Thermoacoustic engine has been proven to be a promising technology for automotive exhaust waste heat recovery to save fossil fuel and reduce emission thanks to its ability to convert heat into acoustic energy which, hence, can be harvested in useful electrical energy. In this paper, based on the practical thermodynamic parameters of the automotive exhaust gas, including mass flow rate and temperature, two traveling-wave thermoacoustic engines are designed and optimized for the typical heavy-duty and light-duty vehicles, respectively, to extract and reutilize their exhaust waste heat. Firstly, nonlinear thermoacoustic models for each component of a thermoacoustic engine are established in the frequency domain, by which any potential steady operating point of the engine is available. Then, a matching between the required heat for the engine and the available heat from the exhaust gas is performed to determine the practical operating point and the performance of the thermoacoustic engine driven by this exhaust gas with a specified thermodynamic state. Furthermore, an outline-size optimization of the thermoacoustic engines is carried out for the heavy-duty and light-duty applications, respectively, to improve the waste heat utilization efficiency and maximize the acoustic power. To consider the impact of the system weight on vehicles, a power-to-weight ratio is adopted in this work as the optimization objective. It's shown that the optimum system outside diameters for the heavy- and light-duty applications are different due to the different thermodynamic characteristics of the exhaust gas. The available acoustic powers with the maximum power-to-weight ratio for heavy- and light-duty applications are approximately 515 Watts and 935 Watts with 8.2% and 18% of thermal efficiency, respectively, corresponding to 29.3% and 42.9% of their respective Carnot efficiencies. The resulted CO2 emission reductions are approximately 0.37 kg/h and 0.77 kg/h, respectively.