Paolo Sementa

Knocking diagnostics in the combustion chamber of boosted port fuel injection spark ignition optical engine

High spatial resolution optical techniques have been used to get information about the timing and the location of knocking and about the chemical species involved in this phenomenon. The experiments were realised in the combustion chamber of a boosted single-cylinder spark ignition port-fuel injection optical engine fuelled with commercial gasoline. Engine conditions with different levels of knock were considered from the borderline case onto standard knocking and heavy knocking. Cycle-resolved digital imaging was used to follow the combustion and the flame propagation in normal combustion and knocking conditions. Moreover, the effects of an abnormal combustion due to the firing of fuel deposition near the intake valves and on the piston surface were investigated. The knocking influence on the flame front propagation and combustion speed was investigated following the time evolution of the mean flame radius in the different engine conditions. The appearance of the auto-ignition centres in the end gas during the knock was evaluated in terms of timing, location and frequency of occurrence. Finally, UV-visible natural emission spectroscopy was applied to detect radical species that marked the knock. HCO and OH were identified as markers of the knocking onset and OH of its intensity.

Optical investigations of the early combustion phase in spark ignition boosted engines

An improved understanding of the thermo-fluid dynamic phenomena that occur during the combustion process in boosted spark ignition engines is necessary for future developments in these engines. In particular, increased understanding of flame kernel evolution is fundamental since the initial growth of the flame contributes significantly to cycle-to-cycle variation in engine performance and emissions. In this work, flame inception and early stages of the combustion process were investigated in an optically accessible single-cylinder ported fuel injection engine. This engine was equipped with the cylinder head of new generation spark ignition turbocharged engine with the same characteristics as the research engine. Cycle-resolved visualization was applied to follow the flame propagation from the spark ignition to the flame kernel growth. A retrieval procedure for the optical data was realized to obtain information about the flame radius evolution. Natural emission spectroscopy in the ultraviolet–visible wavelength range allowed the detection of the chemical markers of the combustion process such as CH, OH, and HCO radicals, and formaldehyde molecules. In-cylinder optical investigations were correlated with engine parameter measurements obtained by conventional methods.

Analysis of flame kinematics and cycle variation in a Port Fuel Injection Spark Ignition Engine

ABSTRACT This paper reports on the analysis of flame kinematics and cycle variation in port fuel injection (PFI) spark ignition (SI) engine. The engine was equipped with a four-valve head and with an external boost device. Different operating conditions were considered. Cycle-resolved digital imaging was used to investigate flame motion and the effects of an abnormal combustion due to the firing of fuel deposition near the intake valves and on the piston surface. Various algorithms are applied to the acquired images. Coefficients of Proper Orthogonal Decomposition (POD) were computed and used for a statistical analysis of cycle variability. The advantage is that the analysis can be run on a small number of scalar coefficients rather than on the full data set of pixel valued luminosity. POD modes are then discriminated by means of normality tests, to separate the mean from the coherent and the incoherent parts of the fluctuation of the luminosity field, in a non truncated representation of the data.

Effects of lubricant oil on particulate emissions from port-fuel and direct-injection spark-ignition engines:

This work presents experimental tests where lubricant oil was added to the engine in order to highlight its contribution to particle emissions from both gasoline and compressed natural gas spark-ignition engines. Three different ways of feeding the extra lubricant oil and two fuel-injection modes—port fuel injection and direct injection—were investigated to mimic the different ways by which lubricant may reach the combustion chamber. In particular, in the tests using compressed natural gas, the oil was injected either into the intake manifold or directly into the combustion chamber, whereas in both the port-fuel-injection and direct-injection tests using gasoline, the oil was premixed with the fuel. The experiments were performed on a single-cylinder, optically accessible spark-ignition engine, running at 2000 r/min under stoichiometric and full-load conditions, and requiring no lubrication. Particle size distribution functions were measured in the range from 5.6 to 560 nm by means of an engine exhaust particle sizer. Particle samples were taken directly from the exhaust flow, just downstream of the valves. Opacity was measured by an AVL 439 opacimeter, and gaseous emissions were measured by means of an exhaust gas analyzer in order to globally monitor the combustion process. Detailed analysis of the recorded total particulate number and particle size distributions allowed to determine the size ranges and relative amounts associated with the lubricant-oil-derived particles. Oil addition produced a significant increase in the particles emitted in the lowest range size, independent of the way lubricant was added. Only when lubricant was injected directly into the combustion chamber (either blended with the fuel or by itself), an increase in the number of particles with sizes larger than 50 nm was recorded.

