Performance Investigation & Gas Exchange Assessment of Exhaust Piston–assisted Turbocharged Engine (EPTE) Concept : A simulation-based assessment

Abstract: There is an increasing stringency in the emission legislation within the transport industry along with ambitious performance demands from the customer side. These have pushed the vehicle manufacturers and relevant component suppliers to develop and integrate a wide portfolio of engine technologies. Turbocharging is one such technical solution deployed by the industry to reduce fuel consumption and thereby CO2 emissions from the tailpipe. The area within which turbocharging has been practiced spans across a variety of vehicle segments including on-road and off-road applications.  It has been well established that standard turbocharging comes with a downside of not having the ability to provide suitable efficiency levels across a broad operating range. Furthermore, four-stroke turbocharged engines with fewer cylinders have uneven exhaust pulsations flowing into the turbine inlet. This implies that the turbocharger is not able to meet the air system demands throughout the engine cycle. There are existing commercial technologies such as twin-scroll turbocharging, Variable Geometry Turbochargers (VGTs) and Electrically-assisted Turbochargers (EAT) to tackle the above highlighted challenges. However, they induce high cost and overall system complexity. A patented concept also referred to as the Exhaust Piston-assisted Turbocharged Engine (EPTE) claims to address the aforementioned drawbacks related to various turbocharging strategies. The EPTE concept uses an extra piston-cylinder which acts solely as a compressor and expander for the exhaust gases flushing out of the combustion cylinders. This extra piston-cylinder is also referred to as the Exhaust Cylinder (EXC) component.  This thesis investigates the performance and gas exchange metrics of the EPTE concept across a broad engine speed range, and further compares those against a baseline engine which does not incorporate the EXC component. Gas exchange metrics were evaluated to assess the impact of the EXC component on the performance of turbocharger and sizing of the muffler and aftertreatment system. Performance analysis was conducted for the EXC component to quantify its contribution to the total brake power produced at the crankshaft. The investigation was performed using the commercial engine performance prediction tool, GT-PowerTM. Additional outcomes of the study included evaluation of Normalized Brake Torque (NBT) fluctuation and the hardware modifications in the intake and exhaust side required while transitioning from baseline engine to EPTE. An evaluation of the EXC component specifications was also conducted in this thesis to provide an overview of the extra piston-cylinder from the design standpoint.  The EPTE concept proved to be more fuel efficient while producing higher power output than the baseline engine at high boost pressure limits. The new concept exhibited disadvantages from the fuel efficiency and power output standpoint at low boost pressure limits. The concept proved to also increase the performance of the turbocharger by providing a smoother exhaust pressure pulse to the turbine across the engine cycle, when compared against the baseline engine. Furthermore, it had a positive impact on the aftertreatment sizing and flywheel inertia. Another observation was that the EPTE concept can produce such benefits while not having the need to radically modify the intake and exhaust geometrical parameters. With a smaller turbocharger, such an engine proved to be beneficial compared to the baseline engine across the whole engine operating range.  The thesis project provides a simulation-based system-level perspective of the EPTE concept which has been patented by Mats Olshammar. Such a system-level perspective will help to gain an understanding of the operation of the EPTE before commencing any hardware development initiatives. The report also provides some recommendations for future work, based on the advantages and disadvantages of the engine concept emanating from the results of the work.