The restrictions imposed by CO2 emission standards in Europe and many countries have promoted the development of more efficient spark ignition engines. The reduced swept volume and number of cylinders of four-stroke engines has significantly improved fuel economy by means of lower pumping and friction losses. This approach, known as engine downsizing, has demonstrated its potential of reducing fuel consumption on its own as well as applied to hybrid vehicles where a low weight engine is desired. However, aggressive engine downsizing is currently constrained by thermal and mechanical stresses and knocking combustion. In order to overcome these limitations, the present work evaluates the application of a conventional poppet valve direct injection engine into the two-stroke cycle. Two-stroke engines have the ability to produce higher power with reduced swept volume and less weight than four-stroke engines thanks to the doubled firing frequency. These advantages, although, are sometimes offset by poorer emissions resulted from fuel short-circuiting; lower thermal efficiency resulted from short expansion process; and reduced engine durability due to lubrication issues. Therefore, in this research the four-stroke engine architecture was employed so these shortcomings could be addressed by the use of direct fuel injection, variable valve actuation and a wet crankcase, respectively. The burnt gases were scavenged during a long valve overlap by means of boosted air supplied by an external compressor. An electrohydraulic fully-variable valve train enabled the optimisation of the gas exchange process in a variety of engine operating conditions. The air-fuel mixture formation was evaluated through computational fluid dynamic simulations and correlated to experimental tests. In addition, the engine operation with ethanol was assessed in a wide range of engine loads and speeds. Finally, the engine performance, combustion process, air-fuel mixing and gas exchange results were presented, discussed and contextualised with current four-stroke engines. Keywords: Two-stroke poppet valve engine; gasoline and ethanol direct injection; engine downsizing; supercharged two-stroke cycle.

Internal combustion engines developments are driven by emissions reduction and energetic efficiency increase. To reach the next standards, downsized/downspeeded engines are required to reduce fuel consumption and CO2 emissions. These techniques place an important demand on the charging system and force the introduction of multistage boosting architectures. With many possible arrangements and large number of parameter to optimize, these architectures present higher complexity than current systems. The objective of this thesis has thus been to investigate the potential of two-stage boosting architectures to establish, for the particular case of passenger car downsized/downspeeded Diesel engines, the most efficient solutions for achieving the forthcoming CO2 emissions targets. To respond to this objective, an exhaustive literature review of all existing solutions has first been performed to determinate the most promising two-stage boosting architectures. Then, a new matching methodology has been defined to optimize the architectures with, on the one hand the development of a new turbine characteristic maps representation allowing straight forward matching calculations and, on the other hand, the development of a complete 0D engine model able to predict, within a reduced computational time, the behavior of any boosting architecture in both steady state and transient operating conditions. Finally, a large parametric study has been carried out to analyze and compare the different architectures on the same base engines, to characterize the impacts of thermo-mechanical limits and turbocharger size on engine performance, and to quantify for different engine development options their potential improvements in term of fuel consumption, maximum power and fun to drive. As main contributions, the thesis provides new modeling tools for efficient matching calculations and synthesizes the main trends in advanced boosting systems to guide future passenger car Diesel engine develop / Varnier ., ON. (2012). Trends and Limits of Two-Stage Boosting Systems for Automotive Diesel Engines [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16880