiency of the Diesel cycle was found to be less than the efficiency of the Otto Cycle for the same compression ratio. However, the compression ratio of Diesel engines is much greater than the compression ratio of the Otto Cycle because the fuel is injected at the end of the compression stroke rather than compressing the fuel– air mixture. The efficiency of the Brayton cycle was found to be proportional to the pressure ratio, r p

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  Advanced Thermodynamics

  Fundamentals, Mathematics, Applications

  • Mehrzad Tabatabaian, R. K. Rajput(Authors)
  • 2017(Publication Date)
  • Mercury Learning and Information

    (Publisher)

  It is evident from Figure 11.26 that the air standard efficiencies increase with the increase in the compression ratio. For a given compression ratio the FIGURE 11.26. Comparison of efficiency at various compression ratios. GAS POWER CYCLES • 579 Otto Cycle is the most efficient while the Diesel cycle is the least efficient (η otto > η dual > η diesel ). The maximum compression ratio for the gasoline engine is limited by detonation. In their respective ratio ranges, the Diesel cycle is more efficient than the Otto Cycle. A comparison of the cycles (Otto, Diesel, and Dual) on the p-v and T-s diagrams for the same compression ratio and heat supplied is shown in Figure 11.27. Heat rejected We know that, Heat supplied η =1− ( ) ... 13 Since all the cycles reject their heat at the same specific volume, shown by the process line from state 4 to 1, the quantity of heat rejected from each cycle is represented by the appropriate area under the line 4 to 1 on the T-s diagram. As is evident from the equation (13), the cycle which has the least heat rejected will have the highest efficiency. Thus, the Otto Cycle is the most efficient and the Diesel cycle is the least efficient of the three cycles. otto dual diesel . ., . i e η > η > η Figure 11.28 shows the Otto and Diesel cycles on p-v and T-s diagrams for constant maximum pressure and heat input, respectively. — For the maximum pressure the points 3 and 3′ must lie on a constant pressure line. — On a T-s diagram the heat rejected from the Diesel cycle is repre- sented by the area under the lines 4 to 1, and this area is less than the FIGURE 11.28. (A) p-v diagram, (b) T-s diagram. 580 • ADVANCED THERMODYNAMICS Otto Cycle area under the curve 4′ to 1; hence the Diesel cycle is more efficient than the Otto Cycle for the condition of maximum pressure and heat supplied.

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  Engineering Principles in Everyday Life for Non-Engineers

  • Saeed Benjamin Niku, Saeed Benjamin(Authors)
  • 2022(Publication Date)
  • Springer

    (Publisher)

  Consequently, the air feels better because it is cooler and also drier. However, like refrigerators, the condensed water has to be drained. You may have noticed that in many air-conditioning systems, it appears that the unit is leaking. That is in fact condensed water and not a leak. The same is true in automobile air- conditioners. Other than these differences, an air-conditioning system and a refrigeration system are thermodynamically very similar. 4.3 SPARK-IGNITION POWER CYCLE A spark-ignition cycle approximates the cycle of power development by an internal combustion engine with spark plugs. This is also similar to what is referred to in thermodynamics as an Otto Cycle which is an ideal cycle (an ideal cycle is approximate. Real cycles differ somewhat from ideal cycles. But to learn the principles, we always start with an ideal cycle, then modify the cycle to a more realistic model). Conversely, a compression ignition cycle approximates a diesel engine, where the air is compressed much more and consequently, it becomes much hotter to the point that when the fuel is injected into it, it explodes and burns without the need for a spark plug. We will discuss the differences between these two engines later. An internal combustion engine in general refers to any type of engine in which the com- bustion of the fuel and air within a closed environment produces the gases that generate the mechanical work, and includes regular gasoline engines, diesel engines, rotary engines, and jet engines. Conversely, steam engines are not internally combusting engines; in steam engines, the fire is outside of the engine and instead, combustion products boil water into steam in a boiler and the steam is used to power the engine. Common gasoline and diesel engines are called recip- rocating IC engines because the piston reciprocates (moves up and down) in a cylinder, rotating a crankshaft that is connected to it via a crank and a connecting rod.

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