High performance gasoline engine development approach for new current requirements

Nowadays the efforts aimed at enhancing the Internal Combustion Engines (ICEs) are mainly focused on the fuel consumption minimization to comply with binding CO2 emission legislation for vehicle homologation. Concerning the Spark-Ignition ICEs, the most widespread path to satisfy the pollutant emission limits is the adoption of a three-way catalyst (TWC) along the exhaust line. As known, this solution poses some issues, such as a low efficiency at cold start or an effectiveness degradation because of aging. In addition, it involves the impossibility to exploit the advantages of lean combustions, since a close to stoichiometric air/fuel mixture is mandatory for efficient TWC operation. For the above reasons, a growing interest towards solutions limiting engine raw emissions is emerging. So future legislation requires new technical measures to increase engine efficiency and reduce pollutant emissions. Here gasoline engines with high specific power have a huge development potential, since, on the one hand, knocking at high Brake Mean Effective Pressure (BMEP) limits thermal efficiency and, on the other hand, high power densities lead to increased thermal loads, which, for component protection reasons, need to be controlled by means of enrichment beyond the stoichiometric air-fuel ratio. This operation leads to increased fuel consumption and to higher pollutant emissions; especially harmful soot particles, hydrocarbons and carbon monoxide are emitted in a higher amount. In addition to known systems, such as exhaust manifolds integrated in the cylinder head for direct cooling of the exhaust gas, extended effective expansion by optimized valve timings (Miller, Atkinson) and external cooled exhaust gas recirculation, also new technologies are being developed for passenger cars. Those technologies primarily aim to widen the lambda one range of the engine in order to maintain the stoichiometric air/fuel ratio throughout the entire engine operating range, which is expected to be required for future Real Driving Emissions (RDE) legislation. The first chapter explains the current situation and future direction of internal combustion engines, with a particular focus on the gasoline engines with high specific power and covers broad regulatory changes in the last year related to tailpipe emissions of criteria pollutants and CO2/fuel economy. Throughout the chapter, a brief overview of internal combustion engines and their future development will be provided so to understand and appreciate why it is still relevant to conduct research in this field, while facilitating the improvement of green technologies in order to achieve a sustainable transportation system. The motivation behind this study and the research direction will also be clarified. Then, a Lamborghini 12-cylinder naturally aspirated spark ignition engine is investigated. The engine is experimentally tested under full and part load operation with two different Air-to-fuel ratio maps. Main performance parameters, in-cylinder pressure cycles and raw pollutant emissions are measured. The engine is schematized in a one-dimensional model (GT-Power™), where “user routines” are employed to simulate turbulence, combustion, knock and pollutant production. 1D model is validated against the experimental data, denoting a good accuracy. The innovative contribution of this section can be hence recognized in the development of a 1D model characterized by a single set of tuning constants allowing for an accurate reproduction of the combustion process in all the engine configurations. As better explained in the following, the combustion model is in fact coupled to a turbulence sub-model, preliminary tuned with reference to 3D-CFD results, in motored operation. This methodology is particularly helpful in the calibration of a VVT engine, where the turbulence levels substantially vary at part load according to the intake/exhaust valve strategies. Combustion and turbulence constants are hence selected through comparisons with few experimental data at full load and 3D results, and then employed at part load and in the optimization process, as well. The results about the raw emissions put into evidence that the numerical approach predicts the experimental data of carbon monoxide (CO) and nitrogen oxides (NO), but it is not enough advanced to reproduce the hydrocarbon (HC) level, although the variations with the engine operating parameters (speed, load, air/fuel ratio) are captured. The model is employed to study the water injection impact to draw the variation trend of the exhaust temperature, performance and the pollutant emissions changing the engine hardware, rather than to predict their absolute levels. The combustion speed takes into account the water presence with a refined correlation of laminar flame velocity and the knock model gives the possibility to set the best spark advance. The water evaporation reduces the in-cylinder temperature and, as a consequence the knock level, is lower. The computed HC, CO and NO maps have been embedded in a vehicle simulation to estimate the impact of the analysed technical solutions on a RDE cycle suggested by Lamborghini. In this way, with a fully numerical approach, a new hardware is analysed and its impact in term of pollutions level is verified on a realistic drive cycle that the new regulations seems to impose. In this way, a new approach to design the future high performance engines has been individuated and the impact of the new regulations can be seen before experimental tests, reducing the time to market and the economic effort. Then, the difference between the engine out emission level and the limit that the regulation imposes can help also to design the after-treatment system. Summarizing, the presented numerical approach showed the potential to predict, on a physical basis, the combined effects of various techniques on the engine performance. This methodology could represent an effective tool to identify the trade-off between engine complexity and expected improvements, contributing to support and drive the development process of new engine/vehicle