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Nowadays internal combustion engines can operate under lean combustion conditions to maximize efficiency, as long as combustion stability is guaranteed. The robustness of combustion initiation is one of the main issues of actual spark-ignition engines, especially at high level of excess-air or dilution. The enhancement of the in-cylinder global motion and local turbulence is an effective way to increase the flame velocity. During the ignition process, the excessive charge motion can hinder the spark discharge and eventually cause a misfire. In this perspective, the interaction between the igniter and the flow field is a fundamental aspect which still needs to be explored in more detail to understand how the combustion originates and develops. In this work, a combined experimental and numerical study is carried out to investigate the flow field around the spark gap, and its effect on the spark discharge evolution. Two different velocities of the flow field are explored and compared to the static configuration under ambient conditions in an air flow channel, investigating the effect of the orientation of a conventional J-type spark plug. Results of numerical simulations predict spark stretching that is in close agreement with experimental observations. The model results are used to provide insight into the spatial and temporal development of the spark discharge. This work provides a numerical tool that can be extremely useful to link the interaction between the in-cylinder charge motion and the spark discharge to the engine cycle-to-cycle variability prediction.

A dielectric barrier discharge igniter (BDI) ensures engine stable combustion at lean and/or diluted conditions by the generation of non-equilibrium low-temperature plasma (LTP), strong promoter of the ignition, in a wide mixture volume. Beside this, a substantial amount of thermal energy is also released in the medium to enhance the chemical reactions of fuel oxidation. Therefore, the measurement of thermal energy is crucial to characterize the igniter capability to guarantee a robust combustion onset and it can be performed only in controlled environments. In this work, a Radio-Frequency BDI was tested via pressure-based calorimetry by evaluating the thermal energy released during the discharge in a controlled environment represented by a constant volume vessel. The corresponding thermal efficiency was evaluated as well. Pure nitrogen and air were exploited at engine-relevant pressure levels by varying the discharge features as its time duration and intensity (the latter by peak electrode voltage variation), to fully characterize the igniter behavior. Both media showed similar trends of absorbed and released energy but differences up to 50% were found for the latter. This trend was found to be linear, unexpectedly in contrast to the quadratic one showed by the LTP corona streamer-type igniter (CSI). Furthermore, the thermal energy dependence from pressure showed a change up to complete independence at a specific electrode value. To provide an explanation for such different behaviors, a comparative analysis between the diverse discharges of the two igniters was carried out. Moreover, by varying voltage and duration, the device was found to be able to release up to 60 mJ in the tested point with very low dispersion, essential to guarantee stable combustion ignition at challenging operating conditions for next-generation internal combustion engines.

Currently, the Radio-Frequency Corona Ignition systems represent an important solution for reducing pollutant emissions and fuel consumption related to Internal Combustion Engines while at the same time ensuring high performance. These igniters are able to extend the lean stable limit by increasing the early flame growth speed. Kinetic, thermal and ionic effects, together with the peculiar configuration of the devices, allow to start the combustion process in a wider region than the one involved by the traditional spark. In this work two corona igniters, namely a Barrier Discharge Igniter and a Corona Streamer Igniter, were tested in a single-cylinder research engine fueled with gasoline at different engine loads in order to investigate the igniters performance through indicated analysis and pollutant emissions analysis. For each operating point, the devices control parameters have been set to ensure maximum energy releasement into the medium with the aim of investigating, at the extreme operating conditions, the capability of the devices to extend the lean stable limit of the engine. The Corona igniters have been tested on a constant volume calorimeter as well, reproducing the engine pressure conditions at the corresponding ignition timing. The target is to give an estimation of the thermal energy released during the discharge and then to compare their capability to provide high-stability energy.

