Engine-out Soot Emissions Research Articles

Characterization of Hydrotreated Vegetable Oil (HVO) in a Euro 6 Diesel Engine as a Drop-In Fuel and With a Dedicated Calibration

Renewable fuels can play an important role in achieving future goals of energy sustainability and CO2 reduction. In particular, hydrotreated vegetable oil (HVO) represents one of the most promising alternatives to petroleum-derived diesel fuels. Several studies have shown that conventional diesel engines can run on 100% HVO without significant modifications to the hardware and control strategies. The current activity has experimentally evaluated the potential of HVO as a “drop-in” fuel, i.e., without changes to the original baseline calibration, comparing it to conventional diesel fuel on a 2.3-litre Euro 6 compression ignition engine.Tests revealed that HVO can significantly reduce engine-out soot (by more than 60%), HC and CO emissions (by about 40%), compared to diesel, while NOx levels and fuel conversion efficiency remain relatively unchanged under steady-state warmed-up conditions. The advantages of HVO proved to be further enhanced when the engine has not yet warmed up.Using statistical techniques of design of experiments (DoE) at three warmed-up steady-state operating points, the main engine control parameters were recalibrated to demonstrate that engine-out emissions can be further optimized with a dedicated calibration.

Exploiting the potential of large eddy simulations (LES) for ducted fuel injection investigation in non-reacting conditions

The diesel combustion research is increasingly focused on ducted fuel injection (DFI), a promising concept to abate engine-out soot emissions in compression-ignition engines. A large set of experiments carried out in constant volume vessel and numerical simulations, at medium-low computational cost, showed that the duct adoption in front of the injector nozzle activates several soot mitigation mechanisms, leading to quasi-zero soot formation in several engine-like operating conditions. However, although the simplified CFD modelling so far played a crucial role for the preliminary understanding of DFI technology, a more accurate turbulence description approach, combined with a large set of numerical experiments for statistical purposes, is of paramount importance for a robust knowledge of the DFI physical behaviour.In this context, the present work exploits the potential of large eddy simulations (LES) to analyse the non-reacting spray of DFI configuration compared with the unconstrained spray. For this purpose, a previously developed spray model, calibrated and validated in the RANS framework against an extensive amount of experimental data related to both free spray and DFI, has been employed. The tests have been carried out considering a single-hole injector in an optical accessible constant volume vessel, properly replicated in the simulation environment. This high-fidelity simulation model has been adapted for LES, firstly selecting the best grid settings, and then carrying out several numerical experiments for both spray configurations until achieving a satisfying statistical convergence. With this aim, the number of independent samples for the averaging procedure has been increased exploiting the axial symmetry characteristics of the present case study.Thanks to this approach, a detailed description of the main DFI-enabled soot mitigation mechanisms has been achieved, shrinking the knowledge gap in the physical understanding of the impact of spray-duct interaction.

Innovative Model-Free Onboard Diagnostics for Diesel Particulate Filter

<div>Recent legislations require very low soot emissions downstream of the particulate filter in diesel vehicles. It will be difficult to meet the new more stringent OBD requirements with standard diagnostic methods based on differential sensors. The use of inexpensive and reliable soot sensors has become the focus of several academic and industrial works over the past decade. In this context, several diagnostic strategies have been developed to detect DPF malfunction based on the soot sensor loading time. This work proposes an advanced online diagnostic method based on soot sensor signal projection. The proposed method is model-free and exclusively uses soot sensor signal without the need for subsystem models or to estimate engine-out soot emissions. It provides a comprehensive and efficient filter monitoring scheme with light calibration efforts. The proposed diagnostic algorithm has been tested on an experimentally validated simulation platform. 2D signatures are generated from soot sensor signal for nominal and faulty configurations. Gaussian dispersions on soot estimator (30%) and sensor model (15%) have been considered. Based on a statistical analysis, a relevant threshold is defined satisfying a compromise between non-detection and false alarm rates. The selected threshold is then used for online DPF diagnostic using NEDC cycle. The obtained results are promising and clearly show the performance of the proposed method in terms of non-detection and false alarm rates. The resulting diagnostic scheme can be easily integrated in the ECU for onboard DPF monitoring.</div>

