Diesel Cycle

6 Key excerpts on "Diesel Cycle"

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  Biofueled Reciprocating Internal Combustion Engines

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

    (Publisher)

  The heat energy is converted to mechanical work by piston movement (change in boundary of the system). The burned product is expelled to the atmosphere during the exhaust stroke and this cycle is completed, and then the next cycle continues in this manner. The details of the Diesel Cycle process are given in Table 4.2. The pressure-volume and temperature-entropy diagram of Diesel Cycle are shown in Figure 4.2. Table 4.2 Process of Constant Pressure Cycle (Diesel Cycle) Process Details Process 1-2 Isentropic compression of air Process 2-3 Heat addition at constant pressure Process 3-4 Isentropic expansion Process 4-1 Heat rejection at constant volume Figure 4.2 Pressure-volume and temperature-entropy diagram for Diesel Cycle. 4.5.1 Derivation of Thermal Efficiency and Mean Effective Pressure for Constant Pressure Cycle 4.5.1.1 Process 1-2 Isentropic compression: T 2 T 1 = (p 2 p 1) γ − 1 γ = (v 1 v 2) γ − 1 = r γ − 1 4.17 4.17 4.5.1.2 Process 2-3 Heat addition at constant. pressure: Q a = m × c p × (T 3 − T 2) 4.18 4.18 4.5.1.3 Process 3-4 Isentropic expansion: T 3 T 4 = (p 3 p 4) γ − 1 γ = (v 4 v 3) γ − 1 4.19 4.19 4.5.1.4 Process 4-1 Heat rejection at constant volume: Q r = m × c v × (T 4 − T 1) 4.20 4.20 The thermal efficiency of the ideal Diesel Cycle is given. below: η = Heat added − Heat rejected Heat added = Q 1 − Q 2 Q 1 η = c p (T 3 − T 2) − c v (T 4 − T 1) c p (T 3 − T 2) η = 1 − 1 γ (T 4 − T 1 T 3 − T 2) η = 1 − T 1 γ T 2 (T 4 T 1 − 1 T 3 T[. --=PLGO-SEPARATOR=--]2 − 1) 4.21 4.21 For reversible adiabatic (isentropic) compression and expansion processes: T 1 T 2 = (v 2 v 1) γ − 1 and T 4 T 3 = (v 3 v 4) γ − 1 For constant pressure heat addition. 2-3, T 3 T 2 = v 3 v 2. Also, v 4 = v 1. Thus, T 4 T 1 = T 3 T 2 (v 3 / v 4 v 2 / v 1) γ − 1 = v 3 v 2 (v 3 v 2) γ − 1 = (v 3 v 2) γ Substituting these values in Equation 4.21, the thermal efficiency can be written

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  Internal Combustion Engines

  Applied Thermosciences

  • Allan T. Kirkpatrick(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  The Diesel Cycle engine is also called a compression ignition engine. As we will see, actual diesel engines do not have a constant pressure combustion process. The cycle for analysis is shown in Figure 2.3. The four basic processes are: 1 to 2 isentropic compression 2 to 3 constant pressure energy addition 3 to 4 isentropic expansion 4 to 1 constant volume energy rejection Figure 2.3 The Diesel Cycle (,). Again assuming constant specific heats, the student should recognize the following equations: Compression stroke (2.21) Energy addition (2.22) Expansion stroke (2.23) where we have defined the parameter, a measure of the combustion duration, as (2.24) In this case, the indicated efficiency is (2.25) The term in brackets in Equation (2.25) is greater than one, so that for the same compression ratio,, the efficiency of the Diesel Cycle is less than that of the Otto cycle. However, since Diesel Cycle engines are not knock limited, they operate at about twice (20:1) the compression ratio of Otto cycle engines. For the same maximum pressure, the efficiency of the Diesel Cycle is greater than that of the Otto cycle. Diesel Cycle efficiencies are shown in Figure 2.4 for a specific heat ratio of 1.30. They illustrate that high compression ratios are desirable and that the efficiency decreases as the energy input increases. As approaches one, the Diesel Cycle efficiency approaches the Otto cycle efficiency. Figure 2.4 Diesel Cycle characteristics as a function of compression ratio and energy addition (). Although Equation (2.25) is correct, its utility suffers somewhat in that is not a natural choice of independent variable. Rather, in engine operation, we think more in terms of the energy transferred in

