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In the effort to achieve the goal of a climate-neutral transportation system, the use of hydrogen and other synthetic fuels plays a key role. As battery electric vehicles become more widespread, e-fuels could be used to defossilize the hard-to-electrify transportation sectors and to store energy produced from renewable and non-continuous energy sources. Among e-fuels, hydrogen and ammonia are very attractive because they are carbon-neutral and their oxidation does not lead to any CO2 emissions. Furthermore, hydrogen/ammonia mixtures overcome the issues that arise as each of the two fuels is separately used. In the automotive sector, the use of either hydrogen, ammonia or their blends require a characterization of such mixtures under engine-like conditions, that is, at high pressures and temperatures. The aim of this work is to evaluate the Laminar Flame Speed (LFS) of hydrogen/ammonia mixtures by varying the thermodynamic conditions and the molar composition of the reactants.

In this work, an ozone/air/gasoline mixture has been used as an alternative strategy to achieve a stable combustion in a spark ignition (SI) single cylinder PFI research engine. The air intake manifold has been modified to include four cells to produce ozone with different concentrations. In the research engine, various operating parameters have been monitored such as the in-cylinder pressure, temperature and composition of the exhaust gases, pressure and temperature of the mixture in the intake manifold, engine power and torque and specific fuel consumption. Experimental tests have been carried out under stoichiometric mixture conditions to observe the influence of ozone addition on the combustion process. The results show an advance and an increase of the in-cylinder pressure compared to the reference test-case, where a gasoline/air mixture is used. It is worth noting that, especially under stoichiometric condition, ozone concentration induces auto-ignition and knock.

The transition to electric vehicles in the transportation sector still faces multiple technological challenges and large investments as regards both vehicle design and vehicle charging infrastructure. Therefore, internal combustion engines still play a role in such a sector, making the engine improvements, in terms of pollutant emissions and efficiency, essential to mitigate the impact of human activities on the environment. One of the possible approaches to improve the efficiency of internal combustion engines is the recovery of the engine exhaust heat, from both the hot exhaust gases and the engine cooling system. In recent years, among the energy recovery strategies, the use of direct injection of H2O under supercritical and superheated thermodynamic states has been explored. Such a technique uses pressurized water recovered from the exhaust gases, heated to high temperature by using the engine exhaust heat and re-injected into the engine combustion chamber.

Nowadays, the vehicle hybridization and the use of non-conventional fuels for heavy-duty applications brings to a new beginning in the use of spark ignition (SI) engines. For a standard intake system, the premixed fuel/air mixture is controlled by the injection of fuel after the throttle valve. Then, the geometry of the intake system, with the intake duct, the intake valves and the cylinder head shape, influences the characteristics of the flow within the cylinder up to the combustion process. The new technology of fluid-power and electrical actuations gives the opportunity to decouple the intake and exhaust valve actuations with respect to the standard cam shaft distribution. The Variable Valve Actuation (VVA) concept is not new, but its application is now affordable and flexible enough to be applied to partial load conditions.

Supercritical water injection for ICEs may be a valid option to recover engine wall heat transfer and energy from exhaust gases, with benefits in terms of efficiency and performances. Water is recovered from exhaust gases and is brought up to supercritical conditions by employing the waste heat during engine operations. A preliminary study of this energy recovery approach has already been performed in an authors’ previous work, by employing a port fuel injection (PFI) internal combustion engine quasi-dimensional model, which has been validated against experimental data, returning satisfactory results in terms of overall efficiency gain. In this work, in order to obtain a more reliable and accurate evaluation of the achievable energy recovery with supercritical water injection, a multidimensional CFD model of the engine has been set and validated. As regards the engine geometry, a simplified axisymmetric engine has been used, in order to reduce the computational time and storage.

The efficiency of ICEs is strongly affected by exhaust gases and engine cooling system heat losses, which account for about 50% of the heat released by combustion. A promising approach is to transfer this exhaust heat to a fluid, like water, and inject it into the combustion chamber under supercritical conditions. In such a way, the recovered energy is partially converted into mechanical work, improving both engine efficiency and performance. A quasi-dimensional model has been implemented to simulate an SI engine with supercritical water injection. Specifically, a spark ignition ICE, four-stroke with Port Fuel Injection (PFI) has been considered. The model accounts for gas species properties, includes valves opening/closing, wall heat transfer, a water injection model and a combustion model. The influence of some injection parameters, i.e.

The more and more stringent regulations on emissions lead the automotive companies to develop innovative solutions for new powertrain concepts, including the employment of advanced combustion strategies and mixture of fuels with different thermochemical properties. HCCI combustion coupled with the partial direct injection of the charge is a promising technique, in order to control the performance and emissions and to extend the operating range. In this work an in-house developed multi-dimensional CFD software package has been used to analyze the behavior of a multi stage direct injection - partially stratified charge compression ignition engine fueled with PRF97. A combustion model based on the partially stirred reactor concept to include the influence of turbulence on chemistry has been employed. Specifically, a skeletal kinetic reaction mechanism for PRF oxidation, with a dynamic adaptive chemistry technique to reduce the computational cost of the simulations has been used.

