Analysis Of Piston Design And Its Effects On Engine Processes

Impact of Piston Design and Material on Engine Processes

Discuss about the Cfd Simulation Of Air Movement Inside The Cylinder Of An Internal Combustion

The engines and piston uses small skirts since they grow shorter and lighter with evolution. Currently, pistons are built from aluminium alloys as opposed to silicon that lowers thermal expansion and improves resistance (Guzzella & Onder, 2009). The introduction of piston “tops” with bowl like tops that is subjected to the combustion chamber giving it different effects in the heating process. Primarily the piston crown act as the burning chamber since the diesel has no ignition phase. This improves the combustion ratio. Gasoline engines have employed the use of advanced crowned pistons. The fuel and air motion are managed by the pistons bowl design during compression stroke before ignition (Stone, 2009)

Bowl shaped piston design creates perfect turbulence mixture of air and fuel for efficient combustion generating more power for the engine processes. Bowls are designed with different shapes for effective fuel consumption (Chellen & Baranescu, 2010). The use of gasoline has led to the spread of bowled pistons. The low conditions in high-speed direct injection engines at the end of compression stoke, near top dead centre have become critical .Air flow into the cylinder and intake valves in the induction process and its evaluation in the compression stroke  is used to determine the combustion process (Rakopoulos & Giakoumis, 2016)

More researches and studies have been conducted on the geometry of the piston to asses on issues affecting the flow distribution of diesel engines. The project chapter is used to analyse previous published works that lays a foundation for more work research in this project. This gives a proper understanding of the research topics and gives a guide throughout the work process. The engine cylinder flow characteristics with different pistons designs were analysed (Ozsezen etal, 2009)

Due to this, the intake and compression stroke analysis have been undertaken at realistic operating conditions and proper turbulence and velocity flows generated in every combustion chamber verified in details (Taylor, 2009). Analysis shows that the geometry of the piston had a small effect on in-cylinder flow at the beginning of the compression stroke and the intake stroke. The bowl shaped piston is of great importance at the TDC and at the start of the expansion stroke as it regulates the turbulence velocity fields and ensemble-average mean (Rakopoulos etal, 2017)

The turbulence or swirl motion of the engine cylinder during the compression and intake strokes of the pistons geometry construction with a single intake valve had been analysed and studied (Som & Datta, 2018), though a small validation presentation has been of the analysis and calculation made. Full calculation of the analysis and solutions of the compression and intake processes have been conducted and analysed by Chen et al who presented the calculations and compared the performance of the engine with experimental results obtained from conducted analysis (Rakopoulos etal, 2011)

Bowl-Shaped Pistons and Efficient Combustion

From the data has been used to indicate that the calculated results were used to predict the turbulence velocity flow. Comparisons of the results with the real geometry were used to explain the error differences in the experimental data and the disadvantages of the standard k–? model. In addition to that, Dillies et al conducted a study and too presented the same calculation analysis of a diesel engine having a single intake valve for a single combustion chamber, the case which data was correctly merging well with the conducted experimental results. A computational review cited on large eddy simulation was done (Benjumea & Agudelo, 2009)

The computational fluid dynamics analysis applies an integration of computational software, mathematical modelling and numerical methods in order to visualize and give a prediction of both the quantitative and the qualitative characteristics of fluid flows (Anderson & Wendt, 2015 )(Niu etal, 2010).  This tool has the capability of providing various solutions such as the multi-phase and single phase conditions isothermal flow, a chemical reaction in fluid flow as well as the incompressible or compressible fluid flow (Chung, 2010).

In our daily experiences, we experience various circumstances involving fluid flow and the computational fluid dynamics analysis tool has been applied while performing the analysis of the air conditioning, engine combustion, ventilation as well as the propulsion system (Versteeg & Malalasekera, 2009). For the internal combustion, there exist four methodologies which differ from one another with regards to the results of the operators. These methods include

• In-cylinder combustion simulation (Kollmann, 2012)
• Port flow analysis (Sayma, 2009)
• Full cycle simulation (Roache, 2011)
• Cold flow analysis (Hu, 2012)

In this method, depending on the selection of the operator, the engine configuration is held frozen at a certain engine cycle angle. This technique is very significant and is very crucial when it comes to the static analysis mostly in problems involving the computational fluid dynamics (Huang etal, 2015). The CFD operators are provided an option of either the full cycle simulation or the combustion simulation that’s for the CFD simulation (Lin etal, 2009).Nevertheless, when it comes to the in-cylinder combustion simulation, the simulation is only done for the power stroke (Versteeg & Malalasekera, 2014). Whereas for the full cycle simulation   the analysis that is obtained has the possibility of offering the full picture of the processes occurring in the engine such as the injection of fuel, exhaust gases  generated from the chemical reaction, modelling of the air that’s  flowing inside the cylinder  and also the combustion (Anderson &Wendt 2015).

