AbstractCylinder head design is one of the most involved disciplines in engine design. Whether designing an altogether new head or revamping an old one, several different coupled and inter-dependent technologies ranging from heat transfer, fluid flow, combustion, material non-linearity, structural and fatigue have to be accounted. Simultaneous designing of ports, jacket and combustion chamber leads to cylinder head design, which is then tested for its strength and durability.

Traditionally a series of analytical, empirical, test-based and simulation based exercises are conducted to design cylinder heads. With increasing pressure on reducing cost and turnaround time, focus on moving towards a quasi-simulation based design and development of cylinder heads is gaining strength.

This paper talks about how a simulation driven process for cylinder head design can be developed. The process would involve ten different interdependent simulations on ports, jackets, combustion chamber and head to eventually come up with the head design. ANSYS products and solutions are used for each of these simulations.

IntroductionThe specific output of modern IC-engines is following a continuously increasing trend, driven mainly by the desire for enhanced performance. This in turn leads to higher engine temperature and hot spots which affects fuel consumption, performance, oil consumption, NVH, emissions and structural integrity of the engine [1]. Therefore accurate thermal prediction of an engine is a key challenge today.

Thermal management of an engine is a highly complex problem and involves simultaneous interplay of a lot of systems, components and physics. The charge gets combusted in the combustion chamber, releasing tremendous amount of energy, used to drive the engine. The heat gets dissipated through the liner, piston and head and is carried away by the coolant flowing through the jackets as well as the oil flowing through piston gallery. The heat released during combustion depends upon the incoming charge and its swirl, injection rate shapes, speed and load conditions apart from the EGR content and the geometric details of the combustion chamber. Accurate prediction of the heat release as well as its spatial distribution, in itself is a complex enough problem. Added to that is the complexity that the heat dissipation time-scales of the gas, metal and coolant are so different. Phase change of the coolant adds another dimension to this seemingly complex problem.

3D simulation technologies, assisted by 1D simulation techniques are virtually replacing testing and prototyping of the engine at early design stages. The ability of these different 3D and 1D simulation technologies to interact with each other seamlessly, the increased computational speed and the usage of HPC and the ability to perform parametric and design exploration studies along with optimization has made this possible.

The present paper describes different types of simulations that form the base of simulation driven design of cylinder heads. Figure-1 shows a typical design process. This process has been simplified where a lot of interdependencies and external factors have been omitted.

Simulation Driven Design of Engine Cylinder Head 2015-01-1739

Published 04/14/2015

Padmesh MandloiANSYS Fluent India Pvt, Ltd.

Sourabh Shrivastava and Chetan PatilANSYS India

Santosh KottalgiANSYS Inc.

CITATION: Mandloi, P., Shrivastava, S., Patil, C., and Kottalgi, S., "Simulation Driven Design of Engine Cylinder Head," SAE Technical Paper 2015-01-1739, 2015, doi:10.4271/2015-01-1739.

Copyright © 2015 ANSYS Inc.

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Figure-1. Simulation driven cylinder head design process

The design process starts with simultaneous design of engine ports, water jacket and combustion chamber. For a diesel engine cylinder head, ports are design for optimum swirl and maximum effective flow area. Cycle simulations are carried out on the engine to optimize the combustion process at different speed-load conditions. Water jacket design starts from an existing design and is optimized for effective cooling, which in turn affects the combustion behavior. As these three components start to take shape a broad outline of cylinder head starts to get created. The head is then designed for durability including thermal stresses, vibrations and strain-life. The color code explains what parts of the head assembly are analyzed at every stage. The three red boxes denote FEA simulation whereas all the other boxes represent a CFD simulation.

A single-cylinder section of an in-line multi-cylinder heavy duty diesel engine is considered for the study. The model is shown in Figure-2.

The following sections will cover each aspect of the design process in more detail.

Figure 2. Single-cylinder head of an in-line multi-cylinder heavy-duty engine

Port DesignThe key design challenges for ports of a diesel engine are optimum flow area and optimum swirl at different lifts without adding too much to the pressure drop. Pressure recovery is an important consideration for the exhaust ports. Traditional techniques involve testing ports on a flow-bench and then redesigning the port and then

testing again. Shape optimization (either free-form or parametric) using 3D CFD tools can significantly reduce the turnaround time and cost and give much detailed insights of the flow behavior in the ports.

