Combustion In Direct-injection Engine Research Articles

Three-Dimensional Simulation Analysis of the Effect of Hydrous Ethanol and Exhaust Gas Recirculation on Gasoline Direct Injection Engine Combustion and Emissions

Abstract To analyze the influence of hydrous ethanol on the performance of the direct-injection engine, the three-dimensional simulation is carried out by using converge software coupled with the combustion mechanism of hydrous ethanol gasoline and the soot model. The combustion and soot generation characteristics of a direct-injection gasoline engine burning hydrous ethanol gasoline using exhaust gas recirculation (EGR) technology are investigated. It is found that the increase of the blending ratio of the hydrous ethanol can accelerate the flame propagation speed, shorten the combustion duration, and improve the combustion isovolumetric. The nucleation and growth of soot are jointly controlled by polycyclic aromatic hydrocarbons (PAHs) and the small molecular components such as C2H2. The oxygen content properties and high reactive OH of the hydrous ethanol-containing gasoline inhibit soot formation. Compared with pure gasoline, the carbon soot precursor mass is reduced by 60%, 54.5%, 73.3%, and 52.4% for 20% anhydrous ethanol blended with gasoline, A1, A2, A3, and A4, respectively, and the carbon soot mass is reduced by 63.6% and the carbon soot volume density is reduced by 40%. The introduction of EGR exhaust reduces the burning rate and leads to an increase in the production of carbon monoxide, hydrocarbon, and soot. However, the combination of EGR with hydrous ethanol gasoline can significantly improve the engine combustion environment and significantly reduce soot and PAHs concentrations. The impact of EGR also includes the ability to reduce combustion chamber temperatures and reduce NOx emissions from hydrous ethanol gasoline by 75%.

Impact of injector tip deposits on gasoline direct injection engine combustion, fuel economy and emissions

Gasoline direct injection (GDI) engine development is facing the great challenges in both fuel economy and particulate emissions. Trade-off is often required in GDI engines to sacrifice fuel economy in order to meet the strict emission regulations. GDI injector deposits have been identified as a potential cause of increased particulate emissions. In this work, a series of experimental tests was conducted on a 1.5 L turbocharged GDI engine to further understand the effect of injector deposits. The deposits formed on the injector tip surface were removed after the 55-hour fouling test and their effects on fuel consumption, in-cylinder combustion, thermal efficiency and engine out emissions were investigated before and after the removal. The spray characteristics of an identical injector under clean and fouled conditions were examined and the deposits inside the injector nozzle holes were observed by a scanning electron microscope. The test injectors were mildly fouled with an average of 1.5% flow rate loss and 1.84% injection pulse width increase. After removing the injector tip surface deposits, the engine combustion phase became advanced and the peak in-cylinder pressure increased. The combustion efficiency was close to 98% and showed no significant change. Although the indicated thermal efficiency was only slightly improved by 0.31–0.44% after removing the tip surface deposits, the particulate emissions were significantly affected and reduced by up to around 45%. At the meantime, NOx emissions moderately increased by approximately 12% after removal and no significant change occurred in the THC and CO emissions.

Impact of intake port injection of water on boosted downsized gasoline direct injection engine combustion, efficiency and emissions

Introduction of ever more stringent emission regulations on internal combustion engines beyond 2020 makes it necessary for original equipment manufacturers to find cost-effective solutions to improve the combustion engine efficiency and decrease its emissions. Highly efficient combustion engines can benefit from technologies such as cooled external exhaust gas recalculation and water injection. Among these technologies water injection can be used as a promising method to mitigate knock and significantly reduce the CO2 emissions. This is particularly important in highly downsized boosted engines which run under much higher intake pressures and are more prone to knocking combustion. In addition to anti-knock behaviour, water injection is also an effective method for reducing NOx emissions and exhaust gas temperature at high loads, which can protect the turbine in turbocharged engines. This study shows the influence of intake port injection of water on efficiency and emissions of a boosted downsized single-cylinder gasoline direct-injection engine in detail. Six different steady-state speed and load combinations were selected to represent the conditions that knocking combustion start to occur. Water ratio sweep tests were performed to find out the optimum water/fuel ratio at each test point and the impact on the combustion and emissions. In addition to gaseous emissions, impact of water injection on particle emissions was also investigated in this study. The results show the net indicated efficiency improved significantly (by a maximum of around 5% at medium load and around 15% at high load) up to a maximum level by increasing the injected water mass. Improvement in efficiency was mainly due to the increased heat capacity of charge and cooling effect of the injected water evaporation which reduced the in-cylinder temperature and pressure. Thus, knock sensitivity was reduced and more advanced spark timings could be used, which shifted the combustion phasing closer to the optimum point. However, increasing the water/fuel ratio further (more than 1 at medium load and more than 1.5 at high load) deteriorated the combustion efficiency, prolonged the flame development angle and combustion duration, and caused a reduction in the net integrated area of the P-V diagram. Efficiency improvements were lower at higher engine speed (3000 r/min) as the knock sensitivity was already reduced intrinsically. In terms of other, harmful, non-CO2 emissions, water injection was effective in reducing the NOx emissions significantly (by a maximum of around 60%) but increased the HC emissions as the water/fuel ratio increased. The results also show a significant reduction in particle emissions by adding water to the mixture and advancing the spark timing at medium and high loads. In addition, water injection also reduced the exhaust gas temperature by around 80°C and 180°C at medium and high loads, respectively.

