A Cold-Start Emissions Model of an Engine and Aftertreatment System for Optimisation Studies

The minimisation of tail-pipe emissions and fuel consumption during cold-start can be viewed as a constrained optimisation problem involving many parameters. Examining this problem mathematically first requires an accurate and computationally practical model of the engine and exhaust system. This paper proposes such a model for use during the cold-start of a conventional spark ignition engine. This model uses as much physics-based modelling as is computationally practical for optimisation and control studies. It takes a given set of engine control inputs to simulate tailpipe CO, HC and NO emissions, and is both calibrated and validated using detailed measurements obtained on a transient engine dynamometer following the New European Drive Cycle (NEDC). INTRODUCTION It is well known that passenger vehicles with spark ignition engines release most of their unburned hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOX) emissions during the first minutes of driving, while the three-way catalyst (TWC) is below its so-called “light-off temperature”. As such, the introduction of catalytic converters was quickly followed by intensive efforts aimed at optimising their performance during this ‘cold-start’ period. These efforts have typically been heavily reliant on the knowledge and judgement of very experienced engine calibrators, designers and catalyst manufacturers. For example, common engine calibration approaches include controlling the engine idle speed and ignition timing to reduce the catalyst light-off time. Increasing the idle speed leads to higher (total) enthalpy exhaust gas reaching the catalyst, thus heating the catalyst more quickly. Similarly, retarding the spark can lead to reduced engine efficiency, raising the exhaust gas temperature and thus affecting the catalyst light-off. Positioning of the catalyst closer to the engine [1; 2], thus increasing its inlet gas temperature, is now commonplace. Many other, more complex approaches have also been studied. For example, Engler et al. [3]; Henein and Tagomori [2] use traps designed to adsorb hydrocarbons at cool temperatures and release them after light-off. Lafyatis et al. [4] implemented a secondary air injection system, taking advantage of the highly exothermic carbon monoxide oxidation reaction. Ma et al. [5]; Kanada et al. [6] studied ways of burning pre-catalyst exhaust gas, whilst Oh et al. [7]; Kirchner and Eigenberger [8] examined the use of electrically heated catalysts. However, all of these strategies have their shortcomings. Raising the idle speed or retarding the ignition timing will increase the fuel consumption. Close coupling of the catalyst requires greater catalyst robustness. The other approaches discussed above also clearly have additional costs. The minimisation of tail-pipe emissions and fuel consumption during cold-start can therefore be viewed as a constrained optimisation problem involving many parameters. A Cold-Start Emissions Model of an Engine and Aftertreatment System for Optimisation Studies 2010-01-1274 Published 04/12/2010 D. I. Andrianov, F. Keynejad and R. J. Dingli The University of Melbourne G. Voice Ford Motor Company of Australia M. J. Brear and C. Manzie The University of Melbourne Copyright © 2010 SAE International Downloaded from SAE International by University of Melbourne, Tuesday, January 07, 2014 06:17:36 PM