Stratified Charge Engine Research Articles

Coupling a Lagrangian–Eulerian Spark-Ignition (LESI) model with LES combustion models for engine simulations

In the United States transportation sector, Light-Duty Vehicles (LDVs) are the largest energy consumers and CO2 emitters. Electrification of LDVs is posed as a potential solution, but SI engines can still contribute to decarbonization. Car manufacturers have turned to unconventional engine operation to increase the efficiency of Spark-Ignition (SI) engines and reduce the carbon emissions of their fleets. Dilute, lean, and stratified-charge engine operation has the potential for engine efficiency improvements at the expense of increased cyclic variability and combustion instability. At such demanding engine conditions, the spark ignition event is key for flame initiation and propagation and for enhanced combustion stability. Reliable and accurate spark ignition models can help design ignition systems that reduce cyclic variability. Multiple computational spark-ignition models exist that perform well under conventional conditions, but the underlying physics needs to be expanded, for unconventional engine operation. In this paper, a hybrid Lagrangian–Eulerian Spark-Ignition (LESI) model is coupled with different turbulent flame propagation models for engine simulations. LESI relies on Lagrangian arc tracking and Eulerian energy deposition. The LESI model is coupled with the Well-Stirred Reactor (WSR), Thickened Flame Model (TFM), and g-equation model and used to simulate several cycles of a Direct-Injection Spark-Ignition (DISI) engine using a commercial Computational Fluid Dynamics (CFD) engine solver. The results showcase the successful coupling of LESI with the combustion models. Global engine metrics, such as pressure and Apparent Heat Release Rate (AHRR), for each simulation setup are compared to experimental engine results, for validation. In addition, results highlight the successful prediction of spark channel movement by comparing simulation images to experimental optical engine images. Finally, the successful coupling of LESI to combustion models, making it a usable model in the engine modeling community, is emphasized and future development details are discussed.

Impact of coolant temperature on the combustion characteristics and emissions of a stratified-charge direct-injection spark-ignition engine fueled with E30

The direct injection spark ignition (DISI) engine has received considerable attention due to its potential to increase the power density of traditional spark ignition engines while significantly improving fuel economy through lean, unthrottled combustion. However, the market introduction of DISI engines operated in a lean combustion mode is inhibited by their unsatisfactory emissions, especially during cold start conditions that make proper mixture formation more challenging. Ethanol-blended gasoline, now a widely used fuel, makes the cold start of a DISI engine more difficult, leading to higher HC and soot emissions because of the high latent heat of vaporization of ethanol relative to gasoline. This work investigated the impact of coolant temperature on the characteristics of combustion and emissions in a stratified-charge DISI engine fueled with an E30 fuel (i.e. 30% ethanol in gasoline), while the coolant temperature was alternated between four levels (45, 60, 75, and 90 °C) to simulate different conditions throughout the warm-up process. The experiments showed that the coolant temperature affected the post-spark inflammation time, as well as the speed, intensity, and stability of the combustion process in the engine. When the coolant temperature rose, the engine produced more NOX and less CO, PM and HC. In addition, high-speed direct photography was used to obtain crank-angle resolved images of fuel sprays and flames in the cylinder. As the coolant temperature rose, the liquid spray lengths became shorter, reducing the possibility of wall wetting, and reduced irradiance from soot particles also indicated less non-premixed combustion. The in-cylinder imaging results are consistent with the observed combustion and emission characteristics and shed light on the underlying processes. Some potential solutions to the emissions challenges faced here could be either raising in-cylinder temperatures by using trapped residuals or modifying the injection schedule, for example by increasing the number of injections or to inject later in the cycle into a higher-density environment.

Development of a three-zone combustion model for stratified-charge spark-ignition engine

