Partially Premixed Combustion Research Articles

An experimental study on in-cylinder soot formation and flame oscillation of renewable fuel blends in an optical engine

Methanol has been widely recognized as a promising alternative fuel for mitigating greenhouse gas emissions. However, its application in internal combustion engines often necessitates blending with a high-reactivity fuel to address challenges related to ignitability and combustion stability. In this study, a two-color method was employed in a single-cylinder optical engine to investigate the characteristics of soot formation and flame oscillation of methanol blended with a high-reactivity fuel, hydrogenated catalytic biodiesel (HCB), under partially premixed combustion (PPC) modes. The blended fuels include M15 (15% methanol, 60% HCB, and 25% n-octanol) and M25 (25% methanol, 60% HCB, and 25% n-octanol), with pure hydrogenated catalytic biodiesel M0 (100% HCB) serving as the reference fuel. N-octanol was utilized as the co-solvent to enhance the stability of the mixture. The results indicate that, employing double-injection strategies, as the methanol content increases in the blended composition, leading to a postponed peak cylinder pressure and heat release rate during combustion. Thanks to the high-level premixed combustion and higher oxygen content, the increases of alcohol blending significantly reduced in-cylinder soot formation. Double-injection strategies reduced soot production effectively and resulted in a more uniform soot distribution compared to single-injection strategies. In terms of flame oscillation, its amplitude primarily correlates with the rate of cylinder pressure rise, while the methanol content within the blends exerts a minor impact on flame oscillation. Double-injection strategies contributes to a more uniform fuel distribution, ensuring a smoother heat release process and significantly reducing flame oscillation amplitudes compared to single-injection strategies.

Numerical modeling of combustion in gas engines with prechamber ignition

A numerical model is developed to predict the combustion processes in large-bore gas engines featuring prechamber ignition systems and being coupled with Computational Fluid Dynamics (CFD). The proposed combustion model is based on the simultaneous modeling of premixed and partially-premixed flames by an extended progress variable approach. By introducing an additional fuel scalar to track the externally injected fuel of the scavenged prechamber and depending on a gradient criteria of the fuel scalar, regions of locally premixed or partially-premixed state can be differentiated. The reaction rate for premixed combustion is described through a turbulent flame speed closure approach, whereas the partially-premixed combustion is described by a pre-tabulated flamelet chemistry approach. For the validation of the combustion model and its performance in the different flame propagation phases, that is, prechamber flame ignition and main-chamber flame propagation, experimental data of two large-bore gas engines with different operating conditions, prechamber configurations and engine geometries are taken into account. A good agreement of the simulations with the experimental results is shown for the variety of operating conditions and engine configurations. The developed combustion model is able to predict the combustion process in the prechamber as well as the ignition of the main chamber charge by means of the protruding flame jets through the prechamber nozzles. The prechamber ignition system accelerates the early flame phase and hence shortens the burning duration due to the deep-penetrating and turbulence-inducing flame jets in comparison to a conventional spark plug engine.

Exploiting a new combustion regime using a split partially premixed compression ignition engine fueled with n-butanol/diesel

An experimental investigation has been carried out to explore a novel combustion regime. This regime involves the utilization of a split injection partially premixed combustion (PPC) strategy in combination with DB20 fuel, which consists of 20 % n-butanol and 80 % diesel. The primary objective of this investigation is to assess how variations in pilot-main intervals (PMIs) and pilot-main injection ratios (PIRs) affect the emissions, combustion characteristics, and overall performance of a 4-cylinder turbocharged CRDI engine at various loads. Experimental results show that a split injection strategy combined with DB20 fuel reduces the maximum pressure rise (or combustion noise) rate while reducing NOx and smoke simultaneously compared to single-injection PPCI engines. A 20.1 % reduction in NOX emissions was observed at 20° CA PMI of 2 bar BMEP load and an 11.1 % reduction at 10° CA PMI of 6 bar BMEP load, respectively, compared to a single injection PPCI engine. There is a greater sensitivity to PMI when calculating the brake thermal efficiency (BTE) of DB20 blended fuel. In addition, it was observed that the split injection PPCI engine, powered by DB20 blend fuel, showed a 4.78 % increase in BTE for 20 % PIR and 20° CA PMI under 6 bar BMEP engine load.

