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Towards the effort of using natural gas as an alternative fuel for a diesel engine, the concept of Diesel Dual Fuel (DDF) engine has been shown as a strong candidate. Typically, DDF's engine-out emission species such as soot and nitrogen oxides are decreased while carbon monoxide and hydrocarbons are increased. The aftertreatment system is required in order to reduce these pollutant emissions from DDF engines. Additionally, DDF engine exhaust has a wide temperature span and is rich in oxygen, which makes HC emissions, especially methane (CH₄), difficult to treat. Until now, it is widely accepted that the key parameter influencing methane oxidation in a catalytic converter is high exhaust temperature. However, a comprehensive understanding of what variables in real DDF engine exhausts most influencing a catalytic converter performance are yet to be explored.

Accurate and useful mathematical models of physical processes can be made when we understand all of the phenomena involved. This paper reviews our understanding of the fluid flow, heat transfer and thermodynamic processes occurring in engines and the status of mathematical models expressing this understanding. Thermodynamic single system rate models are found to be extremely useful in predicting power and fuel consumption performance but of limited value in predicting emission performance. Multiple-zone, nonequilibrium models are essential for predicting emissions but are limited in accuracy by computer capacity and our understanding of engine phenomena which vary rapidly both with space and time. The need for and ability of new types of instrumentation, primarily optical, to increase our understanding of engine phenomena and improve our models is discussed.