Juan José Hernández

Diagnosis of DI Diesel combustion from in-cylinder pressure signal by estimation of mean thermodynamic properties of the gas

Combustion diagnostic methods based on the in-cylinder pressure signal are extensively used for calculating the heat release law or the burned fuel mass as well as the mean gas temperature from combining both the first principle of thermodynamics and the state equation. In both equations the instantaneous gas composition has great influence, even through the internal energy or through the gas constant. In the proposed method the gas is supposed to be composed of pure air, gaseous fuel and products of a stoichiometric combustion, neglecting the effect of local conditions (local mixing ratios and temperatures), but including their bulk temperature dependence. The concentration in the gas of each of these species was related with engine test parameters. A thermodynamic approach, coherent with the mentioned species distinction, is also proposed. The results of temperature correlations of the main thermodynamic properties are presented, as well as some results of the combustion diagnostic procedure from engine tests with different exhaust gas recirculation ratios.

Evaluation of exhaust gas recirculation as a technique for reducing diesel engine NOx emissions

Abstract The aim of this work is to evaluate the potential of exhaust gas recirculation (EGR) to reduce NO x emissions, through the most significant parameters, and to delimit the application range of this technique. Therefore, a detailed analysis of experimental results on NO x formation and emissions is presented, and the mechanisms by which EGR affects them are explained. A collection of five different automotive diesel engines, all of them from Nissan, was tested, so it was possible to distinguish the combined effect of EGR and their main characteristics, such as supercharging, intercooling, size, compression ratio and combustion chamber type [direct injection (DI) and indirect injection (IDI)]. The engines were tested in a set of steady conditions, trying to reproduce the different operating modes of the European transient urban/extra-urban certification cycle. In this way, comparisons between different engine emissions provided conclusions directly related to the emission certification limits.

COMPOSITION AND SIZE OF DIESEL PARTICULATE EMISSIONS FROM A COMMERCIAL EUROPEAN ENGINE TESTED WITH PRESENT AND FUTURE FUELS

Abstract The search for alternative fuels, such as methyl ester from vegetable oils or water-oil emulsions, has become even more pertinent in the current context of fossil fuel shortage, the diesel vehicle population explosion and the new environmental policies. In this paper, the results of experimental research on particulate emissions from a typical indirect injection (IDI) diesel engine running under five different operating conditions and tested with fourteen types of conventional and alternative fuel are reported. The chemical analysis of the emitted particulate matter showed, for example, that the total mass of particulate emissions decreases when using biofuels owing to a reduction in their insoluble fraction. Moreover, composition analysis made it possible to identify the hydrocarbons adsorbed on the soot surface, to quantify their proportion with respect to the total particulate mass and to distinguish between their origins. The results of surface analysis using scanning electron microscopy showed for all the fuels that the number of particles detected per filter surface increased with load. This analysis also showed the effect of some fuel specifications (aromatic and sulphur content) on certain parameters related to the particle size distribution obtained from the filter images.

Autoignition prediction capability of the Livengood–Wu correlation applied to fuels of commercial interest

The integral method proposed by Livengood and Wu has been traditionally used to predict the occurrence of knock on spark-ignition engines. Due to its simplicity and low computational demand, this is a method of great interest for the prediction of another autoignition phenomenon, such as the onset of combustion in compression ignition or homogeneous charged compression ignition engines. However, the simplicity of the method is a consequence of the restrictive assumptions considered during its development, which may limit the applicability of the equation. In this study, the validity of the correlation proposed by Livengood and Wu has been evaluated at different initial operative conditions under pure homogeneous charged compression ignition combustion mode for fuels with practical interest (hydrogen, methane, ethanol and n-heptane). The integral method has shown very good prediction capability for the fuels, which do not present two-stage heat release (hydrogen, methane and ethanol) except in those cases when the onset of combustion is very delayed. When cool flames appear (as in the case of n-heptane), the integral method overpredicts the autoignition times since it does not consider the first stage of heat release. In these cases, the prediction of the integral method may be improved if the whole combustion process is considered as two individual processes. This approach shows fairly good prediction capacity although it is unpractical since the simulation of the second-stage combustion requires the previous calculation of the composition of the mixture and the temperature increase at the end of the first stage. Finally, two alternatives to the original integral method are tested which keep its simplicity and universality while taking into account both first and second heat release, one of them showing better results than the original Livengood and Wu equation.