Numerical investigation of the combustion regime in pilot-ignited high-pressure direct-injection low reactivity fuel combustion with the injection system of eccentric configuration

The pilot-ignited high-pressure direct-injection (HPDI) combustion with the eccentric configuration is a practical approach to achieve high thermal efficiency and clean combustion. However, the combustion process and combustion regime still remain unclear. In this study, the effects of two key parameters “offset” (distance between concentric and eccentric injectors) and “dwell” (interval between concentric and eccentric injection timing) on the combustion process, combustion regime, and the corresponding performance are numerically investigated systematically. Results show that the interaction between the flame kernels of pilot fuel and the plumes of main fuel determines the combustion mode of each main fuel plume, and thereby different combustion regimes in the pilot-ignited HPDI engine can be generated by controlling the offset and dwell. Specifically, under the concentric configuration or the eccentric configuration with very small offset and dwell, all the main fuel plumes are in the non-premixed mode, exhibiting a diffusion combustion (DC) regime. Under the eccentric configuration with medium offset and dwell, some of the main fuel plumes are in the non-premixed and the others in the partially premixed mode, exhibiting a special combustion regime, which is named the multi-mode co-existing combustion (MMCC) in this study. Under the eccentric configuration with large offset and dwell, all the main fuel plumes are in the partially premixed mode, exhibiting a partially premixed combustion (PPC) regime. Among the three combustion regimes, the diffusion combustion regime exhibits the lowest ISFC and unburned HC emission but high NOx and soot emissions. The ISFC of the partially premixed combustion regime is almost double that of the other two regimes which is far from being acceptable. The MMCC regime shows a better comprehensive performance: achieving much lower ringing intensity and soot emission with almost the same ISFC compared to the diffusion combustion regime. In summary, the pilot-ignited HPDI combustion with the eccentric configuration can potentially achieve high fuel economy with low emissions, under the premise of the ratio of non-premixed and partially premixed combustion modes in the MMCC regime is well controlled.

An iterative methodology for REDIM reduced chemistry generation and its validation for partially-premixed combustion

Partially-premixed flames (PPFs) incorporate effects of both premixed and non-premixed types of reaction zones. The modelling of PPFs using manifold-based model reduction methods faces some inherent difficulties due to the underlying assumptions of a-priori identification of the type of combustion system. In this work, the reaction–diffusion manifold (REDIM) model reduction method is applied to study PPFs. The REDIM method requires minimal prior knowledge about the type of combustion system, which makes it a suitable method for studying PPFs. It allows incorporating system-specific diffusion (gradients) terms in a generic way so that the manifold can evolve according to the diffusion related information provided by the combustion system. In this way, a prior identification of the type of combustion system is no longer needed. This work utilises an iterative methodology to generate REDIM chemistry tables so that the reduced manifold can be iteratively converged very close to the detailed manifold according to the gradients of the reduced coordinates provided by the physical combustion system in each iteration step. In addition, a new method is proposed to provide the gradient estimates of the reduced coordinates during the generation of REDIM from the scattered gradient data in REDIM reduced CFD calculations. Laminar triple flames, a special case of PPFs, with two types of mixture fraction gradients are selected as the target cases to assess the presented iterative methodology. REDIM reduced calculations are compared with simulations based on detailed finite-rate kinetics. It is found that in the final iteration steps, temperature and all considered major and minor species mass fraction profiles are very well predicted by the REDIM reduced calculations.

Potential of the reverse-reactivity controlled compression ignition (R-RCCI) combustion for maintaining ultra-low emissions and enhanced thermal efficiency

Focusing on the reverse-reactivity controlled compression ignition (R-RCCI) concept, the intake and fuel supply parameters were optimized using a promoted genetic algorithm combined with the computational fluid dynamics simulation in this investigation. The combustion characteristics and energy utilization of both gasoline partially premixed combustion (PPC) and polyoxymethylene dimethyl ethers (PODEn)/gasoline R-RCCI were further evaluated in detail. The results indicated that R-RCCI exhibits superior performance compared with PPC. The optimized gross indicated thermal efficiency (ITE) of R-RCCI is 3.9% and 2.6% higher than that of PPC under 4 and 7 bar indicated mean effective pressures (IMEPs), respectively. The premixed high-reactivity PODEn in R-RCCI decreases the needed intake temperature, which provides more flexibility for the combustion phasing control. This correspondingly results in improved thermal efficiency for R-RCCI at 4 bar IMEP. Meanwhile, the lower intake temperature helps to decrease the heat transfer loss under 7 bar IMEP. Moreover, R-RCCI exhibits significant superiority over PPC on the unburned hydrocarbon (HC), carbon monoxide (CO), nitrogen oxides (NOx), and soot emissions under the operating conditions investigated in this work. For R-RCCI, the pre-combustion of PODEn can create a thermal atmosphere containing accumulated oxygen radicals, which provides the reaction conditions for the subsequent gasoline oxidation.

