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A new test bench has been set up and equipped in order to analyze the air mean motion and turbulence quantities in the combustion system of an automotive diesel engine with one helicoidal intake duct and a conical type in-piston bowl. A sophisticated HWA technique employing single- and dual-sensor probes was applied to the in-cylinder flow investigation under motored conditions. The anemometric probe was also operated as a thermometric sensor. An analytical-numerical procedure, based on the heat balance equations for both anemometric and thermometric wires, was refined and applied to compute the gas velocity from the anemometer output signal. The gas property influence, the thermometric sensor lag and the prong temperature effects were taken into account with this procedure. The in-cylinder velocity data were reduced using both a cycle-resolved approach and the conventional ensemble-averaging procedure, in order to separate the mean flow from the fluctuating motion.

The combustion propagation and burned-gas expansion processes in a bi-fuel CNG SI engine were characterized by applying a newly developed diagnostic tool, in order to better understand how these processes are related to the fuel composition, to the engine operating variables as well as to the exhaust emissions. The diagnostic tool is based on an original multizone heat-release model that is coupled with a CADmodel of the burned-gas containing surface for the computation of the burning speed and the burned-gas mean expansion velocity. Furthermore, the thermal and prompt NO sub-models, embedded in the diagnostic code, were employed to study the effects ofNO formation mechanisms and thermodynamic parameters on nitric oxide emissions.

The simulation activity for the piston bowl development in a low compression ratio (CR) high-performance diesel engine is described, starting from the calibration of a 3-D CFD commercial code by pressure-based combustion diagnostics data. Calibration was made for the baseline engine built by GMPT-E, matching experimental pressure traces and heat release rates derived from these through the diagnostic tool. Measured pollutant emissions were also applied for calibration at this stage. The engine was susceptible to modifications, according to the outcomes of combined simulation and experimental investigations. The validated CFD model was used for the screening of three new piston bowls featuring a reduced compression ratio. The 3-D code has been integrated with a robustly calibrated 1-D hydraulic model for the injection system simulation and with a 1-D fluidynamic tool for modeling engine flow processes external to the cylinder to provide quite accurate boundary conditions.

A new combustion system with a low compression ratio (CR), specifically oriented towards the exploitment of partially Premixed Charge Compression Ignition (PCCI) diesel engines, has been developed and tested. The work is part of a cooperative research program between Politecnico di Torino (PT) and GM Powertrain Europe (GMPT-E) in the frame of Low Temperature Combustion (LTC) diesel combustion-system design and control. The baseline engine is derived from the GM 2.0L 4-cylinder in-line, 4-valve-per-cylinder EU5 engine. It features a CR of 16.5, a single stage VGT turbocharger and a second generation Common Rail (1600 bar). A newly designed combustion bowl was applied. It features a central dome and a large inlet diameter, in order to maximize the air utilization factor at high load and to tolerate advanced injection timings at partial load. Two different piston prototypes were manufactured by changing the internal volume of the new bowl so as to reach CR targets of 15.5 and 15.

An innovative hydraulic layout for Common Rail (C.R.) fuel injection systems was proposed and realized. The rail was replaced by a high-pressure pipe junction to have faster dynamic system response during engine transients, smaller pressure induced stresses and sensibly reduced production costs. Compared to a commercial rail, whose inside volume ranges from 20 to 40 cm3, such a junction provided a hydraulic capacitance of about 2 cm3 and had the main function of connecting the pump delivery to the electroinjector feeding pipes. In the design of the novel FIS layout, the choice of high-pressure pipe dimensions was critical for system performance optimization. Injector supplying pipes with length and inner diameter out of the actual production range were selected and applied, for stabilizing the system pressure level during an injection event and reduce pressure wave oscillations.

A production SI engine originally designed at Fiat Auto to operate with unleaded gasoline was converted to run on natural gas. To that end, in addition to designing and building the CNG fuel plant, it was necessary to replace the multipoint electronic module for injection-duration and ignition-timing control with an ECM designed to obtain multipoint sequential injection. The engine was modified so as to work either with gasoline or natural gas. For the present investigation, however, the engine configuration was not optimized for running on methane, in order to compare the performance of the engine operated by the two different fuels with the same compression ratio. In fact, the engine is also interesting as a dual-fuel engine because of its relatively high compression ratio ≈10.5 that is almost suitable for CNG operation. The engine had the main features of being a multivalve, fast-burn pent-roof chamber engine with a variable intake-system geometry.

