Engine Bottoming Cycle for Waste Heat Recovery

Heavy Duty Diesel Engines (HDDE) are widely used in several applications, such as vehicles and ship propulsion units, as well as, together with reciprocating gas engines, for small-medium size distributed stationary power generation. They are also between the main contributors to CO2, Green House Gases (GHG) and pollutants emissions. The US EPA reports that the road transport sector, mostly powered by HDDE, has been estimated to contribute for 13% to the world global Green House Gases (GHG) emissions, while the global carbon emissions from fossil fuels have significantly increased since 1900, with a 1.5 factor in the years between 1990 and 2008. Fuel cost and emission reduction in order to fulfil new stringent legislations are pushing engine manufactures and developers in the direction of further increasing energy efficiency. Several strategies are adopted for this purpose, and can be divided basically in 2 categories: engine-powertrain-applied or engine-bottoming technologies, depending if they are directly applied or retrofitted to the engine-powertrain system, or if they recover wasted engine energy. In the last years, Organic Rankine Cycles (ORC) are between the most studied and developed technologies to recover engine waste heat from several sources: exhaust gas, EGR (Exhaust Gas Recirculation), Jacket Cooling Water, CAC (Charge Air Cooler) or Oil Cooler. However, currently, ORC are mostly developed as a retrofit of existing engines, resulting in a non-optimized powertrain. For this reason, in a preliminary phase of an engine-ORC development project, it could be useful to investigate combined optimized concepts in oder to maximize energy efficiency and reduce emissions. For this purpose, first and second law of thermodynamics techniques can help in order to understand where, in the combined system, irreversibilities destroy exergy, thus increasing system entropy production and reducing overall system performance. Furthermore, an accurate assessment of typical applications (e.g. vehicles or ships) engine duty profiles allows to better understand at which operating points is better to design the system components in order to achieve the best performance during operations. Finally, thermo-economic techniques can support the overall analysis procedure and help to estimate a cost effective system even before entering production stages.