Thermodynamic Cycle Analysis - Report Example

Assignment: Thermodynamics   In thermodynamics a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat "source" generates thermal energy that brings the working substance to the high temperature state (Solanki& Arun, 2008). The working substance generates work in the "working body" of the engine while transferring heat to the colder "sink" until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid. Heat engines are often modeled using a standard engineering model such as the Otto cycle. The theoretical model can be refined and augmented with actual data from an operating engine, using tools such as an indicator diagrams. Since very few actual implementations of heat engines exactly match their underlying thermodynamic cycles, one could say that a thermodynamic cycle is an ideal case of a mechanical engine. In any case, fully understanding an engine and its efficiency requires gaining a good understanding of the (possibly simplified or idealized) theoretical model, the practical nuances of an actual mechanical engine, and the discrepancies between the two.          Most automobile engines work in a cyclic thermodynamic process that consists of five steps: 1:Intake Stroke; The piston is pulled out, drawing the fuel air mixture into the cylinder at atmospheric pressure. 2: Compression stroke; the piston is pushed back in, compressing the fuel air mixture and work done on the gas. 3: Ignition; A spark ignites the gases, quickly and dramatically raising the temperature and pressure. 4: Power stroke; the high pressure that results from ignition pushes the piston out. The gases do work on the piston and some heat flows out of the cylinder. 5: Exhaust stroke; A valve is opened and the exhaust gases are pushed out of the cylinder.           An automobile engine is a four stroke engine in the sense that each cycle has four strokes consisting of 1, 2,3,4,5 steps during which the piston moves. Of the energy released by burning gasoline, only about 20 to 25% is turned into mechanical work used to move automobiles forward and run other systems (Solanki& Arun, 2008). The rest is discarded. The hot exhaust gases carry energy out of engine, as does the liquid cooling system Polytrophic: Pressure correction is applied to each point of cycle data by: P(Actual)= P(Measured) + P(Correction) Randolf used a constant polytrophic coefficient n, of 1.32 and quotes Hohenberg and Killmann as using 1.32 for homogeneous-charge engines and 1.27 for Diesel engines). Randolf suggests that to minimize variability from slope computation (of polytrophic coefficient) it is advisable to maximize the number of measurements and the crank angle spread of those measurements when calculating the polytrophic compression coefficient. However, it must remain between intake-valve closure and ignition. Brunt et al quotes AVL as suggesting a polytrophic index of 1.32 between 100 and 65 degrees BTDC, whilst Kistler’s 5219A signal conditioner uses 1.35 between 120 and 70 degrees BTDC. Brunt et al also notes that the polytrophic method is more sensitive to noise spikes than pressure referencing, but this can be reduced by increasing the upper crank angle (Solanki& Arun, 2008). Their conclusions are that polytrophic indexing is the best method for pegging, however it is unsuitable for situations where a polytrophic index is unknown, such as weak mixtures. Thermal shock is a major problem with piezoelectric pressure transducers when trying to make accurate cylinder pressure measurements. From Brunt et al Cyclic exposure of a piezoelectric pressure transducer to combustion results in the expansion and contraction of its diaphragm due to large temperature variations throughout the cycle. This causes the force on the quartz to be different to that applied by the cylinder pressure alone. Thermal shock affects all parameters derived from pressure data, but the greatest is IMEP, which can be affected by over 10%. Thermal shock was found to be most significant at low engine speeds, high loads, advanced ignition timings, slightly rich mixtures and low EGR (Solanki& Arun, 2008). Transducer location did not have a significant effect on thermal shock.RTV coating was found to be effective in reducing thermal shock.IMEP errors for the Kistler 6125A were reduced from a maximum of –4.9% to between –0.4% and +0.8%.Equation 2.5 has been developed for the Ford Zetec engine; further work is required to determine performance of algorithm with other engine designs. Data validation: From Lancaster et al In terms of screening acquired data, Brown is quoted, “consistency is no indication of accuracy.” Motored pressure data exhibits little cycle-to-cycle variability and can therefore yield significant information about the accuracy and reliability of the test set-up and recording procedure. Peak pressure in a motored engine occurs before TDC due to irreversibility caused primarily by heat transfer and the additional lag from measuring pressures in an IDI CI engine. Kim et al calls this the thermodynamic loss angle (TLA) and is calculated as the crank angle difference between the dynamic TDC and the motored peak pressure angle. Kim et alfind the peak pressure angle using best fit curve of cylinder pressure data within 20 degrees before and after the peak. They note that heat loss per engine cycle and mass blow by loss will reduce with increasing engine speed. This results in the motoring peak pressure shifting towards TDC with increased speed. During intake, at the part of the engine cycle where the instantaneous gas flow rate is high, the cylinder pressure should fall below the measured mean intake manifold pressure (Solanki& Arun, 2008). The fact that motored engine IMEP is non-zero is one source of estimating the error in IMEP for a fired engine. An excellent test of motoring pressure data is a direct comparison to computer simulation data. Brown is quoted as indicating that a crank angle phase error of less than 0.1 degrees is required for accurate (less than 1% error) IMEP calculations for a CI engine. This can be relaxed to 0.2 to 0.3 degrees for SI engines. Control Volume When analyzing thermodynamic systems it is essential to define a control volume. For diesel engine the