Piston engine

New engine strategies like direct injection with stratified combustion, high dilution by exhaust gas recirculation (EGR), and engine downsizing are employed to reduce pollutant emissions and automotive engine consumption. However, the use of these strategies has several undesirable consequences such as their large propensity for generating cycle-to-cycle variations. In extreme cases, if the level of these cyclic variations is too large, the drivability is impacted and pollutant emissions increase drastically. Therefore, the prediction of these cyclic variations becomes a crucial issue in the context of the future European environmental regulations.
Determining the sources of these variations is a complex task because of the numerous physical phenomena (acoustic, chemistry, turbulence...) involved. Complementary to experimental studies, 3D CFD simulations are a powerful tool that allows to identify and isolate these different causes. Although RANS approaches are still the most commonly used technique for piston engine computations, the Large Eddy Simulation (LES) technique seems particularly well-tailored to this kind of highly unsteady flows.

MULTI-CYCLE PISTON ENGINE SIMULATIONS

A multi-cycle simulation consists in computing several consecutive complete engine cycles. The objective is to check if LES is able to capture cycle-to-cycle combustion variations observed experimentally and to investigate the root causes of this variability. Initially unthinkable, the fast development of massively parallel machines allows nowadays to compute several tens of cycles. All computations shown below are performed with the AVBP code.

XU10 engine (LESSCO2 project):

The computation performed in this project is one of the first multi-cycle LES

Fig. 1 Velocity magnitude field (left) and equivalence ratio field (right) during the intake stroke (XU10 engine).

S. Richard, O. Colin, O. Vermorel, A. Benkenida, C. Angelberger, D. Veynante. Towards large eddy simulation of combustion in spark ignition engine. Proc. Combust. Inst. 31(2) (2007) 3059-3066.

O. Vermorel, S. Richard, O. Colin, C. Angelberger, A. Benkenida. Multi-cycle LES simulations of flow and combustion in a PFI SI 4-valve production engine. SAE Paper 2007-01-0151, 2007.

O. Vermorel, S. Richard, O. Colin, C. Angelberger, A. Benkenida, Denis Veynante. Towards the understanding of cyclic variability in a spark ignited engine using multi-cycle LES. Combust. Flame, 156 (2009) 1525-1541.

F7P engine (SGEmac project):

The french SGEmac research project aims at acquiring detailed experimental data on stable and unstable operating points for a single-cylinder SI engine and to exploit them for simulation methodologies based on LES. During the definition and acquisition of the experimental database, particular care was given to the cycle-resolved characterisation of the unsteady flow in the whole engine system, by using a specifically designed and well controlled set-up. On the one hand, cycle-resolved data are acquired for an accurate definition of the boundary conditions for LES. On the other hand, flow motion and combustion characterisations are provided by means of optical diagnostics, in order to validate the predictions of LES concerning local flow features in the intake pipe and the combustion chamber.

The LES approach adopted in this work consists in computing the whole system, from the intake plenum to the exhaust plenum, including intake pipes, cylinder, exhaust pipes and flame arresters. Therefore, acoustic effects that have been deliberately neglected in the XU10 configuration can now be taken into account.

Fig. 2 Computational domain for the F7P multi-cycle simulation.

Fig. 3 Tetraedral mesh for the F7P multi-cycle simulation.

B. Enaux, O. Vermorel, L. Thobois, T. Poinsot. Multi-cycle LES of a spark-ignited engine with full intake and exhaust systems: methodology and modelling issues. In LES for Internal Combustion Engine Flows, Rueil-Malmaison, France, December 2008.

C. Lacour, C. Pera, B. Enaux, O. Vermorel, C. Angelberger, T. Poinsot. Exploring cyclic variability in a spark-ignition engine using experimental techniques, system simulation and Large-Eddy simulation. In 4th European Combustion Meeting, Vienne, Austria, April 2009.

B. Enaux, V. Granet, O. Vermorel, C. Lacour, C. Pera, C. Angelberger, T. Poinsot. LES and experimental study of cycle-to-cycle variations in a spark ignition engine. Proc. Combust. Inst. (2010) doi:10.1016/j.proci.2010.07.038.

B. Enaux, V. Granet, O. Vermorel, C. Lacour, L. Thobois, V. Dugue, T. Poinsot. Large Eddy Simulation of a motored single-cylinder piston engine: numerical strategies and validation. Flow. Turb. Combust. (2010) doi:10.1007/s10494-010-9299-7.

STEADY FLOW RIGS AT FIXED VALVE LIFT

A typical way to study the aerodynamics of internal combustion engines is to transform then in a steady-state flow bench problem: piston and exhaust valves are put out, the airflow passes through the intake port(s) and enters into the cylinder which is directly linked to the outlet. This kind of configuration allows in particular to measure the discharge coefficient and the swirl number of a given geometry which are crucial parameters for the design of intake ports.

Fig. 4 Velocity field and iso-surface of Q criterion.

L. Thobois, G. Rymer, T. Souleres. Large-eddy simulation for the prediction of aerodynamics in IC engines, SAE Paper 2004-01-1854, 2004.

L. Thobois, G. Rymer, T. Souleres, T. Poinsot, B. Van den Heuvel. Large-Eddy Simulation in IC engine geometries, Int. J. Vehicle Des. 39 (4) (2005) 368-382.

Contacts: Olivier Vermorel (vermorel@cerfacs.fr)