Influence of fuel injection parameters at low-load conditions in a partially premixed combustion (PPC) based heavy-duty optical engine

The study examined the effects of second injection timings and fuel injection pressures (FIPs) in a double-injection based partially premixed combustion (PPC) using a single-cylinder heavy-duty optical diesel engine equipped with two different configurations, namely all-metal and optical. The thermodynamic analysis of energy balance, exhaust emissions, and particulates characterization was conducted in an all-metal engine configuration. For the same base engine architecture, high-speed natural combustion luminosity, cool-flame, and electronically excited hydroxyl (OH*) chemiluminescence, and planar laser-induced fluorescence (PLIF) imaging of formaldehyde (HCHO-PLIF) were applied in the optical configuration. Additionally, 1D modeling using GT-Power was utilized to examine the distribution of heat transfer losses through the individual cylinder boundaries. The experiments were conducted at low-load conditions at a fixed fuel mean effective pressure (MEPfuel) using a primary reference fuel with a volumetric mixture of 50% n-heptane and 50% iso-octane (PRF50). The results demonstrated that for a fixed first injection, the second injection timing of −6°CA aTDC is the most favorable condition due to increased gross indicated efficiency, reduced exhaust losses, decreased uHC/CO/soot emissions with a slight compromise on heat losses and NOx emissions. Among the tested FIPs of 1200, 1500, and 1800 bar, 1500 bar showed higher efficiency, and overall lower energy losses and exhaust emissions. At higher FIPs, the overall flame development process revealed reduced luminous intensity with dispersed signals formed downstream of the nozzle due to increased premixed combustion. For 1800 bar FIP, both cool-flame and HCHO signals showed an increased rate of reaction zones close to the bowl-wall region from where the OH radicals developed. Not only the initial low- to high-temperature reaction transition was faster for 1800 bar FIP but the signal decay of HCHO signals by OH radicals was also rapid in the late cycle. These findings explained the root cause of increased heat transfer losses and higher uHC/CO emissions at 1800 bar FIP.

Investigation of combustion characteristics, physical and chemical ignition delay of methanol fuel in a heavy-duty turbo-charged compression ignition engine

In previous research, there have been more investigations on methanol blended with other fuels such as diesel, biodiesel, gasoline, etc., but fewer investigations on methanol with ignition additives as a mono-fuel. To better understand the methanol mono-fuel combustion characteristics and to further apply them, a combined experimental and simulation study of methanol in a Scania heavy-duty compression ignition (CI) engine was carried out in this work. The experiments consisted of four groups with variable injection timings, variable fraction of ignition additives, variable charge air temperatures, and variable overall excess air ratios/power sweeps. Heat release rate (HRR), cylinder pressure, ignition delay and indicated efficiency were analyzed for each case. The analysis showed that the combustion type was partially premixed combustion (PPC) in some cases and diesel-like combustion in the rest. By observing all cases, the shortest ignition delay was 14.1°, and the longest was 22.8°. The indicated efficiencies were in the range of 0.35 to 0.43. Simulations and validation analyses were performed for all cases by a multi-packets model. The physical and chemical ignition delays were predicted. The physical ignition delays were in the range of 4.25 to 8.10°, and the chemical ignition delays were in the range of 6.66 to 17.1°. The chemical ignition delay was always longer than the physical one. This indicates that chemical ignition delay has to be prioritized to improve the ignition performance of methanol fuel.