Let's take the case of free form deformation of intake port. As a first step, one would want to know which region in the port to focus on in order to derive maximum improvement. In absence of this knowledge one would define control region which would either be incorrectly located or would encompass the entire port region. Adjoint solver in ANSYS Fluent is used to predict shape sensitivity of the intake port. Flow volume of the engine model with intake and exhaust ports and intake and exhaust valves is taken as the starting point. This is quickly converted into a flow volume containing just the intake ports and a cylindrical plenum with valves at mean lift position.

Figure 3. Intake port model

Figure-3 shows the intake port model used for the study. First the Adjoint solver is used to obtain the shape sensitivity and region of interests are obtained. Mesh Morpher available in Fluent is used to define the free form deformation input parameters for these regions. Figure-4 shows the constraint of maximum deformation allowed for each region. These input parameters are then exposed to Design Explorer, which is a design optimization tool available in ANSYS Workbench. Goal for this optimization study is set to maximize the effective flow area (EFA), define by eq-1.

(1)

Where

m = Mass flow rate through the intake port

ρ = Density of the incoming fuel-air mixture

ΔP = Pressure drop across the intake port

DOEs are generated using Central Composite Design based algorithm. Simulations are performed for all the design points and response surface is generated based on full second order polynomial method, to find out the relative sensitivity of different input parameters on EFA. After this, goal driven optimization is performed using MOGA algorithm to derive best configuration of input parameters to maximize the EFA.

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Figure 4. Input deformation parameters

Similar analysis can be performed by considering different port models and different output parameters, like EFA is used for the present study

Design engineers can also start with a parametric CAD model that they want to optimize. Such CAD models can be read in ANSYS Workbench ICE system to setup CFD simulations, semi-automatically. Using Design exploration within Workbench, the design space can be analyzed for sensitivity of different parameters as well as response of a given output parameter (e.g. swirl) to given input parameters. Using different multi goal driven optimization schemes, a best candidate design can be arrived at which shows the most optimum behavior.

Water Jacket DesignWater jackets are designed for optimum cooling. They are also designed to avoid localized hot spots in the head. From a convective heat transfer stand point, it is important that the flow and turbulence levels are enhanced everywhere in the jacket. Especially the exhaust-exhaust bridge which is most susceptible to overheating has to be designed properly. The location of holes on the gasket which connects the engine block jacket to the head jacket are also critical as play an important role in improving the flow and turbulence levels in the jacket.

Three types of CFD simulations are carried out on water jacket. A single-phase isothermal flow simulation is carried out on in the water jacket to predict flow and turbulence levels and can be used to optimize the shape as well as placement of gasket holes. Here, the turbulent kinetic energy levels are used as indicators for whether jacket would be able to perform its function appropriately or not.

A multiphase Volume-Of-Fluid (VOF) flow simulation is then carried out to check if the jacket is susceptible to air pockets which stay on in the jacket and don't move through the outlet. Presence of air pockets in the jacket can lead to very poor local heat transfer leading to localized hot spots. The VOF simulation also helps to give an idea for the residence time (or the filling time) of a water jacket. The same inference can be drawn from the single-phase flow simulation as well.

Another important metric in water jacket design is the pressure drop across it, as it helps to estimate the pumping power required to drive the coolant. Since water jackets have abstract shapes, they are ideal candidates for free-form shape optimization, where the objective parameters are pressure drop and minimum velocity levels and the constraints on shape deformation are considerations on minimum metal thickness in the block.

