Single-cylinder Optical Engine 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.

Effect of water content in ethanol on flame front wrinkling and curvature in a spark-ignition engine using OH PLIF imaging

The production of ethanol fuel involves separating water and ethanol by distillation and dehydration in an energy intensive process. To reduce the carbon footprint, hydrous ethanol is seen as a promising alternative. This study focused on the effect of water content in ethanol on flame development in a single-cylinder optical spark-ignition engine. Ethanol with 0%, 4% and 10% water content per volume was used with Port Fuel Injection (PFI) and Direct Injection (DI) for homogeneous and heterogeneous mixture preparation. Tests were also conducted with iso-octane for comparison. Flames captured by Planar Laser Induced Fluorescence (PLIF) of OH were spatially filtered to remove scales larger than the flame size, then the flame brush was quantified in terms of thickness, crossing frequency between instantaneous and filtered contours, and curvature. Increasing water content resulted in slower flame growth, with 10% hydrous exhibiting marginally higher flame speed than iso-octane. PFI led to faster flame growth and thicker flame brushes than DI at the same timing after ignition. When the brush thickness was normalised by flame radius, all hydrous fuels and iso-octane exhibited similar values with PFI, whilst anhydrous ethanol exhibited the thickest normalised brush. The alcohols maintained similar normalised thicknesses with DI and iso-octane exhibited the thickest normalised brush. When flames of the same radii were considered with PFI at the same timing, the fuels with sequentially lower laminar burning velocity were associated with larger crossing frequency. For PFI, increasing water content resulted in decreased flame roundness irrespective of size, with the 4% hydrous and iso-octane exhibiting similar values. With DI, anhydrous ethanol flames were rounder than the 4% hydrous when averaged over all cycles, but marginally rounder than the 10% hydrous and iso-octane. Increasing water content widened the curvature distribution and increased the mean with PFI. Flame roundness decreased with increase in mean curvature and Lewis number for both PFI and DI.

In-cylinder spray evolution in a motored central-injection gasoline engine: Imaging and simulating the effects of flash-boiling and intake crossflow

Accurate predictions of fuel spray behavior and mixture formation in simulations of direct-injection spark-ignition (DISI) engines are fundamental to ensure proper description of all subsequent processes including ignition, combustion, and emissions. In this work, the spray evolution in a single-cylinder optical DISI engine was studied experimentally and numerically with the goal of enabling predictive computational fluid dynamics (CFD) modeling of in-cylinder sprays. The authors explored a wide range of operating conditions characterized by several fuel injection temperatures and engine speeds, using a well-characterized nine-component gasoline surrogate known as PACE-20. The effect of flash boiling and intake crossflow on the spray is discussed, with a focus on evaluating the ability of the spray models to capture highly transient spray behavior. In the experiments, the fuel temperature was varied between 20°C and 80°C, allowing for non-flash- to flash-boiling transition to emerge with enhanced flashing intensity at the highest temperatures. Spray collapse resulted in vapor-rich regions, owing to the locally lower inertia of the fluid. Varying the engine speed from 650 to 1950 rpm promoted increasingly more turbulent in-cylinder crossflow which interacted with the spray during the injection event and resulted in enhanced spray dispersion. The CFD model was able to capture the spray morphology transition at different fuel temperatures and engine speeds adequately. It is shown that the spray breakup model could capture the transitional spray behavior induced by flash boiling atomization and intake flow via proper initialization of the spray cone angle and calibration of the spray models’ constants.

