Search

A reduced kinetic model for the combustion of n-heptane, i-octane, n-cetane and heptamethylnonane was developed based on a prior model designed for use with a primary reference fuel consisting of n-heptane and i-octane. The present model, which can be easily employed in conjunction with a conventional computational fluid dynamics code, contains 59 chemical species and 96 reactions. Predicted ignition delay times under high pressure and temperature conditions were generated using this new kinetic model and compared with those obtained from full kinetic models. The results indicate that the general trends exhibited by the ignition delay times as temperature and pressure are varied are accurately predicted with this reduced model. The present model was also combined with a commercial computational fluid dynamics code and used to simulate the ignition of a diesel spray at high pressure and temperature. Finally, the effects of the cetane number of the fuel on the ignition process were investigated.

Numerical calculations were carried out to investigate the formation of a fuel–air mixture as well as ignition and combustion processes associated with a diesel spray impinging on a wall. This was performed by modeling the spray formed by injecting n-heptane into a constant-volume vessel under high temperature and pressure, with the fuel droplets described by a discrete droplet model. The flow and turbulent diffusion processes were calculated based on the large eddy simulation method to simulate the formation of a local non-homogeneous mixture and the accompanying heat release. The flame structure and heat transfer to the wall during impingement were also assessed. The results show that heat transfer to the wall is increased in the peripheral region around the stagnation point, as a result of the high temperature and thin boundary layer. Conversely, in the outer region, the heat transfer decreases as the boundary layer becomes more developed.

In previous study, post injection has shown a significant benefit in the reduction of soot emissions. Among them, the close post injection which only has short dwell between the main injection is supposed to be able to increase the thermal efficiency. The reduction of the cooling loss due to the main flame impingement is considered as the reason to the increase of thermal efficiency. However, there is a lack of study related to the influence of post injection timing and quantity on the combustion characteristics. Therefore, in this study, the engine performance and emission characteristics of injection strategy with close post injection was investigated. The post injection timing was modified from near zero interval to around 4.5°CA from the end of main injection. In addition, the post injection quantity was modified from 3, 9, to 15 mm3/cycle and the intake pressure was modified from 170, 200, to 220 kPa while maintaining the total injection quantity at 39 mm3/cycle. In order to clarify the mechanism of cooling loss by applying post injection and the effect of injection dwell to the cooling loss and combustion characteristics, an optical engine was used to observe the combustion process. As a result, a certain dwell is needed to fully exploit the effect of cooling loss reduction by post injection.

Previously, some efforts were made to investigate the effect of combining premixed charge compression ignition (PCCI) combustion with conventional mixing controlled combustion using two injectors for single cylinder. In this experiment, the PCCI combustion was introduced by a sub injection fixed at early stage and the conventional diesel combustion was introduced by main injection starts near top dead center (TDC). The self-ignitability of sub injection fuel was modified by blending n-heptane and isooctane. Therefore, the ignition delay of PCCI combustion was extended to overlap with conventional diesel combustion near TDC by increasing the isooctane volume percentage in sub fuel. By overlapping PCCI and conventional diesel combustion, the cooling loss could be reduced. Later, the combustion and emission characteristics of the combined combustion strategy was examined. The experimental results showed that with larger PCCI combustion ratio at the fixed load, the degree of constant volume (DCV) increased. At the same time, the cooling loss was decreased slightly.

The direct-injection-premixed charge compression ignition–based combustion process with a high exhaust gas recirculation ratio and early injection timing is simulated using a Reynolds-averaged Navier–Stokes–based commercial computational fluid dynamics code with a nonhomogeneous mixture auto-ignition combustion model. In addition, the formation processes of nitrogen oxide (NO), carbon monoxide (CO), and unburnt hydrocarbon are calculated. The calculation results are compared with the experimental data. According to the calculation results, the formation processes of NO, CO, and hydrocarbon are discussed in relation to the spray development and ignition point. Furthermore, the combustion process of two-stage injection is also calculated. The result shows that the combustion process is described well by this model except in the case where auto-ignition occurs in the squish area. Additionally, the relationship between the combustion process and the mixture distribution is clarified. The main origins of the unburnt hydrocarbon and CO emissions are located in the center region of the combustion chamber, where the mixture becomes excessively lean and low in temperature.