underlying control volume is constrained by the cylinder dimensions. This approach is mainly based on a single zone combustion model are presented within Stapersma. This implies that the dwelling time and fuel droplets after gas after evaporation are both assumed to be zero. Starting with the energy and mass balance for an open control volume, it is shown that the change of temperature in the control volume. The full derivation is dT/dt= Qcomb-Qloss-p.Dv/dt+Ef/m.Cv Where Qcomb= the rate of heat release (ROHR) or combustion heat flow Qloss= heat loss from the gas to the surrounding walls p.Dv= indicated work E= energy of the liquid fuel entering the cylinder M=the in-cylinder Cv = specific heat capacity at constant volume for the in-cylinder gas mixture Heat release The expression for the change of temperature can be rearranged and the corresponding gross apparent heat release rate, which tells how fast the heat from combustion is released. Opposed to the net apparent heat release rate, GAHRR also includes the heat loss. GAHRR= Qcomb+ Ef= m.CV Dt/dt + Qloss+p.dV/dt If the GAHRR is divided by the specific energy content of the fuel, the result is the combustion reaction rate (CRR). The specific energy of the fuel is here specific heat of combustion plus the specific energy ef of the fuel entering in its liquid phase (Solanki& Arun, 2008). Pressure Measurements The pressure measurements were taken for 20 cycles and the averaged results for the generator load and propeller. Since this shows the averaged measurement and the step size in the simulation environment can set as short as desired the graphs show little fluctuations. It is clear that a relatively higher load gives a higher pressure peak (Solanki& Arun, 2008). The pressure was also measured when there was no fuel injected to the cylinder. These pressures increase with higher speeds. Combustion Reaction Rate The reaction rates calculated by the heat release model are depicted on the graph. The ordinate axis shows time instead of crank angle, with the purpose of better illustrating each working point. Nevertheless, when the engine speed is kept constant the graphs lay somewhat on top of each other (Solanki& Arun, 2008). Fluctuations are displayed due to the differentiations within the computation procedure, which amplifies the small fluctuations in the pressure measurements. For the WP11 the premixed combustion phase pressure peak is a little lower than for the other working points, but a peak in the diffusive combustion phase can be noticed. For idle conditions the reactions rate has a lower premixed combustion peak and no diffusive combustion. Method for smoothing the fluctuating nature of the reaction rate signal and the method involves curve fitting of the calculated RCO (Solanki& Arun, 2008). For the RCO signals a dip before SOC can be seen and this is not physical. Since no fuel is present the RCO ought to be zero until the start if injection. In case the RCO is negative it means that fuel is being created, which is impossible. Combustion Heat Flow (ROHR) The instantaneous combustion heat flow or rate of heat release as shown in the graphs and a dip before SOC can be noticed. This is because the fuel is evaporating and thus consumes energy instead of releasing it. The instantaneous heat flow shows the same trend as the reaction rate. When the energy of the entering liquid fuel E is included the effective instantaneous heat flows become a little lower shown by the continuous lines. The mean combustion heat the unphysical dip before SOC can be recognized. It seems that the difference amidst the effective combustion heat and the combustion heat before adding E is greater at higher loads, speeds and thus temperatures. In-Cylinder States The temperatures increase with heavier loads and higher speeds. The cylinder volume calculated from 2.58 for the corresponding Hydra. Calculated in-cylinder masses are normally presented on the graphs (Solanki& Arun, 2008). They normally start at m1 and increases when fuel is injected. More fuel has to be injected with heavier loads and higher speeds. The same dip before SOC as for the RCO can be recognized for the in-cylinder mass. The in-cylinder air fraction gets an increase before the SOC because its calculations mainly include the RCO. It is clear from the graphs that more air is consumed with heavier loads and higher engine speeds. Thermodynamics properties The thermodynamics properties of the cylinder gas lies amidst the properties of the dry air and stoichiometric gas (Solanki& Arun, 2008). Thus, specific internal energies at constant volume are dominated by their temperature dependency. Energies decrease with temperature because energy of the entering liquid fuel. When the specific heats of combustion of the gaseous fuel are added to these energies, the effective specific heats of combustion lines on the produced graphs become continuous. Heat Loss Increased engine speeds and loads give increased heat loss to the walls. Both the heat transfer coefficients and the corresponding heat loss flows follow the trend of the pressure. For the propeller load curve which alters speed it can be seen as the differences at SOC is relatively bigger than for the generator load (Solanki& Arun, 2008). The approximation of the heat transfer coefficient includes the mean piston speed, which again depends on the engine speed. At the start of compression(IC) the heat loss is negative, which means that the gas is picking up heat from the then hotter cylinder walls. Indicated work The instantaneous indicated work for the eight working points in the engine is normally used in computation of the indicated work. Since the compression requires energy, the work is negative until the top dead center (TDC). With increasing speed the required work is greater because the compression ought to happen faster. When the loads increase more work is done in the expansion phase. The mean indicated power of the engine is normally indicated on the PV diagram (Solanki& Arun, 2008). When mechanical losses are accounted for via the mechanical efficiency, it gives actual output power on the engine shaft. Work cited Solanki, Arun. Thermodynamics. Jaipur, India: Oxford Book Co., 2008. Print. Read More