Demonstrating the Fuel Economy Benefit of Exhaust Energy Recovery

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The internal combustion (IC) engine remains the most cost effective device for converting liquid fuels to useful work. Even as bio-fuels become more popular, it is the IC engine that is the practical device for realising their benefits. The IC engine works by ensuring a good flow of fresh air into the engine to support the combustion process. The process of supplying air requires that the products of combustion in the form of exhaust gas are removed quickly creating a hot exhaust gas stream.It is this hot exhaust stream that offers the potential for generating additional useful energy. Generating energy from hot exhaust gas can be done in several ways and attempts have been made with steam cycles and with additional expansion through a turbine. Most methods tend to significantly increase the mechanical complexity of the engine and with it the cost.Thermo-electric (TE) devices use the so called Seebeck effect where using dissimilar metals a potential difference can be created between hot and cold objects. In an engine that temperature difference will be created between the exhaust gases and the external air temperature. This is a large temperature difference and offers the potential for efficient energy conversion. Thermodynamic theory suggests that with a 50kW passenger car engine, there is the potential to regenerate energy in the range 9-12 kW. With the best of modern thermo-electric materials only 0.5-1 kW could be achieved, but this is already enough to consider, for example, replacing the vehicle alternator with a such a thermo-electric device. A thermo-electric device is solid state, with no moving parts and is likely to be more durable than the other methods that have been considered so far.The primary challenge for the successful application of TE methods is the quality of materials. At present, bulk materials deliver a low efficiency. Newer materials offer a great deal of potential, but it is unclear how much extra performance is needed from materials before there is a practical proposition. The primary aim of this project is to demonstrate the best thermo-electric performance using the class of materials known as Skutterudites which are showing great promise in this application. Properly understood and assembled into modules, these materials can produce TE performance competitive with a vehicle alternator. The modules will be tested on the bench then computer based models representing this performance will be used in real time alongside a practical engine to predict the fuel economy of the whole engine system. The model will be adjusted to include hypothetical material properties. The investigation will be directed to identify the set of material properties that will give a strong system performance. The proposed work will use a technique known as component-in-the-loop, signifying that a real engine is in use in an engine test laboratory. At the same time the TE device is represented as a model which is run on a fast computer at the same rate as the physical behaviour of a real device. Its output will be fed back to the engine system to represent the electrical current produced. Component in the loop is an emerging technique and we are proposing this novel application as a secondary research goal.With the two sets of results: a set of proposed material properties and a viable research methodology, this project will set the scene for a detailed investigation into materials whose result will be a device capable of practical application.