A thermodynamic model for calculating the operating process in the cylinder of a spark-ignition engine with internal mixture formation and stratified air-fuel charge based on the volume balance method was developed. The model takes into account the change in the working fluid volume during the piston movement in the cylinder. The equation of volume balance of internal mixture formation processes during direct fuel injection into the engine cylinder was compiled. The equation takes into account the adiabatic change in the volume of the stratified air-fuel charge, consisting of fuel-air mixture volume and air volume. From the heat balance equation, the change in the fuel-air mixture volume during gasoline evaporation in the fuel stream and from the surface of the fuel film due to external heat transfer was determined. Basic equations of combustion-expansion processes of the stratified air-fuel charge were derived, taking into account three zones corresponding to combustion products, fuel-air mixture and air volumes. The equation takes into account the change in the working fluid volume due to heat transfer and heat exchange between the zones and the walls of the above-piston volume. Dependences for determining the temperature in the three considered zones and pressure in the cylinder were obtained. Graphs of changes in the volumes of the combustion products, fuel-air mixture and air zones with the change of the above-piston volume in partial load modes (n=3,000 rpm) were plotted. With increasing load from bmep=0.144 MPa to bmep=0.322 MPa, at the moment of fuel ignition, the volume of the fuel-air mixture increases from 70 % to 92 % of the above-piston volume. At the same time, the air volume decreases from 30 % to 8 %. Analysis of theoretical and experimental indicator diagrams showed that discrepancies in the maximum combustion pressure do not exceed 5 %

Effects of stratification on premixed cool flame propagation and modeling

Low-temperature cool flames play a critical role for the performance of stratified-charge internal combustion engines. Accurate modeling of stratified combustion in engines requires a clear understanding of cool flame propagation in a compositionally stratified mixture. There are many studies on hot flame propagation in fuel-stratified mixtures. However, comparatively little effort has been made to explore stratification effects on cool flame propagation and modeling. To this end, a series of one-dimensional numerical simulations is performed in this work to investigate unsteady stratified dimethyl ether (DME)/air cool flames propagating in six stratification configurations (i.e., lean-to-stoichiometric (L2S), stoichiometric-to-lean (S2L), stoichiometric-to-rich (S2R), rich-to-stoichiometric (R2S), lean-to-rich (L2R), and rich-to-lean (R2L) stratified mixtures). To better understand stratification effects, the above stratified cool flames are compared with compositionally homogeneous cool flames. Furthermore, a well-established premixed flame modeling approach – Artificially Thickened Flame (ATF)/Flamelet-Generated Manifolds (FGM) – is evaluated with respect to its suitability for modeling stratified cool flames. The comparisons between stratified cool flames and homogeneous cool flames demonstrate that the stratification with an increase (e.g., Cases L2S, S2R and L2R) or decrease (e.g., Cases S2L, R2S and R2L) in equivalence ratio can promote or suppress cool flame propagation. Both the positive and negative effects would be augmented when decreasing stratification layer thickness. Moreover, sensitivity analysis and reaction path flux analysis of OH consumption are conducted to identify a cause and effect chain that elucidates how stratification affects cool flame propagation. It is found that stratification can significantly change the competition between reactions CH2O+OH⇌HCO+H2O and CH3OCH3+OH⇌CH3OCH2+H2O and hence affect the fuel consumption rate. Furthermore, a scaling law is proposed based on a stratification Damköhler number, in order to comprehensively characterize stratification effects on cool flame propagation. In addition, the evaluation of ATF/FGM in the laminar context suggests that the FGM method is a good candidate for describing low-temperature chemistry, while the ATF approach may need further consideration for accurately capturing cool flame propagation speeds.

Large-eddy simulations of a stratified-charge direct-injection spark-ignition engine: Comparison with experiment and analysis of cycle-to-cycle variations

Multiple-cycle large-eddy simulations (LES) have been performed for an optically accessible, single-cylinder, four-stroke-cycle, spray-guided direct-injection spark-ignition (SG-DISI) engine operating in a stratified globally fuel-lean mode. The simulations combine a standard Smagorinsky turbulence model, a stochastic Lagrangian parcel method for liquid fuel injection and fuel spray modeling, a simple energy-deposition spark-ignition model, and a modified thickened flame model for turbulent flame propagation through highly stratified reactant mixtures. Comparisons between simulations and experiments include individual-cycle and ensemble-average pressure and apparent-heat-release-rate traces, individual-cycle and ensemble-average indicated mean effective pressures (IMEP), and instantaneous two-dimensional vapor-equivalence-ratio contours. Although the number of LES cycles is small (35), the results show that the simulations are able to capture the global combustion behavior that is observed in the experiments, including cycle-to-cycle variations. The simulation results are then analyzed further to provide insight into the conditions that lead to misfire versus robust combustion. As has been reported in earlier experimental and LES studies for homogeneous-charge SI engines, local conditions in the vicinity of the spark gap at the time of ignition largely determine the subsequent flame development. However, in contrast to homogeneous-charge engines, no single local or global quantity correlates as strongly with the eventual peak pressure or IMEP for each cycle. Rather, it is the interplay among the early flame kernel, the velocity field that it experiences, and the fuel distribution that it encounters that ultimately determines the fate of each combustion event. Deeper analysis and quantitative statistical comparisons between experiments and simulations will require the simulation of larger numbers of engine cycles.