The Radio-Frequency Corona Ignition System is characterised by a wide initial combustion volume and precursors production, via radical insemination by the streamers, in addition to high released thermal energy. These features lead to faster combustion, a higher tolerance for lean mixtures and EGR dilutions and, in general, more adaptability. The thermal energy released by the igniter to the surrounding medium can help to understand the performance, the behaviour and the application range. This paper proposes a systematic experimental analysis of the thermal energy released by the igniter at room temperature, via pressure-based calorimetry. This analysis, carried out at different pressures (up to 10 bar) and medium type (air or nitrogen), is extended to the whole range of the corona igniter control parameters, namely streamer duration and driving voltage. The latter is proportional to the maximum electrode voltage, as shown in the model here presented, and as confirmed by experiments. The results show, for all the vessel pressures, the high energetic efficiency of the ignition system and the high amount of the released energy. The latter is found to increase linearly with the corona streamers duration and quickly with the driving voltage up to the streamer-to-arc transition threshold. The efficiency tends to reach a defined upper limit. For each tested point, the energy released to pure nitrogen is higher than to air, which evidences the impact of the oxygen presence under streamer exposure.

In recent years, radio-frequency corona igniters have been extensively studied for their capability to ensure an effective ignition also in lean or diluted mixtures. Corona ignition is volumetric, with streamers coming from a star-shaped electrode. During the discharge, many radicals and excited species, able to speed up the combustion onset, are generated. At the same time, corona igniters are able to release a considerable amount of thermal energy inside the combustion chamber. The correct determination of such energy is crucial to evaluate the effectiveness of the ignition. In this work, corona discharge is experimentally evaluated inside an optical vessel. In this apparatus, the released thermal energy is measured by means of pressure-based calorimetry, and at the same time the natural luminosity of the streamers is recorded with a high-speed camera. The goal is to find a relationship between thermal energy release and streamers luminosity. Tests are performed using nitrogen as medium, at different pressure levels inside the vessel. The peak electrode voltage is varied to characterize the igniter behaviour in different operating conditions. The results of this work can be used to quantify the corona ignition capabilities to involve a wide amount of medium while releasing a high amount of thermal energy. A repeatability evaluation of streamer evolution is investigated as well.

Charging a motorcycle engine is challenging, since requirements of lightness, system simplicity and engine responsiveness are key factors. This paper reports on a preliminary study on a pressure wave compressor, the “Impulse Drum Charger”. Performances of a 4-stroke motorcycle engine with and without Drum Charger were compared at the test bench and a pressure analysis in the intake manifold was carried out as well. Results show that this system is able to effectively improve engine power (up to 1.4 kW at 9500 rpm WOT) without an ECU recalibration.

Three-dimensional computational fluid dynamics internal combustion engine simulations that use a simplified combustion model based on the flamelet concept provide acceptable results with minimum computational costs and reasonable running times. Moreover, the simulation can neglect small combustion chamber details such as valve crevices, valve recesses, and piston crevices volume. The missing volumes are usually compensated by changes in the squish volume (i.e., by increasing the clearance height of the model compared to the real engine). This paper documents some of the effects that such an approach would have on the simulated results of the combustion phenomena inside a conventional heavy-duty direct injection compression-ignition engine, which was converted to port fuel injection spark ignition operation. Numerical engine simulations with or without crevice volumes were run using the G-equation combustion model. A proper parameter choice ensured that the numerical results agreed well with the experimental pressure trace and the heat release rate. The results show that including the crevice volume affected the mass of a unburned mixture inside the squish region, which in turn influenced the flame behavior and heat release during late-combustion stages.

The robustness of combustion initiation is one of the main issues of actual spark-ignition engines, especially for highly-diluted or lean mixtures. In this work, the effects on combustion stabilization obtained by the usage of a radio-frequency corona igniter were evaluated on a single-cylinder optical engine. The comparison with a conventional spark igniter was carried out using pure methane fuel and a blend of hydrogen and methane. For each combination of fuel and igniter, the combustion stability was explored at different air–fuel ratios, from stoichiometric conditions to the lean stable limit (up to λ = 2.0 with the corona igniter and the hydrogen-methane mixture). The combustion analysis was carried out by using the synchronized indicating and imaging data. The latter is essential to estimate the contribution of the corona igniter, which was found to be considerable only before the 5% of mass fraction burned. The corona effect igniter, with respect to a conventional spark igniter, was able to extend the lean stable limit of about 0.15 λ units with methane fuel, and about 0.10 λ units with the hydrogen-methane blend in the tested engine point. Early flame analysis confirmed the capability of corona igniter to improve combustion onset speed and to obtain a more stable and repeatable flame kernel. The findings of this study can help for a better implementation of corona ignition with gaseous low-carbon fuels, and in particular to achieve a higher lean limit extension without the drawback of a performance decay given by a substantial hydrogen enrichment.

