STAR-CD at Fiat Powertrain Technologies

Angelo Rosetti, Fiat Powertrain Technologies, Arese, Italy.

STAR-CD is used to support the engine design process of Fiat Powertrain Technologies (hereafter FPT) in Arese. Advanced CFD calculations are usually performed, such as 1D-3D coupled analysis, cylinder head and block water jacket, manifolds analysis, intake runner cold flow. Some spray-injection and combustion simulations have been now performed with the latest multi-hole injectors configuration. Here 1D-3D coupled and water cooling jacket analyses will be described.

Manifold Design

1D-3D Coupled analyses are used to design manifolds. The one dimensional code GT-Power v6.1 and STAR-CD v3.24 are used on Linux RH Enterprise3 machines.
Simulations are usually done in full load condition, for the maximum power rpm and sometimes for the maximum torque rpm determination. An amount of seven engine cycles are run, six of them to reach the convergence and the last one to write results. A 0.5 degree crank-angle-step is usually set, to get the best compromise between analysis velocity and accuracy.
In order to provide faster results to the design engineer, a particular cluster has recently been built, made of only one Linux machine and “n” number of WM-Vare Linux virtual machines mounted on Windows technical workstations. Each of these technical workstation have 2 processors; when you want to run a simulation with the cluster you simply have to “turn on” the virtual machine from Windows and you get a Linux machine.

 

 

Then you have only to decide how many processors you want to add to the mini-cluster (only one or both of them). In that way you can work on a machine and use it with one processor with Windows and use the other processor for the Linux cluster: so you can decide how many machines to use among those available on your office! A Linux cluster has been preferred because of Es-Ice and Ice calculations can be run.

The results obtained are analysed and discussed together with the designer. Velocity field, pressure, temperature, pressure waves, Mach number, flow separation and mass flow rate balance between cylinders are the quantities usually observed. A complete analysis for one manifold solution (meshing process, running, post process and report building) takes about 4 or 5 working days, depending on the mesh complexity. Figure 1 shows the mesh of a six cylinder engine intake manifold, the mesh is about 400K cells, with one extrusion layer of about 1mm. The Standard K-e Turbulence Model has been used. Figure 2 shows an instantaneous velocity field of some runner sections; Figure 3 shows the mach number of only one runner section.

Water Cooling Jacket Analysis

The water jacket analyses performed at FPT are usually part of the complete thermo-structural analysis of the whole cylinder head and block. From the CFD code the wall heat transfer coefficients are passed to the FEM code in order to make the thermo-structural calculation. STAR-CD is then used to control all the water flow distribution inside the head and the block; further more the gasket hole position and dimension are optimized in order to keep the metal temperature not too high for all the parts of the engine.

 The standard calculation is usually represented by a steady run in maximum power condition. The mass flow is the inlet condition and the exact coolant properties are specified.

 

 

Both isothermal and non isothermal analysis can be performed and thanks to the flexibility of STAR-CD, boiling effect is taken into account, using an appropriate user subroutine. The Rohsenow nucleate boiling model has been used to correct the one-phase wall heat flux in case of critical wall temperature values.

Figure 4 shows a complete mesh of the water cooling jacket of the cylinder block and head for a four cylinder engine. The mesh consists of about 800K cells. The coolant enters from the block and goes out in two outlets: one going to the radiator and the other to the interior compartment. The position of the gasket holes, those connect cylinder block and head, are very important for the whole thermal balance of the engine: the role of the calculation is to best fit the positioning. As known, the exhaust valve bridge in the cylinder head is the critical point to monitoring for the thermal behaviour of the engine and an organized coolant motion without stagnation on the exhaust side is the starting point for water cooling jacket design.

 

 

Cylinder head pressure, wall heat transfer coefficients and temperature can be observed respectively in Figure 5, 6 and 7.

Conclusions

STAR-CD is the CFD simulation software used in the engine design and analysis department of FPT, Arese Site. The article is an overview of how coupled 1D-3D and water cooling jacket CFD analyses are usually performed and clearly shows the fundamental role of the simulations in the design process of  modern automotive engines.