Partially premixed combustion of diesel-di-ethyl ether blends in light-duty commercial engine

Diethyl ether (DEE) has evolved as a potential alternative for partial diesel replacement in compression ignition (CI) engines. In this study, a light-duty commercial diesel engine was operated in partially premixed combustion (PPC) mode. This work investigated the engine characteristics with lower and higher DEE-diesel blends in PPC mode. The split ratio and the start of the main injection (SoMI) timings were maintained at 70:30 (Main: Pilot) and 9° bTDC, respectively, while the start of the pilot injection (SoPI) timing was varied from 25 to 40° bTDC. The performance, combustion, and regulated emission characteristics of the PPC engine were compared with the conventional diesel combustion (CDC) mode as a baseline. The PPC mode exhibited higher/ comparable brake thermal efficiency (BTE) than the baseline CDC mode. The DEE-diesel blends exhibited higher BTE than the diesel in PPC mode generally. The DEE40 showed greater improvement in BTE than DEE20 at lower brake mean effective pressure (BMEP). PPC mode fuelled with diesel and diesel-DEE blends showed lower brake specific fuel consumption (BSFC) than baseline CDC mode at lower BMEP, but it was comparable at higher BMEP. PPC mode showed higher carbon monoxide (CO) emissions than baseline CDC mode. CO emissions increased with advancing SoPI timing for all BMEPs, except 5.6 bar. Although, the PPC mode exhibited higher hydrocarbon (HC) emissions than the CDC mode. Diesel-DEE blends showed slightly higher HC emissions than diesel in PPC mode at lower engine BMEPs. NOx emissions in the PPC mode were 1.5 to 2 times the baseline CDC mode. This indicated that the PPC mode engine performance requires further optimization similar to CDC mode, which had optimized exhaust gas recirculation (EGR), and significantly reduced the nitrogen oxide (NOx) emissions.

RESEARCH ON NUMERICAL SIMULATION OF PCCI ENGINE: A REVIEW

The global environment is being affected by the use of fossil fuels to provide energy for internal combustion engines (ICE) due to a large amount of CO2 emission. Specific toxic substances are NOx, and CO, soot, particulate matter that exists in the environment at high densities while having a global impact. The automotive industry of countries worldwide is constantly changing and evolving to discover new solutions that reduce fuel use while developing cleaner and more advanced combustion methods, which as a low-temperature combustion engine (LTC). This study uses computational fluid dynamics (CFD) to understand partial pre-mixed compression combustion (PPCI), with HCCI and RCCI being advanced low-temperature combustion concepts. Lowering the average combustion temperature is an advantage of PCCI; it optimizes CO and NOx emission rates while maintaining high thermal efficiency. Investigate the intake pattern, including flame length, mixing pulse, and intake air volume, to determine the importance of that factor for oxidation using the LES. Studies have been performed using RANS and LES models to stimulate mixing and combustion.

Control of low-temperature polyoxymethylene dimethyl ethers (PODEn)/gasoline combustion considering fuel concentration, fuel reactivity, and intake temperature at low loads

This study aims to investigate the control mechanism of fuel properties and intake temperature (Tin) on the low-temperature polyoxymethylene dimethyl ethers (PODEn)/gasoline combustion to maximize the advantages of PODEn in enhancing engine performance. To achieve this goal, two representative combustion modes of partially premixed combustion (PPC) and reactivity-controlled compression ignition with reverse reactivity stratification (R-RCCI) with different fuel regulating methods were investigated. In PPC, the fuel was regulated by blending PODEn and gasoline outside the cylinder, while for R-RCCI, the in-cylinder fuel was tuned by delivering the two fuels into the cylinder with two different fuel supply systems. The main factors dominating the combustion process and pollutant emissions were identified for these two combustion modes. The results show that the combustion process of PPC is dominated by the collaborative organization of fuel reactivity and concentration in the cylinder, and 50% burn point (CA50) plays only a secondary role, but CA50 determines the nitrogen oxides (NOx) emission level. For PPC operated with the start of injection later than −40 °CA ATDC, increasing local fuel concentration is more effective in improving the combustion efficiency and indicated thermal efficiency than increasing Tin. For R-RCCI, the lower local temperature caused by the heat of vaporization of the directly injected gasoline remarkably influences the combustion process, but this cooling effect is not significant in PPC. The comparison of PPC and R-RCCI shows that the in-cylinder local fuel reactivity impacts the ignition more significantly than the overall fuel reactivity. Compared with PPC, R-RCCI can effectively reduce combustion instability and combustion rate, and simultaneously 26% reduction in NOx emissions was accomplished.