The partial fuel stratification, by means of direct fuel injection, is one of the most suitable combustion strategies in order to overcome the limits of ignition control and operating range of HCCI engines. In this work, a multidimensional model, coupled with a detailed kinetic mechanism for ethanol oxidation, is used to investigate the performance of a partially stratified charge CI engine fueled by ethanol. The model, which accounts for turbulence effects on combustion, has been validated in a previous work, against experimental results in terms of both HCCI engine performance and emissions. In this work, computations have been carried out by varying the fraction of the fuel stratified charge and the injection timing and by considering different flow structures within the cylinder.

This work explores the coupling of advanced combustion strategies for engines with bio-based fuels. The characteristics of ethanol combustion in HCCI mode are investigated by using a multidimensional CFD model coupled with an accurate combustion model. In such a model, the chemical source terms are computed by a detailed kinetic mechanism and are corrected in order to take into account the influence of turbulence. The predictive capability of the model is proven by comparing the results with experimental measurements. The sensitivity analysis to initial and boundary conditions gives suggestions in order to increase engine efficiency and reduce pollutant emissions.

The following computational study examines the structure of sonic hydrogen jets using inlet conditions similar to those encountered in direct-injection hydrogen engines. Cases utilizing the same mass and momentum flux while varying exit-to-chamber pressure ratios have been investigated in a constant-volume computational domain. Furthermore, subsonic versus sonic structures have been compared using both hydrogen and ethylene fuel jets. Finally, the accuracy of scaling arguments to characterize an underexpanded jet by a subsonic “equivalent jet” has been assessed. It is shown that far downstream of the expansion region, the overall jet structure conforms to expectations for self-similarity in the far-field of subsonic jets. In the near-field, variations in fuel inlet-to-chamber pressure ratios are shown to influence the mixing properties of sonic hydrogen jets. In general, higher pressure ratios result in longer shock barrel length, though numerical resolution requirements increase.

This paper provides a comparison of wall heat fluxes and quenching distances as one-dimensional hydrogen and heptane flames impinge head-on onto a wall. It is shown that the quenching distances for stoichiometric H2/air and C7H16/air flames under the specified conditions of this study are about the same, but the wall heat flux for the H2/air flames is approximately a factor of two greater. For lean H2/air mixtures, the quenching distance increases substantially and the wall heat flux decreases. To understand more clearly the interplay of flame speed, temperature, thermal diffusivity, and surface kinetics on the results, studies of H2/O2 flames are also carried out.

In this paper, results from multidimensional computations in which a flamelet model is employed to model heat release rates and NO in transient jets under Diesel conditions are presented. These results are compared with those obtained by employing a Local Equilibrium Characteristic Time (LECT) model which is a combination of mixing-limited and kinetic-limited submodels. The LECT model has been widely employed in Diesel engine computations in prior work. Several variables, arising in the implementation of the flamelet model, are considered in detail to determine the sensitivity of the computed results to the variables themselves. These include probability density functions (PDFs), strain rates and kinetics. It is shown that the heat release rate results are not significantly sensitive to the PDFs selected and the strain rates. It is also shown that the heat release rates are relatively insensitive to the choice of detailed or reduced kinetics.

A multidimensional model for flows, sprays and combustion in engines is applied to study the entrainment characteristics of transient jets of relevance to Diesel and direct-injection spark-ignition engines. The following jets will be considered: solid-cone jets, hollow-cone jets and air-assist sprays. The entrainment characteristics will be evaluated by studying the evolution of lean, flammable and rich mixtures in the chamber. The focus of this work is on comparing the entrainment rates of jets from multi-hole solid-cone type jets with hollow-cone type jets. It will be shown that for the conditions considered in this work, the entrainment rate of the hollow-cone jets is less than that of the solid-cone jets. The effects of imparting swirl to the jets and to the air in the ambient will also be briefly discussed.

In this work, a multidimensional model that the authors have previously developed for computing radiant heat loss in an internal combustion engine is applied to study radiant heat loss characteristics in a Diesel engine. The model is applied to study the effects of load and speed on radiant heat transfer in the engine. It is shown that as load is increased the radiant heat loss increases and the fraction of radiant to total heat loss increases from about 12% at an overall equivalence ratio of 0.3 to 16% at an overall equivalence ratio of 0.5. As speed is increased, the radiant and total heat loss again increase but the ratio of radiant to total heat loss remains about the same for the cases considered. It is shown that there is a strong correlation between the radiant heat loss characteristics and soot concentration and temperature in the chamber.

The spray submodel is an important component in multidimensional models for Diesel engines. The satisfactory representation of the spray is dependent on adequate representation of turbulence in the jet which, in part, determines its spread and penetration. In this work, the RNG k-ϵ model is evaluated relative to the standard k-ϵ model for computing turbulent jets. Computations are made for both gas jets and sprays. The gas jet is computed with an adequately high degree of numerical spatial resolution of the order of the orifice diameter. In the case of the spray, achieving such a high resolution would be challenging. Since the spray has similarities to the gas jet, and the gas jet may be computed with such high resolution and adequate accuracy, firm conclusions may be drawn for it and they may be applicable to sprays. It is concluded that the RNG k - ϵ model, in general, results in predictions of greater mixing in the jets relative to the standard model.