Ability of Particulate Matter Index to describe sooting tendency of various gasoline formulations in a stratified-charge spark-ignition engine

This study investigates the ability of Particulate Matter Index (PMI) to describe the sooting behavior of various gasoline formulations in a stratified-charge (SC) spark-ignition engine. The engine was operated at 2000 rpm with an intake pressure of 130 kPa where soot formation is known to primarily occur in the bulk gases. Exhaust soot emissions were measured for nine test fuels at various exhaust gas recirculation levels. A comparison between measured soot levels and PMI shows that PMI is a relatively poor predictor of the sooting tendency of the tested fuels under lean SC combustion. Among the fuels, three fuels, namely the di-isobutylene blend, High Olefin, and E30 fuels exhibit measured soot behavior opposite of that predicted by PMI. Optical diagnostics were utilized to further investigate the in-cylinder phenomena for these three fuels. Analysis of natural luminosity and diffused back-illumination extinction imaging suggests that fuel-induced differences in the amount of soot formed are responsible for a majority of the discrepancy in measured versus predicted sooting tendency. Fuel-induced differences in soot oxidation and spray development seem to play minor roles. Because the combustion and air-fuel mixing processes for lean SC combustion are different from conventional stoichiometric operation it is hypothesized that the PMI correlation needs to be modified to account for differences in stoichiometric air-fuel ratio and level of oxygenation between fuels. Furthermore, the role of fuel volatility in PMI possibly needs to be de-emphasized for SC operation with fuel injection into compression-heated gases.

Optimization on timings of injection and spark of a high compression-ratio stratified-charge methanol engine under ultra-lean burn

High compression-ratio and ultra-lean burn are the two most effective measures to enhance economy of spark-ignition engines. The investigations were performed on a high compression-ratio stratified-charge direct-injection spark-ignition methanol engine with an injector of non-uniform spray-line distribution. The effect of timings of injection and spark on the concentration field of the stratified-charge mixture, combustion and emissions behaviors of methanol engine under ultra-lean burn conditions was studied by simulation. Simulation results demonstrated that an ideal stratified-charge mixture was the guarantee to obtain the best performance of a high compression-ratio spark-ignition methanol engine under ultra-lean burn. The timings of injection and spark had a remarkable effects on mixture formation, combustion and emission of a methanol engine under ultra-lean burn. There existed an optimal timings of injection and spark to obtain the peak in-cylinder pressure, peak heat release rate and peak in-cylinder temperature of methanol engine under ultra-lean burn, as well as the shortest ignition delay and combustion duration. Lower CO (carbon monoxide), unburned methanol and soot emissions, and higher NO (nitric oxide) emission were acquired at optimal timings of injection and spark under high compression-ratio and ultra-lean burn. Hence, realizing an ideal stratified-charge combustion was the key to obtain the best performance of a spark-ignition methanol engine under high compression-ratio and ultra-lean burn.

Assessment of ultra-lean burn characteristics for a stratified-charge direct-injection spark-ignition methanol engine under different high compression ratios

Lean burn is one of the most important characteristics of a stratified-charge direct-injection spark-ignition engine. In this study, a non-uniform 10-hole × 0.30 mm spray-line distribution nozzle was chosen to achieve stratified-charge of the methanol/air mixture. The mixture formation, combustion, and emissions characteristics of a stratified-charge direct-injection spark-ignition methanol engine under five global equivalence ratios and three high-compression ratios were simulated to assess its ultra-lean burn characteristics. The results showed that the equivalence ratio of the stratified-charge direct-injection spark-ignition methanol engine for stable combustion at high compression ratio was as low as 0.20. The ignition delay and combustion duration decreased as the compression ratio increased at high equivalence ratio, and vice versa at low equivalence ratio. The stratified-charge direct-injection spark-ignition methanol engine greatly reduced nitric oxide emissions under ultra-lean combustion. The unburned methanol, carbon monoxide, and soot emissions increased slowly as equivalence ratio decreased at all compression ratios and equivalence ratio >0.25. The indicated thermal efficiency decreased with decreasing compression ratio at high equivalence ratio, and vice versa at low equivalence ratio. For an equivalence ratio of 0.67, the indicated thermal efficiency at compression ratio of 14 was approximately 14.9% lower than at compression ratio of 16. However, for an equivalence ratio of 0.20, the indicated thermal efficiency at compression ratio of 14 was approximately 13.5% higher than at compression ratio of 16. The compression ratio of 14 was more favorable for the stratified-charge direct-injection spark-ignition methanol engine to achieve ultra-lean burn and find practical application.