Engine research community interest in the Radio-Frequency corona-based ignition systems is currently gaining in importance mainly due to their capability to ensure robust combustion at challenging operating conditions such as very lean mixture and/or high EGR dilution. The benefits of Corona low-temperature plasma foster the early flame development thanks to combustion precursors production and to a more energetic and volumetric discharge, resulting in a larger amount of involved mixture. The corona discharge generates ionizing waves, named streamers, whose temporal and spatial variability in orientation, length and branching can affect the combustion onset and, therefore, the engine cycle-to-cycle variability. In this work, the discharge natural luminosity of a RF corona igniter, characterized by four tips electrodes, was recorded in an optically accessible engine via high-speed camera detection. A preliminary statistical analysis of the spatial and temporal streamer variability was performed by operating in motored conditions. Four different engine speeds and two different loads were explored in order to deeply investigate the streamer behaviour at diverse engine operating conditions. A comparison between a motored and a lean operating condition is also proposed to analyse, at a specific engine speed, the mixture influence on the streamers propagation before the start of the combustion.

In recent years, radio-frequency corona ignition gained increasing interest from the engine research community because of its capability to extend the engine stable operating range in terms of lean and EGR dilution. The corona discharge generates streamers coming from a star-shaped electrode, generally consisting of four or five tips. The temporal and spatial variability of such streamers in length, orientation, and branching can be factors that affect the combustion onset and, therefore, engine cycle-to-cycle variability. Generally, the latter is reduced with respect to a conventional spark igniter at the same air&ndash, fuel ratio, but still present. In this work, analysis on the corona discharge and on the subsequent combustion onset was carried out in an optically accessible engine by means of the detection, via high-speed camera, of the natural luminosity of streamers and flames. A method to characterize spatial and temporal variability in motored conditions is firstly presented. A statistical analysis of the streamer behavior was performed, by separately analyzing the streamers generated by each tip of the star-shaped electrode. Finally, an original method aimed at determining the moment of the first flame appearance, caused by the combustion onset, is presented. The outcome of this work can be used to improve the knowledge on corona discharge, in particular on the stochastic behavior that characterizes the streamers. The presented optical analysis can also be adapted to other volumetric, single- or multi-point ignition systems.

3D CFD IC engine simulations that use a simplified combustion model based on the flamelets concept can provide acceptable results with minimum computational costs and reasonable running times. More, the simulation can neglect small combustion chamber details such as valve crevices, valve recesses and piston crevices volume. The missing volumes are usually compensated by changes in the squish volume (i.e., by increasing the clearance height of the model compared to the real engine). This paper documents some of the effects that such an approach would have on the simulated results of the combustion phenomena inside a conventional heavy-duty direct-injection CI engine, which was converted to port-fuel injection SI operation. 3D IC engine simulations with or without crevice volumes were run using the G-equation combustion model. A proper parameter choice ensured that the simulation results agreed well with the experimental pressure trace. The results show that including the crevice volume affected the mass of unburned mixture inside the squish region, which in turn influenced the flame behavior and heat release during late-combustion stages.

