Physics
Engine Cycles
Engine cycles refer to the sequence of events that occur within an internal combustion engine during operation. These events typically include intake, compression, power, and exhaust strokes. The engine cycle is crucial for converting fuel into mechanical energy, and it plays a fundamental role in the functioning of various types of engines, such as those found in automobiles and aircraft.
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11 Key excerpts on "Engine Cycles"
eBook - PDFThermodynamics
A Smart Approach
- Ibrahim Dinçer(Author)
- 2020(Publication Date)
- Wiley(Publisher)
Those that the cycle as a whole is made up from a single system that operates in a closed system behavior, specifically as a piston – cylinder device and the cycle is such that the device undergoes different processes in sequence, are the Otto, Diesel, and the com-bination of both the Dual cycles. However, other cycles are made up of a set of open systems that makes up the whole cycle, in which the systems have multiple devices; each device continuously performs a specific action throughout the cycle. In this chapter, the cycles included in Figure 6.2 are covered and discussed in detail along with their T-s and P-v dia-grams and applications; numerous example problems are provided to better understand the concepts and the way balance equations are written. A heat engine, as outlined in the introduction, is a thermodynamic system that is turned into cycle. It essentially receives energy in the form of heat and converts it to power; for example, heat engines such as the Otto cycle, Diesel cycle, Rankine, and Brayton cycles are common types. If one looks at heat engines or what can also be referred to as power cycles from an input and output point of view, one can present these cycles in a simple in and out form as shown in the schematic in Figure 6.3 for both energy (Figure 6.3a) and exergy flows (Figure 6.3b). Consider a heat engine shown in Figure 6.3, where the heat engine receives a total ther-mal energy of Q H from a heat source at a temperature of T H . Some part of this total amount of heat received by the heat engine from the heat source is converted to useful net work output ( W net ), while the remaining part of that heat, which is denoted in Figure 6.3a as Q L , is rejected to a heat sink at a temperature of T L . In this regard, while Figure 6.3a presents the energy interactions within the heat engine, Figure 6.3b shows the exergy interactions within the heat engine in terms of inputs and outputs.
No longer available |Learn more- Irving Granet, Maurice Bluestein(Authors)
- 2014(Publication Date)
- CRC Press(Publisher)
During the actual processes, both heat and work are interchanged at each portion of the cycle. The mass, specific heats, and state of the working fluid are all variable. Under these circumstances, the analysis of 432 Thermodynamics and Heat Power a cycle becomes quite difficult. To simplify the analysis, certain idealized cycles have been proposed as prototypes of the actual Engine Cycles. These models are considered for the rest of this chapter. In the analysis of each of the cycles, the following assumptions are made: 1. Each process is carried out reversibly. Friction, pressure differences, turbulence, and the like are neglected. 2. The working fluid is an ideal gas, and all relations already derived for the ideal gas are applicable. 3. The necessary energy is added or removed externally to achieve the desired state changes. 4. The working fluid is a gas with constant specific heats. 5. The amount of working fluid in the cycle is constant. In effect, the same air remains in the engine through each cycle. It is apparent from these assumptions (and their implications) that the analysis of an engine cycle based on them is quite artificial. However, certain generalizations can be arrived at from this analysis, called the air-standard analysis , and as such, it can be quite useful. The Otto cycle is the prototype for most internal-combustion engines commonly in use. Regardless of the number of strokes required to complete the cycle, it is conceived of as consisting of four separate and distinct processes. As shown in the T – s and p – v diagrams in Figure 9.5, the Otto cycle consists of (after induction of the gas) an isentropic compres-sion followed by the reversible constant-volume addition of heat, then an isentropic expan-sion from which work is extracted, and finally, a reversible constant-volume rejection of heat.