Parametric evaluation of ducted fuel injection in an optically accessible mixing-controlled compression-ignition engine with two- and four-duct assemblies

Ducted fuel injection (DFI) is a strategy to improve fuel/charge-gas mixing in direct-injection compression-ignition engines. DFI involves injecting fuel along the axis of a small tube in the combustion chamber, which promotes the formation of locally leaner mixtures in the autoignition zone relative to conventional diesel combustion. Previous work has demonstrated that DFI is effective at curtailing engine-out soot emissions across a wide range of operating conditions. This study extends previous investigations, presenting engine-out emissions and efficiency trends between ducted two-orifice and ducted four-orifice injector tip configurations. For each configuration, parameters investigated include injection pressure, injection duration, intake manifold pressure, intake manifold temperature, start of combustion timing, and intake-oxygen mole fraction. For both configurations and across all parameters, DFI reduced engine-out soot emissions compared to conventional diesel combustion, with little effect on other emissions and engine efficiency. Emissions trends for both configurations were qualitatively the same across the parameters investigated. The four-duct configuration had higher thermal efficiency and indicated-specific engine-out nitrogen oxide emissions but lower indicated-specific engine-out hydrocarbon and carbon monoxide emissions than the two-duct assembly. Both configurations achieved indicated-specific engine-out emissions for both soot and nitrogen oxides that comply with current on- and off-road heavy-duty regulations in the United States without exhaust-gas aftertreatment at an intake-oxygen mole fraction of 12%. High-speed in-cylinder imaging of natural soot luminosity shows that some conditions include a second soot-production phase late in the cycle. The probability of these late-cycle events is sensitive to both the number of ducted sprays and the operating conditions.

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.

Strategies for Reduced Engine-Out HC, CO, and NOx Emissions in Diesel-Natural Gas and POMDME-Natural Gas Dual-Fuel Engine

<div class="section abstract"><div class="htmlview paragraph">Dual-fuel engines employ precisely metered amounts of a high reactivity fuel (HRF) such as diesel at high injection pressures to burn a low reactivity fuel (LRF) such as natural gas, which is typically fumigated into the intake manifold. Dual fuel engines have demonstrated the ability to achieve extremely low engine-out oxides of nitrogen (NOx) emissions compared to conventional diesel combustion at the expense of unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. At low engine loads, due to low in-cylinder temperatures, oxidation of HC and CO is very challenging. This results in both compromised combustion and fuel conversion efficiencies. The experimental campaign discussed in this paper involved a set of six engine control parameters that were strategically varied to find the best possible efficiency-emissions trade-offs for both diesel- and poly-oxy methylene dimethyl ether (POMDME)-natural gas dual fuel combustion on the University of Alabama single-cylinder research engine (SCRE) based on a PACCAR MX-11 heavy-duty engine platform. The control parameters investigated include: ((1)) Start of Injection (SOI1) of HRF, ((2)) percentage energy substitution (PES) of LRF, ((3)) introduction of the second HRF injection (SOI2), ((4)) split ratio, i.e., ratio of the duration of the first injection to the duration of the second injection, ((5)) rail pressure, and (6) intake pressure. At a fixed gross indicated mean effective pressure (IMEPg) of 5 bar (representative of typical low load operation) and an engine speed of 1339 rpm (“B speed” of the SCRE), SOI1 was varied to determine the lowest engine-out NOx point. Using that SOI1 as baseline and not-to-exceed (NTE) limits of: ((1)) Indicated specific NOx < 1 g/kWh, ((2)) maximum pressure rise rate (MPRR) < 10 bar/deg, and ((3)) coefficient of variation (COV) of IMEPg < 10%, the rest of the five control parameters were systematically varied. Based on HC and CO vs NOx emissions trade-offs, the best PES, SOI2, and split ratio were determined to achieve the lowest possible HC emissions. Subsequently, rail pressure sweeps showed minimal impact on performance and emissions between 500 bar and 1500 bar. Finally, reducing the intake pressure significantly reduced CO emissions to achieve the best overall set of operating parameters. Compared to the baseline diesel-natural gas case, HC and CO were reduced by ~88% and ~82%, respectively, in addition to ~21% improvement in the indicated fuel conversion efficiency. Whereas for POMDME-natural gas dual fuel combustion, HC and CO were reduced by ~85% and ~92%, respectively, in addition to ~20% improvement in the indicated fuel conversion efficiency. Furthermore, due to the high oxygen content of POMDME (47% m/m), engine-out soot emissions were reduced to zero measurable filter smoke number (FSN) for all conditions investigated.</div></div>

