Mohammad Izadi Najafabadi

Ignition Sensitivity Study of Partially Premixed Combustion by Using Shadowgraphy and OH* Chemiluminescence Methods

Partially Premixed Combustion (PPC) is a promising combustion concept for future IC engines. However, controllability of PPC is still a challenge and needs more investigation. The scope of the present study is to investigate the ignition sensitivity of PPC to the injection timing at different injection pressures. To better understand this, high-speed shadowgraphy is used to visualize fuel injection and evaporation at different Start of Injections (SOI). Spray penetration and injection targeting are derived from shadowgraphy movies. OH∗ chemiluminescence is used to comprehensively study the stratification level of combustion which is helpful for interpretation of ignition sensitivity behavior. Shadowgraphy results confirm that SOI strongly affects the spray penetration and evaporation of fuel. However, spray penetration and ignition sensitivity are barely affected by the injection pressure. There is a critical SOI range, in which a significant amount of fuel is trapped in the crevice volume. Injection in this critical range has a negative influence on the combustion efficiency and ignition sensitivity. Impingement of liquid fuel on the piston crown advances the combustion phasing by providing higher levels of stratification. Moreover, results of combustion stratification study show that stratification level has an inverse correlation with combustion phasing of PPC for late injections. (Less)

Combustion Stratification for Naphtha from CI Combustion to PPC

This study demonstrated the change in combustion homogeneity from conventional diesel combustion via partially premixed combustion towards HCCI. Experiments are performed in an optical diesel engine at a speed of 1200 rpm with diesel fuel. Single injection strategy is employed and the fuel is injected at a pressure of 800 bar. The cylinder pressure at TDC is maintained at 35 bar and a high-speed video of the combustion process is captured through optical piston. The high speed video is processed to study the combustion homogeneity based on an algorithm reported in previous studies. Starting from late fuel injection timings, the combustion homogeneity is investigated by advancing to early fuel injection timings. For late fuel injection timings, a direct link between fuel injection timing and combustion phasing is noticed. At advanced fuel injection timings, the start of combustion is independent of fuel injection timing. The combustion homogeneity for the transition from CI via PPC towards HCCI is also investigated for various levels of dilution by displacing oxygen with nitrogen in the inlet. The start of combustion was retarded with the increase in dilution, while the mixture homogeneity is enhanced due to longer ignition delay. To compensate for the retarded combustion phasing with dilution, the inlet air temperature is increased. The experimental results show that the high speed image is initially blue and then turned yellow, indicating soot oxidation. The images are processed to generate the level of stratification based on the image intensity. This study shows better combustion homogeneity for early fuel injection timing and higher level of dilution and temperature in the inlet.

Effects of Injection Timing on Fluid Flow Characteristics of Partially Premixed Combustion Based on High-Speed Particle Image Velocimetry

Partially Premixed Combustion (PPC) is a promising combustion concept ,based on judicious tuning of the charge stratification, to meet the increasing demands of emission legislation and to improve fuel efficiency. Longer ignition delays of PPC in comparison with conventional diesel combustion provide better fuel/air mixture which decreases soot and NOx emissions. Moreover, a proper injection timing and strategy for PPC can improve the combustion stability as a result of a higher level of fuel stratification in comparison with the Homogeneous Charge Compression Ignition (HCCI) concept. Injection timing is the major parameter with which to affect the level of fuel and combustion stratification and to control the combustion phasing and the heat release behavior. The scope of the present study is to investigate the fluid flow characteristics of PPC at different injection timings. To this end, high-speed Particle Image Velocimetry (PIV) is implemented in a light-duty optical engine to measure fluid flow characteristics, including the flow fields, mean velocity and cycle-resolved turbulence, inside the piston bowl as well as the squish region with a temporal resolution of 1 crank angle degree at 800 rpm. Two injectors, having 5 and 7 holes, were compared to see their effects on fluid flow and heat release behavior for different injection timings. Reactive and non-reactive measurements were performed to distinguish injection-driven and combustion-driven turbulence. Formation of vortices and higher turbulence levels enhance the air/fuel interaction, changing the level of fuel stratification and combustion duration. Results demonstrate clearly how turbulence level correlates with heat release behavior, and provide a quantitative dataset for validation of numerical simulations.

Validation of a Reduced Chemical Mechanism Coupled to CFD Model in a 2-Stroke HCCI Engine

Homogeneous Charge Compression Ignition (HCCI) combustion technology has demonstrated a profound potential to decrease both emissions and fuel consumption. In this way, the significance of the 2-stroke HCCI engine has been underestimated as it can provide more power stroke in comparison to a 4-stroke engine. Moreover, the mass of trapped residual gases is much larger in a 2-stroke engine, causing higher initial charge temperatures, which leads to easier auto-ignition. For controlling 2-stroke HCCI engines, it is vital to find optimized simulation approaches of HCCI combustion with a focus on ignition timing. In this study, a Computational Fluid Dynamic (CFD) model for a 2-stroke gasoline engine was developed coupled to a semi-detailed chemical mechanism of iso-octane to investigate the simulation capability of the considered chemical mechanism and the effects of different simulation parameters such as the turbulence model, grid density and time step size. The validation of numerical results was carried using an experimental study on the 2-stroke engine that was modified to operate in HCCI mode. Results confirm that the considered iso-octane chemical mechanism is able to predict the ignition timing of HCCI combustion in the 2-stroke gasoline engine but special care has to be taken to the numerical setting like grid size, time step size and turbulence model. Furthermore, the k-e RNG model is the best turbulence model for simulation of this case study coupled to the time step size of 0.25 crank angle degree and the average cell size of 1.35 mm.

