Modelling of the auto-ignition angle in diesel HCCI engines through D-optimal design

The HCCI (Homogeneous Charge Compression Ignition) process is one of the most promising combustion processes developed to reduce pollutant emissions from automotive vehicles. However, there are practical difficulties concerning the control of the onset of ignition, and thus the availability of simple models which allows to simulate the auto-ignition phenomena may be very interesting for the development of new HCCI engines. In this work, the onset of ignition in a HCCI engine and the auto-ignition angle were modelled (OAM and AAM respectively) through experimental plans based on the D-optimal criterion. The experimental values were obtained by using the chemical kinetic code CHEMKIN together with an appropriate diesel fuel surrogate. The models developed have an acceptable goodness-of-fit and predictive capability (differences lower than 3 CAD were obtained between modelled and real auto-ignition angles for all the cases). The relative fuel/oxidant ratio and the intake temperature were the most significant engine parameters affecting the onset of auto-ignition, while the intake temperature and pressure appear as the most important parameters determining the auto-ignition angle. These models could be used by the Engine Control Unit (ECU) as an on-board diagnostic technique to control the HCCI combustion in real time. The optimal engine parameters for five specific operating conditions (chosen to cover the most common light duty diesel vehicles operating modes) were also calculated by using the above mentioned models (OAM and AAM) and by solving two non-linear optimization problems. To achieve optimization, a desirability function was defined. The optimization methodology proposed can be used to obtain the optimum engine parameters, which are used by the ECU, matching different vehicle requirements.     

An experimental and numerical analysis of the HCCI auto-ignition process of primary reference fuels,toluene reference fuels and diesel fuel in an engine,varying the engine parameters

For a future HCCI engine to operate under conditions that adhere to environmental restrictions, reducing fuel consumption and maintaining or increasing at the same time the engine efficiency, the choice of the fuel is crucial. For this purpose, this paper presents an auto-ignition investigation concerning the primary reference fuels, toluene reference fuels and diesel fuel, in order to study the effect of linear alkanes, branched alkanes and aromatics on the auto-ignition. The auto-ignition of these fuels has been studied at inlet temperatures from 25 to 120 °C, at equivalence ratios from 0.18 to 0.53 and at compression ratios from 6 to 13.5, in order to extend the range of investigation and to assess the usability of these parameters to control the auto-ignition. It appeared that both iso-octane and toluene delayed the ignition with respect to n-heptane, while toluene has the strongest effect. This means that aromatics have higher inhibiting effects than branched alkanes. In an increasing order, the inlet temperature, equivalence ratio and compression ratio had a promoting effect on the ignition delays. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process.     

A soot formation embedded reduced reaction mechanism for diesel surrogate fuel

A reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene was developed, the reduced mechanism (referred as the “THU mechanism”) includes 60 species and 145 reactions, and it contains soot formation reactions. The THU mechanism was developed from the existing n-heptane/toluene mechanism (70 species and 313 reactions) of Chalmers University of Technology (referred as the “CTH mechanism”). SENKIN and XSENKPLOT were used to analyze the important reactions and species during n-heptane, toluene oxidation and soot formation processes to formulate the reduced mechanism. Ignition delays of n-heptane and toluene predicted by the THU mechanism match well with the CTH mechanism and shock-tube test data under different conditions. The THU and CTH mechanisms also show similar soot concentration prediction. The global reaction of diesel fuel decomposed into n-heptane and toluene with mole fraction 7:3 was built to accelerate the decomposition and advance ignition timing. Kinetic constants of soot oxidation reactions were adjusted to reduce the soot oxidation rate. The THU mechanism was coupled with the KIVA-3V Release 2 code to model diesel combustion processes in the constant-volume combustion vessel and optical diesel engine of Sandia. The predicted ignition delay, in-cylinder pressure and heat release rate match the experimental results well. The predicted spatial and temporal soot concentration distributions have similar trend with the experiments.     