Lift-off behaviors of the partially-premixed jet flame in a supersonic vitiated coflow

Achieving fast and efficient combustion is considered as the primary difficulty in overcoming the high-Mach-number scramjet. A novel injection strategy with coflows has been proposed to improve the combustion efficiency, however, the inherent dynamics that drives the partially-premixed combustion is still not very clear. Numerical simulation is adopted to investigate the supersonic partially-premixed flame formed by the coflows of inner jet and annular vitiated air. Validation of numerical results has been conducted by comparing with the experimental data, and the predicted values show a reasonably good agreement with that in the experiments. Numerical results indicate that the supersonic jet flame is stabilized with a certain of lift-off height, with the dynamics balance of flow transport and flame propagation in the autoignition-dominated flame base. According to the flame index, the supersonic reactive flow fields are divided into three stages, including the no reaction region, premixed flame region and diffusion flame region. On this basis, the influence of equivalence ratio and injecting velocity of inner jet on the lift-off behaviors of jet flame, is explored by a series of contrastive analysis.

Optical diagnostics of methanol active-thermal atmosphere combustion in compression ignition engine

Methanol has become one of the research hotspots of internal combustion engine in recent years. One method of methanol compression ignition is that create in-cylinder active-thermal atmosphere to ignite direct injection (DI) of methanol. However, the combustion essences and related mechanisms in methanol active-thermal atmosphere combustion (ATAC) have not been comprehended. In current study, the characteristics of methanol ATAC were investigated on a light-duty engine using different optical diagnostics. Polyoxymethylene dimethyl ethers (PODE2) and n-heptane were chosen as port injection (PI) fuel respectively, and methanol was used as DI fuel. The experiment results indicate that methanol ATAC with PI of PODE2 is divided into two combustion processes. One process is the premixed combustion of PODE2 and the partially premixed combustion (PPC) of methanol, which appears that the premixed blue flames of PODE2 and the partially premixed yellow flames of methanol. The other is PODE2 premixed combustion and methanol diffusion combustion, which presents that the premixed blue flames of PODE2 and the yellow spray diffusion flames of methanol. Both the heat release rate and the flame structure of methanol ATAC can be adjusted by changing DI timing. The increase of PI energy proportion advances the timing of premixed blue flames of PODE2. The DI of methanol is driven by spray diffusion flames unless the methanol injection timing is earlier than high temperature heat release of PODE2. The capability of PODE2 to make methanol auto-ignited is higher than that of n-heptane. The methanol energy proportion can achieve 70% in PODE2 case, while achieve merely 30% in n-heptane case. Kinetic analysis reveals that the ignition delay times of PODE2 are lower than that of n-heptane even at lower equivalence ratio. The KL factor of methanol diffusion flame ranges from 0.02 to 0.04, which is remarkable lower than diesel spray flames. The methanol ATAC achieves higher substitution rate of methanol and the flexible combustion process can be obtained using PODE2 as PI fuel.

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.

An experimental investigation into the combustion stability and emissions of an n-butanol/diesel blended fueled partially premixed compression ignition (PPCI) engine

Higher cyclic variability in internal combustion engines (ICE) increases driveability problems, specific fuel consumption, and exhaust emissions. Consequently, quantification of the combustion stability in different combustion parameters is essential for the design of closed-loop control for a partially premixed combustion (PPC). This research examines the impacts of a PPC strategy on the spectral features of IMEP combustion parameters for early, and late injection timings of a 4-cylinder CRDI engine using wavelet transforms (WT). Furthermore, operating points were optimized based on the combustion and emissions characteristics of the partially premixed compression ignition engine. Results indicated that the early start of injection (SOI) timing of 12° CA BTDC has a lower standard deviation in the time-series data of IMEP than the late SOI timing. Moreover, when the amount of n-butanol in diesel fuel was increased from 0% to 20% at a load of 2 bar BMEP, COVIMEP decreased from 3.1% to 2.75% at early SOI timing. The WT results reveal that the continuous strong intensity band pattern changes irregularly when the injection timing is advanced. It was also found that at lower cyclic variability conditions (SOI = 12°CA BTDC), CO and smoke emissions decreased, and there was a noticeable increase in NOx emissions.

