Design of an under-bonnet heat exchanger for the improvement of energy efficiency

The present work is aimed at reporting the methodology used to develop an innovative type of heat exchanger to exploit the free space under the upper skin of a car bonnet between the stiffener ribs. The idea is to take advantage of the large surface of the bonnet itself as a radiating area to increase the heat exchange capacity of the cooling system to save energy and improve the overall vehicle efficiency. The heat exchanger can be built with a thin-shell under-bonnet properly shaped that, fixed below the lower surface of the bonnet skin, realizes a closed chamber where the cooling fluid can pass and exchange heat. With the aim of weight reduction, the thin-shell under-bonnet will be made of plastic material and the technology to fix it and create the closed chamber will be, necessarily, structural bonding. To this aim, it has been necessary to develop a methodology to characterize the metal-to-plastic adhesive joint to design and verify the proposed solution. In the work, the experimental method consisting of a C-shaped metal half-specimen (made of steel, and obtained from a sample of a vehicle bonnet) on a plastic plate of the material used for the thin-shell under-bonnet will be illustrated. The adhesion properties of the joint have been then obtained by means of the inverse method with a parametric numerical model reproducing in detail the experimental test in order to identify the parameters of the material model used to describe the adhesive behavior. The necessity of this type of test depends on the type of applied load, mainly direct pull-out, and the type of joined parts and materials. Once the adhesive model parameters obtained, they have been used to virtually study a prototype of the heat exchanger to obtain a suitable solution in terms of thermo-mechanical strength and energetic efficiency.