CFD Modeling of a Mixed Mode Boosted GDI Engine and Performance Optimization for the Avoidance of Knocking

The paper applies simulation techniques for the prediction and optimization of the thermo-fluid-dynamic phenomena characterizing the energy conversion process in a GDI engine. The 3D CFD model validation is realized on the ground of experimental measurements of in-cylinder pressure cycles and optical images collected within the combustion chamber. The model comprehends properly developed submodels for the spray dynamics and its impingement over walls. This last is particularly important due to the nature of the mixture formation mode, being wall-guided. Both homogeneous stoichiometric and lean stratified charge operations are considered. In the case of stoichiometric mixture, the possible occurrence of knocking is also accounted for by means of a submodel able to reproduce the preflame chemical activity. The CFD tool is finally included in a properly formulated optimization problem aimed at minimizing the engine-specific fuel consumption with the avoidance of knocking through a non-evolutionary algorithm.

A comprehensive analysis of the impact of biofuels on the performance and emissions from compression and spark-ignition engines:

The compression ignition and small displacement spark-ignition engines play an important role in the urban air pollution. In particular, vehicles equipped with compression ignition engines are widely used because of their higher performance and fuel efficiency with respect to the spark-ignition ones. Nevertheless, spark-ignition engines with low displacements are even more wide-spreading because of the lower fuel consumption and emissions. They are also used for two-wheeled vehicles, whose easier navigation makes them widely used in heavily congested areas. Their contribution on urban pollution is worsened by the fact that these vehicles have to comply with the Euro 3 standard; light vehicles have, instead, to fulfill the more restrictive Euro 6, which for the compression ignition and gasoline direct injection engines indicates for particle emissions also a number-based regulation. This article aims to characterize the effects of biofuels on engine emissions and performance of compression ignition and spark-ignition engines. The investigation was carried out on different class of engines. Direct injection and a port fuel injection spark-ignition engines fueled with ethanol and its blends, 10 v/v%, 50 v/v% and 85 v/v% of ethanol in gasoline. The compression ignition engine was equipped with a common rail injection system and was fueled with pure rapeseed methyl ester, representative of fatty acid methyl ester, and its blends in diesel, 20 v/v% and 50 v/v%. The gaseous emissions and the particle concentration were measured at the exhaust by means of conventional instruments. Particle size distribution function was measured in the range from 5.6 to 560 nm by means of an engine exhaust particle sizer. A comprehensive characterization of the particulate carbon was performed by means of optical diagnostics in the combustion chamber. In particular, two-dimensional images of flame evolution were detected and processed by two-color pyrometry technique to assess the in-cylinder soot formation and oxidation processes. For both the investigated spark-ignition and compression ignition engines, the use of biofuels shows a partial increase in the specific fuel consumption and a reduction of the soot particles emission. Nevertheless, a further effort on engine technology should be paid to balance the mass with the size and number of the particles.

Modeling and performance optimization of a direct injection spark ignition engine for the avoidance of knocking

The paper applies simulation techniques for the prediction and optimization of the thermo-fluid-dynamic phenomena characterising the energy conversion process in an internal combustion engine. It presents the development and validation of a 3D CFD model for a GDI optically accessible engine operating either under stoichiometric homogeneous charges or under overall lean mixtures. The model validation is realized on the ground of experimental measurements of the in-cylinder pressure cycle and of the available optical images. The model comprehends properly developed sub-models for the spray dynamics and the spray-wall interaction. This last is particularly important due to the nature of the mixture formation mode, being of the wall-guided type. In the stoichiometric mixture case, the possible occurrence of knocking is also considered by means of a sub-model able to reproduce the pre-flame chemical activity. The CFD tool is finally included in a properly formulated optimization problem aimed at minimizing the engine specific fuel consumption with the avoidance of knocking. The optimization, performed through a non-evolutionary algorithm, allows determining the best engine control parameters (spark advance and start of injection).

Optimization of the compressed natural gas direct injection in a small research spark ignition engine

The permanent aim of the automotive industry is the further improvement of the engine efficiency and the simultaneous pollutant emissions reduction. In order to optimize the small internal combustion engines, it is necessary to further improve the basic knowledge of the thermo-fluid dynamic phenomena occurring during the combustion process. In this context, the application of optical diagnostic techniques permits a deep insight into the fundamental processes such as flow development, fuel injection, and combustion process. The aim of this study was the optimization of the compressed natural gas direct injection by means of the analysis of the injection phase and combustion process. This analysis allowed the improvement of the engine efficiency in lean-burn operation condition too. The investigation was carried out in an optically accessible small direct injection spark ignition single-cylinder engine. Two different injectors were tested. The first one was the injector designed according to the results of model simulation, and the second one was a modified prototype obtained using the findings of the optical analysis carried out in the combustion chamber. The characteristic parameters of the gaseous fuel jet were evaluated through an image processing procedure. The fuel rail was modified in order to allow the injection of seeding particles into the gaseous fuel. The gas jet was analyzed using a white light source coupled with a high spatial and temporal resolution fast camera. The combustion process development was investigated for same engine operative condition performing a cycle-resolved visualization of the flame. Moreover, this methodology permitted the evaluation of motion field and turbulence during the injection process directly into the combustion chamber. The results of investigation evidenced that the modification of the injector, designed according to the optical analysis made in the optically accessible engine, allowed an optimization of compressed natural gas direct injection resulting in improved combustion and emissions reduction.

