Mixing-controlled Combustion Research Articles

Evaluation of massive exhaust gas recirculation and Miller cycle strategies for mixing-controlled low temperature combustion in a heavy duty diesel engine

The future of compression ignition engines depends on their ability for keeping their competitiveness in terms of fuel consumption compared to spark-ignition engines. In this competitive framework, the Low Temperature Combustion (LTC) concept is a promising alternative to decrease NOx and soot emissions. Thus, this research focuses on implementing the LTC concept, but keeping the conventional mixing-controlled combustion process to overcome the well-known drawbacks of the highly-premixed combustion concepts, including load limitations and lack of combustion control.Two strategies for implementing the mixing-controlled LTC concept were evaluated. The first strategy relies on decreasing the intake oxygen concentration introducing high rates of cooled EGR (exhaust gas recirculation). The second strategy consists of decreasing the compression temperature by advancing the intake valves closing angle to reduce the effective compression ratio, compensating the air mass losses by increasing boost pressure (Miller cycle). These strategies were tested in a single-cylinder heavy-duty research engine. Additionally, 3D-CFD modeling was used to give insight into local in-cylinder conditions during the injection-combustion process.Results confirm the suitability of both strategies for reducing NOx and soot emissions, while their main drawback is the increment in fuel consumption. However, they present intrinsic differences in terms of local equivalence ratios and temperatures along combustion.

Insights into the hydrocarbon and carbon monoxide emissions in moderately and highly dilute Low Temperature Combustion

Low-temperature combustion in diesel engines offers attractive benefits through simultaneous reduction in the nitrogen oxide emissions and the soot emissions. However, it is known that the in-cylinder conditions typical of low-temperature combustion operation tend to produce high emissions of unburned hydrocarbons and carbon monoxide, reducing the combustion efficiency. The present study develops from the hypothesis that this characteristic poor combustion efficiency is due to the in-cylinder mixture preparation strategies which are non-optimally matched to the requirements of the low-temperature combustion mode. In this work, the effects of three key fuel path parameters, namely the injection fuel quantity ratio, the dwell and the injection timing, on the carbon monoxide and hydrocarbon emissions were examined using a central-composite-design design-of-experiments method. The experiments were performed on a single-cylinder diesel research engine operating in a mixing-controlled low-temperature combustion mode at high and moderate exhaust gas recirculation rates with a split fuel injection for all conditions. The experiments identified the potential of fuel metering control for optimising the hydrocarbon emissions in low-temperature combustion by showing the effects of the fuel control parameters on the fuel mixing quality and the emission formation mechanisms. The response surfaces created from the detailed statistical analysis give a potent visualisation of the constraints on low-temperature combustion operation. This in turn allowed improved prescription of combustion modifications with the potential to moderate the negative effects observed.

Numerical energetic and exergetic analysis of CI diesel engine performance for different fuels of hydrogen, dimethyl ether, and diesel under various engine speeds

The current study addresses engine specification and second thermodynamic law analysis of the CI diesel engine fueled with hydrogen, DME, and diesel at six engine speeds. The 3-D simulation was first carried out and then the results were exploited to calculate availability through a developed in-house code. Availability analysis was performed separately for chemical and thermo-mechanical availability to highlight each fuel'0s capacity in chemical and mechanical efficiency delivery. The results indicate that hydrogen fuel prevails in chemical and thermo-mechanical availability, indicated power, and mean effective in-cylinder pressure under all crank angle and engine speeds. Temperature distribution has more extensive and intensified region developed across the cylinder, although hydrogen demonstrated the lowest ISFC (indicated specific fuel consumption) value. With regard to engine speed, 2000 rpm shows overall better IP (indicated power), IMEP (indicated mean effective pressure), chemical and thermo-mechanical availability, irrespective of fuel type. The mean irreversibility rate for PMC (pre-mixed combustion) and MCC (mixing controlled combustion) combustion phase shows a different trend. Furthermore, hydrogen fueled engine demonstrates the highest temperature distribution of 2736 K and the wall heat flux to the amount of 29160 W. The variance of chemical availability for Hydrogen from 1500 to 4000 rpm decreases by crank-angle evolution from 43.3% to 10.1% corresponding to 10–40°CA after top dead center.

