The paper is focused on the development of a fractal combustion model, included within a whole-engine one-dimensional model (1Dime code). An extensive validation is carried out through the comparison with experimental data. The experimental activity was carried out in the combustion chamber of an optically accessible one-cylinder engine, equipped with a commercial head. Experimental data basically consisted on optical measurements which were also correlated to the instantaneous pressure inside the cylinder. Optical measurements were based on 2D digital imaging and UV chemiluminescence of radical species. The rate of chemical energy release and related parameters were evaluated from the in-cylinder pressure data using interpretation models for heat release analysis. Moreover a post-processing of the optical measurements allowed to define the mean flame radius, and propagation speeds as well, as a function of the crank angle. Theoretical and experimental analyses allowed to fully characterize flame structure and propagation speed as well. In order to estimate prediction levels and limitations of the numerical procedure, different engine speeds and spark timings were experimentally analyzed. A good agreement has been found in the whole tested range.

In this work, a promising technique, consisting of a liquid Water Injection (WI) at the intake ports, is investigated to overcome over-fueling and delayed combustions typical of downsized boosted engines, operating at high loads. In a first stage, experimental tests are carried out in a spark-ignition twin-cylinder turbocharged engine at a fixed rotational speed and medium-high loads. In particular, a spark timing and a water-to-fuel ratio sweep are both specified, to analyze the WI capability in increasing the knock-limited spark advance.In a second stage, the considered engine is schematized in a 1D framework. The model, developed in the GT-Power™ environment, includes user defined procedures for the description of combustion and knock phenomena. Computed results are compared with collected data for all the considered operating conditions, in terms of average performance parameters, in-cylinder pressure cycles, burn rate profiles, and knock propensity, as well.Finally, the validated model is applied to investigate the full potential of water injection in reducing the knock tendency and improving the fuel economy in a wide load range. The numerical results highlight that WI technique involves significant Brake Specific Fuel Consumption (BSFC) advantages, especially at the medium-high loads. These benefits are limited by the maximum allowable levels for the in-cylinder pressure, while additional advantages are obtained in terms of reduced turbine inlet temperature, turbocharger speed, and boost pressure.The developed numerical procedure, based on validated combustion and knock sub-models, is able to take into account the complex interactions among different parameters, which affect the engine behavior. It is hence believed to realistically forecast the WI-related BSFC advantages and constraints, induced by thermo-mechanical stresses. Simultaneously, it underlines the need of a partial engine redesign to fully exploit WI potential.

A wide experimental investigation on a 16 valves, 1242 cm3 SI-engine is reported. Experimental data were collected in correspondence with about 250 different operating conditions of the engine. This allowed to deeply assess the accuracy of a simulation model (1Dime code), developed by the authors, based on a one-dimensional computation of the gas flow in the manifolds, and on a quasi-dimensional fractal approach for combustion simulation. The model is then employed to theoretically verify the advantages that can be exploited from the adoption of various VVT (Variable Valve Timing) or VVA (Variable Valve Actuation) systems, including those realizing a throttle-less operation of the engine. The gains predicted in terms of torque profile at WOT, and of BSFC or NOx emissions at part load are quantified and discussed. The main objective is the development of a methodology, which can substantially reduce experimental activity and help the definition of optimal control strategies of a VVT-equipped engine.

It is well known that the downsizing philosophy allows the improvement of Brake Specific Fuel Consumption (BSFC) at part load operation for spark ignition engines. On the other hand, the BSFC is penalized at high/full load operation because of the knock occurrence and of further limitations on the Turbine Inlet Temperature (TIT). Knock control forces the adoption of a late combustion phasing, causing a deterioration of the thermodynamic efficiency, while TIT control requires enrichment of the Air-to-Fuel (A/F) ratio, with additional BSFC drawbacks.In this work, a promising technique, consisting of the introduction of a low-pressure cooled exhaust gas recirculation (EGR) system, is analyzed by means of a 1D numerical approach with reference to a downsized turbocharged SI engine. Proper “in-house developed” sub-models are used to describe the combustion process, turbulence phenomenon and the knock occurrence.The effects of cooled gas recirculation are studied for various EGR and A/F levels. The presented results highlight that the proposed solution involves significant BSFC improvements. In particular, it is able to reduce the engine knock tendency, resulting in the possibility to advance the combustion phasing. Moreover, it involves a TIT reduction that allows to limit the over-fuelling.The developed numerical procedure is able to take into account the complex interactions among different driving parameters affecting the fuel consumption. It can be hence very useful to define a numerical engine pre-calibration and to realistically predict the EGR-related BSFC advantages which can be gained at different speed and load conditions.