In recent decades, the increasingly tight emissions regulations, along with the ever-increasing price of fuels and the request for more power from the engines, has pushed the world car industry to improve the performances of the applications of electronics, designed to control the internal combustion engines (ICE) and the pollutant emissions systems. At present, one of the main problems, in the development of diesel engines is represented by the achievement of an increasingly strict control on the systems used for the pollutant emission reduction. In particular, as far as NOx gas is concerned, EGR systems are mature and widely used, but increased efficiency in terms of emissions abatement, is necessary in order to determine as best possible the actual oxygen content in the charge at the engine intake manifold. The present work compares the ability of the ANN and Neuro-Fuzzy approach (ANFIS) to predict the volumetric oxygen concentration at the intake, using experimental data acquired on a compression ignition engine in transient operational conditions. In an off-line evaluation of results, both models show good predicting abilities; in particular the ANFIS model presents an absolute error value for the training and test phases respectively equal to 0.7 and 0.9 (as a percentage of 3.5% and 4.5%), while, the same evaluation obtained using the ANN-BP model provides 0.92 and 0.9 (as a percentage of 4.6% and 4.5%). The comparison shows that the ANFIS model produces more accurate solutions in less time, using linear rules that bind the input variables with the output. The linearity of the rules is a key feature to decrease the convergence time especially when the model is used for the on-board periodic training activity due to the aging of the engine.

The paper reports on the optical investigation of a multiple spark ignition system carried out in a closed vessel in inert gas, and in an optical access engine in firing condition. The ignition system features a plug-top ignition coil with integrated electronics which is capable of multi-spark discharges (MSD) with short dwell time. First, the ignition system has been characterized in constant ambient conditions, at different pressure levels. The profile of the energy released by the spark and the cumulated value has been determined by measuring the fundamental electrical parameters. A high speed camera has been used to visualize the time evolution of the electric arc discharge to highlight its shape and position variability. The multiple spark system has then been mounted on an optical access engine with port fuel injection (PFI) to study the combustion characteristics in lean conditions with single and multiple discharges. The natural luminosity of the deflagration front was observed by means of the high speed camera, while correspondingly quantitative combustion analyses were performed based on the indicated pressure signal. The impact of the ignition system on the combustion stability is studied focusing on stable, near-limit and unstable operating conditions, as the relative air fuel ratio is increased. The results indicate that a extension of about 0.10 can be achieved using the MSD ignition. The benefit is associated with the presence of a non-homogeneous mixture preparation produced by the gasoline PFI fueling strategy. The high speed flame imaging allows to quantify cycle resolved equivalent flame radii as a function of time. The lower cycle-to-cycle variability attained with the MSD on the engine correlates to the higher ignition probability. A lower standard deviation of flame radius growth in the early stage of combustion is detected.

The aim of the paper is the comparison of the injection process with two fuels, a standard diesel fuel and a pure biodiesel, methyl ester of soybean oil. Multiphase cavitating flows inside injector nozzles are calculated by means of unsteady CFD simulations on moving grids from needle opening to closure, using an Eulerian–Eulerian two-fluid approach which takes into account bubble dynamics. Afterward, spray evolutions are also evaluated in a Lagrangian framework using results of the first computing step, mapped onto the hole exit area, for the initialization of the primary breakup model. Two nozzles with cylindrical and conical holes are studied and their behaviors are discussed in relation to fuel properties. Nozzle flow simulations highlighted that the extent of cavitation regions is not much affected by the fuel type, whereas it is strongly dependent on the nozzle shape. Biodiesel provides a slightly higher mass flow in highly cavitating nozzles. On the contrary using hole shaped nozzles (to reduce cavitation) diesel provides similar or slightly higher mass flow. Comparing the two fuels, the effects of different viscosities and densities play main role which explains these behaviors. Simulations of the spray evolution are also discussed highlighting the differences between the use of fossil and biodiesel fuels in terms of spray penetration, atomization and cone-angle. Usage of diesel fuel in the conical convergent nozzle gives higher liquid penetration.