The development and experimental validation of a reduced ternary kinetic mechanism for the auto-ignition at HCCI conditions,proposing a global reaction path for ternary gasoline surrogates

To acquire a high amount of information of the behaviour of the Homogeneous Charge Compression Ignition (HCCI) auto-ignition process, a reduced surrogate mechanism has been composed out of reduced n-heptane, iso-octane and toluene mechanisms, containing 62 reactions and 49 species. This mechanism has been validated numerically in a 0D HCCI engine code against more detailed mechanisms (inlet temperature varying from 290 to 500 K, the equivalence ratio from 0.2 to 0.7 and the compression ratio from 8 to 18) and experimentally against experimental shock tube and rapid compression machine data from the literature at pressures between 9 and 55 bar and temperatures between 700 and 1400 K for several fuels: the pure compounds n-heptane, iso-octane and toluene as well as binary and ternary mixtures of these compounds. For this validation, stoichiometric mixtures and mixtures with an equivalence ratio of 0.5 are used. The experimental validation is extended by comparing the surrogate mechanism to experimental data from an HCCI engine. A global reaction pathway is proposed for the auto-ignition of a surrogate gasoline, using the surrogate mechanism, in order to show the interactions that the three compounds can have with one another during the auto-ignition of a ternary mixture.     

An experimental and numerical analysis of the influence of the inlet temperature,equivalence ratio and compression ratio on the HCCI auto-ignition process of Primary Reference Fuels in an engine

In order to understand better the auto-ignition process in an HCCI engine, the influence of some important parameters on the auto-ignition is investigated. The inlet temperature, the equivalence ratio and the compression ratio were varied and their influence on the pressure, the heat release and the ignition delays were measured. The inlet temperature was changed from 25 to 70 °C and the equivalence ratio from 0.18 to 0.41, while the compression ratio varied from 6 to 13.5. The fuels that were investigated were PRF40 and n-heptane. These three parameters appeared to decrease the ignition delays, with the inlet temperature having the least influence and the compression ratio the most. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process. The same kinetic mechanism is used to better understand the underlying chemical and physical phenomena that make the influence of a certain parameter change according to the operating conditions. This can be useful for the control of the auto-ignition process in an HCCI engine.     

Low sooting combustion of narrow-angle wall-guided sprays in an HSDI diesel engine with retarded injection timings

An optically accessible single-cylinder high speed direct-injection (HSDI) diesel engine was used to investigate the spray and combustion processes with narrow-angle wall-guided sprays. Influences of injection timings and injection pressure on combustion characteristics and emissions were studied. In-cylinder pressure was measured and used for heat release analysis. High-speed spray and combustion videos were captured. NO_x emissions were measured in the exhaust pipe. With significantly retarded post-top dead center (TDC) injections, smokeless combustion was achieved for wall-guided diesel spray. Premixed-combustion was observed from the heat release rates and the combustion images. Natural luminosity was found significantly lower for smokeless combustion case. However, NO_x emissions were higher for the low sooting combustion cases. In addition, retarding injection timing lead to more complete combustion with more heat released from the same amount of fuel. Spray images revealed significant fuel impingement for all the conditions and the spray development was controlled and guided by the piston bowl curvature. NO_x and natural luminosity trade-off trend was observed for these conditions. However, quite different from conventional diesel combustion, retarding post-TDC injection timing leads to lower natural luminosity and higher NO_x emissions for narrow-angle wall-guided spray combustion. For the smokeless combustion case under moderate operating load, both homogeneous combustion and low-luminosity pool fires were observed during combustion process and the latter was due to fuel-piston impingement. The findings in this study could be used to solve the smoke issues associated with narrow-angle injection technique under high load conditions. With narrow-angle injectors, ignition could occur for significantly retarded post-TDC injections, which provides a unique mixing approach for diesel engines.     

Investigation of fuel spray atomization in a DI heavy-duty diesel engine and comparison of various spray breakup models

In the last decade 3D-CFD has been successfully established for the simulation of IC-engine fuel spray formation and propagation processes. The accuracy of the calculation results, however, strongly depends on the models adopted for simulation of the primary and secondary atomization processes. Hence, careful validations of the individual models serve as major prerequisites for the successful analysis and optimization of high-pressure sprays in diesel engines. In the present work, a CFD code has been used to study the detailed modeling of spray and mixture formation in a caterpillar heavy-duty diesel engine. With respect to the liquid-phase, spray calculations are based on a statistical method referred to as the Discrete Droplet Method (DDM). This paper presents a comparison of four Lagrangian fuel spray breakup models that are in use with commercial softwares in diesel engine simulation. In this paper, we tried to highlight this models prediction difference for sample case, compare their result and explain some possible reasons for differences. The predicted results are validated by comparing with existing experimental data. A good agreement between the predicted and experimental values ensures the accuracy of the numerical predictions collected with the present work.     

