Climate change concerns demand a drastic reduction in CO2 emissions, tending to what is called carbon neutrality. Even if political guidelines promote electrification, considering the transportation sector, not all applications have the same requirements and boundary conditions, and hence, their optimal solution is not necessarily the same. In this context, in parallel with pure electric powertrains, the internal combustion engine (ICE) still has a relevant role to play, mainly in hybrid powertrains, working together with an electrical motor. In this hybridization context, the spark-ignition (SI) engine uses to be the most adopted solution because of its lower cost and complexity. Consequently, it can be concluded that the SI engine will still play a significant role in the near future. However, when ICEs are considered, the search for carbon neutrality requires the use of fuels other than fossil fuels. At this point, many alternatives arise, from biofuels to synthetic e-fuels, or even H2. This extreme variety of fuels introduces complexity from the combustion modeling point of view. This study proposes a methodology for developing a 0D combustion predictive model that characterizes the engine flame front effective area (FFEA) of the engine based on a reference fuel (gasoline) and extrapolates it to other engine conditions and/or to other fuels (e.g., H2), predicting the in-cylinder pressure evolution. To allow the prediction for any other fuel, the only requirement is to know the laminar flame speed of that fuel at different operating conditions. The results show that, for different engine conditions, the proposed methodology could predict the in-cylinder pressure with an average error of ~2.1% for the reference fuel and an average error of ~2.6% for Hydrogen (H2). These findings indicate that the proposed methodology has good performance and could be used for analyses requiring a limited number of experiments and a short computational time.

Understanding the dynamics of soot-formation pathways is crucial for the foreseeable advancement of gasoline engines, which depend on optimizing a complex array of parameters to meet regulatory standards. This paper employs a comprehensive set of parameter changes to examine the sources of soot formation observed in an optical direct-injection spark-ignition engine. By testing three injection pressures, three intake temperatures, five start of injection timings, and two engine loads, the study assesses their impact on soot formation and combustion characteristics. The analysis benefits from two distinct optical accesses, enabling a detailed portrayal of the flame morphology and the origins of sooting flames. Acquired images were supplemented by measured particle emissions, reinforcing the findings of the optical data. Transitioning from medium to high load led to an increase in particle emissions caused by a higher degree of injector tip wetting, as seen from the in-cylinder imaging. Increasing injection pressure reduced injector tip sooting, albeit leading to more spray-wall impingement at early injection timings. Under the earliest injection timing (-330 degrees CA), employing the lowest injection pressure (60 bar) reduced particle number and soot mass concentrations, with minimal impact on combustion performance. Intake temperature impacts fuel film quantity on both the injector tip and piston surface, with higher temperatures accelerating film evaporation from the bulk gas side. On the downside, it was necessary to retard combustion phasing at elevated intake temperatures to mitigate engine knock, consequently leading to a degradation in flame propagation speed and work output. The experiments also revealed a U-shaped trend in injection timing sweep, where spray-wall impingement dominates at the earliest timing, gas-phase in- homogeneities become more prominent at delayed timings, and injector tip soot is observed under all conditions due to injector fouling. The results of this study illuminate potential strategies for reducing engine-out particle emissions while maintaining satisfactory combustion performance, aiming to possibly obviate the necessity for gasoline particle filters.

Oxy-combustion is a promising concept to achieve an extremely clean combustion, independently of the fuel type, because, on the one hand, it is a NOx-free combustion and, on the other hand, the CO 2 produced during combustion can be easily captured once the water vapour is removed from the exhaust gases stream, consequently allowing also carbon neutral operation. An existing zero-dimensional (0D), mixing-controlled combustion model, developed for a standard diesel combustion scenario, has been adapted to the oxy-fuel combustion scenario. Initially, the model over-predicted the heat release at the end of the combustion process. The main model adaptation was to modify the relationship between the average Y O2 and the effective Y O2 (i.e. the one of the charge actually entrained by the spray), to be consistent with the significant increase in compression ratio needed in the oxy-fuel context. As a result, a model able to correctly predict the combustion behaviour at any operating condition has been obtained, which finally represents a very suitable tool to assist in the concept development.