Modern day jackets are designed for controlled boiling. Controlled nucleate boiling helps in augmenting the convective heat transfer and therefore helps in better cooling. A phase-change study on the water jacket model is carried out using the Semi-Mechanistic Boiling Model (SBM) based on Chen's correlation [2]. The SBM works on mixture multiphase model and is therefore a low-cost approach to accurately predicting nucleate boiling and the resulting change in heat transfer coefficient in components. A validation of this SBM on a rectangular horizontal channel is presented here [3]. Figure-5 shows the computational domain of the 3D rectangular channel which is simulated. The heating surface is at the bottom of the flow channel, 10 × 50 mm2, and located 76 mm downstream from the entrance of the channel. The operating conditions included inlet velocities of 0.25 m/s and 1 m/s, operating pressure of 1 bar and 3 bar and all temperature of 90 C and 170 C. Outlet is maintained at zero gauge pressure in all the cases. All the other walls were maintained as adiabatic walls. Figure-6 shows the comparison of numerical and experimental results at 0.25 m/s inlet velocity and inlet temperature of 90 C at different operating pressure and different wall temperatures. Figure-7 shows similar comparison at higher inlet velocity of 1 m/s and 3 bar operating pressure for cases with different inlet temperature. These two plots suggest that the SBM model gives reasonable prediction of heat transfer augmentation under various operating conditions.

Figure-5. Computational domain to replicate Robinson's experimental work

Figure-6. Comparison of numerical and experimental results of wall heat flux vs wall superheat at different operating pressure for 0.25 m/s inlet velocity and 90 C inlet temperature

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Figure-7. Comparison of numerical and experimental results of wall heat flux vs wall superheat at different inlet temperature for 1.0 m/s inlet velocity and 3 bar operating pressure

The same model is used to predict heat transfer augmentation in the engine cylinder head. A conjugate heat transfer model of the cylinder head along with the coolant channel and intake and exhaust ports is modeled. A time-average heat-flux profile is applied as boundary condition on the fire deck side of the head. This is obtained from a separate IC combustion analysis. The remaining walls of the head were assumed at adiabatic conditions. A 50:50 water and ethylene glycol mixture at 90 C and 1.94 kg/s is assumed to flow through the jacket. Appropriate inlet and outlet conditions were specified to model gas flow through the intake and exhaust ports. The SBM model along with mixture multiphase model is used to model boiling in the jacket. A steady state simulation is performed to derive the results. Figure-8 and Figure-9 show the heat transfer augmentation factor and the vapor volume fraction in the cooling jacket. An augmentation factor of almost 6 is seen in certain regions of the jacket as a result of the nucleate boiling. The higher this factor, the more is the overall heat transfer resulting in better cooling of the jacket.

Figure-8. Heat transfer augmentation factor due to nucleate boiling

Figure-9. Vapor volume fraction in the jacket

Cylinder Head DesignEngine cylinder head design needs to consider design and functional aspects of different components like ports, water jacket, valves, valve-guide, engine fire-deck etc. This should be robust enough to withstand high thermal and mechanical stresses that get generated during the engine operation. Simulations can help is such design process by providing spatial distribution of quantities of interests like temperature, stresses etc. The following section describes simulations that are needed to get the temperature distribution and hence thermal stresses in the cylinder head domain. Also it discuss the approach of using simulation tools to determine fatigue lifecycle of cylinder head model

Thermal Analysis of Cylinder HeadThermal analysis of an engine cylinder head is carried out to understand the temperature distribution within the domain. This information is then used to measure distribution of thermal stresses and structural integrity of system

Thermal analysis of cylinder head involves, modeling heat generation within the engine chamber and modeling heat transfer through cooling jackets. Since time scales involved in the two studies are order of magnitudes different, coupled approach is usually followed. This consists of two types of simulations: Transient in-cylinder combustion simulation and steady state conjugate heat transfer simulation of the head and the cooling jacket along with intake and exhaust ports. These two simulations are run in coupled manner for 4-5 iterative cycles to arrive at steady temperature distribution in the cylinder head.

Figure-11 describes the Simplified coupled approach which is a derivative of the Detailed-approach Figure-10. This approach uses a simplified cylinder head model, without any ports and requires simulation run for only closed part of cycle, i.e. from IVC (Inlet-Valve closed) position to EVO (Exhaust -Valve open) position. Conversion of detailed-approach to simplified-approach is performed with the help of scaling-factor.

