International audienceThe exhaust gas of heavy duty diesel engines can provide an important heat source that may be used in a number of ways to provide additional power and improve overall engine efficiency. The sizing of a heat exchanger that can manage the heat load and still be of reasonable size and weight without excessive pressure drop is of significant importance especially for truck applications. This is the subject of the present work. To approach the problem, a total of five different configurations are investigated and a comparison of conventional and state of the art heat transfer enhancement technologies is included. Two groups of configurations are examined: a) a classical shell and tube heat exchanger using staggered cross flow tube bundles with smooth circular tubes, finned tubes and tubes with dimpled surfaces, and b) a cross flow plate heat exchanger, initially with finned surfaces on the exhaust gas side and then with 10 ppi and 40 ppi metal foam material substituting for the fins. Calculations were performed, using established heat exchanger design methodologies and recently published data from the literature to size the aforementioned configurations. The solutions provided reduce the overall heat exchanger size, with the plate and fin type consisting of plain fins presenting the minimum pressure drop (up to 98% reduction compared to the other configurations), and the 40 ppi metal foam being the most compact in terms of size and weight. Durability of the solutions is another issue which will be examined in a future investigation. However, coupling of the exhaust heat exchanger after a particulate trap appears to be the most promising solution to avoid clogging from soot accumulation

Diesel engine manufacturers are currently intensifying their efforts to meet future emission limits that require a drastic reduction of NOx and particulate matter compared to present values. Even though several after-treatment techniques have been developed for tailpipe NOx reduction in heavy duty diesel engines, the in-cylinder control of NOx formation still remains of utmost importance. Various methods have been used to control NOx formation in diesel engines such as retarded injection timing and EGR providing each one of them very promising results. However, use of these techniques is accompanied by penalties in specific fuel consumption and exhaust soot. A promising technology for NOx reduction especially for heavy-duty diesel engines and mainly large scale ones is the addition of water to the combustion chamber to reduce peak combustion temperature that obviously affects NOx formation. This method has been applied mainly for marine and stationary diesel engines due to the requirement of minor modifications on engine infrastructure. However, up to now, its true potential for diesel engines remains uncertain due to the limited theoretical and experimental knowledge. For this reason, it is examined herein the possibility to use this method to reduce NOx emissions in direct injection heavy-duty diesel engines. Two different technologies are examined for the addition of water into the combustion chamber: use of a water-fuel emulsion or injection of water into the intake manifold. Various percentages of injected water are examined for both technologies. This is attained using a multi-zone simulation model appropriately modified to simulate the use of water/fuel emulsion or injection of water into the intake manifold. The analysis provides information concerning the actual effect of water on the combustion and pollution formation mechanisms. It is revealed a significant reduction of NOx with both techniques compared to conventional engine operation. The reduction is higher when using water-fuel emulsion compared to water injection in the intake manifold for the same water percentage. Information is derived concerning the penalty on fuel consumption, which is a dominating parameter especially in large diesel engines. Furthermore it is examined the effect of water usage on soot formation since in some cases a reduction of soot can be observed due to the dissociation of water, the production of OH radicals and improvement of the air fuel mixing process. Thus, the simulation model is used to estimate the correlation between water percentage, NOx relative reduction and the associating penalty on engine bsfc and soot. The results can be utilized to define an optimum water/fuel ratio, which will provide the maximum reduction of NOx at an acceptable fuel penalty. The results of the present investigation can be used as a pilot for the application of this method on different diesel engine types and sizes.