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In recent years, internal combustion engine (ICE) downsizing coupled with turbocharging was considered the most effective path to improve engine efficiency at low load, without penalizing rated power/torque performance at full load. On the other side, issues related to knocking combustion and excessive exhaust gas temperatures obliged adopting countermeasures that highly affect the efficiency, such as fuel enrichment and delayed combustion. Powertrain electrification allows operating the ICE mostly at medium/high loads, shifting design needs and constraints towards targeting high efficiency under those operating conditions. Conversely, engine efficiency at low loads becomes a less important issue. In this track, the aim of this work is the investigation of the potential of the oversizing of a small Variable Valve ActuationSpark Ignition gasoline engine towards efficiency increase and tailpipe emission reduction.

As known, reliable information about underlying turbulence intensity is a mandatory pre-requisite to predict the burning rate in quasi-dimensional combustion models. Based on 3D results reported in the companion part I paper, a quasi-dimensional turbulence model, embedded under the form of “user routine” in the GT-Power™ software, is here presented in detail. A deep discussion on the model concept is reported, compared to the alternative approaches available in the current literature. The model has the potential to estimate the impact of some geometrical parameters, such as the intake runner orientation, the compression ratio, or the bore-to-stroke ratio, thus opening the possibility to relate the burning rate to the engine architecture. Preliminarily, a well-assessed approach, embedded in GT-Power commercial software v.2016, is utilized to reproduce turbulence characteristics of a VVA engine.

Recently, a growing interest in the development of more accurate phenomenological turbulence models is observed, since this is a key pre-requisite to properly describe the burn rate in quasi-dimensional combustion models. The latter are increasingly utilized to predict engine performance in very different operating conditions, also including unconventional valve control strategies, such as EIVC or LIVC. Therefore, a reliable phenomenological turbulence model should be able to physically relate the actuated valve strategy to turbulence level during the engine cycle, with particular care in the angular phase when the combustion takes place.