The internal combustion engine's (ICE) electrification calls to question ICE concepts nowadays, leading to new engine architectures conceived to operate as series hybrids. This is the case of the unit analyzed in this work; a 2-stroke, spark-ignited, rod-less opposed-piston engine (2S-ROPE). Previous results revealed some efficiency and maximum power drawbacks, which motivated the development of the supercharged or turbocharged engine version. Supercharging the engine results in a straightforward task; however, turbocharging the engine reveals serious stability problems due to the thermo-fluid-dynamic coupling between the intake and exhaust lines. This work presents a method to study and identify the exhaust line geometry since the impact of pressure pulse propagation on the scavenging process is one of the most critical points to be considered from the very first steps. Finally, despite the difficulties of turbocharging the presented 2S engine, the parametric study revealed a suitable geometrical configuration and the engine operative range. Compared to the supercharged engine version, the turbocharged one presents more difficulties but diminishes the mechanical compressor power consumption, implying an advantage in efficiency terms.

Fuel film deposits on combustion chamber walls are understood to be the main source of particle emissions in GDI engines under homogenous charge operation. More precisely, the liquid film that remains on the injector tip after the end of injection is a fuel rich zone that undergoes pyrolysis reactions leading to the formation of poly-aromatic hydrocarbons (PAH) known to be the precursors of soot. The physical phenomena accompanying the fuel film deposit, evaporation, and the chemical reactions associated to the injector film are not yet fully understood and require high fidelity CFD simulations and controlled experimental campaigns in optically accessible engines. To this end, a simplified model based on physical principles is developed in this work, which couples an analytical model for liquid film formation and evaporation on the injector tip with a stochastic particle dynamics model for particle formation. The modeling framework is applicable under steady-state engine operating conditions, and can be extended to transient driving cycle simulations, although the former is only presented in this work. Particle number, mass, and size distributions are validated with experimental measurements performed on a steady-state engine test bench for a 2.0L turbocharged gasoline engine under two engine speeds at part and full load. The model is able to qualitatively capture the main trends in particle number concentration values, especially at the higher load conditions. An underestimation of PN and PM at lower loads is observed, which can be attributed to other sources of particle formation.

Due to its potential to extend the lean limit or overcome knock-limited conditions on spark-ignited (SI) engines, the pre-chamber spark-ignition (PCSI) concept has gained attention as an efficient alternative to the conventional ignition architecture. Nevertheless, especially in engine-like conditions, the effect of the main geometrical parameters on combustion development is still not fully understood and might be a hindrance to the use of this technology. In this sense, the current study aims to evaluate the effect of the orifice diameter on the jet characteristics and main chamber combustion of a novel passive pre-chamber in an optically accessible single-cylinder SI engine, as well as to compare the PCSI with the typical SI ignition method. High-speed broadband chemiluminescence imaging was used to track the jet penetration and flame front propagation of three mixture compositions at various spark timings (ST). Additionally, based on the in-cylinder pressure, a heat-released analysis was performed to assess the overall combustion process. When compared with the typical SI, the pre-chamber concept presented a higher equivalent flame front velocity, which is also corroborated by the higher in-cylinder pressure rise rate and rate of heat released. For all mixtures tested, the 1.2 mm orifice diameter PC presented the shortest flame development angle (crank angle interval between ST and 10 % of mass fraction burned in the MC), followed by 1.5 and lastly 1.0 mm. When comparing different STs, the shortest flame development angle occurred at MBT condition. Despite the highest flame development angle, the PC 1.0 mm case presented the lowest main chamber combustion duration (CA10-90), which increases as the orifice diameter is increased.

