Characterization and Control of Multi-Cylinder Partially Premixed Combustion

University dissertation from Lund University

Abstract: In the last decade diesel combustion has developed in a new direction. Research has been carried out trying to prolong the ignition delay and enhance fuel/air premixing to avoid diffusion combustion as well as lowering the combustion temperature through use of EGR. One of these new combustion concepts is Partially Premixed Combustion (PPC). PPC is aimed to provide combustion with low smoke and NOx without sacrificing fuel consumption. This thesis presents the development of a multi-cylinder PPC concept. It reaches from the basic characterization of this new combustion strategy to the demands on hardware, control and fuels for a realizable PPC solution. In summary it contains a thorough PPC characterization where the results suggest that high EGR, early injection PPC strategies are to prefer over late injection approaches or smokeless rich diesel combustion. Further, a strong connection between mixing period, defined as the period between end of injection and start of combustion, and PPC has been ascertained. Based on this knowledge a combustion controller with feedback control of mixing period was derived. The operating range of multi-cylinder diesel PPC was then evaluated. The study showed that the PPC load range was limited covering only 25% of the operating region for conventional combustion. In order to reach higher loads for PPC the EGR system was rebuilt to a low pressure system. This system improves EGR/air mixing and cooling and enables high EGR and boost pressure simultaneously. Additionally, gasoline fuels were introduced to extend the ignition delay and mitigate soot formation. An extensive fuel comparison was carried out to find the most suitable fuel for PPC operation. With the improved set-up the operating range was reevaluated. By combining the use of a low pressure EGR system and standard gasoline the operating region of PPC has been extended to cover 50% of the engine nominal operating region. The final part of this thesis is dedicated to a novel method of cylinder individual efficiency estimation based on the cylinder pressure trace. With this method, control strategies aiming directly at fuel consumption optimization can be developed. An extremum seeking control algorithm was applied. The results show that the controller manages to find the maximum brake torque region both with and without external excitation. Finally, the estimation error in accumulated fuel consumption from the experiments is around 1% which shows the potential of using the absolute value of the efficiency estimation in other control concepts.