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How to achieve precise clean engine design

16 Nov 2012  | Bernie Rosenthal

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Nowadays, engine designs increasingly depend on the ability to exploit fuel behaviour and control combustion to improve performance and emissions. Engines have to comply with tighter emissions regulations, including soot particle size, changing fuel combinations and global differences in the make-up of fuels as well as future fuels. The ability to accurately predict what happens when fuel is ignited in a combustion chamber still puzzles engine designers. Reaction Design's software is touted to enable car designers and manufacturers to automate the analysis of chemical processes via computer simulation and modelling solutions.

Accurate, fast chemistry simulation is required for predictive to predict ignition calculations, predictive engine knock simulations and predictive emissions calculation sin engines. The RD software package robustly and accurately simulates engine-cylinder combustion performance for reciprocating engines, with virtually any fuel, helping engineers rapidly design clean, high-efficiency, fuel-flexible engines. Engine simulation applications include spark ignition gasoline, diesel, and advanced designs such as Homogeneous Charge Compression Ignition (HCCI), PCI and PPCI, multiple fuel injections and dual-fuel engines.

The process by which fuel ignites and burns of combustion can be modelled effectively using a detailed chemical mechanism of the fuel describing the thousands of short-lived species and chemical reactions that dictate how a fuel ignites, how the flame propagates, and how emissions like NOx, CO, and soot are formed. Accurately modelling real fuel behaviour requires more chemistry than traditional Computational Fluid Dynamics (CFD) approaches can handle with acceptable time-to-solution. As most commercial CFD improvements directed towards better accuracy have focused on enhancing meshing and turbulence modelling, there has been little effort directed towards improving the fundamental chemistry calculations, to reflect the key engine behaviours that are now beginning to dominate the design space. Given that chemistry calculation times in CFD can account for 90% of the total simulation time even when employing severely reduced mechanisms, there is substantial opportunity for decreasing time-to-solution by accelerating these calculations.

Figure 1: Visualisation of the combustion process in a diesel engine.

FORTÉ is a comprehensive CFD package that allows the engine designer to go directly from CAD drawings to simulation results in far less time than other CFD software, while taking advantage of real fuel chemistry models for better results without the need for expert calibration. Accurate modelling of realistic fuel effects is viable with superior time-to-solution metrics that fit in commercial-development timeframes. FORTÉ employs a novel solver approach that takes advantage of the chemical similarity of groups of cells and implements a parallel processing algorithm to dramatically reduce the chemistry calculation time. This technique can reduce simulation run times by almost two orders of magnitude. Chemistry models that previously were thought of as only practical for 0-D simulations are now practical for full 3-D engine simulations complete with moving pistons and valves.

The Model Fuels Consortium (MFC), an industry-led program, has developed the detailed chemical mechanisms needed to simulate real fuel behaviour. Most chemistry models in use today are slow and lack sufficient detail for accurate and predictive results. While it is widely accepted that the use of more detailed chemical mechanisms is required for accurate prediction of important characteristics of the combustion process such as ignition, flame propagation and emissions production, many designers don't believe they have the option of incorporating these mechanisms into their simulations without significant increases in compute time and solution instability. As a result, designers cannot rely on their combustion CFD to predict values or even accurate trends in these critical combustion behaviours.

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