To further understand the processes and phenomena taking place in the pre-chamber (PC) ignition concept, many studies under simplified conditions have been carried out in different experimental facilities (e.g. constant volume chambers and rapid compression machines). However limited information is provided about how the volume, orifice diameter and number of orifices were defined, raising the question whether the results are representative of engine-like conditions or not. This novel study arises from the necessity to determine a methodology to reproduce a reference pre-chamber, preserving as much as possible its jet characteristics. A theoretical development based on the first law of thermodynamics has been performed, and a relationship between the effective flow area, pre-chamber volume and engine speed is proposed as the governing parameter of the mass exchange between chambers. Besides, relaying on the know-how of gas jets, a relationship between the orifice diameter, jet tip penetration and engine speed is suggested as the criterion to preserve the relative jet penetration (respect to the distance from the PC hole to the combustion chamber walls). A numerical validation of these assumptions was carried out using a one-dimensional flow calculator to estimate the thermodynamic properties and mass transfer between chambers, and a one-dimensional spray model to estimate the penetration of the PC combustion products jets. Finally, preserving the ratio between the total area of the PC holes and the product of the PC volume and the engine speed for two pre-chamber geometries, an identical pressure rise rate, in an angular basis, is achieved in both pre-chambers. Furthermore, the same relative jet penetration rate, in an angular basis, can be also achieved, even under different engine speeds, when the ratio between the orifice diameter and the product of the square of the jet free length and the engine speed is preserved.

The approach of this research is to enlarge the knowledge about the methodologies to increase the maximum achievable load degree in the context of gasoline CAI engines. This work is the continuation of a previous work related to the study of the water injection effect on combustion, where this strategy was approached. The operating strategies to introduce the water and the interconnected settings were deeply analyzed in order to optimize combustion and to evaluate its potential to increase the maximum load degree when operating in CAI. During these initial tests, the engine was configured to enhance the mixture autoignition. The compression ratio was high compared to a standard gasoline engine, and suitable fuel injection strategies were selected based on previous studies from the authors to maximize the reactivity of the mixture, and get a stable CAI operation. Once water injection proved to provide encouraging results, the next step dealt in this work, was to go deeper and explore its effects when the engine configuration is more similar to a conventional gasoline engine, trying to get CAI combustion closer to production engines. This means, mainly, lower compression ratios and different fuel injection strategies, which hinders CAI operation. Finally, since all the previous works were performed at constant engine speed, the engine speed was also modified in order to see the applicability of the defined strategies to operate under CAI conditions at other operating conditions. The results obtained show that all these modifications are compatible with CAI operation: the required compression ratio can be reduced, in some cases the injection strategies can be simplified, and the increase of the engine speed leads to better conditions for CAI combustion. Thanks to the analysis of all this data, the different key parameters to manage this combustion mode are identified and shown in the paper.

The combustion diagnostics and subsequent analysis are standardized tools based on the estimation of the heat release law (HRL). From this estimation, the different combustion parameters can be obtained: combustion phasing and duration, heat release rate, and so on. This analysis might be usually enough to study traditional spark ignition (SI) engines. However, with the new upcoming SI engines, this is probably not the case anymore, since different combustion modes can be operated in the same engine, as for instance a combination of SI and controlled auto-ignition (CAI) combustion modes. When different combustion modes are combined, it seems interesting to study in more depth the HRL, trying to get more data and to study the differences among the diverse combustion modes. Toward this end, a methodology to go deeper in the study of the HRL is proposed in this work, consisting of, mainly quantifying and taking into account the most relevant influencing parameters: the fuel properties (mainly its lower heating value), the in-cylinder oxygen content, the density of the burned and unburned zones, the laminar combustion speed, and the turbulence effect. With the proposed methodology, a standard SI combustion, developed by a flame front, can be characterized at any given operating point. This would allow to predict which the combustion developement would be, at this operating point, assuming it to be developed by a flame front. Subsequently, this SI combustion prediction can be compared to the one obtained experimentally, making it possible to identify and analyze abnormal combustion phenomena, as well as to study the differences between a combustion developed by a flame front (SI) and by auto-ignition (CAI). Derived from this work, an alternative equation to experimentally characterize the laminar combustion velocity has also been proposed, in order to improve its applicability in a wider range of fuel/air ratios and dilution degrees.

