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This paper describes a systematic, physics-based approach in deriving engine performance requirements for a Diesel cold-start catalyst warmup strategy that would satisfy low NOx regulations. These requirements are valid for both conventional and hybrid vehicles. The requirements are driven by an understanding of catalyst and engine behavior. The paper defines a metric that can be used to design and evaluate the performance of technologies, controls and calibration strategies. Optimization strategies based on this metric are proposed. Some examples of the optimization are highlighted.