Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng On the effects of increased coolant temperatures of light duty engines on waste heat recovery Vikram Singh a, ⁎ , Jelmer Johannes Rijpkema b , Karin Munch b , Sven B. Andersson b , Sebastian Verhelst a a Faculty of Engineering, Division of Combustion Engines, Lund University, 21100 Lund, Sweden b Chalmers University of Technology, Hörsalsvägen 7B, 41280 Göteborg, Sweden HIGHLIGHTS • Increase in system efficiency using waste heat recovery up to 5.2 percentage points. • Coolant temperature increase shows up to 1.7 percentage points gain in efficiency. • Optimum coolant temperature dependent on engine load and speed was observed. • Best working fluid for coolant heat recovery simulated was cyclopentane. ARTICLE INFO Keywords: Low temperature waste heat recovery Elevated coolant temperatures Light duty engine Rankine cycle Recoverable power Reduced heat losses ABSTRACT In this paper, an investigation is done into the potential of increasing the coolant temperature of an engine to maximize the powertrain efficiency. The study takes a holistic approach by trying to optimise the combined engine and waste heat recovery system. The work was done experimentally on a Volvo 4-cylinder light duty diesel engine in combination with Rankine cycle simulations. For the study, the coolant temperature was swept from 80 °C to 160 °C at different operating points. It was seen that with increased coolant temperatures, the brake efficiency of the engine increased by up to 1 percentage point due to reduced heat losses. An optimum coolant temperature was observed, dependent on the operating point, for maximizing coolant recoverable power. An expansive study was done simulating 48 working fluids for a dual loop waste heat recovery system. From the working fluids simulated, cyclopentane was seen as the best for coolant waste heat recovery, whereas methanol and acetone were better for the exhaust gases. The gain in efficiency seen, was up to 5.2 percentage points, with up to 1.7 percentage points as the effect due to recovered power from the coolant. 1. Introduction Over the past few decades, the internal combustion engine has moved towards higher fuel efficiencies to reflect the trends in the market as well as changes in emission regulations. While engines today are quite fuel efficient with heavy duty (HD) diesel engines having indicated efficiencies of 50% or higher [1], a large part of the fuel energy is still wasted in the form of heat – from the exhaust, coolant, oil and radiative losses to the environment. These losses can be reduced by optimising the combustion chamber, the spray and the gas exchange processes and the combustion mode [2–4]. However, these losses still form a large fraction of the fuel energy that is not trans- mitted to the engine crankshaft. One method to increase the power output at the crankshaft is to use a secondary thermodynamic cycle to recover some of the waste thermal energy from the engine. Thermodynamic cycle based waste heat re- covery devices have been researched quite well and have shown good potential for implementation [5]. It should be noted that waste heat recovery has been used in other applications as well, such as to reduce warm up time for the engine oil [6,7]. However, in these cases, it does not affect the steady state power output at the engine crankshaft. A waste heat recovery system, such as the Rankine cycle, can be used to take heat from any of the engine’s heat sources (intercooler air, oil, water or exhaust) to heat a secondary fluid at higher pressures and expand the fluid through an expander to get some recoverable power which can be transmitted to the crankshaft. A schematic of the Rankine system can be seen in Fig. 1. The Rankine cycle efficiency is dependent on the temperature of the high temperature heat source (heat exchanger temperature in Fig. 1) https://doi.org/10.1016/j.applthermaleng.2020.115157 Received 1 October 2019; Received in revised form 13 February 2020; Accepted 3 March 2020 ⁎ Corresponding author. E-mail address: vikram.singh@energy.lth.se (V. Singh). Applied Thermal Engineering 172 (2020) 115157 Available online 04 March 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved. T