Effects of a split injection in a spray-guided direct-injection spark ignition engine under lean stratified operation

The effects of a split injection were investigated in a spray-guided direct-injection spark ignition engine under lean stratified operation. In-cylinder pressure analysis, exhaust emissions measurement and visualization of the spray and combustion were applied. From the results using a single-injection strategy, stratified combustion showed advanced and rapid combustion characteristics. However, the low work conversion efficiency and the high nitrogen oxide emissions were obstacles to stratified combustion. Furthermore, the smoke emissions were high as a result of the dominant mixing-controlled combustion. A dual-injection strategy was applied to control the level of stratified combustion by modifying the dwell time and the split ratio. With a split injection, the nitrogen oxide emissions and the smoke emissions were reduced because of the moderated combustion speed and the enhanced local homogeneity. The liquid phase of the stratified mixture and the luminous sooting flame were reduced significantly as well. However, a low combustion efficiency and a low indicated mean effective pressure resulted owing to the over-mixing effect when the portion of lean premixed combustion was increased. Via moderately controlled stratified combustion, a split injection could be an effective way to reduce the nitrogen oxide emissions and the smoke emissions with a reasonable indicated mean effective pressure and a reasonable combustion efficiency.

An Experimental Study of Diesel-Fuel Property Effects on Mixing-Controlled Combustion in a Heavy-Duty Optical CI Engine

An Experimental Study of Diesel-Fuel Property Effects on Mixing-Controlled Combustion in a Heavy-Duty Optical CI Engine

Combustion modelling for a diesel engine with multi-stage injection using a stochastic combustion model

This study presents the development of a phenomenological combustion model to simulate the combustion processes in diesel engines with multi-stage fuel injection. A newly developed zero-dimensional spray propagation model and a model of spray-to-spray interaction were combined with a stochastic combustion model, which had been developed for the calculation of diesel combustion in the case of single-stage injection. In this model, the combustion chamber is divided into an ambient air zone and several spray zones, where the spray formed by each injection is treated as a spray zone. The turbulent mixing, the fuel evaporation, the heat loss and the chemical reactions are calculated in each spray zone separately. A zero-dimensional spray propagation model including the spray evolution after the end of injection and a model of interaction between the sprays from sequential injections are developed to describe the spray behaviour for the case of multi-stage injection. Then the developed combustion model is validated against the experimental data from a single-cylinder direct-injection diesel engine with two-stage pilot–main injection, in which the pilot injection conditions are varied with a fixed main-injection timing. Based on the analysis of the heat release rate, the entrainment rate and the microscopic information inside the spray, such as the probability density function of the equivalence ratio, the effects of the wall impingement and the interaction between adjacent sprays on the fuel–air mixing rate and the entrainment rate are formularized and employed to reproduce the measured histories of the heat release rate. The reduction in the fuel–air mixing rate is considered when the spray flows into the squish region after wall impingement, which is effective in obtaining the measured decrease in the heat release of the pilot spray with advancing pilot injection timing. The effects of the wall impingement of the main spray and the interaction between adjacent sprays are modelled to reproduce the heat release rate during the initial part and later part of the mixing-controlled combustion. After these improvements, the heat release rates of the test engine when varying the pilot injection conditions were successfully predicted.

Fuel-Flexible Engine Control of Biodiesel Blends During Mixing-Controlled Combustion

The use of biodiesel blends has the potential to result in many benefits including decreased reliance on imported petroleum, increased sustainability, decreased net carbon dioxide emissions, and decreased particulate matter emissions. There are, however, two major combustion-related challenges to the use of biodiesel blends: (1) decreased torque/power capacity and (2) increased emissions of nitrogen oxides. The work presented in this paper demonstrates that both of these challenges can be met through the use of a physically based fuel-flexible combustion control strategy. The approach consists of two parts: estimation, whereby the engine control module (ECM) detects the biodiesel blend fraction being supplied to the engine, and accommodation, whereby the ECM dynamically changes the control setpoints in order to improve the combustion performance. The proposed control method utilizes only stock engine hardware and does not require the creation of new ECM lookup maps. The proposed framework is incorporated into an already complex engine control system, and as such, it must be ensured that the stability of the overall system is not detrimentally affected. A formal stability analysis is outlined which demonstrates that the engine control system will remain stable. Experimental validation of this control strategy on a 2007 6.7 liter Cummins ISB series engine at several very different operating modes shows that this fuel-flexible control strategy greatly reduced or completely eliminated increases in emissions of nitrogen oxides of up to 30% while largely maintaining the torque/power capacity of the engine when operating with biodiesel.