eBook - PDF- J. Edward Pope(Author)
- 1996(Publication Date)
- Gulf Professional Publishing(Publisher)
61 Brayton Cycle: A Gas Turbine Cycle ............................... 62 Otto Cycle: A Power Cycle .............................................. 63 Diesel Cycle: Another Power Cycle ................................. 63 Gas Power Cycles with Regeneration .............................. 64 51 52 Rules of Thumb for Mechanical Engineers THERMODYNAMIC ESSENTIALS Thermodynamics is the subject of engineering that pre- dicts how much energy can be extracted from a working fluid and the various ways of achieving it. Examples of such areas of engineering interest are steam power plants that gen- erate electricity, internal combustion engines that power au- tomobiles, jet engines that power airplanes, and diesel lo- comotives that pull freight. The working fluid that is the medium of such energy transfer may be either steam or gases generated by fuel-air mixtures. Phases of a Pure Substance The process of energy transfer from one form to anoth- er is dependent on the properties of the fluid medium and phases of this substance. While we are aware of basically three phases of any substance, namely solid, liquid, and gaseous, for the purposes of thermodynamic analysis we must define several other intermediate phases. They are: 9 Solid: The material in solid state does not take the shape of the container that holds it. 9 Subcooled liquid: The liquid at a condition below its boiling point is called subcooled because addition of a little more heat will not cause evaporation. 9 Saturated liquid: The state of liquid at which addition of any extra heat will cause it to vaporize. 9 Saturated vapor: The state of vapor that is at the verge of condensing back to liquid state. An example is steam at 212~ and standard atmospheric pressure. 9 Liquid vapor mix: The state at which both liquid and vapor may coexist at the same temperature and pres- sure.
eBook - PDFModern Diesel Technology
Diesel Engines
- Sean Bennett(Author)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. INTRODUCTION This chapter begins with an explanation of some basic engine terminology. Before you can properly understand how any engine functions, you have to be-come familiar with the language used to describe its operation. After introducing the basic terminology, the chapter describes first the four-stroke diesel cycle and then the two-stroke diesel cycle. The basic engine ter-minology and principles introduced in this chapter are used repeatedly throughout the textbook. As you prog-ress through it, use the glossary to check the definitions of any words with which you are not familiar. KEY ENGINE TERMS The terms explained in this chapter are the building blocks required to understand engine technology. In most cases, technically correct terms are used in this textbook, but remember that the terminology used on the shop floor might differ. When you refer to manu-facturers’ service literature, you will be expected to have a basic understanding of the key terms introduced in this chapter. The following text simply interprets many of the words that are later used to describe the various Engine Cycles and key events within those cycles. We introduce these terms in two sections within this chapter. Building Block Definitions You probably are already familiar with some of the terms introduced in this section, but even if you are, read through the definitions because those given here may differ in small ways from those with which you are familiar. Engine. The term engine describes a machine that converts one energy form to another. A waterwheel driving a mill could be described as an engine. When this book refers to an engine, we will be referring to an engine that converts the heat energy of a fuel into usable mechanical energy.
No longer available |Learn moreAdvanced Thermodynamics
Fundamentals, Mathematics, Applications
- Mehrzad Tabatabaian, R. K. Rajput(Authors)
- 2017(Publication Date)
- Mercury Learning and Information(Publisher)
C H A P T E R 11 GAS POWER CYCLES 11.1. Overview The gas power cycle has the advantage of a lack of phase change com- pared to water/steam plants, but at the cost of lower power capacity. Overall, a gas power plant requires less equipment and is easier to start and stop in operation compared to steam power plants. In this chapter we discuss several gas power cycles. 11.2. Definition of a Cycle A cycle is defined as a repeated series of thermodynamic operations or processes occurring in a certain order. The processes may be repeated in the same order. The cycle may be of an imaginary perfect engine or an actual engine. The former is called an ideal cycle and the latter an actual cycle. In an ideal cycle all accidental heat losses and irreversibility are absent, and the working substance is assumed to behave like a perfect working substance. 11.3. Air Standard Efficiency To compare the effects of different cycles, it is of paramount importance that the effect of the calorific value of the fuel is altogether eliminated, and In This Chapter l Overview l Definition of a Cycle l Air Standard Efficiency l The Carnot Cycle l Otto Cycle l Exercises 510 • ADVANCED THERMODYNAMICS this can be achieved by considering air (which is assumed to behave as a perfect gas) as the working substance in the engine cylinder. The efficiency of an engine using air as the working medium is known as an Air standard efficiency. This efficiency is often called an ideal efficiency. The actual efficiency of a cycle is always less than the air standard efficiency of that cycle under ideal conditions. This is taken into account by introducing a new term, Relative efficiency, which is defined as: relative Actual thermal efficiency Air standard efficiency η = The analysis of all air standard cycles is based upon the following assumptions: 1. The gas in the engine cylinder is a perfect gas, that is, it obeys the ideal gas laws and has constant specific heats.
