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Technical Paper

3-D CFD Analysis of CO Formation in Diesel Combustion - The use of intake air throttling to create reducing atmospheres for NSR catalysts -

The efficiency of the NOx Storage and Reduction (NSR) catalysts used in the aftertreatment of diesel engine exhaust gases can potentially be increased by using reactive reductants such as CO and H₂ that are formed during in-cylinder combustion. In this study, a multi-dimensional computational fluid dynamics (CFD) code coupled with complex chemical analysis was used to study combustion with various fuel after-injection patterns. The results obtained will be useful in designing fuel injection strategies for the efficient formation of CO.
Journal Article

A Numerical Study of the Effects of FAME Blends on Diesel Combustion and Emissions Characteristics Using a 3-D CFD Code Combined with Detailed Kinetics and Phenomenological Soot Formation Models

The objective of the present research is to analyze the effects of using oxygenated fuels (FAMEs) on diesel engine combustion and emission (NOx and soot). We studied methyl oleate (MO), which is an oxygenated fuel representative of major constituents of many types of biodiesels. Engine tests and numerical simulations were performed for 100% MO (MO100), 40% MO blended with JIS#2 diesel (MO40) and JIS#2 diesel (D100). The effects of MO on diesel combustion and emission characteristics were studied under engine operating conditions typically encountered in passenger car diesel engines, focusing on important parameters such as pilot injection, injection pressure and exhaust gas recirculation (EGR) rate. We used a diesel engine complying with the EURO4 emissions regulation, having a displacement of 2.2 L for passenger car applications. In engine tests comparing MO with diesel fuel, no effect on engine combustion pressure was observed for all conditions tested.
Technical Paper

A Numerical Study on Combustion and Exhaust Gas Emissions Characteristics of a Dual Fuel Natural Gas Engine Using a Multi-Dimensional Model Combined With Detailed Kinetics

A numerical study was carried out to investigate combustion characteristics of a dual-fuel gas diesel engine, using a multi-dimensional model combined with detailed chemical kinetics, including 43 chemical species and 173 elementary reactions. In calculations, the effects of initial temperature, EGR ratios on ignition, and combustion were examined. The results indicated EGR combined with intake preheating can favorably reduced NOx and THC emissions simultaneously. This can be explained by the fact that combustion mechanism is changed from flame propagation to HCCl like combustion.
Technical Paper

A Numerical Study on Combustion and Exhaust Gas Emissions Characteristics of a Dual Fuel Natural Gas Engine Using a Multi-Dimensional Model Combined with Detailed Kinetics

Natural gas pre-mixture is ignited by a small amount of pilot fuel in the dual fuel engine. In this paper, numerical studies were carried out to investigate the combustion and exhaust gas emissions formation process of this engine type by using a multi dimensional model combined with the detailed chemical kinetics including 57 chemical species and 290 elementary reactions. In calculation, the effect of the pre-mixture concentration on combustion was examined. The result indicated that the increased concentration of natural gas could improve the burning fraction and THC, CO emissions due to the increased pre-mixture consumption rate and the cylinders gas temperature.
Technical Paper

A Numerical Study on Correlation of Chemiluminescent Species and Heat Release Distributions Using Large Eddy Simulation

A mixed timescale subgrid model of a large eddy simulation was used to simulate the turbulence regime in diesel engine combustion. The combustion model used the direct integration approach with a diesel oil surrogate mechanism (developed at Chalmers University of Technology and consisting of 70 species and 309 reactions). Additional reactions for the generation and consumption of OH*, CO2*, and CH* species were added from recent kinetic studies. Collisional quenching and spontaneous emission resulted in de-excitation of the excited state radical. A phenomenological soot formation model (developed at Waseda University) was combined with the LES code. The following important steps were considered in the soot model: particle inception where naphthalene grows irreversibly to form soot, surface growth with the addition of C2H2, surface oxidation (induced by OH radicals and O2 attack), and particle coagulation.
Technical Paper

A Numerical Study on Ignition and Combustion of a DI Diesel Engine by Using CFD Code Combined with Detailed Chemical Kinetics