Effect of a narrow fuel spray angle and a dual injection configuration on the improvement of exhaust emissions in a HCCI diesel engine

The aim of this work was to investigate the effect of narrow fuel spray angle injection and dual injection strategy on the exhaust emissions of a common-rail diesel engine. To achieve successful homogeneous charge compression ignition by an early timing injection, a narrowed spray cone angle injector and a reduced compression ratio were employed. The combination of homogeneous charge compression ignition (HCCI) combustion and conventional diesel combustion was studied to examine the exhaust emission and combustion characteristics of the engine under various fuel injection parameters, such as injection timings of the first and second spray.The results showed that a dual injection strategy consisting of an early timing for the first injection for HCCI combustion and a late timing for the second injection was effective to reduce the NO_x emissions while it suppress the deterioration of the combustion efficiency caused by the HCCI combustion.     

The formation of deposits from oil under conditions pertinent to diesel engine pistons

The production of deposits from oil on various materials has been studied under conditions pertinent to the ring zone of a diesel engine piston. Deposition is found to occur only as a result of oxidation, and metal catalysed free radical reactions in the liquid phase appear to be responsible for the problem. Metal species removed from the surface are the most active promoters and these may be deactivated by heavy deposition (encapsulation) or by the addition of deactivating complexing agents.     

A neural network approach to the prediction of diesel fuel lubricity

The continuous sulfur reduction in diesel fuel has resulted in poor fuel lubricity and engine pump failure, a fact that led to the development of a number of methods that measure the actual fuel lubricity level. However, lubricity measurement is costly and time consuming, and a number of predictive models have been developed in the past, based mainly on various fuel properties. In the present paper, a black box modeling approach is proposed, where the lubricity is approximated by a radial basis function (RBF) neural network that uses other fuel properties as inputs. The HFRR apparatus was used for lubricity measurements. In the present model, the variables used included the diesel fuel conductivity, density, kinematic viscosity at 40 °C, sulfur content and 90% distillation point, which produced the smallest error in the validation data.     

Influence of nozzle geometry on ignition and combustion for high-speed direct injection diesel engines under cold start conditions

Starting at low temperatures (below 0 °C) is an important issue for current and near future diesel engine technology. Low ambient temperature causes long cranking periods or complete misfiring in small diesel engines and, as a consequence, an increased amount of pollutant emissions. This paper is devoted to study the influence of nozzle geometry on ignition and combustion progression under glow-plug aided cold start conditions. This study has been carried out in an optically accessible engine adapted to reproduce in-cylinder conditions corresponding to those of a real engine during start at low ambient temperature. The cold start problem can be divided in two parts in which nozzle geometry has influence: ignition and main combustion progress. Ignition probability decreases if fuel injection velocity is increased or if the amount of injected mass per orifice is reduced, which is induced by nozzles with smaller hole diameter or higher orifice number, respectively. Combustion rates increase when using nozzles which induce a higher momentum, improving mixture conditions. For these reasons, the solution under these conditions necessarily involves a trade-off between ignition and combustion progress.     

Hydrogen production for fuel cell by oxidative reforming of diesel surrogate: Influence of ceria and/or lanthana over the activity of Pt/Al2O3 catalysts

A series of Pt catalysts supported on Al₂O₃ (Pt/A), Al₂O₃-CeO₂ (Pt/A-C), Al₂O₃-La₂O₃ (Pt/A-L) and Al₂O₃-La₂O₃-CeO₂ (Pt/A-L-C) have been prepared and tested in the oxidative reforming of diesel surrogate with the aim of studying the influence of ceria and lanthana additives over the activity and stability toward hydrogen production for fuel cell application. Several characterization techniques, such as adsorption-desorption of N₂, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, H₂ chemisorption, and thermogravimetric analysis, have been used to define textural, structural, and surface properties of catalysts and to establish relationships with their behaviour in reaction. This physicochemical characterization has shown that lanthana inhibits the formation of α phase in alumina support and decreases ceria dispersion. Activity results show a better performance of ceria-loaded catalysts, being the Pt/A-C sample the system that offers higher H₂ yields after 8 h of reaction. The greater H₂ production for ceria-loaded catalysts, particularly in the case of the system Pt/A-C, is attributed to the Pt-Ce interaction that may change the electronic properties and/or the dispersion of active metal phase. Also, the Ce^III form of Ce^IV/Ce^III redox pair enhances the adsorption of oxygen and water molecules, thus increasing the catalytic activity and also decreasing coke deposition over surface active Pt phases. Stability tests showed that catalysts in which Pt crystallites are deposited on the alumina substrate covered by a lanthana monolayer, give rise to an increase in stability toward H₂ production.     