Ignition delay time correlations for a diesel fuel with application to engine combustion modelling

Abstract New diesel engine combustion concepts, such as the homogeneous charge compression ignition (HCCI), have encouraged the development of ignition delay correlations allowing the reliable prediction of the chemical auto-ignition time of the fuel, which depends on its oxidation kinetics. These correlations permit one to design and optimize the most adequate control strategies leading to both low emission levels of pollutants and proper engine performance. Although some diesel ignition correlations can be found in the literature, most of them consider the physical delay time and have been obtained only for high temperature values, which are usual at the end of the compression stroke in traditional diesel engines. However, there is still a significant lack of information regarding the complex three-stage oxidation (low-, intermediate-, and high-temperature range) of a diesel fuel under HCCI conditions. Thus the proposal of delay time correlations for the three temperature ranges is the objective of the present work. The correlations have been assumed as Arrhenius-type equations and consider the effect of the main parameters affecting the auto-ignition time, i.e. pressure, temperature, and equivalence fuel/air ratio. The adjustment coefficients have been calculated by using multiple linear regression and least-squares techniques, and a very good fit between modelling-predicted and correlation-predicted delay values has been obtained (R2 higher than 0.95 in all cases).

Reduction of kinetic mechanisms for fuel oxidation through genetic algorithms

New automotive engine combustion concepts, such as the Homogeneous Charge Compression Ignition (HCCI) or the Controlled Autoignition (CAI), have been developed in the last years in order to fulfil the very stringent European regulations limiting pollutant emissions (mainly nitrogen oxides and particulate matter). These combustion modes are not controlled by the physical phenomena prior to combustion, as usual in traditional Compression Ignition (CI) and Spark Ignition (SI) engines, but by the fuel chemical kinetics leading to autoignition, which is described by the corresponding reaction mechanism. Since the detailed reaction mechanism of surrogate fuels (which are used instead of the original fuels in order to simulate their oxidation process) consists of hundreds of species and thousands of reactions, it cannot be used for engine simulation purposes due to the very excessive computational time requirements. Thus, reduced reaction mechanisms, which simulate properly some characteristics of the detailed one (autoignition time, combustion temperature, etc.), are used instead. Among the typical reduction techniques (Quasi-Steady-State Assumption (QSSA), Reaction Analysis, etc.), Genetic Algorithms (GA) allow for both an efficient reduction of small/medium size reaction mechanisms and, when combined with other techniques, a more effective reduction of large reaction mechanisms. In this work, a general and innovative GA methodology for the reduction of kinetic mechanisms describing the hydrocarbon oxidation processes has been proposed, and its application to the reduction of two different mechanism sizes (a medium one describing the methane autoignition and a larger one derived from the oxidation of a diesel fuel surrogate) has provided better results when compared to other GA methodologies or reduction techniques. The reduced mechanisms provide very similar autoignition time and combustion temperature values to those obtained from the detailed mechanism under different engine operating conditions (intake pressure and temperature, equivalence fuel/air ratio and exhaust gas recirculation rate).