A CFD code combined with detailed chemical kinetics has been developed, linking with KIVA-3 and subroutines in CHEMKIN-II directly with some modifications. By using this CFD code, formation processes of combustion and exhaust gas emission for a turbo-charged DI diesel engine with common rail fuel injection system were simulated. As a result, formation processes of pollutant including NOx and soot were also considered according to the calculation results. The results show that NO caused by the extended Zeldvich mechanism accounted for about 88% of all NO, and it was found that there is a possibility to predict where and when soot will be formed by considering a simplified soot formation model.
Technical Paper

A Quasi Two Dimensional Model of Transport Phenomena in Diesel Particulate Filters - The Effects of Particle Diameter on the Pressure Drop in DPF Regeneration Mode-

Experimental and numerical studies on the combustion of the particulate matter in the diesel particulate filter with the particulate matter loaded under different particulate matter loading condition were carried out. It was observed that the pressure losses through diesel particulate filter loaded with particulate matter having different mean aggregate particle diameters during both particulate matter loading and combustion periods. Diesel particulate filter regeneration mode was controlled with introducing a hot gas created in Diesel Oxidation Catalyst that oxidized hydrocarbon injected by a fuel injector placed on an exhaust gas pipe. The combustion amount was calculated with using a total diesel particulate filter weight measured by the weight meter both before and after the particulate matter regeneration event.
Technical Paper

A Quasi Two Dimensional Model of Transport Phenomena in Diesel Particulate Filters - The Effects of Particle and Wall Pore Diameter on the Pressure Drop -

Experimental and numerical studies were conducted on diesel particulate filters (DPFs) under different soot loading conditions and DPF configurations. Pressure drops across DPFs with various mean pore diameters loaded with soots having different mean particle diameters were measured by introducing exhaust gases from a 2.2 liter inline four-cylinder, TCI diesel engine designed for use in passenger cars. A mechanistic hypothesis was then proposed to explain the observed trends, accounting for the effects of the soot loading regime in the wall and the soot cake layer on the pressure drop. This hypothesis was used to guide the development and validation of a numerical model for predicting the pressure drop in the DPF. The relationship between the permeability and the porosity of the wall and soot cake layer was modeled under various soot loading conditions.
Journal Article


We have constructed a quasi-2-dimensional NH₃-SCR model with detailed surface reactions to analyze the NOx conversion mechanism and reasons for its inhibition at low temperatures. The model consists of seven detailed surface reactions proposed by Grozzale et al., and calculates longitudinal gas flow, gas phase-catalyst phase mass transfer, and mass diffusion within the catalyst phase in the depth dimension. Using the model, we have analyzed the results of pulsed ammonia (NH₃) feed tests at various catalyst temperatures, and results show that ammonium nitrate (NH₄NO₃) is the inhibitor in NH₃-SCR reactions at low temperatures. In addition, we found that cutting the supply of NH₃ causes decomposition of NH₄NO₃, providing surface ammonia (NH₄+), which rapidly reacts with adjacent NOx, leading to an instantaneous rise in nitrogen (N₂) formation.
Technical Paper

Achievement of Stable and Clean Combustion Over a Wide Operating Range in a Spark-Assisted IDI Diesel Engine with Neat Ethanol

Spark-assisted diesel engines operated with alcohol fuels usually display misfiring or knocking problems. This paper presents an analysis of the factors influencing the ignition characteristics of ethanol in a swirl chamber diesel engine with a multi-spark ignitor. In the experiments, cycle-to-cycle combustion variations and the degree of knocking were investigated by changing engine parameters over a wide operating range. The results of the investigations showed that stable ignition and smooth combustion is achieved when a flammable mixture is formed in the vicinity of the spark plug when only a small amount of the injected fuel has evaporated. By optimizing the design factors, operation with high efficiency and low exhaust emissions was achieved.
Technical Paper

An Investigation of the Transient DPF Pressure Drop under Cold Start Conditions in Diesel Engines