With the aim of reducing pollutant emissions from internal combustion engines (ICE), the application of stoichiometrically operated spark ignition (SI) engines, for light-duty vehicles, has been overcoming the compression ignition (CI) engines market share throughout the past years. The ability of a substantial reduction of the primary harmful emissions (HC, CO, and NOx) through the use of the simple three-way catalyst (TWC) is the main reason for that. Nonetheless, with increasing attention to CO2 emissions, the development of highly efficient downsized SI engines turn to be of enormous interest. The synergies of multiple systems such as direct injection, turbocharger, and variable valve actuation are able to lead the SI efficiencies closer to those of CI engines. However, to enable high load operation on such downsized engines, the compression ratio (CR) must be reduced due to knock limitations, reducing the partial-load operations efficiency. The implementation of two-stage variable compression ratio (VCR) systems enables the extraction of high thermal efficiency with high CR at lower loads and extended knock-free high load operation with low CR. In this study, the evaluation of a two-stage VCR system applied to a state-of-the-art downsized SI engine was made through standard driving cycle simulations. The VCR mechanism is composed of an eccentric element in the small end of the connecting rod, which is rotated to increase/decrease the effective connecting rod length, achieving the CRs of 12.11:1 and 9.56:1. The engine was run in an eddy-current dynamometer test bench throughout the essential operating range to obtain the brake specific fuel consumption (BSFC) map. The VCR mechanism CR switching delay was also experimentally characterized to derive a function of the operating conditions. The measured map was entered into the map-based driving cycle simulation with a sub-model to account for the isolated effects of the transient period encompassing the compression ratio switching. The results show that slow CR transitions lead to fuel consumption penalties, which suggests the need for optimizing the control strategies of the VCR system. Even though this penalty, once the gear up-shift speed is optimized for each driving cycle, the VCR system still enables fuel consumption reductions up to 3% on the WLTC driving cycle, up to 4% on the proposed urban driving cycles and up to 3% on highway driving cycles with respect to the fixed CR.

As noise pollution remains one of the biggest hurdles posed by thermal engines, increasing efforts are made to alleviate the generation of combustion noise from the early design stage of the chamber. Since the complexity of both modern chamber geometries and the combustion process itself precludes robust analytic solutions, and since the resonant, highly three-dimensional pressure field is difficult to be measured experimentally, focus is put on the numerical modeling of the process. However, in order to optimize the resources devoted to this simulation, an informed decision must be made on which formulations are followed. In this work, the experimental cyclic dispersion of the in-cylinder pressure is analyzed in two typical compression-ignited (CI) and spark-ignited (SI) engines. Acoustic signatures and pressure rise rates (PRRs) are derived from these data, showing how while the preponderance of flame front propagation and dependency of previous cycle in SI engine noise usually calls for multicycle, more complex turbulence modeling such as large Eddy simulation (LES), simpler unsteady Reynolds-averaged Navier-Stokes (URANS) formulations can accurately characterize the more consistent pressure spectra of CI thermal engines, which feature sudden autoignition as the main noise source.

CAI (Controlled AutoIgnition) systems, also named HCCI (Homogeneous Charge Compression Ignition), are a promising way to improve gasoline engines. This combustion mode is more efficient than the standard SI (Spark Ignition) combustion and, additionally, it has very low emissions, especially NOx emissions, which represent a source of problems nowadays. The main problem of this combustion mode is the constrained operating range, caused, on the one hand, by the difficulty to ignite the fuel since it has to be auto-ignited by the control of the mixture reactivity, and, on the other hand, by its high heat release rates, causing high pressure gradients and, in some circumstances, knocking combustion. In this paper, the possibility to use directly injected water into the combustion chamber as a reactivity suppressor in order to extend the constrained load range of CAI operation is evaluated. For this study, a four-stroke single-cylinder gasoline engine has been modified to allow CAI combustion by means of adapted valve trains enabling to keep hot residual gases inside the cylinder, which will provoke the fuel autoignition. Additionally, a water direct injection system has been installed in the engine to carry out this study. The results show that water injection is an efficient strategy to increase the maximum affordable load in CAI conditions, since the reactivity of the mixture can be suitably controlled, thus reducing the pressure gradients and the knocking tendency of the combustion process, also keeping good levels of combustion stability. Nevertheless, the engine has to be significantly boosted and the necessary intake pressure compared to a conventional SI operation mode in stoichiometric conditions is much higher.