Comparative analysis of thermal and non-thermal discharge modes on ultra-lean mixtures in an optically accessible engine equipped with a corona ignition system

Radio-Frequency corona-based ignition systems are currently being investigated by the engine research community for their capability to ensure robust combustion at challenging operating conditions such as lean mixture and/or high exhaust gas recirculation (EGR) dilution. The low-temperature plasma (LTP) produced by the corona discharge accelerates the early flame development thanks to the production of combustion precursors like radical and excited species. Plasma benefits can be maintained with extensive management of the control parameters, preventing the transition from the non-thermal discharge mode to a thermal one.However, under ultra-lean conditions, where the ignition probability significantly decreases, the LTP discharge may not be suitable for promoting the combustion process because of insufficient thermal energy transfer and scarcity of available fuel, which is essential for radical production. Therefore, it may be necessary to increase the volume in which the plasma interacts to ensure robust ignitions and stable flame propagations. For this reason, the present work aims to assess and quantify the differences, in terms of combustion performance, between a corona igniter operating at LTP mode and with electric arcs occurring, at ultra-lean operating conditions. In arc-mode, the igniter produces volumetric discharges, facilitating extensive mixing of the mixture and resulting in advantageous effects on kernel formation and propagation. Tests were carried out in a single-cylinder optical access engine under low load and low speed, using gasoline as fuel. Close to the leanest stable limit found, the LTP mode shows a lower cycle-to-cycle variability. Above such a threshold, the thermal discharge can guarantee stable combustion events while the LTP discharge leads to unstable working conditions. Indicating, imaging, and pollutant emission analysis are carried out to evaluate the effects of thermal and non-thermal plasma modes on combustion characteristics and their environmental impact.

Investigating the relationship between butanol molecular structure and combustion performance in an optical SIDI engine

Butanol has a high potential as a renewable substitution for gasoline in spark-ignition direct-injection (SIDI) engines. Different butanol isomers showed various flame characteristics that are strongly related to their molecular structure. However, there has been limited research on the implications of the butanol isomer's molecular structure on SIDI engine combustion, performance, and emissions. This study investigated butanol isomers as gasoline substitutes in SIDI range extender engines, with a focus on the effects of isomer molecular structures. This work employed a single-cylinder optical SIDI research engine and a high-speed camera to examine engine performance, flame kernel stability, flame propagation, and particle number (PN) emissions. The investigated blends comprised 70% toluene reference fuel (TRF) and 30% butanol isomers (1-butanol (n), 2-butanol (s), isobutanol (i), and tertbutanol (t)). Experimental tests are carried out at a 1000 rpm engine speed and a load of 5.7 bar IMEP, while the fuel condition is kept stoichiometric.The results elucidate that adding 1-butanol makes the flame kernel more stable and reduces COVimep compared to pure TRF. On the other hand, TRF-i, TRF-s, and TRF-t decrease flame initiation stability and increase COVimep. 1-butanol exhibited the highest apparent flame speed, IMEP, and peak in-cylinder pressure, followed by TRF, TRF-i, TRF-s, and TRF-t. Over half of the TRF-t cycles exhibited either no or a delayed flame kernel. Flame circularity improved with 1-butanol and diminished with the other blends. Diffusion flame intensity and PN emissions were higher for TRF, TRF-n, and TRF-t compared to TRF-s and TRF-i. In conclusion, linear chain butanol isomers with more internal C-H bonds and terminal C-OH bonds provide a more stable flame kernel and superior engine performance than branched isomers featuring internal C-OH bonds. The results from this study can be useful for expanding the practical applications of butanol isomers as a renewable fuel replacement in SIDI engines.

Extended Coherent Flame Model applied to an optical single-cylinder engine fueled with ethanol

One way to achieve a fast track for the decarbonization of the transportation sector is through the usage of biofuels. Among the many biofuels available for transportation, ethanol is one of the most promising, especially when combined with direct-injection spark-ignited engine technologies. The present work aims to validate 3D Computation Fluid Dynamics (CFD) ethanol spray and combustion models with the calibration of specific model parameters using experimental data obtained with optical measurements. Focus is given on the investigation and determination of the Extended Coherent Flame Model parameters for hydrous ethanol turbulent spray combustion. To characterize the spray produced by the injector, measurements obtained with a Phase Doppler Interferometer system are used. Natural luminosity and in-cylinder pressure are acquired on a single-cylinder research engine with optical access. The work also considers results obtained from 1D and 3D CFD models to supplement the acquired experimental setup. From the comparison between experimental and numerical results, it comes out that a correction of the Extended Coherent Flame Model turbulence stretch parameter can be done according to a ratio of flow and combustion length scales obtained at the spark time. In this sense, an expression is proposed to allow the correction of such a parameter in different engine operating conditions. Accordingly, in-cylinder mean effective pressure calculated with 3D CFD simulations show a good agreement with the experimental data for all studied cases.