Computational Fluid Dynamics Analysis in Engine Studies

The cold flow simulation can either as well categorized as transient simulation (Van wachem etal ,2011). Here, there is the possibility of carrying out a simulation of the full engine cycle without getting a capture of the chemical reaction (Kloss etal, 2012). This technique has the ability to visualize the flow of air throughout the system thereby capturing the air which is being induced and tentatively give a prediction of the swirl formation, tumble formation as well as the formation of squish (Jacobsen etal, 2012).  Besides, the prediction of the mixing of the fuel that has been injected with air can also be done (Li etal,2009)

The assumptions that are made in this simulation includes the changes in the thermodynamics of the engine, the exhaust stroke flow, and the power stroke flow (Karniadakis & Sherwin, 2013). Some of the equation which declines the energy, momentum, k epsilon and mass devoid the chemical reaction and the injection of the fuel include (Lijewski & Suhs, 2014)

 Where  ? is velocity vector,

            I is specific internal energy

            ρ is density

            j is the heat flux vector

           σ is turbulent viscous stress tensor (Versteeg & Malalasekera , 2017)

There should be a clear comprehension of the motion of the fluid inside the combustion chamber since it has a direct impact on the air which is being induced as well as the speed of combustion (Johan, 2016). Nevertheless, when there is efficient mixing of the air, the quality of the gasses that are emitted is improved as well as the better injection of the fuel system (Macpherson & Reese, 2012).  Hence, some of the commonly investigated parameters include tumble, turbulence, and swirl (Guzzella & Onder, 2009)

Turbulence in an engine takes place whenever there is a high speed detected in the engine cycle (Ramos,2011). When the engine is working at an increased velocity, the air that gets in and out of the cylinder is at a higher velocity thereby resulting in the generation of turbulence inside the cylinder (Turbocharging, 2010).  This turbulent flow which is developed plays a significant role when it comes to determining some various factors including the fuel-air mixture, combustion in the engine cycle, evaporation and the heat transfer (Daw etal, 2015). Hence, it is a requirement for the designers to come up with efficient designs which will be able to withstand the pressure even at a top dead center (Han & Reitz, 2013)

Turbulence and Swirl in Engine Processes

The reason as to why is because when the ignition takes place at an increased pace and near the top dead center, there is increased spread of the flame and rapid break up when compared to when there is low turbulence (Cook & Powell, 2012).  In the past, some researchers suggested that the design of the combustion chamber is very significant in influencing the turbulence that will be generated in the engine (Balluchi etal, 2010). Besides, there are other factors which influence the generation of the turbulence (Nikkhajoei & Lasseter, 2009). They include speed of the piston, design of the intake manifold and the condition of the inlet flow. There is also the design of the valves and the tumble flap (Patterson & Reitz,  2013).  The turbulent kinetic energy is one of the actors that can be easily simulated through the CFD (Kokjohn etal,  2010).

The swirl refers to the rotational motion of the air around the vertical axis of the cylinder (Gupta & Syred,  2014). Being one of the factors that influence the transfer of heat, quality of combustion, the in-cylinder fluid movement (Huang & Yang, 2009). When this parameter is combined with tumble, then the overall resulting turbulence in the engine is very high and this can be retained in the compression stroke (Voronkov, 2015). The nature of the swirl often is not easy to determine in prior and one of the techniques which are applied in the determination of the swirl includes the flow bench test. When the swirl is being determined in the operating engine, then one factor that comes clearly is the swirl ratio that is obtained by the following formula (Kishimoto etal, 2011)

 ???????? = ???????? /2????????

 Where

            ωs represents the angular velocity in  the rotating flow at swirl axis,

            Rs is the swirl ratio, (Syred, 2012).

            N is the operating speed of the engine

In the CFD analysis, the swirl ratio is obtained as shown below

???????? = (????.????????/ ????.????????) / (2????/???? 60)

Where

            N is the operating speed of the engine (Voronkov, 2015).    

            I.sa is the inertial momentum of fluid mass with respect to the axis of the swirl,     

            L.sa is the size of the fluid angular momentum with regards to the axis of the swirl,

Immediately that piston nears the top dead center, which is located near the extreme end of the compression stroke there is a radial mixing inside the engine and this activity is known as a squish (DiPietro,2014). It is this squish motion that results in the tumble which is a secondary rotational flow (Humphreys & Smith, 2017). Besides the tumble takes place near the circumferential axis of the outer edge of the piston bowl. It is determined experimentally by the help of steady flow rig (Tailleur & Cates, 2018). The tumble ratios are usually specific for very design hence a comparison of the tumble ratios from different designs cannot be directly compared. The rumble ratio usually is computed automatically in the ANSYS software just be inserting the right command. The ratios are computed as shown below  (Shannon etal, 2012)

Conclusion

???????? = (????.????????/????.????????) /2(???????? /60)

Where

L.ta is an amount of fluid angular momentum with reference to the axis of the tumble,

I.ta is the inertial moment of fluid mass about the axis of the tumble

In addition, to tumble ratio, ANSYS IC Engine introduces another parameter which is the cross tumble ratio which involves the computation of rotational flow at the axis perpendicular to tumble axis which also known as cross tumble axis (Chiodi 2011)..