The need to reduce the emissions coming from automobiles encourages the attempts to study different engine configurations and new combustion strategies. In this case, a two stroke engine able to operate in Controlled Autolgnition (CAI) and Spark Ignition (SI) combustion modes is studied, with the purpose of getting lower NO and CO2 emissions than with other currently employed solutions. The engine configuration retained for the research is a uniflow scavenging configuration with intake ports in the cylinder liner and exhaust valves in the cylinder head. These valves are controlled by a Variable Valve Timing (VVT) system. The scavenging is guaranteed by an external blower driven by the crankshaft. Finally, the fuel supply is performed by a direct injection (DI) air-assisted fuel injection system. Through this paper the adjusting parameters to control the engine operation, as well as their influence on the CAI and SI combustion modes have been studied, providing the most relevant information and knowledge for controlling and optimizing the engine performance. Once these controlling parameters were studied, an EGR system was introduced in order to analyze the effect of this other parameter over the combustion process, as well as to determine the potential benefits of introducing such a system in this type of engines. (C) 2017 Elsevier Ltd. All rights reserved.

Seven different chemical kinetic mechanisms for n-dodecane, two detailed and five reduced, have been evaluated under Engine Combustion Network (ECN) thermodynamic conditions by comparison to experimental measurements in a Rapid Compression-Expansion Machine (RCEM). The target ECN conditions are imposed at Top Dead Center (TDC), which cover a wide range of temperatures (from 850 K to 1000 K), oxygen molar fractions (0.21 and 0.15) and equivalence ratios (0.8, 0.9 and 1), while the pressure is fixed to keep a constant density at TDC equal to 22.8 kg/m(3). The results obtained have been used to validate the chemical kinetic simulations, which have been performed with CHEMKIN, by comparing both cool flames and high temperature ignition delays, as well as the heat released in each stage of the combustion process in case of having a two-stage ignition pattern. The experimental results show good agreement with the chemical kinetic simulations. In fact, the mean relative deviation in ignition delay between experiments and simulations among all the chemical mechanisms is equal to 18.0% (3 CAD) for both cool flames and high temperature ignition. In general, closer correspondence has been obtained for the ignition delay referred to the high-temperature stage of the process, being the cool flames phenomenon more difficult to reproduce. Moreover, the differences between the reduced mechanisms and the most detailed one have been analyzed, concluding that the enhanced specific reaction rates of the most reduced mechanisms cause differences not only on the ignition delays, but also on the Negative Temperature Coefficient (NTC) behavior and on the heat released during cool flames. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A phenomenological explanation about the autoignition propagation under HCCI conditions is developed in this paper. To do so, diffusive effects from the burned zones to the fresh mixture, pressure waves based effects and expansion effects caused by combustion are taken into account. Additionally, different Damkohler numbers have been defined and evaluated in order to characterize the phenomenon and quantify the relevance of each effect. The theoretical explanation has been evaluated by means of chemiluminescence measurements performed in a Rapid Compression Expansion Machine (RCEM), which allow to estimate the velocity of propagation of the autoignition front. The results showed that under HCCI conditions the autoignition propagation is controlled, in general, by the pressure waves established in the combustion chamber, since the characteristic time of the autoignition propagation is too short to assume the absence of pressure gradients in the chamber. Thus, the thermodynamic conditions reached behind the pressure wave promote the autoignition and explain the high propagation velocities associated to the reaction front. Besides, the results also showed that the contribution of diffusive phenomena on the propagation is negligible, since the characteristic time of diffusion is too long compared to the characteristic time of the autoignition propagation. Finally, the experimental measurements showed that the autoignition propagation is affected by a really relevant cycle-to-cycle variation. The turbulence generated by the combustion has, by definition, an aleatory behavior, leading to random heterogeneity distribution and, therefore, to somewhat random autoignition propagation. (C) 2017 Elsevier Ltd. All rights reserved.