Engines are machines designed to provide mechanical energy to a wide variety of systems by converting the chemical energy of a fuel (devices that are capable of producing a mechanical output starting from different types of energy, such as electric or hydraulic energy, are usually called motors). Internal combustion engines (ICEs) exploit the conversion of the chemical energy contained in suitable fuels—typically hydrocarbons (HCs)—into mechanical energy, owing to a combustion process. The heat generated by oxidation reactions of elements such as carbon or hydrogen produces a temperature increase in the fluid that acts as the working fluid in the power plant. The enthalpy increase is then exploited to generate mechanical work in suitable machines, thanks to the fluid expansion. If the combustion takes place within the working fluid itself, the machine is then called ICE; on the contrary, if the working fluid receives the heat from the combustion products remaining separated from them by a solid surface (for instance, in a heat exchanger), the machine is referred to as external combustion engine. Typical examples of external combustion engines are steam turbine plants or Stirling engines, whereas the ICEs category includes gas turbines and reciprocating and rotary ICEs. External combustion engines operate according to a thermodynamic closed cycle, as the working fluid undergoes thermodynamic transformations in a closed loop without any Handbook of Clean Energy Systems, Edited by Jinyue Yan. C2015 John Wiley & Sons, Ltd. ISBN: 978-1-118-38858-7. need of being replaced. On the contrary, the fluid operating in an ICE undergoes chemical transformations that require its periodical replacement with fresh fluid, thus performing a thermodynamic open cycle. Even though gas turbine plants can also be designed to operate through a closed cycle, they are typically operating in open cycle: consequently, strictly speaking, they should be included in the ICEs category. Nevertheless, only reciprocating and rotary engines are commonly designated as ICEs and will therefore be discussed in this chapter

The paper reports on the experimental analysis of a multiple spark ignition system, carried out with conventional and optical non intrusive methods. The system features a plug-top ignition coil with integrated electronics which delivers high ignition energy and high voltage compared to conventional ignition coils, and is capable of multiple discharges with reduced dwell time. The ignition system is characterized in terms of electrical parameters to evaluate the spark power and energy as a function of different hardware configurations and operating conditions. A high speed camera is used to visualize, at different ambient pressures, the time evolution of the electric arc discharge in order to highlight its position variability, which could have an impact on combustion kernel development and deflagration front stability in engines.

The ever more strict pollutant emissions regulations urge to make further steps in the evolution of the internal combustion engines, in particular as far as the emissions generation. A direct observation of the phaenomena taking place inside the cylinder can hence help in better understanding the effects obtainable by using different solutions (injection systems, ignition systems,…). To this end, the use of an optical access engine can be very helpful. This paper presents the first step made at the Department of Industrial Engineering of the University of Perugia for the set-up of an experimental line to be used for the analysis of combustion events observed in a single cylinder optical access engine. The combustion was characterized by using different techniques: thermodynamic (heat release) based on in-cylinder pressure measurements, and non intrusive optical observations by means of images acquisition. The flame images collected by a synchronized CCD camera were post-processed in order to evaluate the rates of the in-cylinder flame front evolution, observing the combustion process from the flame kernel formation on. Different fuels were used in the spark ignition, four valves, optical access engine: pure gasoline, pure ethanol, a 50% gasoline/ethanol blend. The experimental tests were carried out at 900 rpm engine speed, in rich and lean conditions, the latter being of interest for the achievement of significant CO2 reduction.

The optimization of the air-fuel mixture formation plays a very important role in order to reduce the total amount of emissions from an SI engine. To comply with the EURO5 emission restrictions is necessary to understand the influence of injection timing (with respect to engine load) and injector geometry on the air-fuel dynamic interaction. The aim of this paper is to define a CFD methodology for the simulation of a PFI engine. The goals of this analysis are the evaluation of the injector geometry and injection timing influences on the air-fuel mixture preparation and so on the equivalence ratio distribution inside the combustion chamber. Preliminary assessments of the wall-film and droplet-wall interaction sub models have been carried out in order to validate the methodology [1]. In particular a three-step work procedure has been adopted; the first two steps are necessary to properly initialize the secondary breakup model while the third step regards the CFD RANS simulation of the PFI engine considered for this analysis. In particular, a motor-scooter SI engine at a low load operating condition has been considered in the present work. This operating condition has been chosen because of the urban stop-and-go use of a motor-scooter implies that frequently the engine is running at a low load operating condition.