An integrated theoretical and experimental investigation was carried out in order to evaluate the effects that the pump delivery-valve assembly can produce on the performance of a pump-line-nozzle fuel-injection system with a distributor-type pump for automotive diesel engines. Four distinct delivery valves, one constant-pressure valve, one reflux-hole and two relief-volume valves, were separately fitted to the pump and for each configuration of the delivery assembly the system behavior was analyzed under full-load steady-state operations in a wide pump angular-speed range. Fuel injection-rate as well as local pressure time-histories were investigated, paying specific attention to the occurrence and temporal evolution of cavitation phenomena in the pressure pipe and injector nozzle, after the valve closure. The flow across the delivery-valve assembly was theoretically examined in order to ascertain any instability sources as possible causes of cyclic fluctuations.

A numerical model for simulating a Common Rail Piezo Indirect Acting fuel injection-system under steady state as well as transient operating conditions was developed using a commercial code. A 1D flow model of the main hydraulic system components, including the rail, the rail to injector connecting pipe and the injector, was applied in order to predict the influence of the injector layout and of each part of the hydraulic circuit on the injection system performance. The numerical code was validated through the comparison of the numerical results with experimental data obtained on a high performance test bench of the Moehwald-Bosch MEP2000/ CA4000 type. The developed injection-system mathematical model was applied to the analysis of transient flows in the hydraulic circuit paying specific attention to the fluid dynamics internal to the injector.

A combustion simulation code for the prediction of heat release, flame propagation speed and pollutant formation in SI engines was developed and assessed. It is based on a multizone combustion model that takes the non-uniform spatial distribution of the in-cylinder burned-gas thermochemical properties into account. The multizone approach for burning rate calculation is coupled with a CAD procedure for the evaluation of burned-gas front area and radius. Specifically developed sub-models for determining CO and NO formation are included in the code. An original model based on the fractal geometry concept was used to describe the entrainment of fresh mixture through the flame front.

The necessity for further reductions of in-cylinder pollutant formation and the opportunity to minimize engine development and testing times highlight the need of engine thermodynamic cycle simulation tools that are able to accurately predict the effects of fuel, design and operating variables on engine performance. In order to set up reliable codes for indicated cycle simulation in SI engines, an accurate prediction of heat release is required, which, in turn, involves the evaluation of in-cylinder turbulence generation and flame-turbulence interaction. This is generally pursued by the application of a combustion fractal model coupled with semi-empirical correlations of available geometrical and thermodynamical mass-averaged quantities. However, the currently available correlations generally show an unsatisfactory capability to predict the effects of flame-turbulence interaction on burning speed under the overall flame propagation interval.

The burned-gas propagation process has been characterized in two bi-fuel engines by means of a combustion diagnostic tool resulting from the integration of an original multizone heat-release model with a CAD procedure for the burned-gas front geometry simulation. Burned-gas mean expansion speed ub, mean gas speed ug and burning velocity Sb were computed as functions of crank angle and burned-gas radius for a wide range of engine speeds (n = 2000-5500 rpm), loads (bmep = 200-790 kPa), relative air-fuel ratios (RAFR = 0.80-1.60) and spark advances (SA ranging from 8 deg retard to 8 deg advance from MBT), under both gasoline and CNG operations. Finally, the influence of intake runner and combustion chamber geometries on flame propagation process was investigated. Main results show that Sb is generally comparable for the engine running on both gasoline and CNG, at the same engine speed and load, under stoichiometric and MBT operations.

A detailed experimental analysis of the pressure wave propagation phenomena and fuel-injection system dynamics in a last-generation C.R Multijet system was carried out on a high performance new test bench Moehwald-Bosch MEP2000-CA4000 under real engine simulated conditions. Specific attention was paid to the wave propagation induced pressure oscillations as well as to their relationships with system control parameters and multiple-injection performance. In particular the authors aim at illustrating how the geometrical features of each component of the injection-system hydraulic layout influence the aforementioned wave motion providing a key for design optimization in order to obtain efficient multiple injection performance. A parametric analysis on the whole injection system performance when multiple injections are executed in an injection cycle has been carried out, taking the most important geometric features, i.e. pipe lengths, pipe diameters, and rail volume, as test parameters.

