ONE DIMENSIONAL (1D) SIMULATION TOOL: GT-POWER

Transcription

1 CHAPTER 4 ONE DIMENSIONAL (1D) SIMULATION TOOL: GT-POWER 4.1 INTRODUCTION Combustion analysis and optimization of any reciprocating internal combustion engines is too complex and intricate activity. It depends upon several factors and environments. This activity can be well undertaken and handled by using an appropriate predictive simulation software tools very effectively. Depending upon the level complexity involved in the work, suitable tools can be used. (Ferrara et al., 2011) Simulation provides a method to the check various possible experimental options on a computer software tool to arrive at a better results in a shorter period. It is also an efficient communication tool to various engine development groups to synchronize the various paralleled activities very efficiently. One of the principal benefits of adapting the simulation methodology is to begin with a simple approximation of a process and gradually refine the model. As the understanding of the process improves the accuracy of the work also advances. This step-wise refinement enables the simulation engineer to achieve good approximations of very complex problems surprisingly quickly. The refinements with accurate and precise inputs of the engine model, the output of the simulated model becomes more and more exact, errors reduce and co-relation improves. (Bhat et al., 2016). Here in this work one dimensional simulation tool GT-Power is used to predict several engine parameters, help in designing the parts, developing proto parts, analysis of combustion events. 1

2 4.2 SIMULATION SOFTWARE TOOL GT-POWER GT-POWER is an industry standard engine One Dimensional simulation tool used by leading engine and vehicle makers to model the performance of internal combustion engines and additionally it gives a good NOx Predictions. GT-POWER is part of a more comprehensive software family called GT-SUITE. The latter features design analysis capability of engines with emphasis on driveline components (GT-DRIVE), valvetrain components (GT VRAIN), fuel injection and hydraulics (GT FUEL), engine heat management and cooling (GT COOL), and crankshaft dynamics (GT CRANK). GT POWER is based on one dimensional gas dynamics, representing flow and heat transfer in piping and related components. At the heart of GT POWER are two powerful software domains called GT ISE (Integrated Simulation Environment) that builds, executes, and manages the simulation process and, GT POST, a post processing tool that provides access to all the plot data generated by the simulation. Saving in time and development efforts is obtained through such simulation software. For example, before trying anything new on actual engine, we can try out and predict the performance on the software and thus after achieving the required results on the software we can implement them on actual engine GT-POWER can be used for a wide range of activities relating to engine design and development. Typical applications include: Manifold design and tuning Valve profile and timing optimization Turbocharger matching, wastegate, bypasses EGR system performance Manifold wall temperatures 2

3 3-D CFD studies (in conjunction with Star-CD or FLUENT) Thermal analysis of cylinder components Combustion analysis Design of active and passive control systems Intake and exhaust noise analysis Design of resonators and silencers for noise control Transient turbocharger response After treatment systems 4.3 GT-POWER SOLUTION THEORY The governing equations of GT-POWER are Continuity Equation: It simply signifies the law of conservation of mass. It states that the mass of fluid is conserved. = 4.1 Where m is the mass and m is the mass flow rate. Left side indicates change in mass with respect to time Momentum Equation: It is based on Newton s second law which states that the rate of change of momentum of a fluid particle equals the sum of the forces on the particle. The x-component of momentum equation is given below. In similar fashion we can obtain y and z components also. ρ = ( ) S 4.2 3

4 Where p denotes pressure on the surface, τ indicates viscous stress component in j direction on a surface normal to i-direction, u is the velocity vector in x direction, S is the source term accounting body forces. Energy Equation: This is the most common equation based on the law of energy conservation. The law of energy conservation states that the energy can neither be destroyed nor be created. It can just be transformed from one form to the other. +. q = Where E is the local energy density i.e. density per unit volume and q is the energy flux. Based on these governing equations, GT-POWER simulates internal combustion engines in one dimension. It can also simulate NOx emissions since it is majorly temperature dependent. NOx emissions are governed by Zeldovich mechanism. The prediction of this tool is based on two different modes. 4.4 OPERATIONAL DETAILS Speed Mode: This mode is generally used for steady state conditions. In this mode, performance of an engine is determined at an imposed speed by calculating the brake torque. This is fast approach for steady state results since the speed of the engine is imposed for the simulation duration because of which the time required by the engine to come to a steady state reduces marginally. (GT-Power., 2004) Load Mode: This mode facilitates an engine to connect with a vehicle. This mode gives an information regarding the response of an engine to the imposed load. Just like speed, load can be imposed as a constant or a transient function. But it requires many simulation cycles get trustworthy results since it takes some time to bring an engine to the steady state. Hence it is generally preferred to model an engine in speed mode and then switch it to the load mode. 4