In this paper an experimental analysis is carried out to evaluate the dependence of the flow characteristics in the intake system of a high performance 4 valve, Spark Ignition Internal Combustion Engine, on the experimental conditions at the steady flow test bench. Experimental tests are performed at different pressure levels on a Ducati Corse racing engine head, to measure the Discharge Coefficient C d and the Tumble Coefficient N T , expanding the work already presented in a previous work by the same research group: with a new test bench, the maximum test pressure level is increased up to 24 kPa, while differently-shaped tumble adaptors are used to evaluate N t . The study is aimed at determining the influence of the test pressure on C d and N T measurements, and in particular of the tumble adaptor shape. As a matter of fact, researchers usually adopt different shapes, such as the conventional T type or the L one, resulting in a non uniform base to compare results from several investigations. This work is aimed at verifying the quantitative differences that can be found by using these shapes, in particular when the separate contribution to the tumble vortex generation from one only of two intake valves has to be evaluated. The above described analysis is also useful to evaluate the C d values obtained in the different test conditions used.

A performance analysis was fulfilled to compare the steam recompression with the conventional single- and dual-pressure bottoming cycles. The optimal parameters for each cycle were found in order to maximize the exergetic efficiency. Heat transfer surface was also calculated to show the differences in the size of the heat recovery steam generator which have a direct influence on the plant’s costs. The optimal configurations for each cycle were also studied when steam injection is employed. The relative performance when steam extraction is varied between 0 and 20% of the gas turbine’s compressor inlet mass flow rate, was also examined.

Emissions standards for two- and three-wheeled powered vehicles are getting more and more stringent, and measurement procedures require to perform driving cycles with engine cold start. Therefore, a fast activation of the exhaust catalytic converter is of primary importance. In this work a numerical and experimental study of the exhaust system layout of a 125cc scooter has been carried out with the main objective of reducing the catalytic converter light-off time, without affecting engine performance and component cost. First, a 1D engine model has been developed to evaluate the impact of the component modification on engine performance. Then, a CFD-3D analysis has been performed to assess and evaluate the velocity and temperature fields of the gases inside of the muffler. After the numerical study, several prototypes have been designed and built for experimental tests. The engine has been installed on the dynamometric bench and instrumented. The exhaust system prototypes have been tested focusing on the engine brake performance and on the exhaust temperatures during warm-up transients. The latter has been monitored in several points inside the muffler, in order to obtain information about the catalytic converter operating conditions. The best prototype configurations have been installed on the vehicle and further road tests. The vehicle experimental results in terms of exhaust gas temperatures at the catalyst inlet and outlet highlight the improvements with the best exhaust prototype compared to the original configuration.

Nowadays, the most common way to improve energy conversion efficiency is the integration of different systems, thus achieving a better exploitation of the available exergy potential (e.g. combined cycles, cogeneration, etc.). As a means of producing power in hydroelectric plants hydraulic energy is commonly considered to be almost completely exploited. The aim of this paper is to analyse the possible integration of hydraulic energy sources with conventional, fossil fuel based systems; in particular, power plants based on the combination of an hydraulic air compressor (HAC) and a gas turbine are considered. In an HAC, air is entrained in the water flow in a downcomer pipe and compressed. Once separated from the water in a ‘stilling chamber’ at the bottom of the downpipe, the compressed air is supplied to a combustion chamber and then to a conventional gas turbine expander. An attractive characteristic of HACs is the capability, in principle, to perform an isothermal air compression instead of an adiabatic one, as in conventional compressors. In the present work, a thermodynamic analysis is presented of HAC-gas turbine energy conversion systems, which are compared with conventional hydroelectric and gas turbine power plants. The calculated performance levels of such systems are comparable to those of combined cycle plants, making further technical and economical investigations quite interesting.