Fuel combustion under asymmetric piston motion: Tested results

This paper presents the measured characteristics of a single piston hydraulic free-piston engine prototype. The detailed results about the piston motion and the fuel combustion are presented. The results show that there is a strong interaction between the piston motion and the fuel combustion in the free-piston engine. The gas force plays a leading role in the innate asymmetric piston motion around top dead centre. The small moving piston inertia makes the asymmetric characteristics more visible. The combustion energy conversion of the single piston hydraulic free-piston engine is fast. Most of the fuel injected burns in 1.0 ms in the pre-mixed combustion phase. Because of the asymmetric characteristics, the mixing-controlled combustion phase in the single piston hydraulic free-piston engine is much shorter than the one in the conventional diesel engine. The short combustion duration decreases the rated power of the free-piston engine. The asymmetric piston motion is unfavourable for the rated power increase of the free-piston engine. The smaller rated power has a negative effect on the free-piston engine application development.

Commercial Naphtha Blends for Partially Premixed Combustion

Partially Premixed Combustion has shown the potential of low emissions of nitrogen oxides (NOx) and soot with a simultaneous improvement in fuel efficiency. Several research groups have shown that a load range from idle to full load is possible, when using low-octane-number refinery streams, in the gasoline boiling range. As such refinery streams are not expected to be commercially available on the short term, the use of naphtha blends that are commercially available could provide a practical solution. The three blends used in this investigation have been tested in a single-cylinder engine for their emission and efficiency performance. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to legislated emission levels. Conventional diesel combustion benchmarks are used for reference to show possible improvements in indicated efficiency. Analysis of the heat release patterns revealed an interesting and strong correlation between the premixed fraction and the amount of soot produced. To be specific, each of the fuels showed a decrease in this fraction as either fuel pressure was lowered or load was increased, showing a transition from more premixed to mainly mixing-controlled combustion, with the corresponding soot emissions. For one blend, over the whole load range EURO VI PM levels were approached or achieved, combined with a peak gross indicated efficiency of 50% clearly indicating the potential of this concept.

Control-oriented multi-phase combustion model for biodiesel fueled engines

In this paper, the combustion characteristics of Diesel and biodiesel fuels are investigated and compared. Based on the experimental data obtained from a medium-duty Diesel engine, a multi-phase combustion model, which is applicable to both Diesel and biodiesel fuels, is developed and it shows satisfactory accuracy within the test range in this study. A preliminary ignition-delay correlation is established by considering not only the in-cylinder physical factors such as pressure and temperature, but also the fuel property variations. It is also capable of modeling the premixed and mixing-controlled combustion by using two cascaded Wiebe functions. Through a grey-box parameter identification approach, a set of Wiebe coefficients, which are partially physics-based, are found; then the model can have a reasonable accuracy for the range of experiments conducted. Based on the multi-phase model, prediction can be made for the CA50 (crank angle for 50% heat release), which is very important for combustion timing feedback control.

A robust fuel flexible combustion control strategy for biodiesel with variable fatty acid composition during mixing-controlled combustion

Biodiesel is a diesel fuel alternative which is produced from renewable and domestically available sources. The use of biodiesel generally lowers carbon dioxide, carbon monoxide and particulate matter emissions. However, there are certain challenges associated with the use of biodiesel, mainly (1) lower fuel energy density, (2) increased nitrogen oxide (NOx) emissions and (3) fuel variability due to feedstock and processing differences. In prior efforts, the authors have demonstrated that the first two of these challenges can be overcome for different blend fractions of soy-based biodiesel by using a control algorithm incorporating energy-based fueling for torque control and combustible oxygen mass fraction control for NOx regulation. However, in addition to overcoming these combustion-related challenges, in this work, the authors consider the extension of these techniques to biodiesel generated from oils/fats of varying composition. The type of oil/fat from which the biodiesel is derived will impact the ...

FL1-5 Effects of Fuel Composition on Flame Lift-off Length and Pollutant Formation in Dual-component Fuel Spray(FL: Fuels,General Session Papers)

Flame lift-off length in diesel spray is currently recognized as an indicator of sooting tendency during mixing-controlled combustion and is affected by fuel reactivity. This paper reports the experimental investigation of the flame lift-off length and its effects on emissions for dual-component fuels consisting of different less reactivity component. The experiments were conducted at quiescent ambient condition in a Rapid Compression and Expansion Machine. Gasoline-like components contained in dual-component fuel and ambient oxygen concentrations were set as major experimental variables. The experimental results, based on pressure analysis, burned gas analysis and imaging of luminous flame and OH chemiluminescence, show that mixing higher octane number component has lower luminous intensity indicating lower soot because of longer lift-off length and dual-component fuel achieves low soot and NO_x combustion without excessive decrease of ambient oxygen concentration.