The ignition characteristics of six different fuels have been correlated as a function of the temperature, pressure, equivalence ratio and oxygen molar fraction in this investigation. More specifically, the ignition delay referred to cool flames, the high-temperature ignition delay and the critical concentrations and ignition times of HO2 and CH2O have been parameterized for n-dodecane, PRFO, PRF25, PRF50, PRF75 and PRF100. To do so, a wide database of ignition data of the aforementioned fuels has been generated by means of chemical simulations in CHEMKIN, solving a detailed mechanism for PRF mixtures and a reduced mechanism for n-dodecane. In fact, in cylinder engine-like conditions reached in a Rapid Compression Expansion Machine (RCEM) have been replicated. The mathematical correlations have shown a relative deviation around 20% with the database in the low-temperature, low-pressure zone, which is the typical accuracy of usual correlations for the ignition delay. Finally, the ignition delay under transient conditions measured in the RCEM has been predicted by means of different integral methods coupled to both the proposed correlations and the generated database. It has been found that deviations between the predictions obtained with the correlations or with the database are lower than 1%. This means that the correlations are accurate enough to predict the ignition time in spite of showing high deviation with the database, since the low-temperature, low-pressure zone has a minor contribution to the ignition delay.

A study to experimentally analyze the effect of cavitation on the mixing process in diesel nozzles was carried out. The mixing process was studied through the spray cone angle. It was characterized in two different scenarios: with the liquid length (nearly realistic conditions, that is, evaporative but non-reactive spray) and the heat release fraction (fully realistic conditions, that is, evaporative and reactive spray). In both studied scenarios, the increase in spray cone angle caused by the cavitation phenomenon, which leads to a better mixing process, has been confirmed. Nevertheless, when the variations of the effective injection velocity and the spray cone angle obtained by comparing a cylindrical nozzle (i.e. a nozzle that promotes the cavitation phenomenon) with a conical nozzle (i.e. a nozzle that inhibits this phenomenon) were analyzed together, it was found that, for the cases studied here, the mixing process worsens with the cylindrical nozzle.

