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A unique engine, based on the regenerative principle, is being developed with the goal of achieving high brake efficiency over a wide power range. It can be characterized as an internal combustion Stirling engine (ICSE). The engine is a split-cycle configuration with a regenerator between the intake/compression cylinder and the power/exhaust cylinder. The regenerator acts as a counter-flow heat exchanger. During exhaust, the hot gases are cooled by the regenerator. The regenerator stores this heat. On the next cycle, compressed gases flow in the opposite direction and are heated by the regenerator. The gases coming from the regenerator into the power cylinder are very hot (~900°C), which provides the necessary gas temperature for auto-ignition of diesel and other fuels.

The substitution of lightweight materials, such as aluminum or magnesium alloys, to produce lightweight car bodies, has been the subject of intensive research in resent years. It has been established that an aluminum body is lighter than a steel body for constant stiffness. The causes of this weight reduction have not been established. In particular, since the specific modulus (modulus of elasticity/density) of steel, aluminum and magnesium are nearly identical, there is no easy answer for their ability to reduce weight. In this paper, it is shown that there are stress concentrations in thin walled structures, which are dependent on the thickness of the material. These stress concentrations appear in joints and other parts with complex geometry and loading conditions. For example, the flanges on a curved beam in flexure have an effective (load bearing) width, which increases as the material is thickened.

The concept of a structural index, λ, is developed using the simple example of a hollow beam with a cantilever load case. It is assumed that when performing material substitution, only the thickness, t, is changed. It is shown that the stiffness, K, of the beam can be defined as a function of tλ, that 1 ≤ λ ≤ 3, and that λ can be used to predict the weight savings from material substitution where stiffness is held constant. It is then demonstrated that λ can be used to predict the weight savings from material substitution in the more complex cases of the joints of a light truck cab.

This paper reports on the efforts of the initial phase of the IMPACT program to define the underlying structural theory behind selecting the proper material(s) to reduce weight in the most efficient, cost-effective manner. Following this initial phase, the IMPACT program will proceed to design and build, optimized, proprietary, full vehicle platform prototypes that achieve up to a 25 percent weight reduction total without compromising any customer-driven vehicle attributes. Most importantly, the materials and technologies selected must be implementation ready for high volume, low cost, dual-use applications. The purpose of the initial phase and an in-depth discussion on which material properties should most influence material selection are presented.