Performance Analysis of a Dual Fuel Engine Using LPG and Diesel with EGR System

This work is aimed at investigating the performance and emission characteristics of a Dual fuel engine operated with Diesel as primary fuel and ignition source and LPG as secondary fuel with EGR (Exhaust Gas Recirculation) system. In dual fuel operation, much of the energy released comes from the combustion of the gaseous fuel, while only a small amount of diesel fuel provides ignition through timed cylinder injection. The effects of Diesel and LPG substitution ratios, engine speed, and load on the equivalent brake specific fuel consumption and gaseous emissions of hydrocarbons, carbon monoxide and nitrogen oxides are examined for LPG-diesel fuel operation and then compared with the original diesel operation. The concept recirculation of the exhaust gases (EGR), re-burning a part of the exhaust gases is being widely used in many practical situations. The results show that, over a wide range of operating conditions (engine speed, load), the dual fuel engine clearly shows the benefits of reduced NOX emissions. However, under low loads, the results indicate high CO and HC emissions and a higher BSFC when compared to those of the corresponding diesel engine. The LPG addition has been premixed with the engine intake air after the pilot injection of the Diesel in small percentages, in order to improve the engine combustion and to reduce pollutant emissions during the mixing-controlled combustion phase. Experiments show that, using the LPG as an additive fuel, while performing the Diesel fuel main injection keeps the unchanged engine efficiency with respect to the traditional Diesel fuel operation mode.

Second-law analysis of fuel–air mixing and combustion subprocesses in a heterogeneous charge engine

Fuel–air mixing and combustion in engines are complex phenomena involving several subprocesses, wherein the availability (exergy) of the working fluid could be destroyed owing to irreversibilities or could be lost during energy exchanges. In the present work, a quasi-dimensional multi-zone engine model is used to quantify the availability destructions and losses associated with various mixing and combustion subprocesses in a heterogeneous charge compression ignition engine. This second-law analysis helps us to estimate the contributions of individual subprocesses towards the overall availability deficit. The results indicate that most of the availability destruction prior to ignition occurs because of fuel vaporization and mixing, whereas chemical reaction accounts for up to half of the total availability deficit during combustion. In-cylinder pressure equilibration and mixing of the combustion products with spray gases have major roles in availability destruction during the premixed combustion phase and the mixing-controlled combustion phase respectively. The contributions of wall heat transfer, heat transfer from the spray to the surrounding air and mixing of entrained air with spray gases assume significance in the later stages of combustion. Parametric studies indicate that reducing the in-cylinder air swirl and retarding the injection timing effectively curtail the availability deficit, thus improving the exergetic efficiency.

A Physics and Tabulated Chemistry Based Compression Ignition Combustion Model: from Chemistry Limited to Mixing Limited Combustion Modes

This paper presents a new 0D phenomenological approach to predict the combustion process in multi injection Diesel engines operated under a large range of running conditions. The aim of this work is to develop a physical approach in order to improve the prediction of in-cylinder pressure and heat release. Main contributions of this study are the modeling of the premixed part of the Diesel combustion with a further extension of the model for multi-injection strategies. In the present model, the rate of heat release due to the combustion for the premixed phase is related to the mean reaction rate of fuel which is evaluated by an approach based on tabulated local reaction rate of fuel and on the determination of the Probability Density Function (PDF) of the mixture fraction (Z), in order to take into consideration the local variations of the fuel-air ratio. The shape of the PDF is presumed as a standardized <i>β<i/>-function. Mixture fraction fluctuations are described by using a transport equation for the variance of Z. The standard mixture fraction concept established in the case of diffusion flames is here adapted to premixed combustion to describe inhomogeneity of the fuel-air ratio in the control volume. The detailed chemistry is described using a tabulated database for reaction rates and cool flame ignition delay as a function of the progress variable c. The mixing-controlled combustion model is based on the calculation of a characteristic mixing frequency which is a function of the turbulence density, and on the evolution of the available fuel vapor mass in the control volume. The developed combustion model is one sub-model of a thermodynamic model based on the mathematical formulation of the conventional two-zone approach. In addition, an extended sub-model for multi injection is developed to take into account interactions between each spray by describing their impact on the mixture formation. Numerical results from simulations are compared with experimental measurements carried out on a 2 liter Renault Diesel engine and good agreements are found.