Ducted fuel injection with Low-Net-Carbon fuels as a solution for meeting future emissions regulations

Several studies have proven how ducted fuel injection (DFI) reduces soot emissions for compression-ignition engines. Nevertheless, no comprehensive study has investigated how DFI performs over a load range in combination with low-net-carbon fuels. In this study, optical-engine experiments were performed with four different fuels—conventional diesel and three low-net-carbon fuels—at low and moderate load, to measure emissions levels and performance. The 1.7-liter single-cylinder optical engine was equipped with a high-speed camera to capture natural luminosity images of the combustion event. Conventional diesel and DFI combustion were investigated at four different dilution levels (to simulate exhaust-gas recirculation effects), from 14 to 21 mol% oxygen in the intake. At a given dilution level, with commercial diesel fuel, DFI reduced soot by 82 % at medium load, and 75 % at low load without increasing NOx. The results further show how DFI with dilution reduces soot and NOx without compromising engine performance or other emission types, especially when combined with low-net-carbon fuels. DFI with the oxygenated low-net-carbon blend HEA67 simultaneously reduced soot and NOx by as much as 93 % and 82 %, respectively, relative to conventional diesel combustion with commercial diesel fuel. These soot and NOx reductions occurred while lifecycle CO2 was reduced by at least 70 % when using low-net-carbon fuels instead of conventional diesel. All emissions changes were compared with future emissions regulations for different vehicle sectors to investigate how DFI can be used to facilitate achievement of the regulations. The results show how the DFI cases fall below several future emissions regulation levels, rendering less need for aftertreatment systems and giving a possible lower cost of ownership.

Numerical and optical soot characterization through 2-color pyrometry technique for an innovative diesel piston bowl design

The development of innovative diesel piston bowl designs has shown significant improvement of the near-wall flame evolution, resulting in lower fuel consumption and engine-out soot emissions. With this aim, a novel hybrid piston bowl for a 1.6 L light-duty diesel engine was designed, coupling a sharp-stepped bowl and radial-bumps in the inner bowl rim. The effects of the proposed hybrid bowl were analysed through both single-cylinder optical engine and 3D-CFD models, which feature a detailed chemical kinetic mechanism and the Particulate Mimic (PM) soot model. The 2-color pyrometry optical technique was adopted to obtain the optical soot density (KL) and the temperature of the soot surface. Then, a line-of-sight integration of the numerical soot distribution was adopted to obtain a planar KL distribution, which is directly comparable with the experimental KL images. The results showed a good agreement in terms of soot distribution between 3D-CFD and experiments, confirming the high prediction capabilities of the developed numerical methodology. The synergetic application of numerical and optical techniques highlighted that the hybrid bowl strongly mitigates the flame-to-flame interaction with respect to a conventional re-entrant bowl, leading to lower soot formation in the flame collision area. Moreover, faster flame propagation toward the cylinder axis is highlighted with a consequent higher soot oxidation rate in the late combustion phase.

Optical and computational investigations of the effect of Spray-Swirl interactions on autoignition and soot formation in a compression ignition engine fuelled by Diesel, dieseline and diesohol