In computational fluid dynamics exists continuity, momentum and energy equations that are solved? Numerically there are two equations can be applied in an attempt of computing fluid flow. To begin with is continuity equation stating that the fluid getting to a fixed control capacity either get out or stay in it therefore mass balance is a scalar equation and in a mathematical form. Navier-stokes/ momentum equation is the second one and has a momentum balance. It is a vector equation i.e. it has a distinct equation for every coordinate directions (Chmela & Orthaber, 2009)

While conducting analysis of IC engine dynamics, model of meshed manifold together with burning chamber is inserted to ANSYS Fluent 13.0. The CFD simulation is for cold flow where heat is not needed (Dinler & Bucel, 2016)..

The intake and exhaust manifolds need a constant pressure. Next to cells above/ below the valve are the attach boundaries specified on coincident cell face. Logarithmic law is applied to reduce the slipping and heat loss by the walls of the engine without a boundary (Rakopoulos et al, 2010).

Mathematical model

The RNG k- ε with k as the variance symbol of changes in the velocity in the turbulence kinetic energy with dimensions of (L2 T-2) for the rate of change in velocity and k per unit time (L2 T-3). The turbulent kinetic energy equation as demonstrated has a number of simplifications from the rigorous equation I.E (Gajula & Bari, 2017)

The first term on the RHS is the production of ‘k’, the second term (?) is the specific dissipation per unit mass. The last terms describe the transport of ‘k’ by molecular and turbulent diffusion. The standard k-? model is the default turbulence model in Fluent. Rather than solving for a length scale it solves a second transport equation for the dissipation rate

The flows with high Reynold’s numbers or iso-tropic has to be fitted with this model since the energy released is above the normal equilibrium in respect to generation

The engine system which has been taken as a sample for study in this work is bent roof having two valves at the intake and two at the exhaust. Originally, the model which was used is the ANSYS model that has modified into certain configurations but then the design of the engine system has been retained. The variations are done with respect to the shapes of the bowl which are commonly applied to the automobile engine. Besides, the redesigning is also with regards to the simple process of fabrication which is retained.  The engine parameters which are computationally modelled includes the piston bowl, the cylinder, the exhaust valve and intake valve. The analysis will be done symmetrically in order to help in minimizing the grid cells as well as the time used in performing the computational analysis

The shapes that are modified in the piston from the very first stage are just the common shapes of the piston which are available in the cylinder.  The diameter of the bowl and the piston depth was held constant for the reason of effectively analysing the geometrical characteristics of the bowls of the air flowing in the cylinder The original piston bowl is represented by piston a while the piston B is a bit similar to the first piston, only that it has a wide throat. The third piston which is piston c is almost similar to the first piston, the only difference is that it has not been modified. The first figure represents the original model of the engine whereas the second figure represents the engine model that has been analysed using the CFD model. Besides, the table below shows the specifications of the engine

The purpose of decomposition into the various zone was for the purpose of correctly visualizing the engine system as a temporary simulation. The advantage of this is that there will be a desirable regulation of the mesh model. Usually, the zone which is decomposed ion the ANYS meshing is indicated by dissimilar labeling of the various parts as shown in the figures below

The various reason in which the modelling was decomposed includes the exhaust port, the combustion chamber, the intake port and the valves seat. From the below figure, it is clear that various zones pose various shapes of the grid cell as well as the quality.  The region where there is the combustion chamber has the finer grid cell and this is also realized ion the valve seat. On the other side, the grid cells are coarse on the sides containing the exhaust port and the intake port

The finer grid cells are significant in ensuring that the visualization of the opening and the closing of the wire that is majorly concentrated on the opening of the valve and the closing of the induction of the air leading into the chamber. The motion of the piston bowl is along the stroke line towards the BDC from the TDC during the compression stroke and the intake stroke. Hence, this provides the treason as to why the combustion chamber is usually separated in different zones with the piston as the only moving part. The hexahedral cell is utilized in the moving zones to achieve accurate analysis whereas the tetrahedral l cell is used in meshing the static zones to minimize the computational time.  An independence grid test is carried out to obtain the correct results by the use of minimum number of the cells in the mesh

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