Influence of fuel sulfur on the characterization of PM10 from a diesel engine

To study the effects of fuel sulfur content on the characteristics of diesel particle emitted from a typical engine used in China, two types of diesel fuel with sulfur content of 30 ppm and 500 ppm were used in this engine dynamometer test under six operation conditions corresponding to 20%, 50% and 80% load at 1400 rpm and 2300 rpm engine speeds, respectively. Gaseous pollutants and particulate matter (PM) emissions were sampled with AVL AMA4000 and Model 130 High-Flow Impactor (MSP Corp), respectively. More specifically, the PM mass, total carbon (TC), organic carbon (OC), elemental carbon (EC) and water-soluble ion distribution were also measured. Compared with high sulfur diesel, the application of low sulfur diesel can lower fuel-based PM emissions by 9.2-56.6%. At 1400 rpm, the low sulfur diesel decreased both OC and EC by 5-34% and about 20%; while at 2300 rpm, the low sulfur fuel decreased OC by 33-57% and increased EC emission, resulting in a lower OC/EC ratio. The evidence implicating that OC oxidation was promoted by low sulfur diesel, but the effect on EC oxidation was dependent on engine speed. The linear regression has been conducted between TC and PM₁₀, and the slopes were 0.88 and 0.80 for low sulfur diesel and high sulfur one, respectively. Higher sulfate content was detected in the 0.13 μm particles when using the high sulfur diesel, but the percentage of sulfate was 0.9% for PM₁₀ from both diesel fuels. Comparing with that of 500 ppm, EC increased sharply to a maximum of 114% in particles of 0.13 μm when using 30 ppm sulfur diesel at 2300 rpm.     

Performance, combustion and emissions of a diesel engine operated with reformed EGR. Comparison of diesel and GTL fuelling

In this work, the effects of a standard ultra-low sulphur diesel (ULSD) fuel and a new, ultra-clean synthetic GTL (gas-to-liquid) fuel on the performance, combustion and emissions of a single-cylinder, direct injection, diesel engine were studied under different operating conditions with addition of simulated reformer product gas, referred to as reformed EGR (REGR). For this purpose various levels of REGR of two different compositions were tested. Tests with standard EGR were also carried out for comparison. Experiments were performed at four steady state operating conditions and the brake thermal efficiency, combustion process and engine emission data are presented and discussed. In general, GTL fuel resulted in a higher brake thermal efficiency compared to ULSD but the differences depended on the engine condition and EGR/REGR level and composition. The combustion pattern was significantly modified when the REGR level was increased. Although the extent of the effects of REGR on emissions depended on the engine load, it can be generally concluded that an optimal combination of GTL and REGR significantly improved both NO_x and smoke emissions. In some cases, NO_x and smoke emission reductions of 75% and 60%, respectively, were achieved compared to operation with ULSD without REGR. This offers a great potential for engine manufacturers to meet the requirements of future emission regulations.     

Towards a single brick solution for the abatement of NOx and soot from diesel engine exhausts

Nano-structured perovskite-type lanthanum ferrites La_1 − xA_xFe_1 − yB_yO₃ (where A = Na, K, Rb and B = Cu), prepared by the solution combustion synthesis (SCS) method and characterized by BET, XRD, FESEM, AAS and catalytic activity tests in microreactors as well as on an engine bench, proved to be effective in the simultaneous removal of soot and NO, the two prevalent pollutants in diesel exhaust gases in the temperature range 350–450 °C. The best compromise between soot and nitrogen oxide abatement was shown by the La-K-Cu-FeO₃ catalyst which displayed the highest catalytic activity towards carbon combustion and the highest NO conversion activity.     