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Figure 10. Coupled approach to perform engine cylinder head CHT analysis using full engine model

Figure 11. Coupled approach to perform engine cylinder head CHT analysis using simplified (flat) head engine model

Simplified-ApproachFirst the transient IC combustion simulation is performed from IVC to EVO condition, by taking fluid volume of engine-chamber. For the simulation, constant wall temperatures are assumed for cylinder head, liner and piston boundaries. During the run time, heat flux is time averaged on the ‘cylinder-head’. After the completion of run, i.e. at EVO, this time average heat flux on head is saved in the form of profile. This heat flux profile is then scaled down using the scaling-factor (discussed later) and then applied on the ‘fire deck’ boundary of the steady state simulation, which is performed using ‘cylinder head domain’ to get the temperature distribution in the system. After convergence, temperature on the fire deck surface is recorded as a profile. This profile is then applied to ‘cylinder-head’ boundary of ‘engine-chamber’ domain and then next transient simulation is performed from IVC to EVO. At the end, again time-averaged profile is taken out and then used in next steady-state CHT simulation. Such iterative coupled cycles are performed in loop between transient and steady system, till the variation of temperature distribution from one cycle to next reaches to a minimum threshold value

Transient In-Cylinder SimulationANSYS Workbench ICE System is used to prepare the model for combustion simulation. Figure-12 shows mesh on the full model and Figure-13 shows mesh on the simplified flat head model.

Figure 12. Full engine model meshed using WB-ICE

Figure 13. Flat head engine model used for ‘simplified-approach’

Combustion simulation is performed for the full 720 CA using Laminar Finite Rate model with ERC n-heptane mechanism. Chemistry Agglomeration algorithm is applied to speed up the chemistry calculation process. Simulations are performed using second order upwind scheme for momentum, energy and species equations. PRESTO for pressure, PISO for pressure-velocity coupling and gradient calculations are performed using Green-Gauss cell based discretization scheme.

Diesel spray is modeled using Discrete-Phase Model. To model liquid-spray, properties of N-tetradecane liquid are used. Temperature dependent surface tension and vapor pressure data are implemented to accurately model breakup and evaporation. KH-RT break-up model is used to capture atomization. Turbulent dispersion of particles is used to capture the particle trajectories accurately. Turbulence modeling is performed using SST-kw model.

Scaling FactorFigure-14 describes the process of calculating scaling factor. Simulation for full 720 crank rotations with constant temperature wall boundary condition for engine walls is performed, starting from IVC condition. Time averaged heat flux at the cylinder head is recorded at EVO and IVC+720 (i.e. after complete 720 crank rotations) crank position. These values are then used to derive the Scaling factor.

This scaling factor is used to remove the need of running transient engine simulation for full 720 CA. Instead transient simulations are performed from IVC to EVO condition to get the time average value and then this scaling factor is used to get the time average heat flux value for full 720 CA.

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Figure 14. Steps to calculate scaling factor

Assumed value of time-averaged heat flux at 720 CA = time-average heat flux at EVO × scaling factor

For the given engine after making adjustment for other losses this scaling factor came out to be 0.35

Steady State Cylinder Head SimulationFigure-2 shows the CFD model considered for steady state simulation. The domain consists of the fluid regions of intake and exhaust port, fluid region of the cylinder head water jacket and the surrounding solid parts of the cylinder head. This is meshed using polyhedral cells to get the domain shown in Figure-15. Table-1 describes the flow boundary condition used for the system.

Figure 15. CFD model considered for steady state simulation of Engine cylinder head system

Table 1. Details of the water jacket model

A single phase calculation with four different materials is carried out. Both intake and exhaust valves are modeled in open position, with valve height set to mean value. Inlet flow condition for intake and exhaust gases are derived by assuming that valve remains open for full 720 CA with the net flow equivalent to flow occurs in actual 720 deg. cycle. That is flow conditions are time averaged and applied to the inlet boundaries.

Figure -16. Work flow of engine cylinder head cooling analysis

ResultsIterative coupled cycles are performed using workflow shown in Figure-16 for the given IC engine. Converged solution is obtained after 5 iterative cycles. Figure-17 shows pressure and AHRR plots from the last transient combustion simulation (5th transient simulation) of the flat head engine model. Pressure and AHRR plots show similar behavior as is observed with experimental data. Slight mismatch in AHRR plot can be attributed to the reaction mechanism used for the analysis and small uncertainty of EGR fraction at IVC condition. Use of better mechanism and more accurate data on EGR fraction can help in improving the prediction further. Also some changes in pressure plots can be anticipated to the simplified model used for this approach.