An experimental investigation into the soot particle emissions at early injection timings in a single-cylinder research diesel engine

Engine experiments were performed to examine the effect of early-injection on smoke emissions and engine performance under typical low temperature combustion (LTC) and reactivity controlled compression ignition (RCCI) operating conditions. On that basis, the influence of exhaust gas recirculation (EGR), intake pressure, fuel properties and fuel injection strategy on the engine-out soot emissions at early injection timings was also assessed. At last, the morphology, nanostructure characteristics of exhaust soot particles were analyzed. The test results showed when the injection timing was advanced before −30° crank angle after top dead center (CA ATDC) and earlier, the fuel spray impinged onto the piston crown surface, and a part of fuel entered into the squish zone and expanded to reach the cylinder liner wall and entered into the crevice volume, leading to a continuous increase in soot emissions, the peak smoke emission was found to occur at −55° CA ATDC. By adopting fuels with high volatilities and a RCCI fuel delivery strategy, soot emissions can be effectively reduced at early timings. Most of the high-resolution transmission electron microscopy (HRTEM) images of the sampled soot particles showed small spherical particles with a wide range of size distribution connected, and “onion-like” fringe arrangement was found in most of the images. As the fuel injection timing was retarded from −55° to −35° CA ATDC, the samples exhibited more highly organized carbon nanostructures; the fringe tortuosity and separation distance increased, while the fringe length decreased. The formation of more ordered arrangement of graphene layers led to significant resistance to oxidation.

Toward co-optimization of renewable fuel blend production and combustion in ultra-high efficiency SI engines

The shift from fossil to renewable fuels presents an opportunity to tailor a fuel’s molecular structure and composition to the needs of advanced internal combustion engine concepts, while simultaneously aiming for economic and sustainable fuel production. We have recently proposed a method for computer-aided design of tailor-made fuels that integrates aspects of both product and production pathway design. The present paper sets out to sequentially combine that method with experimental investigation on a single cylinder research engine and model-based early-stage process evaluation to create, validate, and benchmark a rationally designed multi-component biofuel for highly boosted spark-ignition engines. To this end, the computer-aided design approach is applied to a network of possible fuel components and their production pathways. The resulting optimal four-component fuel EBCC (50 mol% ethanol, 21 mol% 2-butanone, 15 mol% cyclopentane, and 14 mol% cyclopentanone) is analyzed with regard to combustion performance and estimated fuel production cost. Variations of both the indicated mean effective pressure and the relative air/fuel ratio were performed on an engine equipped with a compression ratio of 14.7. EBCC achieves indicated efficiencies that are significantly higher than those of RON 102 gasoline fuel and comparable to those of pure 2-butanone, an extremely knock-resistant fuel identified in a previous round of model-based fuel design. Furthermore, a strong reduction in engine-out soot emissions is observed compared to RON 102 gasoline. Early-stage process evaluation shows EBCC to have lower estimated fuel production costs than 2-butanone. Production costs of pure ethanol, however, are estimated to be even lower, mainly due to lower plant investment costs and a synthesis pathway that does not require hydrogen. The paper concludes with a brief perspective on further integration of the proposed sequential approach with the goal of co-optimizing the production and combustion of renewable fuel blends.