Prediction of the Rate of Heat Release of Mixing-Controlled Combustion in a Common-Rail Engine with Pilot and Post Injections

A one-dimensional model for the prediction of the rate of heat release (ROHR) in common-rail direct-injection diesel engines with pilot, main, and post injections is developed. The model is an improvement on the previously developed mixing-controlled combustion model extended by including the effect of the wall jet. The decay of the ROHR curve has been improved in the second regime of combustion. It was observed that the input kinetic energy of the wall spray is retarded only after the growth of spray along the bowl wall exceeds the diameter of the spray cone. It is shown that this model can be extended for engines with multiple injections, i.e. pilot, main, and post injections. The ROHR was modelled as a function of the turbulence kinetic energy of the fuel mass and the dissipation rate of this energy in different stages of spray propagation. The inputs required for the model are the nozzle discharge coefficient, the injection pressure, the injection timing, and the injected fuel quantity for each injection stage that are readily controlled in common-rail engines. The predicted heat release histories corresponding to the pilot injection, the main injection, and the post injection were found to be in agreement with the experiments.

Evaluation of non-premixed combustion and fuel spray models for in-cylinder diesel engine simulation

Computational Fluid Dynamics (CFD) study of light-duty automotive diesel engines affords invaluable insights into in-cylinder conditions and processes, which greatly expands on the very limited detail provided by engine-out measurements of exhaust emissions. For the simulation to be successful, accuracy and robustness of the physical sub-models are crucial. The purpose of this feasibility study is to appraise the non-premixed combustion and fuel spray CFD sub-models for in-cylinder diesel engine simulation on a FLUENT 6.3.26 platform. Simulation results are compared against data from parallel experimental test-bed studies in terms of pressure traces, heat release curves and tailpipe values of NO x and soot levels. Heat release rate and pressure trace from the computations are found to be within a reasonable error limit of 10%. Simulated heat released plots are able to capture the general trend of premixed and mixing-controlled diesel combustion phases. Ignition delay period for the main combustion event is well predicted, although the calculated ignition delay period for pilot combustion is 7–10° crank angle degrees earlier for all the test cases. The variations for exhaust soot and NO when the timing of start of injection is changed are reproduced successfully. CFD modelling, when used in conjunction with experimental studies is proven to be particularly effective in elucidating physical details of in-cylinder processes.

Study on Low Temperature Combustion for Light-Duty Diesel Engines

This paper presents a study on the feasibility of obtaining low temperature mixing-controlled combustion (mixing-controlled LTC) in a small HSDI engine with the objective of avoiding simultaneous NOx and soot formation. This mixing-controlled LTC strategy is based on reducing the equivalence ratio at the lift-off cross section and also the local combustion temperatures, but maintaining the conventional diesel spray structure. A parametric study has been carried out to evaluate the effects of in-cylinder gas density, temperature, and oxygen concentration on the characteristics of the mixing-controlled LTC characteristics. Low NOx and low soot mixing-controlled diesel combustion has been attained by combining low in-cylinder gas temperatures together with high air densities and low oxygen concentrations. However, the mixing-controlled LTC concept also presents an important drawback related to the engine efficiency deterioration.

Visualization of Diesel and GTL Spray Combustion and Soot Formation in a Rapid Charging Combustion Vessel with Shadowgraph Method

In this study, visual investigation of sprays and flames has been performed and soot formation in Diesel and GTL fuels has been compared in a specially designed Rapid Charging Combustion Vessel (RCCV) under various ambient gas O₂ concentrations and two injection pressures. It has been concluded that soot in the mixing-controlled combustion of Diesel and GTL fuels has similar tendency to be formed in the leading portion of the jet boundaries. Auto-ignition delay for GTL fuel is shorter than that for diesel fuel. The temporal and special variation of soot concentration in the diesel flame jets at various O₂ concentrations was correlated with the heat release rate. Soot concentration appears in the regions when diffusion combustion starts, and its concentration reaches maximum at the peak of heat release curve and then decreases due to oxidation. Visualization by shadowgraph method showed that soot decreases with lower O₂ concentration, and higher injection pressure.

Investigation of soot formation in Diesel-GTL fuel blends under quiescent conditions

In this study, a visual investigation of sprays and flames is performed, and soot formation in Diesel-GTL fuel blends is studied in a specially designed quiescent constant-volume chamber under various ambient gas temperatures and O2 concentrations. Similar to the case of soot formation during diesel fuel combustion, the sooting zone during the mixing-controlled combustion of Diesel-GTL blends is located in the leading portion of the jet boundaries. Auto-ignition delay and soot concentration decrease with an increase of GTL content in the fuel blend. Soot also decreases with lower O2 concentration, higher injection pressure, and lower ambient gas temperature. The lack of soot formation at lower O2 concentrations and lower temperatures suggests that Diesel-GTL fuel blends can be successfully utilized in low-temperature diesel combustion technologies that are currently being developed. Furthermore, this mixing controlled combustion method with Diesel-GTL blends can be used to modulate various engine operation parameters, and therefore to simultaneously reduce the formation of soot and NOx within a wide range of diesel engine loads.