Diesel engine injection strategy plays a crucial role in fuel–air mixing and consequent soot formation, requiring optimisation. Fuel properties affect the combustion characteristics of the engine significantly. In this study, the fundamental mechanisms of pilot-main injection interactions and the effects of fuel properties have been investigated using a combined approach of optical diagnostics in an optical engine and computational fluid dynamics (CFD) tools. A single-cylinder optical research engine (SCORE) with a cylindrical piston bowl was used for the optical engine investigations. Simulations were carried out using a re-entrant piston geometry model. Reynolds-Averaged Navier-Stokes (RANS) based Re-Normalization Group (RNG) k-ε model was adopted and solved using CONVERGE CFD solver. SAGE detailed chemistry solver was used for the calculations of combustion kinetics. The adaptive mesh refinement (AMR) strategy was used for improved grid resolution. The investigation results highlighted a striking difference between the combustion mechanisms of the pilot injection and the main injection. The pilot-main injection interactions became crucial for igniting the main fuel spray. The swirl intensity was dominant in the pilot-main injection interactions and the combustion initiation. The impinging spray jet and swirl interactions improved the radial mixing and reduced the soot in the up-swirl direction. Dieseline (a blend of diesel and gasoline) and diesohol (a blend of diesel and alcohol) resulted in lower soot formation than baseline diesel. This study demonstrated that in-cylinder swirl and oxygen concentration could be optimised to simultaneously reduce oxides of nitrogen (NOx) and soot emissions using oxygenated fuels.

The Interaction Between In-Cylinder Flow And Flame Propagation In An Optical SI Engine Measured By High-Speed PIV

The relationship between the flow field and flame propagation is essential in determining the dynamics and effects of turbulent flow in an optical SI engine. In this study, the high-speed PIV technique was applied to evaluate the incylinder velocity and turbulence distribution under firing condition. The simultaneous measurement technique of incylinder flow and flame propagation characterized the relationship between flow field and flame propagation in the same cycle. A 500 cc single-cylinder optical engine was used for this experiment. The bore diameter and stroke length were 86 mm and 86 mm respectively, and compression ratio was 10.4. The flame front configuration was extracted from the PIV image based on planar laser tomography method. The strong contrast between the intensity of burned and unburned region in the captured images due to low gas and seed density of the former was utilized to derive this flame front. In order to evaluate the interaction between flame and flow with high resolution, the interrogation area was set to 16 × 16 pixels. This corresponds to 0.75 × 0.75 mm. The spatial filter of 6 mm was used to separate the instantaneous flow velocity into a low frequency component and a high frequency component. The engine speed was 1200 rpm and absolute intake pressure was 60 kPa. The equivalence ratio was stoichiometry condition. The ignition timing were set to 19 deg.BTDC. The characteristic cycles in the same test condition were extracted and discussed. The bulk flow with the large scale as large as the size of the combustion chamber influences the overall shape of the flame propagation, and the state of the flame propagation greatly fluctuates from cycle to cycle. The flame propagates with enhancing the large scale existing tumble flow that exists in earlier crank angle locations. Focusing on the local flame structure, the high-frequency velocity component was strong near the local flame peak and weak near the local flame valley. The flame at the local peak region propagates while pulling the valley region.

Analysis of the in-cylinder flow of a DISI engine using high-speed particle image velocimetry

In addition to exploring electric drives, it is crucial to minimize emissions and improve the efficiency of transport based on internal combustion engines (ICE). For these, in particular, a significant reduction of emissions as well as a considerable efficiency of ICE are required. A reliable formation of an effective mixture prior to ignition and a fast, steady combustion process are two key elements that influence emissions and efficiency. While on the one hand, the air-fuel mixture is formed by strongly three-dimensional, time-dependent flow phenomena during the intake and compression stroke, the turbulence level of the flow near the approaching flame front has a direct impact on turbulent flame propagation. The flow field in the combustion chamber, on the other hand, is highly turbulent and sensitive to cycle-to-cycle variations (CCV). In order to understand the fundamental processes and interactions of the flow field, flow measurements with high spatial and temporal resolution are required. To analyze CCV, the resulting flow field data must be decomposed into its scales, since CCVs are different from random fluctuations due to turbulent flow motions. Besides velocity decomposition based on traditional Reynolds decomposition, which provides an average flow velocity and a fluctuation velocity, most studies include statistical and linear approaches such as proper orthogonal decomposition (POD) and Fast Fourier transformation (FFT). In this paper, time-resolved high-speed stereoscopic PIV (HS-SPIV) measurements on an optical single-cylinder engine are processed using a new nonlinear and transient decomposition method, namely the multivariate empirical mode decomposition (NA-MEMD) based on a data-driven decomposition algorithm. The decomposition is based on the inherent scales of the data and generates physically valid modal representations known as intrinsic mode functions (IMFs), which are arranged with respect to the contained scale values. The approach and the IMFs generated are described in detail using one engine cycle as an example. According to the results, three modes are obtained, the measurement noise and extremely tiny short-term scales, the turbulent scales, and the large-scale tumble vortex.

