Direct and Indirect Exhaust Heat Recovery from Turbocharged Heavy-Duty Engine

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.