Analysis of Piston Cooling Jets by means of advanced CAE software

In the framework of emission reduction, one of the most common trends in engine developments is downsizing. To mitigate engine performance reduction, downsizing is frequently coupled with an increase of specific power output, which leads to higher thermal stress in several engine components and piston in particular. One of the most successful methodologies which are used in high performance engines to control piston temperature is Piston Cooling Jet (PCJ), in which one or more jets of lubricant oil are sprayed towards the bottom of the piston.

Unfortunately, it’s extremely difficult to study and characterise PCJ experimentally, due to difficulties in extracting piston data in running engines. Because of this, simulation tools are frequently used to evaluate how the oil jets develop, which areas of the piston are mainly hit and what is the resulting piston temperature field. Conventional 3D-CFD software, however, present several limitations in simulating PCJ, due to complexity of the phenomena involved and the necessity of a moving grid.

In the last years innovative particle-based software were developed which introduced significant advantages in terms of simulation time and computational requirements for applications like PCJ.

In this context, the present thesis work, conducted at POWERTECH Engineering S.r.l., aimed at simulating piston cooling jets in a high-performance engine using the 3D-CFD particle-based simulation software Particleworks.

The first part of this work was focused on the application of Particleworks to representative case studies, such as fluid flow within a pipe and liquid jets impinging on flat surfaces. Results of the impinging jet case showed good agreement with experimental literature data and allowed the definition of a model setup which was used in the following PCJ study.

The second part of the thesis was focused on the simulation of piston cooling jets for a new high-performance engine. Using Particleworks, it was possible to evaluate the Heat Transfer Coefficient (HTC) map in the region of the piston where the oil impinges. The resulting HTC map was then used in a standalone piston simulation, performed in Star-CCM+, in which the temperature distribution of the piston was calculated. Piston temperature results were compared with available literature data for similar high-performance engines, showing comparable results especially for the maximum piston temperature.

Additionally, a particle-size sensitivity study was also performed, and results showed that particles of 0.15 mm diameter gave the best compromise between accuracy and computational costs. Lastly, the necessity to account for all the engine components which connect the piston to the cranktrain (e.g. piston pin, connecting rod and crankshaft) was also explored. Simulations accounting for all the engine components showed significant differences in average HTC values along the engine cycle, compared to simulations without engine crankshaft and connecting rod, with variations in the maximum piston temperature of about 8°C for the PCJ system under study.