5 Case Set-Ups: It is a part of GT-POWER where we can set up number of different cases and set up input parameters. The cycle can be run for number of cases depending on the inputs. GT-POST: This is the output part of the tool. It gives all the results in a separate window for different components and different systems for each case separately. It also shows the results in terms of graphs and we can compare these results graphically and also in tabular form. Predictive vs. Non-predictive combustion models: For SI and CI engines, we have three different combustion modelling options predictive, non-predictive and semi-predictive combustion models. Depending on the model, the choice is being made. A non-predictive combustion model imposes the burn rate as a function of crank angle. It is assumed that the burn rate is not affected by the residual fraction or the injection timing. This assumption and thus the model works good until it is used for the study of variable which will have zero or very less effect on the burn rate. But in case, there exists a variable which has a direct effect on the burn rate, the choice changes to predictive or semi-predictive combustion model so that the burn rate will immediately respond to the changes in the variable under interest. Theoretically, predictive combustion model is a good choice for all kind of simulations however some practical factors force us to go for a non-predictive model like the slower speed of predictive models. It also demands calibration to the measurement data in order to provide good results. A semi-predictive model turns out to be good choice in some cases against predictive model. It is significantly sensitive to the variable which have direct effect on the engine burn rate and provides quick response to the changes in those variables. These models use nonpredictive combustion model strategies and generally run faster than the predictive model. (GT-Power., 2004) Non-predictive combustion model: It uses Wiebe function to impose burn rate. Imposed Combustion Profile (EngCylCombProfile): This facilitates to impose burn rate as a function of the crank angle. This is useful when the cylinder pressure is known and measured since it can calculate the burn rate. There is a provision in GT-POWER which directly analyzes cylinder pressure and calculates the burn rate. 5

6 Direct Injection Diesel Wiebe Model (EngCylCombDIWiebe): This function imposes burn rate for direct injection CI engines using three term Wiebe function. It is used for direct injection only and when cylinder pressure is known. But it requires number of inputs variables such as start of injection and combustion efficiency. Predictive combustion model Direct Injection Diesel Multi-Pulse Model (EngCylCombDIPulse): This model predicts combustion rate and emissions for DI diesel engines with multi-pulse injection events. It is comparatively faster and more accurate model, with the condition that very accurate injection profile must be provided. Direct Injection Diesel Jet Model (EngCylCombDIJet): This model also needs an accurate injection profile. Besides that it has low speed. This model needs proper tracking of a droplet as it breaks, evaporates and burns. Model Set-Up: The software library itself contains different manuals and tutorials to help user build a model. Different manuals and tutorials are available according to the application like, Graphical Application and Modelling Application. Out of them, for our purpose, we can use Engine Simulation from the modelling application directory which is having a step by step procedure for building an engine model. Readymade models are also available for some general cases which can directly be used with real time data. The engine model was built and simulated with real time data to simulate the actual engine. All the simulations are done according to the standard test cycles mentioned in standards according to the applications. The elements used to build a model are given in the next table. (GT-Power., 2004) 6

7 Table 4.1 Elements in GT-POWER used to build engine models (GT-Power., 2004) Sr. No. Element Details 1. For every engine model, this part starts and ends the model. It describes environmental boundary conditions like pressure, temperature and air composition. It is also having a provision of End Environment adding altitude and relative humidity as an input for the pressure and temperature correction. 2. This object is used to specify all the flow pipes having circular cross-section and an optional bend. There is a special provision in the element that the data used to specify the bend in the pipe is used to Pipe Round calculate pressure loss coefficients and the head losses. 3. This object is used to specify engine inlet and exhaust valves driven by cam. It needs valve geometry, flow array and lift profile as an input. In addition to that it also needs valve lash and cam Cam Driven Valve Connection timing angle. There are some in built formulae in the element which will calculate other necessary parameters based on these inputs. 4. Type of fluid used as a fuel, its properties and the injection profile along with timing are some of the inputs to this object. This forms the injector section of the engine. It is generally used for DI Injection profile Connection diesel engines only however it can be used to inject fuel into cylinder, pipe or flow split also. 5. This object contains all the attributes related to the engine cylinder and not the geometry. This object decides the combustion model approach from Engine Cylinder input data i.e. whether to go for predictive or non- 7

8 Engine Crank-Train Flow Split General Compressor Shaft Gear Connection predictive combustion model. Both the valve objects and injector objects are connected to this element. This object contains the data like, Cylinder geometry, engine firing order, Type of engine and mode of operation (Speed Mode or Load Mode). This object is used to model the kinematics and rigid dynamics of the engine. It is used to describe the flow connected to one or more other flow components. In our model, this object is used to describe the inlet and outlet of an intercooler. In our model, this object is used to mention a supercharger. It takes compressor map as an input so as to predict mass flow rate, outlet temperature and power consumed by the compressor. The same template can be used to model a turbocharger also. This template is used for describing the compressor connection to the engine output shaft. In general, this object is used to attributes of any shaft. This template is a part of 1-D rotational mechanics. It generally describes the motion transfer connection between two elements via gear with no slip. In our case, we have used it to specify the belt drive system. 8