Numerical investigation of PPCI combustion at low and high charge stratification levels

Partially premixed compression ignition combustion is one of the low temperature combustion techniques which is being actively investigated. This approach provides a significant reduction of both soot and NOx emissions. Comparing to the homogeneous charge compression ignition mode, PPCI combustion provides better control on ignition timing and noise reduction through air- fuel mixture stratification which lowers heat release rate com- pared to other advanced combustion modes. In this work, CFD simulations were conducted for a low and a high air-fuel mixture stratification cases on a light-duty optical engine operating in PPCI mode. Such conditions for PRF70 as fuel were experimentally achieved by injection timing and spray targeting at similar thermodynamic conditions. After validating the computed results of cylinder pressure, apparent heat release rate, and OH ∗ spatial distributions, differences in engine thermal load and mixture fraction distributions at first stage and second stage ignition were compared. Assuming similar second stage ignition timing which is provided by intake air heating, experimental and simulation results reveal that the time between first and second stage ignition shortens and combustion phases to the main stage ignition faster in the high stratification case. Using flame structure diagrams, this was attributed to availability of a larger range of mixture fractions with higher reactivity. Creating optimum air-fuel stratification then can be considered as a useful and additional controlling parameter for a PPCI engine combustion phasing and subsequent emission formation.

Effect of aromatics on combustion stratification and particulate emissions from low octane gasoline fuels in PPC and HCCI mode

The objective of this study was to investigate the effect of aromatic on combustion stratification and particulate emissions for PRF60. Experiments were performed in an optical CI engine at a speed of 1200 rpm for TPRF0 (100% v/v PRF60), TPRF20 (20% v/v toluene + 80% PRF60) and TPRF40 (40% v/v toluene + 60% PRF60). TPRF mixtures were prepared in such a way that the RON of all test blends was same (RON = 60). Single injection strategy with a fuel injection pressure of 800 bar was adopted for all test fuels. Start of injection (SOI) was changed from early to late fuel injection timings, representing various modes of combustion viz HCCI, PPC and CDC. High-speed video of the in-cylinder combustion process was captured and one-dimensional stratification analysis was performed from the intensity of images. Particle size, distribution and concentration were measured and linked with the in-cylinder combustion images. Results showed that combustion advanced from CDC to PPC and then attained a constant value in HCCI mode. In PPC and HCCI region, the soot mass concentration was significantly reduced as premixing was improved due to longer ignition delay. The particle number was lower for the late injection and becomes higher as the injection timing advanced to PPC and HCCI mode. While the soot particles were almost nuclear model with the size range of 5nm~17nm and as combustion transited from CDC via PPC to HCCI, the particle size became larger. For TPRF blends, the increased intake air temperature was required to maintain same combustion phasing as that of PRF60. With the addition of toluene to PRF60, the soot concentration increased, which was in-line with the increased intensity (yellow) of combustion images. The degree of stratification was higher for TPRF20 and TPRF40 when compared to PRF60.

Fuel effect on combustion stratification in partially premixed combustion

The literature study on PPC in optical engine reveals investigations on OH chemiluminescence and combustion stratification. So far, mostly PRF fuel is studied and it is worthwhile to examine the effect of fuel properties on PPC. Therefore, in this work, fuel having different octane rating and physical properties are selected and PPC is studied in an optical engine. The fuels considered in this study are diesel, heavy naphtha, light naphtha and their corresponding surrogates such as heptane, PRF50 and PRF65 respectively. Without EGR (Intake O2 = 21%), these fuels are tested at an engine speed of 1200 rpm, fuel injection pressure of 800 bar and pressure at TDC = 35 bar. SOI is changed from late to early fuel injection timings to study PPC and the shift in combustion regime from CI to PPC is explored for all fuels. An increased understanding on the effect of fuel octane number, physical properties and chemical composition on combustion and emission formation is obtained. High-speed images of the combustion process are analyzed for each and every fuel and in-cylinder phenomenon is associated with rate of heat release and in-cylinder pressure. Based on the intensity of the images, stratification analysis is performed.The results of the analysis show that CA50 decreases for all fuels from late to early SOI wherein PPC is realized. According to the reactivity of fuels, intake air temperature is increased to comply with the combustion phasing of baseline diesel. When studying the effect of physical properties of fuels, premixed effect and lean combustion are observed for PRF0 compared to diesel. The engine emissions of THC and CO are higher for PRF0 than diesel, while soot concentration is reduced. Diesel showed more stratified combustion than PRF0 despite having same RON due to the effect of physical properties. The effect of fuel octane number on PPC is suppressed due to temperature effect; intake air temperature is increased to 140°C and 90°C for PRF65 and PRF50. PRF0 lacked LTR phase and combustion was noted to be more premixed than PRF50 and PRF65 at SOI = -10 CAD (aTDC). The intensity of the combustion images is brighter for high RON fuels than PRF0 due to physical effects, while octane number effect is not realized due to higher intake air temperature. While THC and CO emissions decreased with the increase in RON, NOX emission increased due to increased intake air temperature. When comparing real fuels, soot concentration is lower for light naphtha when compared to diesel and heavy naphtha.