Solid particulate mass and number from ducted fuel injection in an optically accessible diesel engine in skip-fired operation

Ducted fuel injection (DFI) is a novel combustion strategy that has been shown to significantly attenuate soot formation in diesel engines. While previous studies have used optical diagnostics and optical filter smoke number methods to show that DFI reduces in-cylinder soot formation and engine-out soot emissions, respectively, this is the first study to measure solid particle number (PN) emissions in addition to particle mass (PM). Furthermore, this study quantitatively evaluates the use of transient particle instruments for measuring particles from skip-fired operation in an optical single cylinder research engine (SCRE). Engine-out PN was measured using an engine exhaust particle sizer following a catalytic stripper, and PM was measured using a photoacoustic analyzer. The study improves on earlier preliminary emissions studies by clearly showing that DFI reduces overall PM by 76%–79% and PN for particles larger than 23 nm by 77% relative to conventional diesel combustion at a 1200-rpm, 13.3-bar gross indicated mean effective pressure operating condition. The degree of engine-out PM reduction with DFI was similar across both particulate measurement instruments used in the work. Through the use of bimodal distribution fitting, DFI was also shown to reduce the geometric mean diameter of accumulation mode particles by 26%, similar to the effects of increased injection pressure in conventional diesel combustion systems. This work clearly shows the significant solid particulate matter reductions enabled by DFI while also demonstrating that engine-out PN can be accurately measured from an optical SCRE operating in a skip-fired mode. Based on these results, it is believed that DFI has the potential to enable fuel savings when implemented in multi-cylinder engines, both by lowering the required frequency of active diesel particulate filter regeneration, and by reducing the backpressure imposed by exhaust filtration systems.

Exhaust valve profile modulation for improved diesel engine curb idle aftertreatment thermal management

Rapid warm-up of a diesel engine aftertreatment system (ATS) is a challenge at low loads. Modulating exhaust manifold pressure (EMP) to increase engine pumping work, fuel consumption, and as a result, engine-outlet temperature, is a commonly used technique for ATS thermal management at low loads. This paper introduces exhaust valve profile modulation as a technique to increase engine-outlet temperature for ATS thermal management, without requiring modulation of exhaust manifold pressure. Experimental steady state results at 800 RPM/1.3 bar BMEP (curb idle) demonstrate that early exhaust valve opening with negative valve overlap (EEVO+NVO) can achieve engine-outlet temperature in excess of 255°C with 5.7% lower fuel consumption, 12% lower engine out NOx and 20% lower engine-out soot than the conventional thermal management strategy. Late exhaust valve opening with internal EGR via reinduction (LEVO+Reinduction) resulted in engine-outlet temperature in excess of 280°C, while meeting emission constraints at no fuel consumption penalty. This work also demonstrates that LEVO in conjunction with modulation of exhaust manifold pressure results in engine-outlet temperature in excess of 340°C while satisfying desired emission constraints. Aggressive use of LEVO can result in engine-outlet temperatures of 460°C, capable of active regeneration of DPF at curb idle, without the significant increase in engine-out soot emissions seen in previously studied strategies.

Effects of fuel oxygenation and ducted fuel injection on the performance of a mixing-controlled compression-ignition optical engine with a two-orifice fuel injector

This paper describes results from an optical-engine investigation of oxygenated fuel effects on ducted fuel injection (DFI) relative to conventional diesel combustion (CDC). Three fuels were tested: a baseline, non-oxygenated No. 2 emissions certification diesel (denoted CFB), and two blends containing potential renewable oxygenates. The first oxygenated blend contained 25 vol% methyl decanoate in CFB (denoted MD25), and the second contained 25 vol% tri-propylene glycol mono-methyl ether in CFB (denoted T25). Whereas DFI and fuel oxygenation primarily curtail soot emissions, intake-oxygen mole fractions of 21% and 16% were employed to explore the potential additional beneficial impact of dilution on engine-out emissions of nitrogen oxides (NOx). It was found that DFI with an oxygenated fuel can attenuate soot incandescence by ~100X (~10X from DFI and an additional ~10X from fuel oxygenation) relative to CDC with conventional diesel fuel, regardless of dilution level and without large effects on other emissions or efficiency. This breaks the soot/NOx trade-off with dilution, enabling simultaneous reductions in both soot and NOx emissions, even with conventional diesel fuel. Significant cyclic variability in soot incandescence for both CDC and DFI suggests that additional improvements in engine-out soot emissions may be possible via improved control of in-cylinder mixture formation and evolution.

