The transport sector is making a significant contribution to the global CO2 emissions causing Global Warming. A large portion of this is caused by heavy-duty vehicles like tractor semi-trailer combinations. Since tractor semi-trailer combinations are often operated at relatively high speeds for long periods of time their fuel consumption, and with that their emissions, can be strongly reduced by reducing the aerodynamic drag. This can be done by improving the aerodynamics of individual vehicles, by carefully rounding their leading edges or application of drag reduction devices, like boat tails and side skirts. Another option is to use the benefits of drafting by operating two or more vehicles closely together in a platoon. The benefits of this have already been proven in multiple studies and real world experiments. When platooning will be implemented on a large scale, it will be beneficial to optimise the platoons for maximum drag reduction. To be able to do this, the effect of different truck design parameters have to be investigated. This study focuses on the influence of underhood flow on the drag of a tractor semi-trailer in isolation and in a platoon. This is done by using simplified models adapted to have a underhood model consisting of one porous medium and four ducts to replicate the mass flow, pressure drop and flow field of a real underhood area. Simulations were performed on full-size and highway speeds using the commercially available PowerFLOW solver, based on the Lattice Boltzmann Method. For an isolated vehicle it was found that an increased underhood mass flow gives a higher total drag, mainly because of the drag contribution of the porous medium. Due to the mass flow entering the underhood, the suction over the leading edges is slightly reduced. On the other hand, smaller leading edge radii with higher suction give less mass flow through the underhood. Besides this the underhood flow actually has beneficial effects on the parts surrounding the tractor-trailer gap and in the trailer underbody area. The highest total drag was found for the models with the most underhood flow and the smallest leading edge radius. Platoons of two vehicles were tested with three different inter vehicle distances, 3.75, 7.5 and 15 meters. The leading vehicle, which did not have underhood flow in all cases, has the strongest drag reduction for the shortest distance. The trailing vehicle has the lowest drag at the largest tested distance, while it is highest for the middle distance. This can be explained by the reduction in pressure in front of the vehicle. This reduces the drag contribution of the front surface, but also reduces the suction over the leading edges. The models with underhood flow experienced a stronger drag reduction, meaning that the absolute drag values were closer than for the isolated vehicle. This is caused by the reduced underhood mass flow in a platoon. At the shortest inter vehicle distance only 35% of the mass flow of an isolated vehicle is available, while this is 50 and 70% when the distance is increased. The beneficial effects of underhood flow on an isolated vehicle are still present in a platoon, although they are reduced in strength. When a boat tail is mounted on the back of the trailer of the leading vehicle the drag of this vehicle is strongly reduced due to the increased back pressure. However, this might not be beneficial for the trailing vehicle. The increased stagnation pressure indeed increases the total drag of the trailing vehicle at an inter vehicle distance of 3.75 m. As discussed before, an increased stagnation pressure also gives increased leading edge suction. Therefore the drag is actually reduced for the two larger inter vehicle distances. It was also found that the tail gives higher flow speeds over the top of the trailing vehicle, and lower flow speeds around the bottom. This reduces the drag for the underbody parts like the wheels and slightly decreases the underhood mass flow. When the platoon is placed at a yaw angle, the total drag of the leading vehicle is increased. The contribution of the front part is decreased, but this is more than compensated for by the drag increase for the rear and all other parts, which are no longer perfectly aligned with the flow. The drag increase of the trailing vehicle is stronger, due to an increased contribution of the front part. The underhood mass flow is also increased compared to the platoons without yaw, this effect is strongest at small inter vehicle distances. The leading vehicle causes the flow to be more aligned for the trailing vehicle, therefore the drag increase for most other parts is less strong. The same effect can be seen for the side force, which is way lower for the trailing vehicle and increases for increasing inter vehicle distance.