Experimental Investigation of the Influence of Engine Operating Parameters on a Rankine Based Waste Heat Recovery System in a SI Engine

One of the most promising techniques to reduce carbon dioxide (CO₂) emissions of future combustion engines is the use of waste heat from exhaust gas with a Rankine cycle. The target of this study was to investigate the influence of engine operating parameters such as ignition timing, coolant temperature and injection parameters on the efficiency and performance of Rankine based waste heat recovery systems (WHR). This was done to gain basic knowledge about the influences of the engine operating parameters which helps to explain the system behavior under different operating conditions and second to identify the operating parameters with the highest overall system efficiency which can be used to highlight the impact of changes in engine application on the car. These first of a kind tests were performed on a state-of-the-art gasoline engine equipped with a prototype Rankine-system.

Each tested engine parameter had an impact on the WHR-system performance; ignition timing was found to have the highest impact. The power output could be more than tripled, but at largely increased fuel consumption. Yet this opens up new options to WHR-systems, which allow enhancing the power output without changing the engine operating point. Other engine parameters increase WHR power output at unfavorable engine operating points. This means that a WHR-system can partially regain loses if the engine has to be operated in such states and thus the WHR-system can reduce fuel consumption.