The automotive engine exhaust emission norms in India have become more stringent and to comply with it, even light-duty diesel engines need to deploy a suitable combination of emission control devices. Arriving at an optimum size and layout of emission control devices for a particular engine configuration through experiments is both time and cost intensive. Hence, robust yet simple simulation models tuned and validated using experimental data prove to be useful in developing and evaluating such systems. Towards this objective, one dimensional exhaust after treatment device (EATD) models were developed in the present work using a commercial code. The data required to tune and validate the EATD models were generated based on experiments done in a light duty diesel engine fitted with a diesel oxidation catalyst (DOC) coated Diesel Particulate Filter (cDPF) and Lean NOx Trap (LNT). Ten representative operating conditions including low load low speed, low load high speed, maximum load high speed, maximum load low speed and maximum torque conditions were chosen for the experiments. The model constants including Arrhenius rate parameters, friction loss and heat transfer coefficients were tuned with 50% of data set and the remaining data set was used for validating the models. The model could predict the pressure drop in the DOC, LNT and cDPF with a maximum error of 1.2 mbar which is well below the actual pressure drop of 250 mbar. The conversion efficiency of the DOC and the storage efficiency of the LNT could be predicted with a maximum error of 1.5% and 3.7% respectively. The validated models were then used to arrive at optimal positioning of EATDs in the exhaust system for achieving maximum conversion efficiency. Subsequently, parametric investigations were done to examine the effect of length and diameter of EATDs on pressure drop, cost and conversion efficiency. Based on a concept called total desirability index, the optimum length and diameter were fixed that resulted in minimum pressure drop and maximum conversion efficiency without any appreciable increase in the cost.