Strategies for active diesel particulate filter regeneration based on late injection and exhaust recirculation with different fuels

Wall flow–type particulate filters are used in diesel vehicle engines to reduce particulate emissions below the limits established in regulations Euro 5 and Euro 6. The soot accumulated in the trap is eliminated during regeneration processes, often combining passive strategies with active ones. Active regeneration is conducted by modifications of the engine control parameters with respect to those set for normal vehicle operation. In this work, three of these parameters were modified to look for an optimized regeneration strategy, considering fuel consumption, gaseous emissions and rate of regeneration. The selected parameters were injection timing (affecting all injection events), exhaust gas recirculation and amount of postinjected fuel. The fuel tested was considered as an additional variable, and therefore, tests were performed with three different fuels: an ultra-low sulfur diesel fuel, a biodiesel fuel produced from animal fat and a gas-to-liquid fuel from low-temperature Fischer–Tropsch process. It was proved that eliminating the gas recirculation is the optimal option for a fast regeneration and reduced fuel consumption and that the late postinjection is essential to keep the temperature conditions needed for an efficient regeneration. Some limits were also proposed for these parameters to prevent from excessively fast or uncontrolled regeneration, to avoid excessive increase in fuel consumption and to reduce the probability of fuel dilution. Finally, the fuel properties were proved to be very relevant to the regeneration process, and therefore, both biodiesel and gas-to-liquid fuels (especially the former) showed un-optimized regeneration processes under the conditions set for regeneration process optimized with diesel fuel.

A combustion kinetic model for estimating diesel engine NO x emissions

A phenomenological combustion model, which considers the space and time evolutions of a reacting diesel fuel jet, has been developed in order to estimate the instantaneous NO x concentration in a diesel engine cylinder from the start of the injection until the exhaust valve opening. The total injected fuel mass has been divided into different fuel packages, through the fuel injection rate file, to take into account the heterogeneous nature of the diesel combustion process. Owing to the importance of the kinetics on the formation and destruction mechanisms of the main pollutant species and radicals, the instantaneous composition of each fuel package has been calculated by using a chemical reaction mechanism which considers 27 species and 59 reactions. The main input data are those resulting from the application of the combustion diagnostic procedure to the instantaneous cylinder pressure signal obtained during the engine tests, such as the heat release law (HRL) and the mean temperature. A single-cylinder diesel engine was tested to validate the model and to analyse the influence of the injection parameters (injection pressure, injection timing and injected fuel mass) on the NO x emissions. A good agreement between the theoretical results and the experimental ones was found when the engine conditions were modified. The model proposed also allows a better knowledge of the local mixing fuel/air processes, which represent one of the most important uncertainties when modelling diesel combustion.

An easy correlation to determine soluble and insoluble fractions in diesel particulate matter

Mathematical fitting of experimental emission data from five different fuels and three mixtures of biodiesel and reference diesel operating under diverse conditions allows to propose an easy equation to calculate both the insoluble and soluble fractions in Diesel Particulate Matter. The relative importance of each mechanism involved in the DPM formation process is also mathematically quantified. Two different mechanisms are principally found to take place during DPM formation. In a first stage (scrubbing effect), adsorbed sulphuric acid reacts with organic compounds in the exhaust to form heavy hydrocarbons which are likely to remain in the condensed phase. The remaining hydrocarbons undergo an absorption process and are retained on particles in the liquid phase. However, some light hydrocarbons may escape from the filter system and are not quantified as soluble organic fraction (SOF). The presence of two retention mechanisms makes it necessary to develop an accurate extraction technique to quantify the soluble organic fraction.

Estimation of Diesel Particulate Emissions from Hydrocarbon Emissions and Smoke Opacity

Experimental estimations of Diesel particulate emissions are very valuable tools because of the difficulties in collection, together with the high cost and trouble of the calibration and maintenance of the necessary equipment. In this work, an estimation involving smoke opacity and total hydrocarbon emissions measurements is proposed. The inclusion of some parameters related to the fuel composition and to the engine type and geometry anticipates some information about the phenomena taking place between the smoke, THC measurements and the particulate formation and emissions. A set of twelve fuels was tested in an automotive IDI engine under typical road operating conditions. A commercial fuel fulfilling the EN-590 European Directive was used as reference fuel, which was also used in other different engine (automotive and agricultural DI engines). The differences in the proposed parameters for these engine types were justified through its oil consumption, quality of the mixing process and exhaust temperatures.