3D CFD analyses are used to analyse the effects of port-injection of water in a high performance turbocharged GDI engine. Particularly, water injection is adopted to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock resistance. A full-load / maximum power engine operation of a currently made turbocharged GDI engine is investigated comparing the actual adopted fuel-only rich mixture to stoichiometric-to-lean mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-injected engine allows the spark advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A relatively simple yet effective solution to increase knock resistance is investigated by means of 3-D CFD analyses in the paper: water is port injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. Full-load engine operations of a currently made turbocharged GDI engine are investigated comparing the adopted fuel-only rich mixture with stoichiometric mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-injected engine allows the Spark Advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption.

New SI engine generations are characterized by a simultaneous reduction of the engine displacement and an increase of the brake power; such targets are achieved through the adoption of several techniques such as turbocharging, direct fuel injection, variable valve timing and variable port lengths. This design approach, called “downsizing”, leads to a marked increase in the thermal loads acting on the engine components, in particular on those facing the combustion chamber. Hence, an accurate evaluation of the thermal field is of primary importance in order to avoid mechanical failures. Moreover, the correct evaluation of the temperature distribution improves the prediction of pointwise abnormal combustion onset.The paper proposes an evolution of the CFD methodology previously developed by the authors for the prediction of the engine thermal field, which is applied to two different high performance turbocharged DISI engines: the methodology employs both in-cylinder 3D-CFD combustion simulations and CHT (Conjugate Heat Transfer) simulations of the whole engine, inclusive of both the solid components and the coolant circuit. In-cylinder analyses are used as thermal boundary conditions for the CHT simulations, which are in turn a fundamental benchmark to evaluate the accuracy of the combustion heat flux estimation by means of a combination of global engine thermal survey and local temperature measurements.A preliminary evaluation of some consolidated heat transfer models is carried out to evaluate the accuracy of the predicted gas-to-wall heat fluxes. Then, a modified heat transfer model is proposed, critically motivated and applied to the specific engine conditions under investigations. The proposed model strongly improves the predictive capability of the combined in-cylinder/CHT methodology in terms of both global thermal balance and pointwise temperature distribution for both the investigated engines.

It is widely recognized that spatial and temporal evolution of both macro- and micro- turbulent scales inside internal combustion engines affect air-fuel mixing, combustion and pollutants formation. Particularly, in spark ignition engines, tumbling macro-structure induces the generation of a proper turbulence level to sustain the development and propagation of the flame front.As known, 3D-CFD codes are able to describe the evolution of the in-cylinder flow and turbulence fields with good accuracy, although a high computational effort is required. For this reason, only a limited set of operating conditions is usually investigated. On the other hand, thanks to a lower computational burden, 1D codes can be employed to study engine performance in the whole operating domain, despite of a less detailed description of in-cylinder processes. The integration of 1D and 3D approaches appears hence a promising path to combine the advantages of both.In the present paper, a 0D phenomenological mean flow and turbulence model belonging to the K-k model family is presented in detail. The latter is implemented in the GT-Power™ software under the form of “user routine”. The model is tuned against in-cylinder results provided by 3D-CFD analyses carried out by the Star-CD™ code at two engine speeds under motored operation. In particular, a currently produced twin-cylinder turbocharged VVA engine is analyzed. The 0D model is then validated against further 3D results at various engine speeds and intake valve lifts, including early closure strategies, both under motored and fired operation.The proposed 0D mean flow and turbulence model shows the capability to accurately estimate the temporal evolution of the incylinder turbulence for all the considered operating conditions, without requiring any case-dependent tuning, proving its generality and reliability.