Effects of intake air conditions and micro-pilot (MP) injection timing on micro-pilot dual fuel (MPDF) combustion characteristics in a single cylinder optical engine

The effects of intake air conditions and micro-pilot injection timing on the characteristics of micro-pilot dual fuel combustion were examined to enhance the combustion stability in marine engines. A minimum amount of diesel fuel was injected as the ignition source, and most of the fuel energy was obtained from methane gas. First, the intake air flow rate was varied. In addition, the methane gas flow rate and micro-pilot injection timing were changed depending on the intake air flow rate. Second, the micro-pilot injection timing was varied under intake air temperatures of 35 °C and 55 °C. The intake air and methane gas flow rates were maintained. Results indicated that increasing the intake air and methane gas flow rates promoted autoignition in the end-gas region. Nevertheless, the standard deviation of the peak cylinder pressure was within 3.1 bar. Under the intake air temperature of 35 °C, advancing and retarding the micro-pilot injection led to misfiring, which increased the standard deviation of the peak cylinder pressure to 10.2 bar and 9.6 bar, respectively. Knocking combustion was occurred when the micro-pilot injection was retarded under the intake air temperature of 55 °C. This phenomenon increased the standard deviation of the peak cylinder pressure to 6.9 bar. Premixed ignition in the end-gas region combustion corresponded to the optimal engine operating conditions for marine engines because of the low combustion variation even under high cylinder pressures of approximately 170 bar to 220 bar. However, high NOx emissions occurred under these conditions.

A combined experimental (PIV) and numerical (LES) study of the tumble formation during the intake stroke of an experimental single-cylinder optical engine

A combined experimental (PIV) and numerical (LES) study of the tumble formation during the intake stroke of an experimental single-cylinder optical engine

Optical diagnostics and multi-point pressure sensing on the knocking combustion with multiple spark ignition

Engine knock is an abnormal combustion phenomenon that limits the thermal efficiency and service life of spark-ignition engines. A better understanding of the knock mechanisms and characteristics is beneficial to knock alleviation and engine efficiency improvement. In this study, a metal liner with four evenly-spaced spark plugs in the periphery of the combustion chamber is designed to initiate knock from different positions. Four spark strategies are applied to the single-cylinder optical research engine and six pressure sensors are utilized to analyze the local pressure oscillations in the cylinder. The knocking combustion is investigated by simultaneous 72 kHz high-speed imaging and 6-point pressure sensing. The experimental results indicate that using multiple spark-ignition could promote knock intensity, advance the start of auto-ignition and introduce more acoustic resonance modes. The center pressure sensor is more sensitive to the first radial resonant mode (0, 1) of the knock pressure oscillation, while the side sensors are more sensitive to the first and second circumferential resonant modes (1, 0) and (2, 0). The knock onset judged by natural flame photography is earlier than that by pressure analysis because the auto-ignition event happens first and induces the subsequent pressure fluctuation. Natural flame luminosity analysis demonstrates that the initial auto-ignition sites only cause weak pressure oscillations, and the instantaneous combustion of the remaining end-gas increases the heat release rate significantly and gives rise to more violent pressure oscillations. Statistically, the maximum amplitude of pressure oscillation follows an exponential relationship with the peak mean flame luminosity. The end-gas resides in the gaps among the flame fronts generated by different spark strategies while the first auto-ignition sites are not evenly distributed in the end-gas zone. This fact gives insights into the local temperature non-uniformity of the end gas zone that affects the spatial distributions of the initial auto-ignition sites in the cylinder.