Analytical conditions for field ionization mass spectrometry of diesel fuel

Field ionization mass spectrometry (FIMS) was investigated to establish a method for clarifying the compositions of hydrocarbons in diesel fuels. Firstly, the influences of reservoir temperature, ion source temperature, emitter current, cathode voltage and ion focusing mode on ion intensities and double bond equivalence value (DBE) distributions were examined to define the analytical conditions for obtaining almost the same carbon number distribution of n-paraffins (DBE=0) as that obtained by gas chromatography. Secondly, the origin of the memory background and the measures to minimize it were examined to obtain the ion intensities of high reproducibility. As a result, variation coefficients of less than 6.4 and 5.1% were obtained for the ion intensity of each hydrocarbon and the sums of the ion intensities of the hydrocarbons with the same DBE, respectively. Finally, two fuels, which were similar in H/C but considerably different in the backend fraction at a distillation temperature of 290 °C (R₂₉₀), were analyzed by FIMS established in this study, to explain the reasons why these fuels yielded nearly the same particulate emissions. FIMS results showed that a fuel with low R₂₉₀ consisted of low carbon number aliphatic hydrocarbons and high carbon number aromatic hydrocarbons, both of which have low inflammability. The fuel was found to yield more HC emission than another fuel with high R₂₉₀. The amount of the particulate emission was larger than that expected from R₂₉₀.     

Study on the application of DME/diesel blends in a diesel engine

In this paper fuels, based on various DME to diesel ratios are investigated. Physical and chemical properties of DME and diesel display mutual solubility at any ratio. The vapor pressure of DME/diesel blends is lower than that of pure DME at the same temperatures and it decreases with an increase of diesel mass fraction in blends, which is beneficial to the elimination of vapor lock in the fuel supply system on CI engines. Performance, emission and other features of three kinds of DME/diesel blend fuels and diesels are evaluated in a four-cylinder test engine. By taking relative advantages of DME and diesel, the DME/diesel blends could achieve satisfactory properties in lubricity and atomization, which contributed to improvements in spray and combustion characteristics. Simultaneously, smoke emission could be reduced significantly with a little penalty on CO and HC emissions for DME/diesel blended engine at high loads, in comparison to diesel engine. NO_x emissions of the engine powered by DME/diesel blends are decreased somewhat. Moreover, the power output would be improved a little and NO_x emission could be reduced further if the fuel supply advance angle is retarded appropriately.     

A study on the reduction of exhaust emissions through HCCI combustion by using a narrow spray angle and advanced injection timing in a DME engine

This paper describes the combustion and emission characteristics as well as engine performance according to the narrow spray angle and advanced injection timing for homogeneous charge compression ignition (HCCI) combustion in dimethyl ether (DME) fueled diesel engine. The bowl shape of the piston head was modified to apply the narrow spray angle and advanced injection timing. The spray, combustion and emission characteristics in a DME HCCI engine were calculated by using numerical method of the KIVA-3 V code coupled with the detailed chemical kinetic model of DME oxidation. Model validation was conducted by a comparison of experimental results for the accurate prediction. The injection timing ranging from BTDC 80° to BTDC 10° and two fuel masses were selected to evaluate the combustion, emission and engine performance. The calculated results were in good accordance with the experimental results of the combustion and emissions of the engine. Nitrogen oxide (NOx) emissions at injection timing before BTDC 30° remarkably decreased, while hydrocarbon (HC) and carbon monoxide (CO) emissions at an injection timing of BTDC 70° showed high levels. Also, the IMEP and ISFC have decreasing and increasing patterns respectively as the injection timing was advanced.     

Cold start and fuel consumption of a vehicle fuelled with blends of diesel oil–soybean biodiesel–ethanol

Fuel consumption and cold start characteristics of a production vehicle fuelled with blends of N. 2 diesel oil (500 ppm sulfur content), soybean biodiesel (3%, 5%, 10%, and 20%) and hydrous ethanol (2% and 5%) were compared. A wagon-type vehicle equipped with a four-cylinder, 1.3-l, 63 kW diesel engine was tested in a cold chamber at the temperature of −5 °C for the cold start tests. Fuel consumption tests were performed following the 1975 US Federal Test Procedure (FTP-75). The results showed that the cold start time was satisfactory for all fuel blends tested, but it was longer for the blend containing 20% of soybean biodiesel (B20) in comparison with the blends with lower biodiesel concentration. The cold start time also increased with increasing with increasing ethanol content in the fuel blend. Specific fuel consumption was not affected by increasing biodiesel concentration in the blend or by the use of 2% of ethanol as an additive. However, the use of 5% of ethanol concentration in the B20 blend resulted in increased specific fuel consumption.