Experimental data for temperature distribution is available for thermocouple location shown in Figure-18. After the 5th cycle, steady state temperature distribution of cylinder head model on thermocouple locations is compared with experimental data. Figure-19 shows the comparison of temperature predicted by coupled simulation on thermocouple locations, with respect to experimental data. Maximum deviation between simulation and experimental data is found to be 24.9 K at thermocouple location 6. Error bars in

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Figure-19 shows the maximum range of 25 K, as can be seen all other temperature values are well within this range. Results thus validate the approach and hence this approach similarly can be extended for the CHT analysis of other IC engines.

Figure -17 (a). Pressure Vs CA, for 5th Transient IC engine simulation

Figure -17 (b). AHRR Vs CA, for 5th Transient IC engine simulation

Figure -18. Thermocouple locations on engine cylinder head domain

Figure -19. Experimental Vs Simulations results for temperature on thermocouple locations. Error-bars = 25K

Thermal Stress and Fatigue ModelingSubsequent to predicting the thermal distribution in the head, focus now shifts to checking the reliability and durability of the head. Increased combustion pressure and higher temperature makes durability testing of the head even more critical.

As a first step, thermal stress analysis is performed to understand the distribution of equivalent stress. Temperature distribution in the head, calculated from the Conjugate Heat Transfer analysis is mapped to the FEA model. Non-linearity of the material properties is considered. Plasticity model for Cast Iron grade 150 is considered. Bolt pre-tension load was applied. Average cylinder pressure was applied on the fire deck side and the model was properly constrained.

Figure 20 and Figure 21 shows the deformation and maximum principal stress in the head. A closer look at the maximum principal stress (Figure 22) in the head shows severe stress localization at a very sharp corner inside the head. This severe stress localization can be catastrophic and therefore the head design was modified in this region of stress localization so as to bring the stress values to permissible limit. Figure 23 shows principal stress on the modified design. The modified design shows principal stresses in the permissible limits now.

Figure -20. Deformation in the Cylinder Head

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Figure -21. Principal Stress in the Cylinder Head

Figure -22. Localized Principal Stresses in the Head

Figure -23. Principal Stresses in the Modified Head

To perform a subsequent Fatigue analysis, nCode DesignLife system is used in conjunction with ANSYS Mechanical. The Fatigue analysis is done to consider two aspects of loading,

1. Dominant Pressure loading cycle and 2. Dominant Creep cycle, to check the effect of the individual

loading characteristics.

In accordance with the above, the Mechanical analysis is divided into 3 load steps, 1st LS - Bolt Pretension load is applied, 2nd LS - Imported Temperature load is applied and 3rd LS - Imported Temperature load is kept constant and Pressure load is applied.

Second LS loading (Temperature rise) and subsequent constant operation of engine (constant Temperature load) for 1 hour and then cool down (ramp down of Temperature load) for another 1 hour. A standard Test Data for Multi-Temperature S-N life curve and Larson

Miller Parameter curve is considered for the cylinder head. Figure-24 shows the life results predicted using the strain-life (S-N) engine. The head was found to have life above the minimum limit of 10,000 cycles. Focus was on an engine start-stop load cycle. Similar cycles like high torque cycle, idling cycle, etc. can be combined to create a total duty cycle

Figure -24. Life Results

ConclusionSimulation driven design of a heavy-duty diesel engine cylinder head s discussed in this paper. The entire design process is replaced with simulation steps to demonstrate the process. Port, Coolant Jacket and Head designs are discussed with analysis ranging from port optimization to in-cylinder combustion and thermal analysis, boiling, and thermal-stress/fatigue analysis. Validation studies were described for some parts of the simulation process. Changes in head based on the simulations were suggested and even tried. Such simulations can help designers to make corrective action in cylinder head design much ahead in overall product development process.

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Definitions/AbbreviationsIC engine - Internal Combustion engine

CHT - Conjugate Heat Transfer

IVC - Inlet Valve Closed

EVO - Exhaust Valve Closed

SOI - Start of Injection

FEA - Finite Element Analysis

CFD - Computational Fluid Dynamics

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