Microscopic investigation of the atomization and mixing processes of diesel sprays injected into high pressure and temperature environments

Atomization and mixing of sprays are key parameters to successfully describe and predict combustion in direct-injection engines. Understanding these processes at the conditions most relevant to engines (high pressures and temperatures) is of primary interest. The present work investigates the atomization and mixing processes of sprays injected into environments with progressively higher pressure and temperature. An Engine Combustion Network Spray A single-hole injector is used to inject n-dodecane fuel into an optically accessible combustion chamber. High-speed imaging is performed using long-distance microscopy and diffused back-illumination to resolve ligament structures and droplets in the near-nozzle region. A unique aspect of this study is the application of high-speed imaging of sprays in the near-field under realistic diesel conditions, particularly those near top-dead-center. Imaging showed droplet and ligament dynamics at low pressure and temperature, but not at high pressure and temperature typical of engine operation, indicating diminished effects of surface tension. Relating the ambient conditions to the fuel critical pressure and temperature showed that when the ambient conditions are slightly above the critical point of the fuel, surface tension remains in effect at gas–liquid interfaces. However, for higher pressure and temperature conditions, the surface tension appears to diminish significantly as expected for supercritical fuel–air mixtures.

Cycle-Resolved Computations of Stratified-Charge Turbulent Combustion in Direct Injection Engine

An extremely lean burning engine has been expected to improve fuel consumption rate of engines. To achieve this, stable combustion should be realized for a wide range of operating conditions at air-fuel ratio over 40.0. In direct injection gasoline engines, cyclic variations of combustion derive from some main factors such as those of air flows and spray motions. In this report, we examine the influence of cyclic variations of "Air flows" and "Spray motions" on combustion instabilities, while other variation factors are fixed for each cycle of engine, by using our simulation model. The cyclic variations of stratified-charge turbulent combustion in direct injection gasoline engine can be simulated during ten continuous cycles based on the multi-level formation for the stochastic compressible Navier-Stokes equation and also a spray model. Computational results agree with cycle-averaged experimental data fairly well, while the cyclic variations computed are comparable to experimental ones reported by the other research group. Consequently, the present computational model will reveal an essential factor which generates the cyclic variations and will offer us an effective way to control combustion instabilities on very lean burning conditions.

A Spray-Interactive Flamelet Model for Direct Injection Engine Combustion

Toward higher efficiency and lower emissions, modern direct injection (DI) engines employ various injection strategies. This leads to more complex in-cylinder spray evaporation and combustion processes, requiring more comprehensive modeling approaches. In this study, an extended flamelet model is developed to describe DI engine combustion over a wide range of injection timings. A key feature of the model is to fully incorporate the interaction between spray evaporation and gas-phase combustion. Additional source terms representing the effect of evaporation were incorporated in the flamelet equation solved in the reactive space. A simple test problem demonstrated that the new formulation successfully accounts for the history of the spray evaporation. The extended formulation was implemented into a multidimensional computational fluid dynamics (CFD) code KIVA3v for full cycle engine simulation. The modeling results were successfully validated against available experimental data obtained from a rapid compression facility.

Acceleration of the chemistry solver for modeling DI engine combustion using dynamic adaptive chemistry (DAC) schemes

Acceleration of the chemistry solver for engine combustion is of much interest due to the fact that in practical engine simulations extensive computational time is spent solving the fuel oxidation and emission formation chemistry. A dynamic adaptive chemistry (DAC) scheme based on a directed relation graph error propagation (DRGEP) method has been applied to study homogeneous charge compression ignition (HCCI) engine combustion with detailed chemistry (over 500 species) previously using an R-value-based breadth-first search (RBFS) algorithm, which significantly reduced computational times (by as much as 30-fold). The present paper extends the use of this on-the-fly kinetic mechanism reduction scheme to model combustion in direct-injection (DI) engines. It was found that the DAC scheme becomes less efficient when applied to DI engine simulations using a kinetic mechanism of relatively small size and the accuracy of the original DAC scheme decreases for conventional non-premixed combustion engine. The present study also focuses on determination of search-initiating species, involvement of the NOx chemistry, selection of a proper error tolerance, as well as treatment of the interaction of chemical heat release and the fuel spray. Both the DAC schemes were integrated into the ERC KIVA-3v2 code, and simulations were conducted to compare the two schemes. In general, the present DAC scheme has better efficiency and similar accuracy compared to the previous DAC scheme. The efficiency depends on the size of the chemical kinetics mechanism used and the engine operating conditions. For cases using a small n-heptane kinetic mechanism of 34 species, 30% of the computational time is saved, and 50% for a larger n-heptane kinetic mechanism of 61 species. The paper also demonstrates that by combining the present DAC scheme with an adaptive multi-grid chemistry (AMC) solver, it is feasible to simulate a direct-injection engine using a detailed n-heptane mechanism with 543 species with practical computer time.

