• Energy Research
• 2016-2025close

<div class="section abstract"><div class="htmlview paragraph">Engine thermal management systems represent a promising solution to improve the efficiency of current Internal Combustion Engines (ICE) and sustain the transition towards a net zero scenario. The core component of an engine thermal management system is the electric pump, which can adjust the coolant flow rate according to the engine thermal needs. This possibility opens to newer design choices, which can contribute to non-negligible energy savings. In this study, three electric coolant pumps with different maximum efficiencies have been investigated to understand the influence of the design operating conditions on the pump energy absorption. A reference vehicle equipping a 130 HP downsized gasoline engine has been considered. An experimental test bench with a copy of the engine and its cooling circuit has been reproduced, and the electric pumps have been tested at a wide range of rotational speeds and thermostat lifts to obtain their characteristic maps. Once their performances were known, the vehicle was run in three driving cycles consisting of different shares of rural, urban and highway sections, acquiring data from the Electronic Control Unit (ECU). These data have been used to calculate the operating condition and energy absorption of the mechanical pump originally equipped by the vehicle and the electric pumps. The results have been evaluated using a statistical approach, normalizing the instantaneous efficiency by using their maximum efficiency values. The results show that all the electric pumps have lower energy absorption compared to the conventional mechanical actuation, with a reduction of up to 77% of the energy absorption. Considering the vehicle's fuel consumption and the lower heating value of gasoline, the potential reduction of CO₂ specific emissions is 1 g/km. The statistical analysis approach showed that the design operating conditions have a higher influence than the maximum pump efficiency. The best performances are achieved through the electric pump with the lowest efficiency, showing a decrease in energy absorption between 10 % and 50% compared to the other electric prototypes, depending on the driving profile.</div></div>

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.

In modern turbocharged internal combustion engines the cooling of the air after the compression stage is the standard technique to reduce temperature of the engine intake air aimed at improving cylinder filling (volumetric efficiency) and, therefore, overall global efficiency. At present, standard values for the intake air temperature are in the range 30-70°C, dependently on engine load, external air conditions and vehicle speed and the adoption of a dedicated cooling fluid operating at low temperatures (-10-0°C) is addressed as the most viable option to achieve an effective temperature reduction. This paper investigates a pilot engine set-up, featuring an evaporator on the intake line of a turbocharged diesel engine, tested on a high speed dynamometer bench: the evaporator was a part of an air refrigeration unit - the same used for cabin cooling - composed also by a compressor, a condenser and a thermostatic expansion valve. The effects of the undercooling of the charge air have been experimentally assessed in terms of fuel consumption and regulated emission reduction, evaluated on the most common engine operating points. Mechanical power needed by the compressor was obviously taken into account in order to assess the overall benefits. A fuel consumption reduction has been demonstrated in the order of 2.5% when the intake air subcooling is turned on. A benefit on the regulated emissions has been observed (NOx, PM). HC and CO behavior, on the contrary, deserves some more attention and involves engine control parameters (for instance, EGR rate) and combustion performances.

<div class="section abstract"><div class="htmlview paragraph">Waste Heat Recovery (WHR) is one of the most viable opportunities to reduce fuel consumption and CO2 emissions from internal combustion engines in the transportation sector. Hybrid thermal and electrical propulsion systems appear particularly interesting because of the presence of an electric battery that simplifies the management of the electrical energy produced by the recovery system. The different technologies proposed for WHR can be categorized into direct and indirect ones, if the working fluid operating inside the recovery system is the exhaust gas itself or a different one whose sequence of transformations follows a thermodynamic cycle. In this paper, a turbocharged diesel engine (F1C Iveco) equipped with a Variable Geometry Turbine (VGT) has been tested to assess the energy recoverable from the exhaust gases both for direct and indirect recovery. A direct technology based on an auxiliary turbine placed in the exhaust pipe (turbo-compounding) has been considered and compared with an Organic Rankine cycle (ORC)-based recovery unit fed by the exhaust gases. A model-based comparison between the two technologies has been assessed in this paper. The input data were the result of an experimental campaign done on the exhaust gases of the F1C Iveco operated on a high-speed dynamometer test bench. Data on exhaust gas properties, turbocharger equilibrium and engine performances were collected for a wide range of engine operating conditions. Concerning the ORC-based power unit, the model uses the significant research experience done on the sector that set up the most relevant machine performances (expander and pump efficiency, engine backpressure produced, pinch points at the two heat exchangers) so giving the model high reliability. Preliminary data on a turbo-compounding system operated on the same engine were also measured so resolving the most important uncertainties of the recovery unit (engine backpressure produced, turbine and electrical generator efficiency, matching between the turbocharging unit). A preliminary assessment of the overall potential recovery when both technologies were present has been done, focusing the attention on heavy-duty engines.</div></div>

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).

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.

