Low Temperature Waste Heat Recovery in Internal Combustion Engines
Abstract: Over the past few decades, the automotive industry has increasingly looked towards increasing the efficiency of the internal combustion engine to meet more stringent emission norms and as a measure to meet demands for improved air quality in cities. One method to improve the internal combustion engine efficiency is to recover some of the energy lost to the coolant and the exhaust using a secondary thermodynamic cycle such as an Organic Rankine Cycle. Organic Rankine Cycle systems have been shown to be some of the most efficient systems for waste heat recovery in automotive applications.While most research into Organic Rankine Cycle waste heat recovery systems studies the recovery of heat rejected to the exhaust gases, the coolant represents a large source of waste heat which is largely overlooked. This is because of the lower temperature of the coolant in comparison to the exhaust gases, which means a lower quality of energy and hence, lower recoverable power from the system.This thesis aims to investigate methods to increase the energy quality for low temperature waste heat in the engine and optimise the waste heat recovery system to improve the recoverable power and powertrain efficiency. The work done was a combination of experiments and simulations. The engines studied were primarily the Scania D13 heavy duty engine, both in single cylinder and multi cylinder configurations, and the Volvo D4 light duty engine.The thesis first numerically investigates the use of elevated coolant temperatures for a single cylinder Scania D13 engine to increase the recoverable power from the engine waste heat. The system simulated used two separate recovery loops for the recovery of waste heat from the exhaust gases and the coolant with ethanol as the working fluid. It was seen that there exists an optimum coolant temperature, depending on engine operating conditions, to maximise recoverable power from the coolant. On the other hand, the recoverable power from the exhaust increased consistently with increasing coolant temperatures due to higher exhaust gas temperatures. The gross indicated efficiency and the combustion were seen to be largely unaffected by the coolant temperature in the study.The thesis then studies the use of an integrated waste heat recovery cooling circuit using simulations for the multi-cylinder Scania D13 engine. This is where the coolant is also used as the working fluid in the Organic Rankine Cycle and the engine cooling channels act as a part of the evaporator. Here, a significant increase in system gross indicated efficiency was seen with the use of evaporative cooling. On comparison, while the integrated waste heat recovery system was seen to perform better than using a dual loop Rankine cycle system with elevated coolant temperatures, there is also increased mechanical complexity in implementing such a system, which lowers its viability for application.The next study uses experimental data from the Volvo D4 light duty engine to optimise the waste heat recovery process for different operating points. An expansive study of working fluids was done to see the fluids that could recover the maximum power from the different waste heat sources. It was seen that for the low temperature heat, cyclopentane performed the best, whereas for high temperature heat, methanol and acetone were the best performing working fluids. An analysis of the working fluids showed that this was due to a combination of thermodynamic properties of these fluids and the constraints imposed for the simulations. The system brake efficiency was increased by 5.2 percentage points when using heat from both the exhaust gases and the coolant. From this, the increase in brake efficiency solely as the effect of increasing the coolant temperature was 1.7 percentage points.Finally, the thesis evaluates the use of an optimised coolant temperature strategy to see the reduction in fuel consumption over a standard World Harmonized Transient Cycle for the multi-cylinder Scania D13 engine. It was seen that cyclopentane and methanol, again, performed the best, showing a total potential reduction of 9% in fuel consumption over the cycle.This thesis thus gives insights on the optimisation process of implementing low temperature waste heat recovery systems for internal combustion engines and improving the powertrain efficiency.
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