A critical review of the impacts of split injection strategies on the performance, combustion and emissions characteristics of a diesel engine fueled with low carbon alcohol fuels

This paper reviews the impacts of split (pilot/main) injection tactics on the exhaust pollutants, performance, and combustion of compression ignition (CI) engines operated with low carbon alcohols fuels. For the past several years, scholars have been attempting to simultaneously reduce NOx and soot emissions by enhancing combustion, which is accomplished by optimizing the fuel injection technique by using high amount of EGR or low carbon alcohols fuels. Split injection tactics are used in the diesel engines to accomplish the partially premixed combustion (PPC). The prolonged ignition delay (ID) in PPCI engines improves air/fuel mixing, resulting in lower soot and NOx emissions. Furthermore, due to the more substantial charge cooling effect of low carbon alcohol fuels reduce the combustion temperature, resulting in lower NOX emissions. Thus, combining a split injection approach with low-carbon alcohol fuels in a modern CI engine might be a promising option to satisfy stringent exhaust pollution regulations as well as reducing dependency on fossil fuels. The present paper focuses on the work done in the area of split injection strategies with various low-carbon alcohol fuels and the effects of injection pressures on engine performance.

Optical diagnostics of misfire in partially premixed combustion under low load conditions

To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion.

An attempt to implement partially premixed combustion strategy in a multi-cylinder CRDI engine: A detailed experimental and wavelet transform analysis

The main objective of this research was to effectively employ the n-butanol fuel in advanced combustion strategy ‘partially premixed combustion (PPC)’ and provide the maximum possible replacement of diesel fuel. An experimental study was conducted on a multi-cylinder turbocharged intercooler CRDI engine to optimize the injection timings and diesel/n-butanol blended fuels based on performance, emissions, and combustion characteristics, including cyclic variability, in order to achieve the PPC strategy without compromising thermal efficiency significantly. The combustion characteristics analysis indicate that adding n-butanol fuel into diesel fuel combined with advanced injection (16° CA BTDC) timing leads to a drastic decrease in smoke emission as well as increases in NOX emission than diesel fuel. However, DB20 fuel at 8° CA BTDC injection timing achieved low-temperature PPC mode and reduced NOX (8.08%↓) and smoke (48%↓) emissions simultaneously compared to diesel fuel at 2 bar BMEP engine load. In addition, wavelet power spectrum (WPS) and global wavelet spectrum (GWS) analysis were utilized to investigate the temporal and spectral fluctuations in the partially premixed combustion parameter (indicated mean effective pressure) at optimum injection timing (8° CA BTDC). Results reveal that DB20 possesses lesser cyclic variations with higher frequency (at period 2–6 cycles) intermittent fluctuation in partially premixed combustion at 2 bar BMEP engine load than other tested fuels. A statistical analysis of IMEP variations was also performed to validate the temporal variation in the IMEP time-series signals of the WPS results. The results of WPS could be used to establish an efficient engine control method based on in-cylinder pressure feedback.

A numerical investigation of gasoline/diesel direct dual fuel stratification (DDFS) combustion at high loads

High-load extension is one of the biggest challenges for the low-temperature premixed combustion strategies such as partially premixed combustion (PPC) and reactivity-controlled compression ignition (RCCI). Alternatively, a different dual-fuel combustion strategy, direct dual fuel stratification (DDFS) mode, which combines the benefits of PPC and RCCI by injecting gasoline and diesel into cylinder directly, is investigated in the present study. A comprehensive numerical comparison of PPC, RCCI and DDFS was conducted, and the effects of fuel property of the near top dead center (TDC) injection as well as different injection parameters on the combustion and emission characteristics of DDFS at a typical high load were revealed. The results indicated that DDFS can effectively control the combustion rate while maintain the thermal efficiency comparable to PPC and RCCI at high loads. For DDFS combustion, the near-TDC injection is the key parameter to control the heat release rate. However, soot emissions remarkably increase owing to the diffusion-combustion nature of the near-TDC injection, and adopting lower reactivity fuel such as methanol is a good approach to further reduce the peak pressure rise rate (PPRR) and suppress soot emissions. In addition, the diesel fraction plays a dominant role in controlling combustion phasing while the gasoline fraction serves as a more important role in combustion duration, and the effects of the start of injection of gasoline and diesel on the DDFS performance become more obvious as the gasoline and diesel fractions increase. From the perspective of load extension, a lower diesel fraction should be employed to avoid excessive PPRR, however, the diesel fraction and the start of injection should be coupled properly to avoid misfire.

Partially-Premixed Combustion Characteristics and Thermal Performance of Micro Jet Array Burners with Different Nozzle Spacings

Partially-Premixed Combustion Characteristics and Thermal Performance of Micro Jet Array Burners with Different Nozzle Spacings