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Technical Paper

Turbulence Length Scale Measurements by Two-Probe-Volume LDA Technique in a Diesel Engine

Cycle resolved LDA measurements of the tangential velocity component, made along a diameter of two combustion chambers (toroidal and square) during the compression stroke of a diesel engine operating at 600 and 1000 rpm, are discussed. Indirect measurements of lateral integral length scales determined by single point autocorrelation technique are presented. Finally direct measurements of lateral integral length scales made by a new laser doppler velocimeter system based on two probe volume technique are reported.
Technical Paper

Thermal Barriers Adoption in D.I. Diesel Engines: Effect on Smoke and Gaseous Emissions

The paper describes some experiments carried out on two d.i. Diesel engines running with insulated pistons. Three different thermal barriers were tested; namely, a stainless steel cup, a Si3N4 cup and a stainless steel piston crown. The combustion process was characterized by heat release calculation and ignition delay measurements. The experiments showed that the indicated efficiency is not affected by thermal insulation adoption, Nox level increases while smoke level decreases consistently.
Technical Paper

The Role of Radical Species in Diesel Engine Auto-Ignition Detection

Ignition delay in diesel engine combustion comprehends both a chemical and a physical amount, the first depending on fuel composition and charge temperature and pressure, the last resulting of time needed for the fuel to atomize, vaporize and mix with air. Control of this parameter, which is mandatory to weight the relative amount of premixed to diffusive stage of the hydrocarbon combustion, is here considered. Experimental measurements of flame intensity spectra obtained by in situ measurements on an optically accessible test device show the presence of peaks corresponding to radicals as OH and CH appearing at the pressure start of combustion. Since OH radicals result from chain branching reactions, a numerical simulation is performed based on a reduced kinetic scheme which allows to measure the branching agent concentration, and whose approximate nature is adequate to the proportion chemical aspects contribute to the overall delay.
Technical Paper

Spectral Analysis of Combustion Process of Common Rail Diesel Engine

Polychromatic extinction and chemiluminescence techniques, from ultraviolet to visible, were applied in an optical diesel engine, in order to analyze the temporal and spatial evolution of a high pressure fuel jet interacting with a swirling air motion. A fully flexible Common Rail fuel injection system equipped with a single hole nozzle was used. The experiments were performed at fixed engine speed and air/fuel ratio for three injection strategies. The first one consisted of a main injection to compare with those operating at low pressure injection. The other ones were based on a pilot and main injections, typical of current direct injection diesel engines, with different dwell time. A detailed investigation of the mixture formation process inside the combustion chamber during the ignition delay time was performed. The liquid and vapor fuel distribution in the combustion chamber was obtained analyzing the polychromatic extinction spectra.
Technical Paper

Numerical Study of the Combustion Chamber Shape for Common Rail H.S.D.I. Diesel Engines

The Common-rail injection system has allowed achieving a more flexible fuel injection control in DI-diesel engines by permitting a free mapping of the start of injection, injection pressure, rate of injection. All these benefits have been gained by installing this device in combustion chambers born to work with the conventional distributor and in-line-pump injection systems. Their design was aimed to improve air-fuel mixing and therefore they were characterized by the adoption of high-swirl ports and re-entrant bowls. Experiments have shown that the high injection velocities induced by common rail systems determine an enhancement of the air fuel mixing. By contrast, they cause a strong wall impingement too. The present paper aims to exploit a new configuration of the combustion chamber more suited to CR injection systems and characterized by low-swirl ports and larger bowl diameter in order to reduce the wall impingement.
Technical Paper

Numerical Study Towards Smoke-Less and NOx-Less HSDI Diesel Engine Combustion

This paper explores the possibility to extend the low-temperature combustion concept developed for low load conditions to medium load conditions of HSDI DI Diesel engines. The aim is to understand which is the limit of conventional Diesel combustion towards smoke-lees and NOx-less conditions. The present research is based on numerical simulations performed by using the Kiva-3 code updated with physical sub-models. The combined influence of EGR cooling and EGR rate on combustion characteristics and emission formation is analyzed. Then, possible improvements to mixture formation are discussed with particularly emphasis on the use of multiple injection. The calculations show that smoke-less conditions by low-temperature combustion cannot be achieved at medium load and therefore a great role is played by mixture formation.
Technical Paper

