An early-design methodology for predicting both expected fuel economy and catalyst-out CO, HC and NOx concentrations during arbitrarily-defined transient cycles is presented. The methodology is based on utilizing a vehicle-powertrain model with embedded maps of fully warmed up engine-out performance and emissions, and appropriate temperature-dependent correction factors to account for not fully warmed up conditions during transients. Similarly, engine-out emissions are converted to catalyst-out emissions using conversion efficiencies based on the catalyst brick temperature. A crucial element of the methodology is hence the ability to predict heat flows and component temperatures in the engine and the exhaust system during transients, consistent with the data available during concept definition and early design phases. This is accomplished by deriving an equivalent resistor-capacitor (R-C) network of heat flows reduced from a comprehensive R-C model of the various component and fluid thermal interactions throughout the engine and exhaust system . The methodology has been validated for an FTP urban driving schedule with known fueling rates and engine speeds for a 4-cylinder engine. Predictions have been found in very good overall agreement with transient measurements of exhaust gas, cooling fluid and catalyst brick temperatures, as well as recorded FTP cycle emissions and fuel economy. The application of the methodology to an intermittent engine duty cycle, as might be used in a hybrid vehicle, is also illustrated.