Optimization of a GDI engine operation in the absence of knocking through numerical 1D and 3D modeling

Various solutions are being proposed and adopted by manufactures and researchers to improve the energetic and environmental performance of internal combustion engines within the transportation sector. For automotive spark ignition engines, gasoline direct injection is one of the presently preferred technologies, in conjunction with turbocharging and downsizing. One of the limiting phenomena of this kind of engines, however, still remains the occurrence of knocking, namely the self-ignition of the so-called end-gas zones of the mixture, not yet reached by the flame front. This phenomenon causes strong in-cylinder pressure oscillations, high stress levels and even damage to engine components.Present work focuses on a numerical and experimental study of a turbocharged GDI engine and is aimed at assessing CFD-O (computational fluid dynamics optimization) procedures to be used in the phase of design as a decision making tool for the development of control strategies for a smooth and efficient operation. A preliminary experimental analysis is performed in order to characterize the considered engine and to investigate the phenomenon of knocking that occurs under some circumstances as the spark advance is increased. The collected data are employed to elaborate a predictive criterion for the appearance of this kind of abnormal combustion, as well as to validate both a 1D and a 3D model for the simulation of the engine working cycle. Various numerical optimization procedures are then realized to increase the engine power output and simultaneously avoid conditions leading to undesired self-ignitions. These are either based on the use of a non-evolutionary algorithm or employ a genetic algorithm in the case multiple contrasting objectives are set. The response surface methodology is also explored as a way to reduce the computational effort.

Optical Characterization of Methane Combustion in a Four Stroke Engine for Two Wheel Application

