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Abstract Rotary Vane Expander is an interesting solution for small-scale ORC power unit due to its reliability, flexibility and competitive cost. As demonstrated by the authors in previous works, the introduction of a secondary intake port leads to an increase of the aspirated mass flow rate and consequently of the mechanical power produced by the machine. In this paper, theoretical and experimental studies were conducted in order to assess the potential benefits in terms of efficiency introduced by the dual intake expander and the trade-off with the produced power for a given pressure-drop. The theoretical results showed that if the relative gain of mechanical power produced by the dual intake technology is higher than that of working fluid mass flow rate, the efficiency grows when the same machines operate at the same upstream and downstream pressures. Two expanders have been designated, built and tested giving the possibility to experimentally verify the performances of a single and a double intake machine. From measured data a mathematical model of the expander was validated, allowing to use it as a virtual platform for further machine optimization and improvement. It was observed that the efficiency gain introduced by the dual intake device depends on the OEM volumetric efficiency and on the pressure ratio. The global efficiency of the dual intake expander grows up to 30% if the volumetric efficiency is 50% and the pressure ratio is 2.3 while the efficiency benefit decreases to 5% if the volumetric efficiency is 70% and the pressure ratio is 3. Nevertheless, even if the global efficiency would be equal for the two machines, the dual intake technology always allows to increase the delivered mechanical power.

Abstract Volumetric machines are the most suitable candidates to be used as expander in power units based on the Organic Rankine Cycle (ORC) thermodynamic concept, for waste heat recovery of Internal Combustion Engine in the on-the-road transportation sector. In particular, the technology of the Sliding Rotary Vane Expander (SRVE) shows intrinsic advantages thanks to their lower cost, shaping features, easier manufacturing and reliability and, generally speaking, very suitable operative conditions. Nevertheless, they show some disadvantages which are typical of volumetric machines, such as low capacity, limited expansion ratio and power lost by friction. Among the different technologies available to reduce the effects of these aspects (revolution speed increase, elliptical stators or with more complex geometries, tip blade optimization, rolling stators, etc.), the Dual-Intake Port (DIP) was assessed as a promising solution to enhance SVREs performance and operability. However, dual-intake port was still not conceived as design option. In this paper, a novel Sliding Vane Rotary Expander design concept involving a Dual Intake Port expander (DIP) option was developed and compared to the conventional Single Intake Port (SIP) one, under the same operating conditions. Thus, after an experimental comparison between SIP and DIP expanders, under the same conditions of mass flowrate, a theoretical expander model of the latter was validated on a set of experimental results. The model allows to outline the advantages of considering the dual port as a design option: for a specific flow rate and revolution speed, this option allows to reduce the friction losses, which represent a weak point of this volumetric machine. For a given mass flow rate of the working fluid, an additional feature is that it allows a sensible downsizing with respect to the SIP, with an axial dimension reduction up to 50%, ensuring additional weight and space saving. Despite the reduction of the dimensions, DIP produces a comparable mechanical power with respect to SIP, limiting the reduction to 8–10% of the SIP one with a 50% lower mechanical power loss due to friction. For the same flow rate, the added intake port determines a decrease of the intake pressure which is completely beneficial in terms of operating conditions of the overall recovery unit.

Abstract The stringent regulations on fuel saving and emissions reduction in the transportation sector have become game-raisers in the development of present internal combustion engines for road applications, even if under-the-hood space constraints, downsizing and down-weighting prevent from adopting radical changes in the engine layout. Charge air cooling is the standard in present turbocharged diesel engines, to the point that a dedicated heat exchanger, fed by environmental air, is located downstream the compressor. The paper proves the option of an additional cooling through the cabin-heating unit - usually over-sized with respect to normal operation - very effective to increase charge air density and improve cylinder filling. The intercooler downstream the compressor would be provided with a lower thermal load, hence calling for smaller heat exchange surfaces, leading to reduced weight, space saving and no increased layout complexity. By pushing this idea forward, a properly sized cabin-heating unit could even supply enough air cooling to replace the intercooler instead of just assisting it, further raising the weight/space/layout advantages. In presence of an additional heat exchanger, the cooling efficiency would be no longer related to the vehicle speed and the benefit in terms of cylinder filling could be kept to the desired value on a wider operating range for the engine. Plus, the lower combustion temperatures associated with both a colder air and a more diluted charge approaching the chamber would also result in a more regular combustion process, in spite of a moderate penalty on the thermodynamic efficiency. The additional fuel consumption to compress the cooling fluid is always offset by the fuel saving with respect to normal operation and a beneficial effect is appreciated on emissions. Nonetheless, major variables to account for when evaluating the feasibility of such a layout are (i) the impact it has on the equilibrium of the turbocharger, i.e. on the efficiency at each operating point, (ii) to what extent the presence of a colder air affects the turbine/compressor matching and (iii) the rack position the ECU fixes at the VGT, to face both pressure losses at the additional evaporator and the different enthalpy content for both the air at the compressor outlet and the exhaust gases at the turbine inlet. A comprehensive experimental activity supported by a detailed 1D model of the engine unit, aimed at assessing the benefits of the air under-cooling, allowed to select the most appropriate cooling layout and, once validated based on the experimental evidence, to investigate the equilibrium at the turbocharger.