Combustion performance improvement of a diesel fueled Wankel stratified-charge combustion engine by optimizing assisted ignition strategy

This work aimed to further improve the combustion performance in a diesel Wankel engine by optimizing the assisted ignition strategy. Based on the established and verified dynamic simulation model, the effects of some key assisted ignition parameters, such as the number of assisted spark plug, assisted ignition position (AIP) and assisted ignition timing (AIT) on combustion performance were analyzed systematically. Simulation results indicated that the reaction process occurred in the mid-anterior region of the rotary chamber due to the stratified-charge mixture and forward-propagate flame. The combustion performance of the double assisted spark plug schemes was better than that of the single assisted spark plug scheme. The double assisted spark plugs symmetrically distributed relative to the major axis (X-axis) of the rotary chamber, the combustion process was more reasonable. When changing AIP, peak combustion pressure (Pmax) and combustion zone varied greatly, while they changed little when advancing AIT. For the optimized assisted ignition strategies, the optimal AIP scheme was Case5 (0, −88.5, ±15 mm) and the optimum AIT scheme was Case7 (35°CA BTDC). Comparing with the original scheme (Case1), their Pmax increased by 16.68% and 17.83%, respectively. Meanwhile, the CA50 and CA10-90 were shortened obviously.

In-cylinder flow and combustion modeling of a CNG-fuelled stratified charge engine

In-cylinder air-fuel stratification and combustion is evaluated in a CNG-fuelled IC engine using a transient, three-dimensional, computational fluid dynamic (CFD) model. Performance of the stratified mode of operation is compared with the premixed mode. The combustion process is modeled using the extended coherent flame model (ECFM-3Z). For simulating the ignition process, the arc and kernel tracking ignition model (AKTIM) is used. The combustion model is first validated with measured in-cylinder pressure data and other derived quantities such as heat release rate and mass burn fraction for the premixed case. A good agreement is observed between measured and simulated values. It is observed that there is a marginal improvement in terms of overall engine efficiency when the stoichiometric premixed case is compared with the lean stratified condition. However, a major improvement in performance is observed when the lean stratified case is compared with lean premixed condition. The stratified case shows a faster heat release rate which could potentially translate to lower cycle-to-cycle variations in actual engine operation. Also, the stratified cases show as much as 30% lower in-cylinder NOx emissions when compared with the premixed case at the same engine load and speed, underscoring the potential of in-cylinder stratification to achieve improved performance and lower NOx emissions.

Impact of coolant temperature on piston wall-wetting and smoke generation in a stratified-charge DISI engine operated on E30 fuel

A late-injection strategy is typically adopted in stratified-charge direct injection spark ignition (DISI) engines to improve combustion stability for lean operation, but this may induce wall wetting on the piston surface and result in high soot emissions. E30 fuel, i.e., gasoline with 30% ethanol, is a potential alternative fuel that can offer a high Research Octane Number. However, the relatively high ethanol content increases the heat of vaporization, potentially exacerbating wall-wetting issues in DISI engines. In this study, the Refractive Index Matching (RIM) technique is used to measure fuel wall films in the piston bowl. The RIM implementation uses a novel LED illumination, integrated in the piston assembly and providing side illumination of the piston-bowl window. This RIM diagnostics in combination with high-speed imaging was used to investigate the impact of coolant temperature on the characteristics of wall wetting and combustion in an optical DISI engine fueled with E30. The experiments reveal that the smoke emissions increase drastically from 0.068 FSN to 1.14 FSN when the coolant temperature is reduced from 90 °C to 45 °C. Consistent with this finding, natural flame luminosity imaging reveals elevated soot incandescence with a reduction of the coolant temperature, indicative of pool fires. The RIM diagnostics show that a lower coolant temperature also leads to increased fuel film thickness, area, and volume, explaining the onset of pool fires and smoke.