In the urban area the internal combustion engines are the main source of CO2, NOx and particulate matter (PM) emissions. The reduction of these emissions is no more an option, but a necessity highlighted by the even stricter emission standards. In the last years, even more attention was paid to the alternative fuels. They allows both reducing the fuel consumption and the pollutant emissions. Regarding the gaseous fuels, methane is considered one of the most interesting in terms of engine application. It represents an immediate advantage over other hydrocarbon fuels because of the lower C/H ratio. In this paper the effect of the methane on the combustion process, the pollutant emissions and the engine performance was analyzed. The measurements were carried out in an optically accessible singlecylinder, Port Fuel Injection, four-stroke SI engine equipped with the cylinder head of a commercial 250 cc motorcycles engine and fuelled both with gasoline and methane. Optical measurements were performed to analyze the combustion process with a high spatial and temporal resolution. In particular, optical techniques based on 2D-digital imaging were used to follow the flame propagation in the combustion chamber. UV-visible spectroscopy allows detecting the chemical markers of combustion process such as the radicals OH and CH. The exhaust emissions were characterized by means of a gaseous analyzer and an opacimeter. The measurements were performed under steady state conditions, at 2000rpm at minimum and full load. Introduction In the urban area the internal combustion engines are the main source of CO2, NOx and particulate matter (PM) emissions. The reduction of these emissions is no more an option, but a necessity highlighted by the even stricter emission standards. In the last years, even more attention was paid to the alternative fuels that allow both reducing the fuel consumption and the pollutant emissions. Methane is a promising alternative fuel to petrol for internal combustion engines [1]. It represents an immediate advantage over other hydrocarbon fuels because of the lower C/H ratio. Moreover, it has a higher Lower Heating Value (LHV) and stoichiometric air/fuel ratio and a higher Research Octane Number (RON) which permits higher compression ratios, higher boost in turbocharged engines, and better knocks limited XXXIV Meeting of the Italian Section of the Combustion Institute 2 spark advances, as reduces the knock sensitivity. The major drawback of the use of methane in the spark ignition or compression ignition engines is the low flame propagation speed. The flame front propagation speed depends mainly on the turbulence and the air/fuel ratio. In particular, it increases at the increasing of the turbulence and at the decreasing of the air/fuel ratio [2]. In this paper the effect of the methane on the combustion process, the pollutant emissions and the engine performance was analyzed. The measurements were carried out in an optically accessible single-cylinder, Port Fuel Injection, fourstroke SI engine equipped with the cylinder head of a commercial 250 cc motorcycles engine. Optical measurements were performed to analyze the combustion process with a high spatial and temporal resolution. In particular, optical techniques based on 2D-digital imaging were used to follow the flame propagation in the combustion chamber. UV-visible spectroscopy allows detecting the chemical markers of combustion process such as the radicals OH and CH. The measurements were performed under steady state conditions, at 2000 rpm at minimum and full load. The engine was fuelled with commercial gasoline and methane. Experimental Apparatus Transparent Engine The experimental activity was performed in an optically accessible single-cylinder, Port Fuel Injection, four-stroke SI engine [3]. The engine bore and stroke were 72 mm and 60 mm, respectively. The geometric compression ratio was 11:1. The engine was equipped with the cylinder head of a commercial 250 cc motorcycles engine. A four-valve, pent-roof chamber engine was mounted on an elongated piston. The engine reached a maximum speed of 5000 rpm. The maximum performance is: 7.9 kW and 14.7 Nm at 5000 rpm. The head had a centrally located spark plug and a quartz pressure transducer was flush-installed in the combustion chamber to measure the combustion pressure. The in-cylinder pressure, the rate of chemical energy release and the related parameters were evaluated on an individual cycle basis and/or averaged on 400 cycles [2]. The optical engine was characterized by an elongated cylinder and a piston provided with a sapphire window which replaces the flat-bottom piston bowl. The engine is also equipped with a quartz cylinder in order to have a lateral point of view of the combustion chamber. This system enables the passage of optical signals coming from the combustion chamber. To reduce the window contamination by lubricating oil, the elongated piston arrangement was used together with self-lubricating Teflonbronze composite piston rings in the optical section. Setup for Spectroscopic Measurements During the combustion process, the light passed through the sapphire window and it was reflected toward the optical detection assembly by a 45° inclined UV-visible XXXIV Meeting of the Italian Section of the Combustion Institute 3 mirror located in bottom of the engine. Chemiluminescence signals were collected and focused on the entrance slit of a spectrograph through an UV-Visible objective. The slit was 250 μm wide open and it was located in front of the combustion chamber. Spectrograph was 15 cm focal length, f/4 luminous, and equipped with a grating of 300 g/mm, blazed at 300 nm, with a dispersion of 3.1 nm/mm. The spectral image formed on the spectrograph exit plane was matched with a gated intensified CCD camera. Data were detected with the spectrograph placed at two central wavelengths, 375 and 625 nm, respectively, and the intensifier-gate duration was set to 166.6 μs in order to have a good accuracy in the timing of the different investigated events. Chemiluminescence signals, due to radical emission species, were detected in the central and lateral locations of the combustion chamber with high spatial and temporal resolution. Engine synchronization with ICCD camera was obtained by the unit delay connected to the signal coming from the engine shaft encoder. In this way, it was possible to determine the crank angles where optical data were detected. Engine Operating Conditions All the experimental investigations were carried out at 2000 rpm. The intake air temperature was fixed at 298 K and the cooling water temperature was set at 333 K. Commercial gasoline and methane fuels were used. For all the test cases, the injection-duration (DOI) was chosen to obtain a stoichiometric equivalence ratio. Two different fuel injection strategies were tested for both fuels: minimum load (closed throttle) and full load (wide open throttle). The coefficient of lambda value variation was measured on 400 consecutive cycles. It was lower than 1.8% for all the selected conditions. The spark timing (SOS) was always fixed to operate at the maximum brake torque. More details about the operating conditions are reported in Table 1. Table 1. Engine operating conditions Test label Fuel Pinj [bar] DOI [cad] SOS [cad] Minimum load Gasoline 3.5 29.5 -29.5 Full load Gasoline 3.5 71 -71 Minimum load Methane 1.5 128.7 -378.7 Full load Methane 1.5 250.6 -500.6 Results and Discussion The measurements were performed from the Start of Spark (SOS) until exhaust valve opening. The spectroscopic measurements were binned along space direction in order to obtain three typical locations: in correspondence of the exhaust valves, the spark plug and the intake valves. The development of the combustion process was identified by means of the analysis of digital images. In particular, the flame front propagation speed, an XXXIV Meeting of the Italian Section of the Combustion Institute 4 important parameter in the study of combustion in spark-ignition engines, was evaluated. The comparison between the flame front propagation speed, for the gasoline and methane fuels at minimum and full load is reported in Figure 1. -20 -10 0 10 crank angle [degree] 0 4 8 12 16 fla m e fro nt p ro pa ga tio n sp ee d [m /s ] minimum load Gasoline