Rapid Compression-Expansion Machines (RCEMs) have been used for years in combustion studies due to their capability to replicate in-cylinder engine phenomena under fully controlled initial and boundary conditions. In this work, the potential of the RCEM available at CMT Motores Térmicos to study several combustion phenomena has been analyzed. First, the repeatability of the RCEM has been checked for three different geometrical configurations, showing very good results. Afterwards, the ignition characteristics of two different fuels have been tested and the combustion analysis under HCCI conditions has been compared to results under standard Diesel and spark-ignition conditions. The adiabatic combustion temperature has been calculated for each mode in order to estimate the NOx emissions, while the combustion efficiency has been obtained in order to estimate CO and UHC emissions. Furthermore, different optical techniques have been applied for combustion diagnosis. Chemiluminescence and spectroscopy measurements have been carried out to analyze the autoignition under HCCI conditions, while two-color pyrometry and diffuse backlighting have been applied in standard direct-injection Diesel conditions to study diffusion flames. The RCEM has shown to be a very flexible facility to perform combustion diagnosis under engine-like conditions, leading to reliable results that can be used to validate several combustion models and to better understand different combustion phenomena. Introduction, justification and objective Internal Combustion Engines (ICE) have demonstrated to have a main role in the frame of propulsive systems for transport media. They have been used for years as the best solution for passenger and merchandise mobility because of their unbeatable power-to-weight ratio, their well-know technology and their easily storable energy source [1]. Engine efficiency has been improved during the last years up to reach hardly improvable values. Thus, nowadays the engine research is focused on reducing pollutant emissions but keeping, or even improving, the high efficiency reached in modern engines. Regulations about pollutant emissions in ICEs have become increasingly restrictive during the last years. Fig. 1 shows the evolution of the European emission standards for passenger cars and light-duty commercial vehicles (<1305kg) for both Compression-Ignition (CI) and SparkIgnition (SI) engines. It can be seen that the trend from Euro III (January, 2000) to Euro VI (September, 2014) is to decrease the limits of unburned and pollutant species, specially for nitrogen oxides (NOx) and particulate matter (PM). Moreover, this trend is not only present in automotive engines, but also for naval and stationary engines. New European emission standards (Euro VI) for heavy-duty vehicles equipped with Diesel engines, for instance, have reduced the NOx limits in 80%, while the maximum soot emissions have been reduced in 50% [2]. In conventional CI-engines, both NOx and soot emissions cannot be simultaneously reduced due to the opposite effects of conventional Diesel combustion strategies on these two pollutant species. As for SI-engines, the combustion efficiency is limited by quenching effects that appear near to the walls because of temperature gradients, which imply the existence of a volume of fuel that cannot be burned by the flame front [3]. Figure 1. Evolution of the European emission standards for passenger cars and light-duty commercial vehicles (<1305kg) for both CI and SI-engines from Euro III (January, 2000) to Euro VI (September, 2014). There are two ways to reduce pollutant emissions in ICEs. On the one hand, pollutant emissions can be reduced by means of after-treatment systems located in the exhaust line. Despite the fact that after-treatment systems for unburned hydrocarbons (UHC), carbon monoxide (CO) and PM are well-known and highly efficient techniques, NOx reduction methods have shown some limitations. Three-way catalytic converters (TWC) have the advantage of working as both oxidation and reduction catalysts. However, their operating range is limited to stoichiometric air-fuel ratios, which precludes the optimization of the equivalence ratio in terms of fuel consumption. Selective Catalytic Reduction (SCR) aftertreatment systems for the reduction of NOx species have shown to have some disadvantages, including high cost and high maintenance. SCR systems need additional injection systems to supply a reductant (typically urea) to the exhaust flow, which increase the complexity of the engine. Moreover, the reductant, which has to be replenished, as well as the reduction products, can cause corrosion in the injector, reducing the useful life of the system [4]. On the other hand, the formation of pollutant emissions can be avoided directly during the combustion process itself. In this sense, Exhaust Gas Recirculation (EGR) is a widely used technique to reduce the formation of NOx through the thermal pathway [5]. The working principle of EGR is to recirculate a certain amount of exhaust gases to the intake manifold, mixing them with the fresh air. The burned gases act as a diluent of the unburned mixture, thus a lower initial oxygen molar fraction is reached and lower combustion temperatures are obtained. In fact, the temperature reached after combustion varies inversely with the exhaust gas mass fraction. Hence increasing the exhaust gas fraction reduces NOx emissions levels. This is a compulsory strategy for conventional CI-engines and its use is also widespread in current SI-engines, where it is also used as a powerful knock mitigation strategy at high loads, and as a fuel efficiency enhancer at low loads. Moreover, the relevance of EGR is even higher in the frame of new combustion modes, which use massive amounts of EGR