A systematic experimental investigation was undertaken to compare the fuel consumption and exhaust emissions of a production SI engine fueled by either gasoline or compressed natural gas (CNG). The investigation was carried out on a two-liter four-cylinder engine featuring a fast-burn pent-roof chamber, one centrally located spark plug, four valves per cylinder and variable intake-system geometry. The engine was originally designed at Fiat to operate with unleaded gasoline and was then converted at Politecnico di Torino to run on CNG. A Magneti Marelli IAW electronic module for injection-duration and spark-advance setting was used to obtain a carefully controlled multipoint sequential injection for both fuels.

An experimental investigation of fuel consumption, exhaust emissions and heat release was performed on a prototype 1.2 liter 4 cylinder turbocharged CNG engine, which has been specifically developed and optimized in order to fully exploit natural gas potential. More specifically, the combination of a high CR of 10.1:1 and a Garrett high-performance turbocharger featuring selectable levels of boost produced a favorable efficiency map, with peak values exceeding 35%. The experimental tests were carried out in order to assess the engine performance improvement attainable through turbocharging and to define the best control strategies for this latter. The investigation included ample variations of engine speed and load, RAFR as well as trade-offs between boost level and throttle position. At each test point, in-cylinder pressure, fuel consumption and ‘engine-out’ pollutant emissions, including methane unburned hydrocarbons concentration, were measured.

The present work concerns the study of the potentialities of high-boost small-displacement C.R. (Common Rail) diesel engines where the compressor and the expander are mechanically disengaged for the purpose of power cogeneration from the exhaust gas. This objective can be achieved by means of advanced concept electrical devices capable of delivering the energy produced by the expander either to the drivetrain transmission or to the even more power-demanding auxiliary equipment of both the engine and the vehicle. The performance of a small-displacement boosted diesel engine with a common-rail injection system has been predicted by means of a computational code obtained by integrating different in-house non-commercial codes that simulate the intake, combustion and exhaust processes. The model validation has been carried out by means of the experimental data obtained at Fiat Research Center on a commercial small-displacement C.R. turbocharged diesel engine.

A comparative investigation of both fuel consumption and exhaust emissions has been carried out on a production SI engine operated by either gasoline or compressed natural gas (CNG). The engine had the main features of being a multivalve, fast-burn pent-roof chamber engine with a variable intake-system geometry. It was originally designed at Fiat Auto to operate with unleaded gasoline and was then converted at Politecnico di Torino to run on CNG. To that end, in addition to designing and building the CNG fuel plant, the multipoint Bosch Motronic M1.7 electronic module for injection-duration and spark-timing control was replaced with a Magneti-Marelli IAW ECM designed to obtain a multipoint sequential injection. With this new ECM, the engine was modified so as to work with either natural gas or gasoline.

The variation of turbulent flow quantities with engine speed has been investigated in the combustion chamber of an automotive diesel engine with a high-squish conical-type in-piston bowl and one helicoidal intake duct, at speeds covering the wide range of 600-3000 rpm, under motored conditions. The investigation had the main purpose of studying the engine speed effect on the structure of both cycle-resolved and conventional turbulence over the induction, the compression and the early stage of the expansion stroke. The low frequency component of the fluctuating motion was also investigated.

Temperature variations due to compressibility effects of the liquid fuel were evaluated, for the first time in high-pressure injection system simulation, by employing the energy conservation equation, in addition to the mass-continuity and momentum-balance equations, as well as the constitutive state equation of the liquid. To that end, the physical properties (bulk elasticity modulus, thermal expansivity, kinematic viscosity) of the fluid were used as analytic functions of pressure and temperature obtained by interpolating carefully determined experimental data. Consistent with negligible thermal effects of heat transfer and viscous power losses involved in the flow process, the equation of energy was reduced to a state relation among the fluid thermodynamic properties, leading to a barotropic flow model.

A contribution has been given to the thermodynamics approach usually used for analyzing the combustion process in IC engines on the basis of cylinder pressure data reduction. A survey of heat release type combustion models and of their calibration methods has first been carried out with specific attention paid to the bulk gas-wall heat transfer correlations used. Experimental results have given evidence that most of these correlations are incapable of predicting the phase shift occurring between the gas-wall temperature difference and the heat transfer during the engine compression and expansion strokes, owing to the transient properties of the fluid directly in contact with the wall. This work develops and applies a refined procedure for heat release analysis of cylinder pressure data including the unsteadiness effects of the convective heat transfer process.