Numerical Analysis of Passenger Car HSDI Diesel Engines with the 2nd Generation of Common Rail Injection Systems: The Effect of Multiple Injections on Emissions

A second generation of Common-Rail injection systems is coming into production making feasible multiple injection strategies. This paper aims to assess the capability of multiple injection in reducing NOx and soot emissions of HSDI Diesel engines. The analysis has been carried out at a characteristic point of the EUDC emission test cycle by using a customized version of the CFD code Kiva3, with updated sub-models developed by University of Bologna and University of Wisconsin. In particular, a recent modification has been introduced in the fuel conversion rate calculation in order to account for turbulence non-equilibrium effects. Different multiple injection profiles and combustion chamber configurations have been simulated and their effects on mixture formation, heat release rate and NOx and soot formation have been analyzed. The main target was to comply with emission standards without significant loss in engine performance.
Technical Paper

Nanometric Particle Formation in Optically Accessible Engine Diesel

In the last years, there has been an increasing concern on the emission of ultrafine particles in the atmosphere. A detailed study of formation and oxidation of these particles in the environment of the diesel cylinder presents many experimental difficulties due to the high temperatures, pressures and extremely reactive intermediate species. In this paper, in order to follow the different phases of diesel combustion process, high temporal and spatial resolution optical techniques were applied in the optically accessible chamber of diesel engine, at 2000 rpm and A/F=80:1 and 60:1. Simultaneous extinction, scattering and flame chemiluminescence measurements from UV to visible were carried out, in order to study the diesel combustion process from the droplet ignition to the formation of soot, through the growth of its precursors.
Technical Paper

LDV Measurements of Integral Length Scales in an IC Engine

Tangential component of velocity and turbulence were measured in three locations in the re-entrant combustion chamber of a motored single-cylinder d.i. Diesel engine (0.435 liter, 21:1 compression ratio) using a Laser Doppler Velocimetry system. Moreover, a modified LDV system with two-probe volume was used to measure directly lateral integral length scales of the velocity tangential component at two engine speeds. The measurements were made on a horizontal plane at 5 mm below the engine head from 100 degrees before TDC to 60 degrees after TDC of both the compression and expansion strokes. The engine was motored at 1,000 and 1,500 rpm respectively. An ensemble-averaging technique was performed to analyze the instantaneous velocity information supplied by two Burst Spectrum Analyzers. The lateral integral length scale was obtained from the integral of the spatial correlation coefficient of the velocity fluctuation for different separation.
Technical Paper

Investigation of the intake tumble flow in a prototype GDI engine using a steady-state test rig

An experimental and numerical investigation, using the Laser Doppler Anemometry (LDA) technique and a 3D fluid-dynamic code (KIVA 3V), was carried out in a prototype engine under steady-state conditions. The aim of the present activity was the flow field characterization and the effect of the intake geometry on the in-cylinder tumble flow. A new steady flow test rig designed for capturing the tumble motion within a test cylinder, made by a blower and an engine head, was assembled to simulate the intake flow. The engine head was mounted on an aluminum cylinder, having the same bore as the real engine. The cylinder was provided with optical accesses on the periphery and a flat optical window located at the bottom to a depth equal to the stroke of the engine. The cylinder was also equipped with two cylindrical ducts, used as air outflow ports.
Technical Paper

Investigation of Mixture Formation Process in a HDDI Diesel Engine by CFD and Imaging Technique

