Ian Postlethwaite, Executive Engineer (Powertrain), Lotus Engineering, UK

Lotus has developed a method that combines data from its own 1D engine simulation code (‘LES’), a 1D fluid system analysis (FLOWMASTER) and STAR-CD CFD code to predict the 3D temperature field in an internal combustion engine (ICE). The solution uses STAR-CD as the core tool to orchestrate the analysis.

As powertrain analysis becomes more sophisticated the need to produce an accurate temperature field of the complete engine becomes increasingly important. Traditional methods use empirical correlations to create the boundary conditions for such analyses. Lotus has developed a method that uses established basic analysis methods to feed boundary conditions for the more complex downstream CFD analysis (Fig 1).

The initial problem is how to obtain the gas side temperatures and heat transfer coefficients (HTCs). We could build a complete 3D CFD model of the gas exchange process, including a moving mesh and combustion, but this would be very computationally intensive and time consuming. Whilst feasible, it would not have been practical within project constraints. Luckily, a simpler approach can be adopted which utilizes data already created from the engine performance simulation carried out at an earlier stage of the project. ‘LES’ is primarily used for optimizing the engine performance within the design constraints of the project. However, data is also produced on gas temperatures and HTCs throughout the engine. These data are used to provide the boundary conditions for the cylinders, the inlet and exhaust ports, and the exhaust runners.

What about the heat transfer between contacting components (i.e. valves to seats)? Here, empirical correlations for the heat transfer between materials of different hardness, surface finish and contact pressure are used.
STAR-CD is used to predict the 3D flow conditions, including temperatures, on the coolant side. However, the coolant heat transfer coefficient is modified to account for nucleate boiling effects. The full cooling system can be added using the link to FLOWMASTER, Starlink.

The CFD analysis uses a computational mesh that incorporates all the major engine components (cylinder head and block, liners, valves, seats and guides). All these components are meshed simultaneously to eliminate mesh discontinuities at baffle boundaries.

The analysis provides the temperature field for the cylinder head (Fig 2) and block as well as other components such as valves, seats and guides. Also calculated is the coolant temperature distribution and heat rejection.Varying the boundary conditions can simulate various scenarios. These include different engine power levels and speeds, varying heat rejection to ambient and also transient simulations. Close agreement with measured data was achieved with this analysis technique (Fig 3).

The result is a process that creates an accurate temperature field of the engine that can be mapped to FEA models for subsequent structural analysis. CFD analysis has proven to be a valuable tool to predict heat rejection to the coolant, steady state temperature fields and transient warm up conditions as well as the flow structure within the cooling jacket, providing confidence in engine designs early in the product development cycle and minimizing engineering risk.

For more information contact: ipostlethwaite@lotuscars.co.uk

Fig 1: Engine thermal analysis
Fig 2: Cylinder head temperature distribution
Fig 3: Comparison of measured and predicted temperatures