An adaptive multi-grid chemistry (AMC) model for efficient simulation of HCCI and DI engine combustion

There is a need to reduce the computational expense of practical multidimensional combustion simulations. Simulation of Homogeneous Charge Compression Ignition (HCCI) engine processes requires consideration of detailed chemistry in order to capture the ignition and combustion characteristics. Even with relatively coarse numerical meshes and reduced chemistry mechanisms, calculation times are still unacceptably long. For the simulation of Direct Injection (DI) engines, fine meshes are needed to achieve the resolution required by the spray and mixing models, and they are computationally expensive even with reduced chemistry. In addition, the increasing application of CFD for engine design optimization is pushing the demand to reduce computational time. In current design optimizations, depending on the size of the parametric space, hundreds of individual simulations are needed. This work presents an efficient Adaptive Multi-grid Chemistry (AMC) model that can be used in engine CFD codes for simulations of HCCI and DI engines with detailed chemistry. It was found that the number of cells computed with the chemistry solver can be reduced by two orders of magnitude for HCCI engines. The results predicted by the present KIVA AMC code are also consistent with those calculated by the original code using every cell. In the method, progressively coarser grids are used for cells with similar gas properties in the chemistry calculation (up to four neighbour levels) or in the global method, cells are grouped without regard for their locations in the cylinder. Averaged and gradient-preserving remapping techniques used in multi-zone engine simulations were also explored. A parametric study was conducted for determining the model variables, such as the degree of local homogeneity for the multi-grid solvers. The simulation results were compared with experimental data obtained from a Honda engine operated with n-heptane under HCCI conditions for which directly measured in-cylinder temperature and H2O mole fraction data are available. In addition, simulation results were found to agree well with experimental data from a DI diesel engine operated under PCCI conditions with ultra-high EGR rates. It was found that computer time was reduced by a factor of ten for HCCI cases and two to three for DI cases without losing prediction accuracy.

Algorithms for improving accuracy of spray simulation

Fuel spray is the pivotal process of direct injection engine combustion. The accuracy of spray simulation determines the reliability of combustion calculation. However, the traditional techniques of spray simulation in KIVA and commercial CFD codes are very susceptible to grid resolution. As a consequence, predicted engine performance and emission can depend on the computational mesh. The two main causes of this problem are the droplet collision algorithm and coupling between gas and liquid phases. In order to improve the accuracy of spray simulation, the original KIVA code is modified using the cross mesh droplet collision (CMC) algorithm and gas phase velocity interpolation algorithm. In the constant volume apparatus and D.I. diesel engine, the improvements of the modified KIVA code in spray simulation accuracy are checked from spray structure, predicted average drop size and spray tip penetration, respectively. The results show a dramatic decrease in grid dependency. With these changes, the distorted phenomenon of spray structure is vanished. The uncertainty in predicted average drop size is reduced from 30 to 5 μm in constant volume apparatus calculation, and the uncertainty is further reduced to 2 μm in an engine simulation. The predicted spray tip penetrations in engine simulation also have better consistency in medium and fine meshes.

Gasoline Direct-Injection Engine Combustion and Emissions Measurements and Modelling

Experimental measurements of exhaust emissions and combustion pressure are presented for a piston-guided direct-injection spark-ignition engine over a wide range of injection timings and ignition timings. The measurements show that the engine can be operated satisfactorily in very lean conditions (λ = 4.0) at wide-open throttle. High hydrocarbon emissions are measured at late start-of-injection conditions and with late ignition. High emissions of nitrogen oxides (NO x) occur with early start of injection and with early ignition. Modelling results show acceptable agreement with the measurements for the indicated mean effective pressure and show that high levels of NO x are produced from the inhomogeneous zones that burn close to stoichiometric conditions. It is argued that early start of injection gives rise to high NO x because of fuel mixing to conditions close to stoichiometric and burning early in the combustion process.

Measurement of droplet size distribution of gasolinedirect injection spray by droplet generator and planar imagetechnique

To understand the spray characteristics that are related tothe characteristics of emissions and the combustion of directinjection engines, the planar image technique was applied to measurethe distribution of droplet size. This technique was based on theprinciples of laser-induced fluorescence (LIF) and the Mie scatteringof droplets in the spray. We decided to apply the LIF and Miescattering intensity indices for unleaded gasoline to this techniquewith a droplet generator. The fluorescence and scattering signalswere imaged simultaneously by an optical system composed of adoubling prism, optical filters and an ICCD camera. Relative Sautermean diameter distributions according to space and time were obtainedby calculating the ratio between two light intensities, and thedistributions were quantified with a laser diffraction particleanalyser. Experimental results have shown that the characteristics ofthe gasoline direct injection injector varied with injectionpressure.

A Study on Combustion in Direct-Injection Engines : In the Case of Deep-Bowl Type

A Study on Combustion in Direct-Injection Engines : In the Case of Deep-Bowl Type