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  Energy, Entropy and Engines

  An Introduction to Thermodynamics

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

    (Publisher)

  While the combustion is heating the gas and increasing its pressure the piston has begun the expansion stroke and is expanding the gas. The net result is that the pressure in the cylinder remains approximately constant during fuel combustion. We can model a compression ignition engine using a Diesel Cycle, which is shown in Figure 10.6 on both P-v and T-s diagrams. The only difference between the Diesel and Otto cycles is that heat addition (process 2→3) is at constant pressure in a Diesel Cycle while it is at constant volume in an Otto cycle. Figure 10.6 The Diesel Cycle on (a) a P-v diagram and (b) on a T-s diagram. The heat added to the engine per unit mass of air in a constant pressure process, assuming constant specific heat is (10.10) The efficiency of a Diesel Cycle is (10.11) To simplify this relation, we note that for the isentropic process 1→2, (10.12) Applying the ideal gas equation to the constant pressure process 2→3, (10.13) and defining the cut-off ratio as the ratio of gas volumes at the end (V 3) and start (V 2) of heat addition, (10.14) Applying the ideal gas equation to the constant volume process 1→4 and noting that the processes 1→2 and 3→4 are isentropic, (10.15) Substituting Equations (10.12), (10.14) and (10.15) into Equation (10.11) gives (10.16) The term in parentheses represents the difference between the efficiencies of Otto and Diesel Cycles. This term is always greater than unity, so in principle the Diesel Cycle has a lower efficiency than the Otto cycle for the same compression ratio. However r can be much larger in compression ignition engines than it is in spark-ignition engines, and therefore in practice diesel engines are more efficient. In spite of this they have several disadvantages that have prevented them from completely replacing spark-ignition engines

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  eBook - ePub

  Thermodynamics and Heat Power, Ninth Edition

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

    (Publisher)

  Fuel is distributed through a distribution valve, which directs the metered fuel to the individual injection valves. The injection valves have mechanically operated plungers to raise the oil pressure to the required injection pressure. In this system, the injection valve pressurizes the fuel and also times the fuel injection. However, it does not meter the fuel. Because of the exacting requirements for fuel injection in a Diesel engine, these engines run in a narrow range of speeds. Thus, Diesel engines used for transportation require more transmission gearing than comparable Otto cycle engines. 9.4 Air-Standard Analysis of the Diesel Cycle All the assumptions made for the air-standard analysis of the Otto cycle regarding the working fluid and its properties apply to the present analysis of the idealized Diesel Cycle. The idealized air-standard Diesel Cycle consists of four processes. The first is an isentropic compression of the air after it has been inducted into the cylinder. At the end of the compression process, fuel is injected, and combustion is assumed to occur at constant pressure. Subsequent to the heat release by combustion, the gas is expanded isentropically to produce work, and finally, heat is rejected at constant volume. The gas is assumed to be recycled rather than rejected. Figure 9.11 shows the ideal Diesel Cycle on both p–v and T–s coordinates. FIGURE 9.11 Diesel Cycle. Heat is received (reversibly) during the nonflow, constant-pressure process, © to ®. The energy equation for a constant-pressure (flow or nonflow) yields q in = c p (T 4 − T 3) Btu/lb m, kJ/kg (9.12) The energy rejected during the constant-volume process is q r = c v (T 5 − T 2) Btu/lb m, kJ/kg (9.13) The net work available from the cycle is W = q in − q r = c p (T 4 − T 3) − c v (T 5 − T 2) Btu/lb m, kJ/kg (9.14) The efficiency thus. becomes η Diesel = c p (T 4 − T 3) − c v (T 5 − T 2) c p (T 4 − T 3) = 1 − 1 k T 5 − T 2 T 4 − T 3 (9.15) Where k = c p / c v

