Riassunto analitico
In the last decade, automotive world has been subjected to big changes. The driving force behind that evolution is certainly the demand for lower pollutant emissions without loss of performance. Brake specific fuel consumption needs to be reduced as much as possible. There are two main aspect to work on to fit the pollution regulation: reduce fuel consumption and increase specific power thanks to a higher efficiency.
CFD has a fundamental role in design of future and present engine: if models are precise enough, can provide very detailed results concerning fuel flow, combustion and pollutant formation. Novel concept of combustion such as HCCI can be developed thanks to computer simulation.
Obviously, the only way to reduce fuel consumption is to use less fuel per cycle (lower RPM, leaner combustion, smaller engine, etc.), while it’s a bit more complicated in case of efficiency.
To maintain performances, current trend in engine design is the adoption of highly supercharged downsized engine. Subjected to heavy thermal load and high intake pressure, these types of engines suffer from a phenomenon called knock or detonation. Fuel, mixed with air, tends to auto ignite producing strong pressure oscillations, leading to serious structural damages, if this process is not restrained. Moreover, engines are forced to run with leaner mixture, close to stoichiometry, that has reduced knock tolerance, since extra fuel helps in keeping temperature under control.
Exhaust gases recirculation, “EGR”, is an effective an efficient way to reduce knock tendency, reducing mixture reactivity and keeping end gas temperature lower thanks to higher specific heat of exhaust gases with respect to pure fresh air.
EGR interaction with combustion and knock are a complex and multidisciplinary phenomena, ranging from thermophysics to chemistry and fluid dynamics. CFD suits simulating such demanding environment.
Further complication comes from the nature of knocking events, difficult to be spotted and analyzed, being highly stochastic and affected by cycle-by-cycle variation. Ensemble average experimental approach can’t lead to satisfactory results and an innovative technique, to be applied to every singular pressure trace, will be presented.
In this work, EGR effect will be studied only adopting CFD simulations so confidence with knock, related parameter and quantities is mandatory to be as predictive as possible. For this reason, models will be previously tuned according to out coming results from the experimental campaign.
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