Assessment of a complete truck operating under dual-mode dual-fuel combustion in real life applications: Performance and emissions analysis

The dual-mode dual-fuel (DMDF) strategy has been demonstrated to be a potential combustion mode to cover all the engine map with low-to-moderate NOx and soot emissions and high efficiency simultaneously. This can be accomplished by modifying the injection strategy to promote a fully premixed or a dual-fuel diffusive combustion depending on the operating conditions. The main limitation of the DMDF are the high concentrations of unburned hydrocarbons and carbon monoxide coupled with low exhaust temperatures, which can be a challenge for the stock diesel oxidation catalyst (DOC). Moreover, the use of a diffusive combustion combined with high EGR rates to avoid mechanical issues at high load enhances the soot formation, which can compromise the final soot levels in a homologation cycle. To evaluate these aspects, this work studies the performance and emissions of a DMDF truck concept along a WHVC and different in-service conformity cycles through vehicle systems simulations. For both types of cycles, five payloads were tested (0%, 25%, 50%, 75% and 100%) to evaluate the impact of this parameter on the operating points distribution inside the DMDF map. The first results show that the DMDF concept provides engine-out NOx levels below the EUVI regulation at normative payload (50%) with similar fuel consumption than the conventional diesel truck. On the other hand, the engine-out HC and CO emissions exceed their respective limits in all the cases, while the engine-out soot emissions only reach the EUVI levels up to 25% payload. By this reason, the stock DOC and diesel particulate filter from the conventional diesel truck were modelled and fitted to the DMDF truck model. The results evidenced that the use of these two ATS allows to achieve the EUVI limits in terms of tailpipe HC, CO and soot independently on the cycle and payload analyzed. Moreover, considering the tailpipe emissions values achieved with ATS at 50% payload, it can be inferred that both devices could be downsized for the DMDF application as compared to the conventional ATS for diesel applications.

Effects of inversed-delta injection rate on late combustion and soot emissions from diesel combustion in a constant-volume chamber

The aim of this study is to clarify the effect of an inversed-delta injection rate, which is a ramp-down injection rate achieved with a tandem injector, on the late combustion period, which affects thermal efficiency and soot emissions. Diesel combustion was simulated in a constant-volume chamber combined with a rapid compression machine. Two types of injection rate profile, namely rectangular and ramp-down, were tested in our experimental trials. To eliminate the significant effect of the maximum injection rate on mixture formation, we compared the results for the two types of injection rate profile under a given rail pressure. The time histories of the heat release rate demonstrated that the ramp-down fuel injection profile shortened the late combustion period by 0.2 ms. As a result, combustion was maintained despite the longer injection duration. The non-dimensional fuel consumption rate, defined in terms of the heat release rate and the fuel injection rate, revealed that high fuel consumption in the injection period reduced the quantity of fuel carried over to the late combustion period, resulting in a short duration of late combustion. When the ramp-down injection profile was employed, the temporal variation of total soot (spatially integrated KL factor) exhibited a rapid decrease after the end of injection. This is due to the abundant oxygen in soot regions that originated from the significant quantity of ambient air entrained into the spray during the injection period. This result indicates that ramp-down injection may reduce engine-out soot emissions.