To monitor emission-related components/systems and to evaluate the presence of malfunctioning or failures that can affect emissions, current diesel engine regulations require the use of on-board diagnostics (OBD). For diesel particulate filters (DPF), the pressure drop across the DPF is monitored by the OBD as the pressure drop is approximately linear related to the soot mass deposited in a filter. However, sudden acceleration may cause a sudden decrease in DPF pressure drop under cold start conditions. This appears to be caused by water that has condensed in the exhaust pipe, but no detailed mechanism for this decrease has been established. The present study developed an experimental apparatus that reproduces rapid increases of the exhaust gas flow under cold start conditions and enables independent control of the amount of water as well as the gas flow rate supplied to the DPF.
Technical Paper

Analysis and Modeling of NOx Reduction Based on the Reactivity of Cu Active Sites and Brønsted Acid Sites in a Cu-Chabazite SCR Catalyst

The NOx-reducing activity of a Cu-chabazite selective catalytic reduction (SCR) catalyst was analyzed over a wide temperature range. The analysis was based on the ammonia SCR (NH3-SCR) mechanism and accounted for Cu redox chemistry and reactions at Brønsted acid sites. The reduction of NOx to N2 (De-NOx) at Cu sites was found to proceed via different paths at low and high temperatures. Consequently, the rate-limiting step of the SCR reaction at Cu sites varied with the temperature. The rate of NOx reduction at Cu sites below 200°C was determined by the rate of Cu oxidation. Conversely, the rate of NOx reduction above 300°C was determined by the rate of NH3 adsorption on Cu sites. Moreover, the redox state of the active Cu sites differed at low and high temperatures. To clarify the role of the chabazite Brønsted acid sites, experiments were also performed using a H-chabazite catalyst that lacks Cu sites.
Technical Paper

Analysis of Diesel Soot Formation under Varied Ignition Lag with a Laser Light Extinction Method

Soot emission from diesel engines generally increases with shorter ignition lags. However, the detailed process and mechanism of this phenomenon has not been well understood. This investigation attempts to observe and analyze the in-chamber soot formation process at various ignition lags by high-speed photography of the direct flame images and laser shadowgraphs as well as the laser light extinction. In the experiment, the separation of soot concentration from the soot-fuel mixture concentration was established by subtracting the laser light extinction intensity through a non-firing chamber from that through a firing chamber. It was found that the soot concentration in the swirl chamber reached a maximum value immediately after the start of combustion, and then decreased rapidly. With shorter ignition lags, the maximum and final soot concentrations in the chamber increased.
Journal Article

Analysis of the Trade-off between Soot and Nitrogen Oxides in Diesel-Like Combustion by Chemical Kinetic Calculation

This study makes use of the detailed mechanisms of n-heptane combustion, from gas reactions to soot particle formation and oxidation, and a two-stage model based on the CHEMKIN reactor network is developed and used to investigate the trade-off between soot and NOx emissions. The effects of the equivalence ratio, EGR, ambient pressure and temperature, and initial particle diameter are observed for various residence times. The results show that high rates of NOx formation are unavoidable under conditions where high reduction rates of soot particles are obtained. This suggests that suppression of the amount of soot during the formation stage is essential for simultaneous reductions in engine-out soot and NOx emissions.
Technical Paper

Catalytic Effects of Metallic Fuel Additives on Oxidation Characteristics of Trapped Diesel Soot

The oxidations of Crapped diesel soots containing catalytic metals such as Ca, Ba, Fe, or Ni were characterized through thermogravimetric analysis with a thermobalance. Soot particles were generated by a single cylinder IDI diesel engine with metallic fuel additives. A two-stage oxidation process was observed with the metalcontalning soots. It was found that the first stage of oxidation is catalytically promoted by metal additives resulting in an enhanced reaction rate and a reduced activation energy. Soot reduction in the rapid first stage increases with increases in metal content. Soots containing Ba and Ca are oxidized most rapidly due to the larger reduction during the first stage. The second stage of oxidation is also slightly promoted by metal addition. The ignition temperature of the collected soot is substantially reduced by the metal additives.
Technical Paper