Mixture formation and combustion process analysis of an innovative diesel piston bowl design through the synergetic application of numerical and optical techniques

The optimization of diesel engine piston bowl geometries has a crucial role in improving the near-wall flame evolution for better air/fuel mixing and soot reduction. With these aims, an innovative piston bowl for a light-duty diesel engine was designed, coupling both a sharp-stepped lip and radial bumps in the inner bowl rim. The impact of the proposed design was investigated through both 3D-CFD and single-cylinder optical engine. The numerical model, featuring a calibrated spray model and a detailed combustion mechanism, was used to analyse the non-reactive air/fuel mixing and the combustion processes. Results highlighted a reduced jet-to-jet interaction and better air/fuel mixing for the innovative bowl with respect to a conventional re-entrant design, thus enabling faster combustion process after the end of main injection. Numerical results in terms of flame’s kinematic and oxidation process were compared with the combustion image velocimetry (CIV) and OH* chemiluminescence from the optical engine, showing higher velocity near the radial bumps, and faster flame recirculation towards the piston center. Moreover, both experiments and simulations showed a more intense OH distribution in the radial-bumps region and above the step during the first stage of the combustion process, thanks to the enhanced air/fuel mixing.

Optical experiments on the effect of turbulent jet ignition on lean burning and engine knocking

Recently the turbulent jet ignition (TJI) configured by pre-chamber has been paid more attention as it can effectively achieve rapid combustion thus high thermal efficiency. In this work, using a single-cylinder optical engine with high compression ratios, the effects of turbulent jet ignition on lean burning and engine knocking were comprehensively investigated, with the conventional spark-ignition scenarios for comparisons. The results show that the pre-chamber with larger volume results in stronger flame jets and a higher promoting effect on the combustion. However, due to the higher heat dissipation and throttling loss, the pre-chamber with larger volume results in lower IMEP. For lean combustion, the optimized TJI (small volume) can reduce the combustion instability and improve the thermal efficiency when compared to spark ignition. The main reason is that TJI can increase the in-cylinder flame speed and achieve a concentrated HRR. For knocking combustion, TJI increases the knock tendency and knock intensity. The main reason is that the accelerated flame advances the auto-ignition and induces a high energy density. In summary, the effect of TJI on engine performance differs under different load conditions and the TJI strategy should be optimized especially under heavy load conditions.

A wall-adapted zonal URANS/LES methodology for the scale-resolving simulation of engine flows

In the present paper, a comprehensive, wall-adapted zonal URANS/LES methodology is shown for the multidimensional simulation of modern direct-injection engines. This work is the latest update of a zonal hybrid turbulence modeling approach, specifically developed by the authors for a flexible description of in-cylinder turbulent flow features with an optimal balance between computational costs and accuracy. Compared to the previous developments, a specific near-wall treatment is added, in order to preserve full-URANS behavior in the first near-wall cells, having in mind typically available mesh resolution in this part of the fluid domain. The updated methodology is applied to the multi-cycle simulation of a reference single-cylinder optical engine, which features a twin-cam, overhead-valve pent-roof cylinder head, and is representative of the current generation of spark-ignited direct-injection thermal power units. Results based on phase-specific flow field statistics and synthetic quality indices demonstrate the consistency and effectiveness of the proposed methodology, which is then qualified as a suitable candidate for affordable scale-resolving analyses of cycle to cycle variability (CCV) phenomena in direct-injection engines.