Numerical calculation based on large eddy simulation was carried out to investigate mixture formation process in a spray formed by injecting n-heptane into a constant volume vessel under high-temperature and high-pressure conditions. Liquid phase of fuel was described by discrete droplet model. The results show that the shape of spray is similar to that of visualized image and spray tip penetration corresponds well to experimental results. In addition, distributions of fuel concentration and local non-homogeneous distribution were discussed by comparing the calculation results with the mixture distributions measured using planer laser induced fluorescence technique. The local distributions were well described except around downstream spray boundary. Calculations were also carried out for lower injection pressure condition and smaller nozzle orifice diameter condition. Although fuel concentration in a spray does not quantitatively reproduce the experimental results, the effects of the injection conditions qualitatively agreed with the experiments. Furthermore, ignition and combustion processes were investigated using Schreiber's model for calculating the progress of oxidation reactions.

To achieve further reductions in fossil fuel consumption and carbon dioxide (CO2) emissions, the transportation sector will need to assume a key role. Based on the BLUE Map scenarios for 50 % reduction in CO2 emissions by 2050 in the Energy Technology Protocol 2010 of the International Energy Agency, conventional engine vehicles and hybrid electric vehicles (HEVs)—including plug-in hybrid electric vehicles (PHEVs)—are expected to account for 90 % of the market and will still account for 50 % of the market by 2050. This means that it is important that thermal efficiency of the internal combustion engine (ICE) be increased. Research and development into high-efficiency ICEs has been progressing in Japan, Europe, and the United States. In this paper, current research status and future prospects of ICEs and HEVs (PHEVs) are described.

In order to extend the available load range and obtain higher thermal efficiency in natural gas premixed charge compression ignition (PCCI) engines, a strategy for controlling direct injection combustion is discussed. Experimental results from single-cylinder engine tests demonstrate the possibility to extend load range by direct fuel injection. Reduced nozzle orifice size and reduced injection angle provide higher combustion efficiency; however, this promotes the tendency to knock because of the formation of a locally rich mixture. Arising from discussions based on prediction by computational fluid dynamics (CFD) code, considering mixture heterogeneity, it is suggested that controlling probability density functions (PDFs) of fuel concentration could be a means to control the rate of pressure rise. Restricted air utilization is useful to activate combustion at low overall equivalence ratios; on the other hand, full utilization of in-cylinder air and formation of a quantity of lean mixture can provide mild combustion.

Effect of hydrogen addition to hydrocarbon fuels on combustion characteristics were investigated using chemical full kinetics. The changes of ignition delay time and laminar flame velocity when hydrogen is added to methane or propane fuels were calculated. The result shows that for ambient pressure condition, ignition delay time decreases corresponding to the amount of hydrogen addition. However for high pressure condition, when 10% hydrogen is added to hydrocarbon fuel, ignition delay time decreases as short as that of hydrogen itself. Laminar flame velocity increases gradually with adding hydrogen, and when more than 70% hydrogen is added, laminar flame velocity rapidly increases. Furthermore, the production rate of OH is an important factor for the change of ignition delay time and laminar flame velocity, when hydrogen is added.

The combustion process of natural gas in a premixed charged compression ignition (PCCI) engine is analyzed using computational fluid dynamics included in heterogeneity of mixture concentration and temperature with stochastic approach. The non-uniform states of turbulence mixing, ignition process is statistically described using the probability density functions (PDF). The results show that the course of in-cylinder pressure is good agreement with experimental data and the effect of mixture heterogeneity on the ignition delay and the rate of heat release is revealed.