Computational study of intake temperature effects on mixture formation, combustion and unregulated emissions of a DISI methanol engine during cold start

Direct injection spark ignition (DISI) methanol engines have an excellent lean-burn properties but with more unregulated emissions of formaldehyde and unburned methanol and cold start difficulties. In this study, intake temperature effects on mixture formation, combustion processes, and unregulated emissions of formaldehyde and unburned methanol were numerically simulated for a stratified-charge DISI methanol engine during cold start. Computational results show that increasing the intake temperature during cold start boosts the evaporation of methanol and improves the in-cylinder distribution of methanol-air mixture, thereby accelerating the combustion and mitigating formaldehyde and unburned methanol emissions. When the intake temperature increases from 283 K to 293 K, the ignition delay is reduced by 1.3°CA (crank angle). Meanwhile, the maximum cylinder pressure, maximum heat release rate, and maximum cylinder temperature increase by approximately 0.59 MPa, 20 J/°CA and 165 K, respectively; formaldehyde and unburned methanol emissions are reduced by 27% and 48%. When the intake temperature is further increased to 313 K, the effects on the engine operations as well as emissions become weaker. There were almost no improvements on combustion or unregulated emissions when the intake temperature was above 313 K.

Exploration of carbon deposit on spark plug of stratified charged gasoline direct injection engine

ABSTRACT Gasoline direct injection (GDI) engines are operating under high temperature and hence fouling of spark plug can cause misfiring followed by carbon deposits in the vicinity of the spark plug. This effect can cause declined engine power output, unburned hydrocarbon emission and also unacceptable high NO x near the lean flammability limit of gasoline. The objective was to explore carbon deposit on the spark plug and to identify the most suitable operating variable for the stratified charge mode of a GDI. Performance study was conducted on GDI engine on Start of Injection (SOI) angles of 60°, 100° and 140° bTDC with spark advance of MBT of 16°, 18° and 20°, respectively. Carbon deposit on spark plug at each injection angle was studied through scanning electron microscope (SEM) analysis at specified fuel air ratio. It was revealed that from performance and SEM analysis SOI 100° was chosen as the most suitable operating variable for the stratified charged GDI engine operation.

Characteristics of near-nozzle spray development from a fouled GDI injector

The near-nozzle spray development of a typical fouled gasoline direct injection (GDI) injector was investigated. The fouled injector had been used in a stratified-charge combustion GDI engine and showed typical characteristics, such as accumulated deposits inside and around the nozzles and a reduced flow rate of 2.9–5.7%. Back-illumination and Mie-scattering methods were employed in spray experiments, in conjunction with a high speed camera and a macro lens, to assess the near-nozzle spray behaviors. The experimental results show that at all injection pressures tested, the interaction between deposits and spray led to several poor spray behaviors during the full injection evolution, including spray distortion, residual fuel storage in the nozzles and deposits layer, liquid splashing, the formation of ligament and large droplets and tip wetting/dripping. These effects all may result in high soot emissions. The after-injection stage of the fouled injector produced more liquid ligaments than that of the new injector. It was also found that high injection pressures did not improve atomization during after-injection, nor reduce the amounts of ligaments and droplet clusters beyond the main spray boundary. The plume width and projected spray area of a single nozzle in the fouled injector were decreased by 5–7% and 17–20%, respectively, due to fuel flow losses. The delays in the start of injection and end of injection were approximately 20 μs and 30–40 μs, respectively.

The Investigation of velocity and temperature changes by CFD analysis for intake stroke of LPG fuel stratified engine

This study numerically investigates the in-cylinder speed and temperature variations for fuel-air mixture intake at the induction stroke by the LPG cylinder of an engine running on the stratified charge principle. The present study is based on the CFD model created using experimental data. Examinations were performed between the top dead center and the bottom dead center during the induction stroke. In-cylinder speed and temperature variations were obtained by k-e turbulence model depending on the crankshaft angle and revolutions per minute (900, 3000, 5500 rpm). The obtained results were compared with the known theoretical values for stratified charge engines, and consistent results were obtained.

Utilizing boost and double injections for enhanced stratified-charge direct-injection spark-ignition engine operation with gasoline and E30 fuels

This study systematically explores the effects of injection strategy and exhaust-gas recirculation on boosted stratified-charge operation of a direct-injection spark-ignition engine using gasoline ...