Characteristics of Diesel Combustion in Low Oxygen Mixtures with Ultra-High EGR

Ultra-low NOx and smokeless operation at higher loads up to half of the rated torque is attempted with large ratios of cold EGR. NOx decreases below 6 ppm (0.05 g/(kW·h)) and soot significantly increases when first decreasing the oxygen concentration to 16% with cold EGR, but after peaking at 12-14% oxygen, soot then deceases sharply to essentially zero at 9-10% oxygen while maintaining ultra low NOx and regardless of fuel injection quantity. However, at higher loads, with the oxygen concentration below 9-10%, the air/fuel ratio has to be over-rich to exceed half of rated torque, and thermal efficiency, CO, and THC deteriorate significantly. As EGR rate increases, exhaust gas emissions and thermal efficiency vary with the intake oxygen content rather than with the excess air ratio.
Technical Paper

Characteristics of Diesel Soot Suppression with Soluble Fuel Additives

Experiments on a large number of soluble fuel additives were systematically conducted for diesel soot reduction. It was found that Ca and Ba were the most effective soot suppressors. The main determinants of soot reduction were: the metal mol-content of the fuel, the excess air factor, and the gas turbulence in the combustion chamber. The soot reduction ratio was expressed by an exponential function of the metal mol-content in the fuel, depending on the metal but independent of the metal compound. A rise in excess air factor or gas turbulence increased the value of a coefficient in the function, resulting in larger reductions in soot with the fuel additives. High-speed soot sampling from the cylinder showed that with the metal additive, the soot concentration in the combustion chamber was substantially reduced during the whole period of combustion. It is thought that the additive acts as a catalyst not only to improve soot oxidation but also to suppress soot formation.
Technical Paper

Characteristics of Smokeless Low Temperature Diesel Combustion in Various Fuel-Air Mixing and Expansion of Operating Load Range

The characteristics of smokeless low temperature diesel combustion in various fuel-air mixing was investigated by engine tests with high rates of cooled exhaust gas recirculation (EGR), three compression ratios, and fuels of various cetane numbers, as well as by computational fluid dynamics (CFD) simulation of the in-cylinder distributions of mixture concentration and temperature. The results show that besides combustion temperature, fuel-air mixing is also vital to efficient, smokeless, and low NOx diesel combustion. Smokeless and low NOx diesel combustion can be realized even with insufficient fuel-air mixing as long as the combustion temperature is sufficiently low. However low combustion temperature and insufficient oxygen due to ultra-high EGR cause very high UHC and CO emissions, and a severe deterioration in combustion efficiency.
Technical Paper

Characteristics of Unburned Hydrocarbon Emissions in a Low Compression Ratio DI Diesel Engine

In a DI diesel engine, THC emissions increase significantly with lower compression ratios, a low coolant temperature, or during the transient state. During the transient after a load increase, THC emissions are increased significantly to very high concentrations from just after the start of the load increase until around the 10th cycle, then rapidly decreased until the 20th cycle, before gradually decreasing to a steady state value after 1000 cycles. In the fully-warmed steady state operation with a compression ratio of 16 and diesel fuel, THC is reasonably low, but THC increases with lower coolant temperatures or during the transient period just after increasing the load. This THC increase is due to the formation of over-lean mixture with the longer ignition delay and also due to the fuel adhering to the combustion chamber walls. A low distillation temperature fuel such as normal heptane can eliminate the THC increase.
Technical Paper

Characterization of Low Temperature Diesel Combustion with Various Dilution Gases

The effects of intake dilution with various dilution gases including nitrogen, argon, and carbon dioxide on low temperature diesel combustion were investigated in a naturally aspirated DI diesel engine to understand the mechanism of the simultaneous reductions in smoke and NOx with ultra-high EGR. NOx almost completely disappears with the intake oxygen concentration diluted below 16% regardless of the kind of dilution gas. Smoke emissions decrease with increased heat capacity of the charged gas due to promotion of mixture homogeneity with longer ignition delays. Intake dilution with the 36% CO2 + 64% Ar mixture which has a similar specific heat capacity as N2 shows lower smoke emissions than with N2. Chemical kinetics analysis shows that carbon dioxide may help to reduce NOx and soot by lowering the reaction temperature as well as by changing the concentrations of some radicals or/and species related to soot and NOx formation.