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  Thermodynamic Cycles

  Computer-Aided Design and Optimization

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

    (Publisher)

  Combustion in the Otto cycle is based on a constant-volume process; in the Diesel Cycle, it is based on a constant-pressure process. However, combustion in actual spark-ignition engine requires a finite amount of time if the process is to be complete. For this reason, combustion in the Otto cycle does not actually occur under the constant-volume condition. Similarly, in compression-ignition engines, combustion in the Diesel Cycle does not actually occur under the constant-pressure condition, because of the rapid and uncontrolled combustion process.

  The operation of the reciprocating internal combustion engines represents a compromise between the Otto and the Diesel Cycles, and can be described as a dual combustion cycle. Heat transfer to the system may be considered to occur first at constant volume and then at constant pressure. Such a cycle is called a dual cycle.

  The Dual cycle as shown in Fig. 3.20 is composed of the following five processes:

  Figure 3.20

  Dual cycle.

  1-2 Isentropic compression 2-3 Constant-volume heat addition 3-4 Constant-pressure heat addition 4-5 Isentropic expansion 5-1 Constant-volume heat removal

  Figure 3.21 shows the dual cycle on p–v and T–s diagrams.

  Figure 3.21

  Dual cycle on p–v and T–s diagrams.

  Applying the first and second laws of thermodynamics of the closed system to each of the five processes of the cycle yields: and

  The net work (Wnet ), which is also equal to net heat (Qnet ), is

  The thermal efficiency of the cycle is This expression for the thermal efficiency of an ideal Otto cycle can be simplified if air is assumed to be the working fluid with constant specific heat. Equation (3.48) is reduced to:

  Example 3.11

  Pressure and temperature at the start of compression in a dual cycle are 14.7 psia and 540°R. The compression ratio is 15. Heat addition at constant volume is 300 Btu/lbm of air, while heat addition at constant pressure is 500 Btu/lbm of air. The mass of air contained in the cylinder is 0.03 lbm. Determine (1) the maximum cycle pressure and maximum cycle temperature, (2) the efficiency and work output per kilogram of air, and (3) the MEP. Show the cycle on T–s

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  Engineering Thermodynamics with Worked Examples

  • Nihal E Wijeysundera(Author)
  • 2016(Publication Date)
  • WSPC

    (Publisher)

  example 10.6 .

  Fig. 10.6 Dual cycle: P-v diagram

  10.3   Gas Turbine Engine Cycles

  Gas turbine engines are used for electricity generation and transport applications, especially in aircraft propulsion. Unlike the SI and CI engines, the gas turbine engine operates in a continuous manner with the working fluid flowing steadily through the system.

  The simple gas turbine plant, shown schematically in Fig. 10.7 , consists of a compressor, a turbine and a combustion chamber. Atmospheric air flows in steadily through the compressor where it is pressurized and then enters the combustion chamber. Fuel flows steadily to the combustion chamber where continuous combustion occurs. As the products of combustion, with a high pressure and temperature, flow through the turbine a part of its enthalpy is converted to work. Of the total work produced by the turbine a significant fraction is used to drive the compressor. The ratio of the compressor work input to the total turbine work output is called the back-work-ratio .

  Fig. 10.7 Simple gas turbine plant

  In the case of a steam power plant, the work required by the feed-pump is only a very small fraction of the work output of the turbine. In contrast, the compressor work input in a gas turbine plant could be more than half the work produced by the turbine. This is because the compression of a gas requires much more work than pressurizing a liquid as it happens in the steam plant. The efficiency of the compressor is therefore of prime importance in the efficient operation of gas turbine plants.

  The actual gas turbine plant does not operate in a cycle. Moreover, the working fluid passing through the compressor and the turbine is not the same pure substance due to the combustion process. The analysis of the actual gas turbine plant is therefore very complex, and requires detailed knowledge of the processes in the different components. As with SI and CI engines, we shall analyze the gas turbine cycles by resorting to the somewhat simplified approach based on air-standard

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