- Saeed Benjamin Niku, Saeed Benjamin(Authors)
- 2022(Publication Date)
- Springer(Publisher)
Unlike 4-stroke cycles, more than one thing happens simultaneously during each stroke of the 2-stroke cycles. Therefore, we need to start at some arbitrary point, follow all that happens, and eventually end at the same point in order to see how this engine works. Imagine that the piston is moving down and is close to its bottom-dead-center as in Fig- ure 4.15a. By this time, the intake valve in the crankcase is closed due to the increased pressure in the crankcase, the exhaust port is opened and the consumed fuel-air mixture is mostly out, and as the piston continues its downward motion, the intake opening on the cylinder opens as well, and the somewhat-compressed fuel-oil-air mixture in the crankcase is pushed into the cylinder. As the piston starts its upward motion (Figure 4.15b), it closes the intake and exhaust openings, opens the crankcase intake valve bringing new fuel-oil-air mixture into the crankcase for the next cycle, and compresses the mixture in the cylinder until it reaches near the top-dead- center. At that point, a spark combusts the mixture, starting the downward power cycle. Once again, as the piston moves down, it closes the crankcase intake valve, opens the ex- haust and the intake openings in short sequence, and repeating the cycle until the engine stops. Clearly, before all the exhaust gases escape, a new fuel-oil-air mixture starts entering the cham- ber and mixing with it. This reduces the efficiency of the engine and also lets some of the new unburned mixture out the exhaust, polluting the air. As was mentioned earlier, these engines are simple, with fewer moving parts, and with more frequent power cycles, but are more polluting and less efficient. 4.4 THERMODYNAMIC REPRESENTATION OF THE SPARK-IGNITION POWER CYCLE Similar to the refrigeration cycle, where we compared the actual work of the system with its thermodynamic representation, we can do the same for power cycles. This can help engineers
eBook - PDF- Bernard Desmet(Author)
- 2022(Publication Date)
- Wiley-ISTE(Publisher)
However, improvements are still possible, such as having the operating cycle being close to that of the Miller–Atkinson cycle, which is energetically more efficient with the gas by extending the expansion phase. This can be obtained by shifting the cycle timings, and which can be further combined with supercharging – an improvement that is already implemented in automotive engines. Among their various evolutions, variable displacement engines are limited by their mechanical complexity as well the opening and closing of the valves (“camless” engines), controlled electromagnetically. This makes it possible to remove the geometric constraints of the camshafts, which are difficult to fine-tune. Internal Combustion Engines 67 The zero-dimensional models, with one or more zones, make it possible to represent fairly precisely the operation of internal combustion engines. Their major drawback is that they require values for the coefficients, which are necessary for the heat transfer and combustion heat release models, and require experimental validation. The quantity of the exhaust gas energy which can be converted into mechanical energy is characterized by its exergy. This conversion into mechanical energy is used to power the turbine, driving the compressor in the turbocharger supercharging system, and also in the waste heat recovery system using a combined cycle (ORC: Organic Rankine cycle). The thermal energy from the exhaust gases is used as a heat source for cogeneration systems. We will go into further detail on these points in Chapter 6. Numerical modeling (CFD) may be implemented to study the local effects during the cycles within an internal combustion engine cylinder, but requires significant energy and computer resources given the complexity of the situation, for example, volume variability of the cylinder–piston cavity, complex flows with heat transfers, intake and exhaust pipe coupling, and chemical combustion reactions.
eBook - PDF- Donald Olander(Author)
- 2007(Publication Date)
- CRC Press(Publisher)
109 4 Heat Engines, Power Cycles, and the Thermodynamics of Open Systems 4.1 HEAT ENGINES In Section 1.9, it was noted that the first law regards heat and work as completely interchangeable; if a certain number of Joules of heat added to a system increases the internal energy of a body by, say, Δ U , the same number of Joules of work performed on the body would produce the same Δ U . In addition, work can be completely converted to heat, as everyday experience with friction attests. However, the reverse is not true; heat cannot be completely transformed into work. This limitation, which is a consequence of the second law, is best demonstrated by studying the properties of heat engines . A heat engine is a system operating in a cycle that receives heat from a high-temperature source (called a thermal reservoir) and produces useful work. However, since the efficiency of conversion must be less than 100%, some of the input heat is rejected to a cold reservoir. Figure 4.1 shows a schematic of a heat engine/heat pump and their associated thermal reservoirs. The reservoirs supply or receive heat without alteration of their temperatures. Heat flows in the reservoirs are reversible whether or not the engine is. FIGURE 4.1 A schematic of a heat engine or heat pump. The heat pump is a heat engine running in reverse. Hot Reservoir T H Cold Reservoir T L Heat Pump Heat Engine Cold Reservoir T L Hot Reservoir T H W W Q H Q L Q L Q H 110 General Thermodynamics The circle with the arrows in Figure 4.1 is a shorthand representation of the heat engine. It is intended to signify that the working substance (a fluid such as an ideal gas or water) moves through many thermodynamic states in a never-ending cyclic process. The detailed structure of the heat engine can vary greatly, but the simplest version contains the following four steps: 1. One in which heat is absorbed isothermally from the high-temperature reservoir. 2. The next, in which work is produced adiabatically.