Abstract Among the wide technological opportunities to reduce CO2 emissions in the transportation sector, Waste Heat Recovery (WHR), done by Organic Rankine Cycle (ORC) power units, was subject of a great scientific interest. However, several complications apply for an on-board operation and management, which reduce its real benefit in terms of net fuel consumption reduction. Weight increase, encumbrances, engine backpressure, intrinsic off design operating conditions are all negative aspects which should be still solved. In this paper, the interest is focused on the limiting factors of an ORC-based unit operating on board: engine backpressure due to Heat Recovery Vapor Generator (HRVG) presence, surface of the condenser and additional weight. All these aspects behave as limiting factors of the WHR. Considering the mechanical power produced by the unit as a desired input value, the paper discusses the HRVG and condenser sizing as a function of engine backpressure, weight increase and vehicle frontal area required by the condenser. The engine considered was an IVECO F1C turbocharged diesel engine and the reference vehicle was a light duty one. Experimental data have been used for gas temperature and flow rates. The paper considers also the possible presence of an internal regeneration within the ORC which increases plant efficiency (reducing the dimensions of the HRVG and condenser) but introduces an additional heat exchanger. A correlation between weight of the heat exchangers and condenser frontal area as a function of the mechanical power recovered has been presented, without and with the regeneration stage.

Abstract Rotary Vane Expanders (RVE) are very suitable prime movers for ORC-based power units in on-the-road transportation sector. RVEs suffer volumetric efficiency deficits due to leakages which limit the overall expander efficiency and can vanish their intrinsic benefits with respect to the other prime movers. Making reference to a 2 kW Sliding RVE type (SRVE), the paper presents a theoretical and experimental contribution which goes deep into the effect of leakages inside the machine and aims to quantify their amount and effects on the expander performances. The results showed that the volumetric losses increase the mass flow rate aspirated by the machine if the intake pressure is kept constant. This increase favors a greater recovery from the hot source (up to 50%) but part of it bypasses the vanes, producing a volumetric loss. An interesting feature is that part of this additional mass is exchanged among vanes and this produces a beneficial effect on the indicated power (16.6% increase with respect the ideal case). The resulting knowledge further supported the effectiveness of dual intake expander technology which allows to theoretically reduce the leakages between adjacent vane up to 60–70% ensuring an improvement of expander efficiency.

Abstract Sliding vane rotary expanders (SVREs) are widely used in organic Rankine cycle (ORC)-based power units for low-grade heat recovery because of their capability to deal with severe off-design working conditions. In particular, the speed of SVREs is a very effective operating parameter, together with the speed of the pump, to regulate the recovery unit and to lead the involved components in an acceptable operating behaviour when they are far from the design conditions. In this study, a control strategy based on the variation in revolution speed of a SVRE was developed, where the inlet pressure of the expander is the main controlled property, which must be verified when the flow rate of the working fluid is changed to match the thermal power recovery at the hot source. In fact, pressure level control is a key point of the recovery unit for thermodynamic reasons and for the safety and reliability of the expander and, more generally, of the whole recovery unit. The proposed control strategy is based on an original theoretical procedure that relates the expander speed, inlet pressure, volumetric efficiency, and working fluid mass flow rate in an analytical form. This analytical formulation is widely nonlinear and is simplified for use as a tool for the model-based control of the inlet expander pressure. An experimental activity performed on a SVRE operating in an ORC-based power unit, fed by the exhaust gases of a supercharged diesel engine, was the base of the analytical formulation. This provided the possibility of deriving a simplified model-based control of the expander inlet pressure and assessing its effectiveness and limits during off-design conditions. Higher expander global efficiencies were obtained (up to 45%), allowing a greater mechanical energy recovery (up to 2 kW).