The paper aims at providing information about the spray structure and its evolution within the combustion chamber of a heavy duty direct injection (HDDI) diesel engine. The spray penetration is investigated, firstly under quiescent conditions, injecting the fuel in a vessel under ambient temperature and controlled back pressure by both numerical and experimental analyses using the STAR-CD code and the imaging technique, respectively. Experimental results of fuel injection rate, fuel penetration, and spray cone angle are used as initial conditions to the code and for the comparison of predictions. The experimental investigation is carried out using a mechanical injection pump equipped by the heavy duty eight cylinder engine. Only one of its plungers has been activated and the fuel is discharged through a seven holes mechanical injector, 0.40 mm in diameter.
Technical Paper

Interpretation of Air Motion in Reentrant Bowl in-Piston Engine by Estimating Reynolds Stresses

The present paper aims at investigating the flow field behavior within a reciprocating engine under motoring conditions. Simultaneous two velocity components of the air velocity were acquired at different engine speeds within the cylinder at different radii from the cylinder axis. Mean motion, integral time scales and Reynolds shear stresses, for the radial and tangential components, were estimated from the instantaneous velocity data by applying an ensemble averaging technique. The integral time scale was obtained from the single point time autocorrelation function whereas, the Reynolds shear stresses were computed through the estimate of the degree of the fluctuations correlation. Tests, carried out at 1,000, 1,500, and 2,000 rpm, showed that the tangential mean motion scales approximately with engine speed whereas a radial inward motion can be observed during the last part of compression.
Technical Paper

In-cylinder optical analysis of CRDI diesel engine combustion

The optimization of diesel engine performance and emissions can be achieved through a better understanding of the in-cylinder combustion process. Advanced non-intrusive optical techniques are providing new tools for investigating the thermo-fluid dynamics processes as well as they are contributing to develop predictive models for DI diesel combustion. High-speed images of spray and flame evolution as well as UV-visible chemiluminescence measurements were carried out in an optical 0.5-liter, single-cylinder, four-stroke, direct- injection diesel engine equipped with a prototype four valves cylinder head and a fully flexible CR injection system. In order to evaluate the effect of different injection strategies on the combustion process, measurements were performed varying injection parameters. The ignition location and time were individuated by combustion visualization and detection of radical species, obtained by chemiluminescence measurements.
Technical Paper

Fuel Composition Effects on Air-Fuel Mixing and Self-Ignition in a Divided Chamber Diesel System by Optical Diagnostics

The influence of fuel composition on mixture formation and first stage of combustion, occurring in a small high swirl combustion chamber of an IDI Diesel engine, was analyzed from measurements of spectral extinction and flame emissivity. Measurements were carried out in an optically accessible combustion chamber in which an air swirling flow is forced from the main chamber through a tangential passage. A conventional injection system was used to inject Tetradecane, N-heptane and Diesel fuel. The distribution of liquid and vapor and the interaction of the jet with air swirl were detected by UV-visible extinction measurements. The autoignition phase was characterized by UV-visible chemiluminescence measurements. For all fuels examined, it was observed that initially the liquid fuel penetrates almost linearly with time until reaching a maximum characteristic length, slightly dependent on the fuel.
Technical Paper

Fluid-Dynamic Analysis of the Intake System for a HDDI Diesel Engine by STAR-CD Code and LDA Technique

The paper illustrates an experimental and numerical investigation of the flow generated by an intake port model for a heavy duty direct injection (HDDI) Diesel engine. Tests were carried out on a steady state air flow test rig to evaluate the global fluid-dynamic efficiency of the intake system, made by a swirled and a directed port, in terms of mass flow rate, flow coefficients and swirl number. In addition, because the global coefficients are not able to give flow details, the Laser Doppler Anemometry (LDA) technique was applied to obtain the local distribution of the air velocity within a test cylinder. The steady state air flow rig, made by a blower and the intake port model mounted on a plexiglas cylinder with optical accesses, was assembled to supply the actual intake flow rate of the engine, setting the pressure drop across the intake ports atûP=300 and 500 mm of H2O.
Technical Paper