eBook - PDFEnergy Systems
A Project-Based Approach to Sustainability Thinking for Energy Conversion Systems
- Leon Liebenberg(Author)
- 2024(Publication Date)
- Wiley(Publisher)
The main applications are in vehicles, trains, ships, and aircraft. Internal combustion engines are also used for stationary applications, such as emer- gency power generators. Internal combustion gas engines (or just “internal combustion engines”) comprise primarily a cylinder, almost always stationary, and a piston. Together, these form a combustion chamber of variable volume. A fuel-air mixture is burned in the combustion chamber and the expanding gases operate on the piston, which performs work. It is therefore called an internal combustion engine. (In contrast with an external combustion engine, such as a gas turbine, the combustion process occurs externally of the prime mover.) Air is brought to the cylinder. Depending on the cycle, fuel is introduced into the air before, dur- ing, or after compression, and mixed with it. Upon ignition of the fuel-air mixture, the heat devel- oped raises the pressure of the products of combustion during one of the strokes of the piston. The pressure imbalance on the piston causes the piston to move, with the motion transferred to a connecting rod and crankshaft, which is linked to the vehicle’s wheels via a transmission (or “gearbox”). The events of the cycle (such as an Otto cycle or a Diesel cycle) on which an internal combustion engine operates are mainly controlled by valves located in ports leading to and from a cylinder. In general, a cylinder comprises one or more inlet valves and one or more exhaust valves. These valves are usually mechanically operated by linking with the crankshaft via the valve gear train (Figure 10.1). As the internal combustion engine develops power in a cyclical manner, the power fluctuations are lessened using a heavy flywheel and with overlapping power impulses by employ- ing multi-cylinder arrangements. An internal combustion engine cycle comprises either two or four strokes. A two-stroke cycle has an intake stroke and a power stroke.
eBook - PDFFundamentals of Heat Engines
Reciprocating and Gas Turbine Internal Combustion Engines
- Jamil Ghojel(Author)
- 2020(Publication Date)
- Wiley-ASME Press Series(Publisher)
We will first consider the case for the dual fuel-air cycle shown in Figure 5.1. On the surface, this cycle looks identical to the air-standard dual cycle discussed before; however, in reality it is one step closer to real compression ignition Engine Cycles. Once the performance equations are developed for this cycle, the constant-volume and constant-pressure cycles can be deduced as special cases. 5.1 Fuel-Air Cycle Assumptions A number of assumptions are made for this cycle: ● Gas-exchange processes at the start and end of the cycle are ignored, i.e. there are no induction and exhaust processes and the cycle is a closed cycle. ● The working fluid trapped in the cylinder is air during the compression process 1–2 and products of complete combustion of the fuel during processes 2–3, 3–4, and 4–5. ● The compression process 1–2 is polytropic (heat to and from the surroundings is accounted for) with an averaged polytropic index n 1 ( n 1 < 𝛾 ). ● The heat-addition profile in fuel-air cycles is exactly the same as in the theoretical (air-standard) cycle; however, the input heat is not taken from an external source but is generated internally as a result of the chemical reaction between the compressed air and the fuel in the cylinder. Additionally, the air-fuel ratio and thermal properties of the products of combustion are accounted for in the calculations. ● The expansion process 4–5 is polytropic, with an averaged polytropic index n 2 ( n 2 < 𝛾 ). Fundamentals of Heat Engines: Reciprocating and Gas Turbine Internal Combustion Engines, First Edition. Jamil Ghojel. © 2020 John Wiley & Sons Ltd. This Work is a co-publication between John Wiley & Sons Ltd and ASME Press. Companion website: www.wiley.com/go/JamilGhojel_Fundamentals of Heat Engines 144 5 Fuel-Air Cycles for Reciprocating Engines 5.2 Compression Process This process starts at point 1 and ends at point 2 (Figure 5.1).
eBook - PDF- 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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