The integration of a Micro-Organic Rankine Cycle (ORC) power unit with conventional solar flat plate collectors ensures the simultaneous fulfillment of electricity and domestic hot water (DHW) demands. Due to the variability of the solar source, despite the introduction of a thermal storage unit, the plant is subject to severe off design operating conditions. A small-scale ORC-based was designed, built and fully tested, to experimentally assess the performance and operating robustness of the plant in steady and dynamic off-design condition. The unit is fed by hot water from a 135 L reservoir. Dedicated electric heaters (12 kW each one) reproduce the thermal availability from 15 m2 of standard solar thermal collectors for domestic applications. The test bench underwent an extensive experimental assessment in both stationary conditions of the hot source and in presence of a variable thermal load at the evaporator. Due to the plant architecture and components, the control of the unit is based on the variation of the mass flow rate of the working fluid (R245fa) matching the thermal equilibrium at the evaporator in each operating condition of the system. The variation of the flow rate, in fact, must fit with thermal power available. The off-design steady state assessment allows the understanding of the wide operability of the plant (17–62 g/s), with power and efficiency varying between 150 and 500 W and 2.4–4%, respectively. The dynamic testing of the pilot unit points out the plant consistency and robustness to severe off-design operation, mostly due to the 1 kW scroll expander, very suitable for time-varying operating conditions. Both the option of a full discharge of the thermal energy of the reservoir and the option of a split discharge to the evaporator were investigated and provided a clear indication on the CHP feasibility when no addition of thermal energy takes place at the reservoir. It was observed that during a complete reservoir thermal discharge, the plant works for 3000–3300 s continuously, with a slow power decrease from 500 W to 100 W. Considering the thermal energy recharging time, the plant could be averagely operated up to 7 times during the day if a partial discharge was performed. Each plant activation lasts 900 s producing a power ranging from 500 W to 200 W. This operating strategy allows to cover up to 11.2% of the whole electric energy yearly required by an average European household.

<div class="section abstract"><div class="htmlview paragraph">Within automotive sector, there are several high-performance applications, like, for instance, those referred to racing and motorsport, where cooling needs are usually fulfilled by simple circuits with conventional low-efficiency pumps. The cooling needs in these applications are represented by low flow rates delivered (in the range of 10 - 50 L/min). The operating conditions of these small pumps are usually characterized by very high revolution speeds, which intrinsically cause low efficiency and critical intake phenomena (cavitation) if the design is not specifically optimized to address these concerns.</div><div class="htmlview paragraph">Hence, in this paper a small-size pump operating in the racing sector has been designed using a model-based approach, built and tested having reached both high efficiency (aimed to 50%) and absence of intake operational problems (cavitation). Starting from the specific cooling request (design flow rate equal to 14.0 L/min and pressure rise equal to 2.5 bar), the very limited space available on board oriented the design to an operational revolution speed of 12000 RPM. The interest of this study was to introduce a so high revolution speed in more conventional automotive cooling pumps electrically assisted, keeping high efficiency. In fact, the strong reduction of the size of the pump allows an easy and correct positioning on board.</div><div class="htmlview paragraph">The model-based design was done by a two-steps procedure. The first made use of a 0D model which, catching main physical phenomena of the flow even in simplified form, leads to an optimum geometrical design for the impeller and the volute. A final refinement has been done with a CFD code predicting the off-design performance and limiting cavitation zones. Cavitation, which is one of the most critical issues of high-speed pumps, was completely investigated through a CFD numerical analysis.</div><div class="htmlview paragraph">The pump has been prototyped and tested on a dynamic test bench for pumps, which reproduces homologation cycles and real driving. A good agreement has been reached between theoretical and experimental results, being the mean relative error on pressure rise for all operating point close to 4 %. This model-based procedure opens the way to support the development of electric water pumps for more conventional applications (automotive, light duty engines) in which a redesign will be focused to manage the thermal state of the engine and reduce the energy absorbed during the homologation cycle.</div></div>

Abstract Charge air cooling is the typical technique to reduce temperature of the engine intake air, increasing air density and improving cylinder filling and engine volumetric efficiency in present turbocharged diesel engines. Usually, charge air is cooled by environmental air that crosses a heat exchanger placed in front of the vehicle: its cooling capacity is related to the vehicle speed and other constraints (presence of the main radiator). This leads to an intake air temperature from 30 to 80 °C, depending on engine load, external air conditions and vehicle speed again. If the intake air was more cooled down, the engine volumetric efficiency would be further increased. This can only be done by the use of a dedicated cooling fluid, operating at lower temperature with respect to external air. In this paper, therefore, an evaporator, mounted in parallel with the one of the refrigeration unit used for cabin cooling, was placed on the intake line of a turbocharged diesel engine (F1C IVECO engine), tested on a high speed dynamometer bench: the air refrigeration unit is also composed by a compressor, a condenser and a thermostatic expansion valve. The effects of the undercooling of the charge air have been experimental assessed in terms of fuel consumption and regulated emission reduction, evaluated on the most common engine operating points. Mechanical power demand of the compressor has obviously taken into account in order to assess overall benefits. Achieved net fuel consumption is in the order of 1% at fixed conditions operating the engine as a light duty type, when the intake air sub-cooling is turned on. A benefit on the regulated emissions has been observed (Nitrogen Oxides, Soot) regardless of the setup of the engine combustion processes (injection time, fuel distribution for each injection, Exhaust Gas Recirculated rate). Unburned hydrocarbon and Carbon monoxide behavior, on the other hand, deserves some more attention and call for the re-calibration of the previously cited combustion control parameters.