La conversione dell’energia termica persa nei motori a combustione interna in energia meccanica

From 2030, emissions from new passenger cars in the European Union will have to be below 49,5 gCO2/km and those from new light commercial vehicles 90,6 gCO2/km. Although electrification is the best way to decarbonize road transport, improving conventional engine technologies is also useful in the transition process. One technology that can improve efficiency and reduce fuel consumption of a conventional thermal engine is the recovery of thermal energy usually lost and its transformation into mechanical energy. This is particularly challenging considering that the share of fossil fuels corresponding to the thermal energy lost is of the order of 60%, significantly greater than that useful for propulsion. This thesis studied the potential benefits of two recovery technologies: Organic Rankine Cycle (ORC) plants and turbo-compound. For ORC technology, a numerical model was developed to simulate the dynamic behavior of the evaporator, on which the dynamics of the plant depend to a greater extent. The model can be used to develop a control system for the ORC unit. An experimental campaign was conducted on a small-scale ORC plant whose upper thermal source's maximum temperature is comparable to the engine coolant temperature. The performance of two plant configurations was compared considering or not a regeneration stage. At the same working fluid flow rate, the power obtained by the expander in the unit with a regenerator is higher, because the degree of superheating and the maximum temperature in the cycle (and pressure) are higher. In addition, the efficiency of the unit with a regenerator is higher throughout the mass flow rate range considered, because the working fluid arrives preheated in the evaporator: at 40 g/s, the efficiency is 5 percent for the unit with a regenerator and 4 percent for the one without. The system performance with two different types of expanders was also compared: a scroll expander and an SVRE expander. The power output of the system varies between 100 and 500 W in the case of the scroll expander, and between 200 and 700 W in the case of the SVRE expander. The efficiency varies between 2% and 6% in the case of the SVRE expander; between 2% and 4%, in the case of the scroll expander. The second recovery technology was based on a direct recovery via a turbo-compounding system. A numerical model was developed to evaluate the potential benefits of installing a power turbine downstream of a turbocharger to further recovery in terms of pressure ratio (and temperature). The model considers an F1C IVECO turbocharged 3L Diesel engine operated on a high-speed dynamic test bench. According to the results obtained from the model, for high engine loads and speeds, the recovered power can be as high as 18 kW; for lower loads, it can be as low as about 4-5 kW. Considering the combined effects of the increase in specific fuel consumption and the increase in power, a reduction in fuel consumption of 8% is obtained when the engine speed is above 3400 rpm. These data were based on an extensive measurement campaign done on the cited engine which characterized all the variables in input to the turbo-compounding system.