Combustion homogeneity and emission analysis during the transition from CI to HCCI for FACE I gasoline

Low temperature combustion concepts are studied recently to simultaneously reduce NOX and soot emissions. Optical studies are performed to study gasoline PPC in CI engines to investigate in-cylinder combustion and stratification. It is imperative to perform emission measurements and interpret the results with combustion images. In this work, we attempt to investigate this during the transition from CI to HCCI mode for FACE I gasoline (RON = 70) and its surrogate, PRF70. The experiments are performed in a single cylinder optical engine that runs at a speed of 1200 rpm. Considering the safety of engine, testing was done at lower IMEP (3 bar) and combustion is visualized using a high-speed camera through a window in the bottom of the bowl.From the engine experiments, it is clear that intake air temperature requirement is different at various combustion modes to maintain the same combustion phasing. While a fixed intake air temperature is required at HCCI condition, it varies at PPC and CI conditions between FACE I gasoline and PRF70. Three zones are identified 1) SOI = -180 to -80 CAD (aTDC) is HCCI zone 2) SOI = -40 to -20 CAD (aTDC) is PPC zone 3) After SOI = -15 CAD (aTDC) is CI zone. Combustion duration, ignition delay, start of combustion and CA90 (crank angle at which 90% of fuel burnt) are comparable between FACE I gasoline and PRF70. The combustion images show a prominent soot flame at CI condition, while only blue coloured premixed flames are visible at PPC condition for both the fuels. PRF70 seems to have a pronounced premixed effect when compared to FACE I gasoline at early injections, showing a decreased level of stratification. NOX emission and soot concentration decreases from CI condition and attains a constant zero value at HCCI condition for both FACE I gasoline and PRF70. CO and CO2 emissions matches between FACE I gasoline and PRF70 at PPC and CI condition, while CO emission is lower for PRF70 at HCCI condition.

Analysis of transition from HCCI to CI via PPC with low octane gasoline fuels using optical diagnostics and soot particle analysis

In-cylinder visualization, combustion stratification, and engine-out particulate matter (PM) emissions were investigated in an optical engine fueled with Haltermann straight-run naphtha fuel and corresponding surrogate fuel. The combustion mode was transited from homogeneous charge compression ignition (HCCI) to conventional compression ignition (CI) via partially premixed combustion (PPC). Single injection strategy with the change of start of injection (SOI) from early to late injections was employed. The high-speed color camera was used to capture the in-cylinder combustion images. The combustion stratification was analyzed based on the natural luminosity of the combustion images. The regulated emission of unburned hydrocarbon (UHC), carbon monoxide (CO) and nitrogen oxides (NOX) were measured to evaluate the combustion efficiency together with the in-cylinder rate of heat release. Soot mass concentration was measured and linked with the combustion stratification and the integrated red channel intensity of the high-speed images for the soot emissions. The nucleation nanoscale particle number and the particle size distribution were sampled to understand the effect of combustion mode switch.

Combustion stratification study of partially premixed combustion using Fourier transform analysis of OH* chemiluminescence images

A relatively high level of stratification (qualitatively: lack of homogeneity) is one of the main advantages of partially premixed combustion over the homogeneous charge compression ignition concept. Stratification can smooth the heat release rate and improve the controllability of combustion. In order to compare stratification levels of different partially premixed combustion strategies or other combustion concepts, an objective and meaningful definition of “stratification level” is required. Such a definition is currently lacking; qualitative/quantitative definitions in the literature cannot properly distinguish various levels of stratification. The main purpose of this study is to objectively define combustion stratification (not to be confused with fuel stratification) based on high-speed OH* chemiluminescence imaging, which is assumed to provide spatial information regarding heat release. Stratification essentially being equivalent to spatial structure, we base our definition on two-dimensional Fourier transforms of photographs of OH* chemiluminescence. A light-duty optical diesel engine has been used to perform the OH* bandpass imaging on. Four experimental points are evaluated, with injection timings in the homogeneous regime as well as in the stratified partially premixed combustion regime. Two-dimensional Fourier transforms translate these chemiluminescence images into a range of spatial frequencies. The frequency information is used to define combustion stratification, using a novel normalization procedure. The results indicate that this new definition, based on Fourier analysis of OH* bandpass images, overcomes the drawbacks of previous definitions used in the literature and is a promising method to compare the level of combustion stratification between different experiments.