High-speed PIV, spray, combustion luminosity, and infrared fuel-vapor imaging for probing tumble-flow-induced asymmetry of gasoline distribution in a spray-guided stratified-charge DISI engine

In this study, the influence of intake-generated swirl and tumble flow on fuel–air mixing and combustion is investigated in a spray-guided stratified-charge direct-injection spark-ignited engine. Previously, it was demonstrated that the introduction of a combined swirl–tumble flow recovered combustion stability, which was otherwise lost when increasing the engine speed from 1000 to 2000rpm. However, the improved combustion came at the expense of elevated engine-out soot emissions. Here, high-speed combustion luminosity and PIV measurements at 2000rpm confirm that soot incandescence is more prevalent with high swirl and tumble. The application of high-speed infrared (IR) gasoline-vapor imaging introduced here provides unique insights, revealing that operation with a combination of swirl and tumble generates an asymmetric fuel distribution that spatially correlates with highly luminous sooting combustion. The IR fuel-vapor imaging technique collects line-of-sight mid-infrared thermal emission from the CH stretch band of the heated fuel near a wavelength of 3.4µm. The IR images resolve the penetrating vapor plumes distinctly, demonstrating that the 3.4µm band is suitable for quantitative measurements of vapor penetration during injection. After injection, the IR images provide a qualitative description of fuel-vapor spread without combustion. It is found that the no-swirl case has a symmetric fuel-vapor development during the latter part of the compression stroke. In contrast, for operation with strong swirl and tumble, vapor rotation and the development of an asymmetric and non-uniform fuel-vapor distribution is observed. PIV measurements reveal that the swirl flow dominates the vapor rotation, while the tumble flow appears to be a major reason for the asymmetric fuel-vapor distribution.

The role of spray-enhanced swirl flow for combustion stabilization in a stratified-charge DISI engine

Implementation of spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle.The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angular-momentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum.This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. In contrast, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel–air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles.

Effect of compression ratio on performance and emissions of a stratified-charge DISI (direct injection spark ignition) methanol engine

Experiments were conducted on a single-cylinder, four-stroke, water cooled, stratified-charge DISI (direct injection spark ignition) methanol engine which was modified from a diesel engine. The effects of compression ratio on the torque, power, brake thermal efficiency and exhaust emissions of a DISI stratified-charge methanol engine were investigated and the all aforementioned parameters were compared with the diesel counterpart. The analysis of compression ratio on combustion behavior of methanol engine was performed. The torque and power of methanol engine decrease not more than 3.5% when the compression ratio decreases from 16 to 14. Its brake thermal efficiency increases at low load and decreases at high load with the compression ratio decreases. Decreasing compression ratio can decrease obviously its maximum pressure rise rate and improve the brake thermal efficiency at low load. As a result, decreasing compression ratio from 16 to 14, brake thermal efficiency of methanol engine increases by 18% at low load, and brake thermal efficiency decreases by 6% and maximum pressure rise rate decreases up to 60% at high load and maximum torque engine speed. Moreover, HC and NOX emissions of methanol engine increase with an increase in the compression ratio, and vice verse on CO emission.

Three-dimensional spray–flow interaction in a spark-ignition direct-injection engine

Large efforts are currently being made toward improving internal combustion engine efficiency without degrading overall performance. To this end, advanced combustion strategies that require in-cylinder fuel/air mixtures to be prepared with unprecedented care and exactness are being implemented. Spray-guided stratified-charge operation is an example of one such strategy that offers high efficiency under light-load operation but requires precise fuel injector and combustion chamber design to ensure robust engine performance. Understanding the mixture formation processes aids in the success of such strategies and is therefore an important goal for internal combustion engine research. Direct visualization via optical diagnostics remains one of the most powerful tools for gaining insight into in-cylinder mixing processes, such as liquid fuel spray atomization. Although substantial efforts have been made over the last few decades to develop and implement liquid fuel spray diagnostics in the challenging in-cylinder environment, such techniques have been almost exclusively two-dimensional or too complex to be used regularly in design practices. Only now are practical, commercially available three-dimensional visualization technologies beginning to mature to a level for which they can be adapted to perform in-cylinder diagnostics. In this study, the three-dimensional fuel spray structure and its variations are visualized by a single plenoptic camera in an optically accessible spark-ignition direct-injection engine. The engine is operated under a variety of conditions relevant to conventional homogeneous-charge and spray-guided stratified-charge engine strategies. The spray and flow field interaction under different bulk flow conditions as well as engine speeds are clearly discernable in the instantaneous three-dimensional spray images, establishing the imaging technique as a viable and unique new tool for complex three-dimensional engine in-cylinder processes research.