to reduce the maximum temperature reached in the cycle. Figure 2. Operating conditions reached in different combustion modes represented in an equivalence ratio versus temperature diagram. The soot and NOx formation peninsulas are also represented (from [7]). Advanced combustion modes based on the autoignition of an air-fuel mixture with a certain degree of homogeneity and high EGR rates, such as Homogeneous Charge Compression Ignition (HCCI), Premixed Charge Compression Ignition (PCCI), Controlled Autoignition (CAI) and others, have been studied for the simultaneous reduction of soot and NOx in CIengines and for the improvement of the combustion efficiency in SI-engines. Their working principle is based on Low Temperature Combustion (LTC) and their effectiveness has been widely proved in previous studies [6]. Fig. 2 shows the conditions reached in these new combustion strategies, as well as in conventional Diesel and spark-ignition combustion, in an equivalence ratio temperature diagram [7], where it can be seen that the simultaneous reduction of soot and NOx is possible by avoiding the soot and NOx formation peninsulas. Regarding CI-engines, LTC modes show virtually zero emissions of soot and NOx, but high UHC and CO emissions that can be easily eliminated with well-known low-cost aftertreatment techniques. Since the maximum temperature reached in the cycle is low and it decreases further after Top Dead Center (TDC), most of the fuel located in the thermal boundary layer and in other crevice volumes cannot diffuse out into the bulk gas and burn, which results in products of incomplete combustion [8]. As for SI-engines, CAI mode is based on the autoignition of a premixed air-fuel mixture to avoid the establishment of a flame front, leading to higher combustion efficiencies and lower UHC, CO and NOx emissions [9]. However, two main challenges appear with the implementation of these combustion strategies in commercial engines: the lack of control over the autoignition process and, therefore, over the heat release rate [10]; and the operating range, which is limited to low-to-medium loads [11]. On the one hand, ignition is controlled by the chemical kinetics of the charge in these combustion modes. This control entails higher complexity because of the absence of an explicit ignition-controlling event, such as a spark or an injection process, when very reactive conditions are reached in the combustion chamber (near TDC). The reactivity of the mixture can be modified by adjusting the engine operating parameters, such as the EGR rate and the inlet temperature. Therefore, improving the knowledge about the autoignition phenomenon under low temperature conditions is mandatory to properly modify the operating conditions of the engine in order to control the heat release. On the other hand, the operating range in LTC modes is restricted to low-to-medium loads due to the fast combustion velocity of the autoignition process when the engine load is increased, which results in high pressure rise rates and, therefore, in high combustion noise and mechanical strains. Therefore, conventional spark-ignited premixed combustion and conventional Diesel combustion should be taken into account nowadays as the method to reach high loads in these engines. Dual-fuel combustion based on Diesel/gasoline mixtures has shown to be a good method to increase the operating range of LTC modes. Bessonette et al. [12] showed that different incylinder reactivities are required for a proper LTC operation under different operating conditions. Specifically, low octane fuels are required at low loads, while high octane fuels are needed at medium-to-high loads. A wide range of octane numbers can be provided by using premixed gasoline mixtures in which a direct Diesel injection causes a stratification of reactivities. Thus, a flexible operation over a wide operating range is possible by modifying both the blend ratio between fuels and the direct injection settings. However, the phenomena that control the combustion in such dual-fuel modes are not completely clear, and more efforts should be made in studying this technology. There is consider

Soot emissions from diesel engines are an important concern in meeting emissions regulations. Soot emissions are the result of two competing processes: soot formation and soot oxidation. Mechanisms of soot formation are discussed extensively in the literature. Equivalence ratio at lift-off length along with residence time and gas temperature play an important role for soot formation in a diffusion flame. Mixing capability and bulk gas temperature are the most important parameters that influence the in-cylinder soot oxidation process. Normally, research studies of soot formation-oxidation processes have been developed under controlled and not completely representative conditions of engine operation in the field. Therefore, the main objective of this work was to develop a simplified methodology to evaluate in cylinder soot oxidation under 'real' engine conditions. In particular the impact of mixing process and bulk gas temperature on late cycle soot oxidation was evaluated. The experimental measurements were made in a production light-duty diesel engine varying those parameters that have been demonstrated in the literature as the most relevant in soot formation - oxidation processes; injection pressure, ambient density and intake air temperature. To measure soot, two color method was applied by means of an optoelectronic pyrometer. To evaluate the mixing capability a specific "tracer" Apparent Combustion Time (ACT(-1)) based on the experimental heat release and injection parameters was defined. The relationship between both parameters was used to explain the soot oxidation process. (C) 2016 Elsevier Ltd. All rights reserved.