CFD optimization for GDI spray model tuning and enhancement of engine performance

M. Costa, U. Sorge, L. Allocca (CNR – Istituto Motori)

The worldwide concern about the environmental impact of energy conversion systems has, as a relapse in the automotive field, the imposition of strict government regulations relevant to pollutant emissions and fuel-efficiency standards of vehicles. Nowadays, the preferred route towards the reduction of both engine exhaust noxious emissions and fuel consumption remains the control of the mixture formation and combustion processes taking place within the engine combustion chamber, a complicated task, affected by many variables. The high cost and time needed to achieve optimization through bench testing alone has drawn interest of engine developers towards the use of Computational Fluid Dynamics (CFD) analyses. 

Coupling a 3D Computational Fluid Dynamics (CFD) tool with a rigorous method of decision making is becoming indispensable in the design process of complex systems, as internal combustion engines. CFD based optimization (CFD-O) is here carried out on a single cylinder, four-valve, four-stroke gasoline direct injection (GDI) engine, to enhance mixture formation under stratified charge operation, hence to choose between the single or double injection strategy maximizing the engine power output. The optimization problem intended to the reduction of the fuel consumption of a GDI engine is aimed at the choice of the injection strategy most proper to realize an effective energy conversion process under a lean-mixture, moderate-load, moderate-speed case. A 3D engine model is coupled with the Simplex algorithm to find the optimal synchronization of both injection and spark timing within the working cycle. CFD-O is also addressed to perform the validation of the gasoline spray model, that otherwise reveals tedious and time-consuming. The Simplex algorithm is used to tune the constants entering a model developed by authors, as applied to three different high pressure GDI injectors, preliminary experimentally characterized.

Two fully automatic procedures are developed, suitable to be used in the phase of design of innovative internal combustion engines. The first one is finalized to the validation of a 3D GDI spray model. In particular, it is applied to three different high pressure multi-hole injectors, preliminary experimentally characterized at both the mass flow rate test bench and in an optically accessible vessel. The automatic tuning of the entering constants serves to prove the predictive capability of a model proposed by authors. The second automatic procedure consists in a CFD optimization of the mixture formation process in a GDI engine, finalized to the minimization of the fuel consumption.