FE Fluid-Structure Interaction/Experimental Transmission Loss Factor Comparison of an Exhaust System

The purpose of this paper is to present a 3-D Finite Element Method able to simulate and predict exhaust transmission noise. The simulation takes into account fluid flow pulsation, aeroacoustic noise sources, flow-induced structural vibration as well as noise radiation in the far field of the exhaust system of a single-cylinder diesel engine. In order to study problems on fluid-structure interaction a 3-dimensional model has been created through a FEM simulation. The study includes the calculation of the transmission loss factor and the determination of which modes dominate the noise transmission. Simultaneously, an experimental measurement has been performed, in order to determine the transmission loss factor of the tested system. Finally, an experimental-numerical comparison has been performed. Experimental and numerical results have been compared and a good agreement has been found.
Technical Paper

Evaluation of the Effects of a New Combustion System and Catalyst on Engine Emissions

The present paper reports the results of an experimental investigation carried out on a four-stroke single- cylinder D.I. diesel engine (100 x 95mm bore x stroke) with the aim to evaluate the effects of a four-lobe square combustion chamber on the gaseous and particulate emissions. Fluid-dynamic behaviour of the axisymmetric toroidal and four-lobe square chambers was investigated by Laser Doppler Anemometry. Engine tests at 2000 and 3000 rpm for different start of combustion (SOC) and A/F ratio are reported. Particulate, HC and NOx emission index measured under different operating conditions are given. In addition, the volatile content of the particulates produced from the two chambers at various engine operative conditions was measured by thermogravimetric analysis (TGA). Finally, the catalytic activity of a metal-oxide-based catalyst in the combustion of particulate was also evaluated by TGA.
Technical Paper

Evaluation of Fluid-Mechanic Behavior of Toroidal and Square, Four-Lobe Combustion Chamber by LDA

LDV measurements of the tangential component of the flow field velocity within the cylinder of a motored, real diesel engine are reported. A comparison of the fluid-dynamic behavior between toroidal and four-lobes square combustion chambers is shown. Tests were carried out over a range of engine speed of 500 and 1500 rpm. An ensemble-average data processing technique was used to analyze the velocity data recorded at 30 CAD BTDC and at TDC during compression stroke. The measurements were made at a depth of 3.0 mm from the piston head in two axial sections of the four-lobe combustion chamber. The results show that the four-lobe, square combustion chamber reduces the bulk swirl and increases the turbulence at the two engine speeds. The square cup transforms more of the kinetic energy of the bulk flows into turbulent kinetic energy than toroidal cup. At TDC the tangential velocity profile tends to solid body along the short section and to flat profile along the long section.
Technical Paper

Effect of Fuel Quality on the Performance of High-Speed Direct Injection Diesel Engines

Two fuels having different aromatics content and different cetane numbers were tested in a direct injection diesel engine with thermally insulated pistons. Actually tests were carried out with a full aluminum piston, an aluminum piston modified to accept a stainless steel crown and a similar one coated with ceramic. Higher combustion noise and emissions were detected using the degraded fuel, having fixed the type of piston. Furthermore, the experiments showed that thermal barrier adoption has a positive effect on the combustion noise.
Technical Paper

Effect of Combustion Chamber Shape on Air Flow Field in a D.I. Diesel Engine

The behaviour of two combustion chambers, a toroidal and a turbulent one, has been compared. The engine performance in terms of imep and exhaust emissions were measured. Laser Doppler Anemometry technique was used to characterize the fluids dynamic aspect of combustion system. The axial asymmetry introduced in combustion chamber shape causes strong differences in the air flow field at the end of compression stroke. The tangential velocity profile is flattened to that obtained with toroidal chamber. Moreover the rms values of tangential velocity measured in turbulent combustion chamber are about three times higher than that measured in the toroidal chamber. At low engine speed the turbulent chamber allows to operate with low NOx levels without penalties of smoke emissions and fuel consumption as happens by using conventional toroidal chamber.