Vehicle exhaust pipe is an excellent heat source and its integration with thermoelectric generators for thermo-electrical conversion has been recently attracting much attention. The methods of increasing power output include the parameter optimizations of the thermoelectric module material, thermoelectric module geometrical shape and exhaust gas property. Because of the coupling nature of these parameters, however, there is a lack of lateral comparison providing with the idea of which parameter gives the largest effect and which parameter combination gives the best power output performance. For this purpose, a novel hexagonal shaped housing has been developed for an engine exhaust thermoelectric generator unit to contain thermoelectric modules with higher temperature difference. The concept of hexagonal shaped housing provides a larger contact area which will increase the temperature differences applied on the hot and cold sides of the thermoelectric module. The thermoelectric generator unit performance has also been improved by the concept of the cross flow which will enlarge the high temperature difference area. Built with these two new concepts, an innovative engine exhaust thermoelectric generator unit has been designed, fabricated and tested. The performance optimization of the thermoelectric generator unit has been conducted based on the validated simulation model. The identified key parameters include the number of thermocouples, geometric shape/curvature of the thermoelectric module, thickness of the thermocouples, thermoelectric module material, number of layers of thermoelectric modules, exhaust gas temperature and the thickness of the ceramic substrate. It suggests that four parameters need to be focused for optimization, which are the number of thermocouples, exhaust gas temperature, thermoelectric material and geometric shape/curvature. In order to reduce the complication brought by modifying other parameters, further, Optimized Systems 1, 2 and 3 with different optimized parameters are compared. It is found that a good trade-off between achieving high power output and avoiding unnecessary technical complexities and related cost increase can be reached for Optimized System 3 with 152 thermocouples and 480 °C exhaust gas temperature. Display omitted •The concepts of hexagonal shape and cross flow are applied in the TE generator.•The prototype generates up to 23 V on a vehicle engine with series connection.•Lateral comparison includes the TE material property, TE geometry and temperature.•Optimized systems with the easy implementation are proposed and compared.