Effects of fuel trapping in piston crevice on unburned hydrocarbon emissions in early-injection compression ignition engines

Early injection strategy is employed in many advanced combustion concepts of modern compression-ignition engines to promote the fuel-air premixing, but it could result in fuel spray impingement on the cylinder wall and potential fuel trapping in the crevice regions. The impact of the fuel trapping on the formation and distribution of unburned hydrocarbon (UHC) in the advanced compression-ignition concepts is not well understood. In this study, the planar laser-induced fluorescence (PLIF) technique was applied in a single-cylinder optical engine to visualize the fuel distribution before combustion and the formaldehyde formation during combustion. A three-dimensional computational model was established to explore the detailed mechanism of UHC formation in the piston crevice. Three cases with injection timings of 20°, 40° (SOI-40), and 100° (SOI-100) were compared. The PLIF and simulation results indicate that, under early injection conditions, the squish region and piston crevice trap a considerable amount of fuel, resulting in increased UHC emission and reduced engine work, and the main combustion zone resides in the squish region. The simulation shows that the amount of UHC formation from the trapped fuel is highly dependent on the local equivalence ratio distribution formed by different injection timings. The local equivalence ratio within the piston crevice region for the SOI-40 case exceeds 2 before combustion; the charge sequentially undergoes both low- and high-temperature heat release (LTHR and HTHR) processes and produces less UHC in the crevice region. However, the SOI-100 case produces an overall leaner mixture with a local equivalence ratio lower than 1 before combustion; the charge undergoes LTHR without noticeable HTHR and becomes UHC near the cylinder wall. The injector dribbling results in more UHC formation in the central part of the cylinder under very early injection timing, and thus an accurate injector dribbling model is required to better reproduce the UHC emissions.

Soot Development in an Optical Direct Injection Spark Ignition Engine Fueled with Isooctane

To better understand the formation and evolution processes of soot, the two-color laser induced incandescence diagnostic method was applied on a single cylinder optical direct injection spark ignition engine. Soot volume fraction was measured, and soot distribution was imaged as cyclic fuel quantity changes. The results show that 45.5 mg/cycle generates the most soot at the same measure plane. Pool fire dominates the formation of soot in the tested engine and generates more soot on the top surface of the piston near the injector. In-cylinder soot increases until 42°CA ATDC and then reduces due to oxidation. Pool fire continues through the expansion stroke till 52°CA ATDC, and then soot cloud gathers near the 10 mm plane. After 82°CA ATDC, in-cylinder soot basically in equilibrium, and residual soot moves follow the in-cylinder flow randomly and evenly distributes within the whole combustion chamber. With increasing cyclic fuel quantity, particles number concentration gradually increases and their distribution present dual-peak shape. In detail, 45.5 mg/cycle emits the most accumulation mode particles while 52 mg/cycle emits the most nucleation mode particles.

The comparison between traditional spark ignition and micro flame ignition in gasoline high dilution combustion

Gasoline spark ignition (SI) – Controlled auto-ignition (CAI) hybrid combustion had previously been shown to expanding the operational range of high-efficiency low-temperature combustion and reducing fuel consumption. However, the spark ignition became ineffective when the mixture became highly diluted and the large cyclic variation and even misfire would occur. To achieve high-efficiency combustion in extended engine operational range and overcome the limitation of SI-CAI hybrid combustion, Micro Flame Ignition (MFI) was proposed and researched as a mean to providing multiple auto-ignition sites to initiate the combustion process of the diluted mixture. In this research, both engine experiments and Computational Fluid Dynamics (CFD) simulations were carried out to study the MFI combustion and SI-CAI hybrid combustion in a single-cylinder optical engine. Compared to the SI-CAI hybrid combustion, the flame propagation in MFI hybrid combustion was initiated by a large number of reaction fronts produced by the DME auto-ignition at multiple sites. MFI was found to deliver substantially more heat and ignition energy to the premixed mixture than the single spark ignition, enabling much faster initial heat release. However, the flame front expansion speed of MFI hybrid combustion dropped significantly to a similar value to that of the SI-CAI case because of the slower flame speed of diluted gasoline mixture. The MFI combustion exhibited three phases of autoignition stage, flame propagation stage and fast heat release stage. It is characterized by a higher peak heat release rate and shorter duration of the main combustion than those of the SI-CAI combustion. Besides, the use of spark ignition in the MFI operation promoted the autoignition of DME, leading to a shorter combustion duration and faster combustion than the MFI combustion without spark ignition. As a result, the spark assisted MFI strategy could be used to control the combustion phasing and optimize the MFI combustion process.