A fully three dimensional numerical model for the combustion process in a spark-ignition engine was established using the combustion submodel described of the flame area evolution. Computations carried out the process of premixed combustion and they were compared with the results using the combustion submodel of the eddy break-up. The combustion in a disc chamber of constant volume was simulated. The results show that the course of flame propagation is well predicted. The calculations for different conditions in a natural-gas engine are performed. It is shown that the effects of fuel-air equivalence ratio and swirl intensity on flame propagation are described. The cylinder pressure histories are agreed with measured one, and the distributions of velocity vectors and temperature in a cylinder are revealed.

An auto-ignition process of a non-homogeneous mixture is investigated using a numerical calculation based on chemical kinetics and the stochastic approach. This type of autoignition phenomenon is considered as a fundamental process of the initial stage of diesel combustion (Ishiyama, et al., 2003) or homogeneous charged compression ignition (Shimasaki, et al., 2003). In order to investigate these combustion processes, many numerical calculations have been performed and for many of those it has been assumed that the ignition process is dominated by the turbulent mixing (Kong and Reitz, 2000).

An auto-ignition process of non-homogeneous mixture was fundamentally investigated using a numerical calculation based on the chemical kinetics and stochastic approach. In the present study, the auto-ignition process of n-heptane is calculated using a reduced mechanism developed by Seiser et al. The non-uniform states of turbulent mixing are statistically described using probability density functions and the stochastic method, which was originally developed based on Curl's model. The results show that the starting points of the low temperature oxidation and ignition delay period are hardly affected by the equivalence-ratio variation, however, the combustion duration increases with increasing variance of equivalence ratio. Furthermore, the combustion duration is mainly affected by the non-homogeneity at the ignition and not very much affected by the mixing rate.

The biodiesel surrogate fuels are realistic kinetic tools to study the combustion of actual biodiesel fuels in diesel engines. The knowledge of fuel chemistry aids in the development of combustion modelling. In order to numerically simulate the diesel combustion, it is necessary to construct a compact reaction model for describing the chemical reaction. This study developed a skeletal kinetic model of methyl decanoate (MD) and n-heptane as a biodiesel surrogate blend for the chemical combustion reactions. The skeletal kinetic model is simply composed of 45 chemical species and 74 reactions based on the full kinetic models which have been developed by Lawrance Livermore National Laboratory (LLNL) [1] and Knowledge-basing Utilities for Complex Reaction Systems (KUCRS) which is built by Miyoshi [2] under the diesel like engine conditions. The model in this study is generated by using CHEMKIN and then it is used to produce the ignition delay data and the related chemical species. The model predicted good reasonable agreement for the ignition delays and most of the reaction products at various conditions. The chemical species are well reproduced by this skeletal kinetic model while the good temperature dependency is found under constant pressure conditions 2MPa and 4MPa. The ignition delay time of present model is slightly shorter than the full kinetic model near negative temperature coefficient (NTC) regime. This skeletal model can provide the chemical kinetics to apply in the simulation codes for diesel-engine combustion.

The aim of this study is to find strategies for fully utilizing the advantage of diesel-ethanol blend fuel in recent diesel engines. For this purpose, experiments were performed using a single-cylinder direct injection diesel engine equipped with a high-pressure common rail injection and a cold EGR system. The results indicate that significant PM reduction at high engine loads can be achieved using 15% ethanol-diesel blend fuel. Increasing injection pressure promotes PM reduction. However, poor ignitability of ethanol blended fuel results in higher rate of pressure rise at high engine loads and unstable and incomplete combustion at lower engine loads. Using pilot injection with proper amount and timing solves above problems. NOx increase due to the high injection pressure can be controlled employing cold EGR. Weak sooting tendency of ethanol-blend fuel enables to use high EGR rates for significant NOx reduction. Above finding indicates that low level of PM and NOx emission with no fuel consumption penalty is achievable when diesel-ethanol blend is used with combination of modern combustion control methods.