International Journal of Engine Research

A methodology for internal combustion engine design has been formulated which incorporates multidimensional modelling and experiments to optimize and simulate direct injection diesel engine combustion and emissions formation. The computer code KIVA-GA performs full-cycle engine simulations within the framework of a genetic algorithm (GA) global optimization code. The methodology is applied to optimize a heavy-duty diesel truck engine. The study simultaneously investigated the effects of six engine input parameters on emissions and performance for a high-speed medium-load operating point. The start of injection (SOI), injection pressure, amount of exhaust gas recirculation (EGR), boost pressure and split injection rate shape were optimized. The convergence of the GA optimization process is demonstrated and the results were compared to those of the experimental optimization study employing a response surface method (RSM), which uses statistically designed experiments to determine an optimum design. In addition, the parameters of the computationally predicted optimum were run experimentally and good agreement was obtained.

The potential for ultra-low emissions levels was assessed through additional computational GA runs that included higher maximum EGR levels (up to 50 per cent). The predicted optimum results in significantly lower soot and NO_x emissions together with improved fuel consumption compared to the baseline design. The present results indicate that an efficient design methodology has been developed for optimization of internal combustion engines, one that allows simultaneous optimization of a large number of parameters.

An overview over flamelet modelling for turbulent non-premixed combustion is given. A short review of previous contributions to simulations of direct injection (DI) diesel engine combustion using the representative interactive flamelet concept is presented. A surrogate fuel consisting of 70 per cent (liquid volume) n-decane and 30 per cent α-methyl-naphthalene is experimentally compared to real diesel fuel. The resemblance of their physical and chemical properties is shown to result in very similar combustion and pollutant formation for both fuels.

In order to account for variations of the scalar dissipation rate within the computational domain, a method using multiple flamelets, called the Eulerian particle flamelet model, is used. A strategy is described for subdividing the computational domain and assigning the resulting subdomains to different flamelet histories represented by Eulerian marker particles. Experiments conducted with an Audi DI diesel engine and diesel fuel are compared to simulations using the surrogate fuel. The use of multiple flamelets, each having a different history, significantly improves the description of the ignition phase, leading to a better prediction of pressure, heat release and exhaust emissions of soot and NO_x. The effect of the number of flamelet particles on the predictions is discussed.

Ignition control is an important issue in homogeneous charge compression ignition (HCCI) engines, which have the advantages of low NO_x emission and high thermal efficiency. In this study, the effect of the exhaust gas recirculation (EGR) on the ignition control of HCCI engines is discussed using an engine cycle simulation in which a homogeneous mixture is assumed. Auto-ignition of 65 per cent iso-octane + 25 per cent toluene + 10 per cent n-heptane, which is used as a fuel to evaluate the characteristics of a gasoline-like fuel, is represented by a detailed reaction model.

The dilution by EGR delays the ignition timing when the charged gas temperature is not changed by EGR. The temperature rise of the charged gas promotes auto-ignition. Based on these characteristics, it was suggested that the ignition timing could be controlled by EGR with temperature control, when the amount of fuel supply is constant.

This control method can also be applied to control of the air-fuel ratio (A/F) in the cylinder while maintaining the optimum ignition timing. In spite of the difference in the A/F and the EGR ratios, no significant difference was found in the pressure rise rate at combustion and the NO_x emission when the ignition timing was the same.

The homogeneous charge compression ignition (HCCI) combustion has been attracting growing attention in recent years due to its potential for simultaneous improvement of exhaust gas emissions and fuel consumption in diesel engines. For practical application of HCCI to internal combustion (IC) engines, precise control of auto-ignition of pre-mixtures during the compression stroke is inevitable. This paper discusses the auto-ignition processes in an HCCI engine operated with n-heptane/air mixtures using a zero-dimensional combustion model including a detailed kinetics. The model proposed is validated first by a comparison between calculated and experimental pressure diagrams, and then the effects of initial charge conditions, compression ratio and excess air ratio on ignition and combustion are investigated. It was found from the parametric study that HCCI combustion of n-heptane/air mixtures is classified into three types of combustion: complete combustion, only low-temperature reaction and misfire, depending on the compression ratio and excess air ratio at which the engine is operated. Finally, the major paths of the HCCI reaction occurring in the engine cylinder were clarified by a sensitivity analysis of chemical reactions involved in the HCCI reaction scheme.