Experimental study of the combustion and emission characteristics of oxygenated fuels on a heavy-duty diesel engine

Bio-derived alcohol fuels and oxygenated aromatic hydrocarbons are interesting alternative fuels applicable in low temperature combustion concepts due to their low reactivity. In this paper, three oxygenated fuels are mixed with n-heptane respectively as binary fuel blends to investigate combustion and emission characteristics of the oxygenated fuels with different molecular structures. The n-butanol/n-heptane blend has an identical stoichiometric air/fuel ratio as benzaldehyde/n-heptane blend while anisole is used to assess the importance of the aromatic structure. Standard diesel is also used as a baseline fuel. Experiments are performed on a single-cylinder heavy-duty diesel engine at medium and high loads. It is found that the n-butanol/n-heptane blend presents a higher premixed burn fraction and a higher pressure rise rate at both loads. Increasing injection pressure leads to lower soot emissions and higher NOx emissions are found regardless of fuel type. The n-butanol/n-heptane blend achieves 0.027 g/kWh and 0.2 g/kWh for engine-out soot and NOx emissions with high exhaust gas recirculation rate at high load. For the benzaldehyde/n-heptane blend and diesel, the classical NOx/soot tradeoff is observed under the same operating conditions. The particle size distribution mainly shows accumulation mode particles increase at a high exhaust gas recirculation rate, irrespective of the fuel structure. Only the n-butanol/n-heptane blend emits more nucleation mode particles and much less accumulation mode particles compared to the benzaldehyde/n-heptane and anisole/n-heptane blends. Clearly, the fuel structure influences the soot formation and is therefore not fully determined by the stoichiometric air/fuel ratio of the base fuel.

Optical Diagnostics Investigation on the Effect of Fuel Injection Timing on Partially Premixed Combustion Stratification and Soot Formation in a Single-Cylinder Optical Compression Ignition Engine

<div class="section abstract"><div class="htmlview paragraph">The present work investigates the effect of fuel injection timing on combustion stratification and soot formation in an optically accessible, single cylinder light duty diesel engine. The engine operated under low load and low engine speed conditions, employing a single injection scheme. The conducted experiments considered three different injection timings, which promoted Partially Premixed Combustion (PPC) operation. The fuel quantity of the main injection was adjusted to maintain the same Indicated Mean Effective Pressure (IMEP) value among all cases considered. Findings were analysed via means of pressure trace and apparent heat transfer rate (AHTR) analyses, as well as a series of optical diagnostics techniques, namely flame natural luminosity, CH<sup>*</sup> and C<sub>2</sub><sup>*</sup> chemiluminescence high-speed imaging, as well as planar Laser Induced Incandescence (pLII). For the chemiluminescence high-speed imaging a novel methodology was developed to acquire the net spatially resolved chemiluminescence signal, using combustion spectroscopy to remove the background chemiluminescence and thermal radiation signal. The analysis suggests that when the injection timing advanced, the stratification of the combustion became weaker, as indicated by the natural luminosity signal, due to the better pre-mixing between the injected fuel and the surrounding air. The enhanced premixing resulted in lower levels of in-cylinder soot late in the combustion cycle and, presumably the lowest engine-out soot emissions, because of the limited formation of locally fuel-rich areas inside the cylinder. This came at the price of increased fuel consumption.</div></div>

Refinement of the two-color pyrometry method for application in a direct injection diesel and natural gas compression-ignition engine

The soot emissions from internal combustion engines have significant health and environmental impacts and, as such, are subject to increasingly stringent regulations. Two-color pyrometry provides the in-cylinder soot cloud temperature and soot volume fraction and can provide insight to the in-cylinder soot formation and oxidation processes to guide research for reducing engine-out soot emissions. This work demonstrates improvements to the two-color pyrometry methodology, with a focus on low-temperature, low-soot regimes such as low-temperature combustion or combustion of direct injected natural gas. Through selection of a fast and robust numerical algorithm, characterizing and increasing the detection envelope, performing static and dynamic perspective adjustments, accounting for non-uniform and non-linear system response, as well as localized signal-to-noise ratio enhancement through image filtering, the performance of the pyrometric method was improved by a 40% increase in the resolved signal fraction. The refined two-color method was evaluated for both direct injected diesel and natural gas fueling strategies using a pilot-ignited direct injected natural gas fuel system and facilitated evaluation of local temperatures and soot concentrations in pilot-ignited direct injected natural gas combustion, despite the generally low soot levels in this combustion strategy.

Study of the characteristics of PM and the correlation of soot and smoke opacity on the diesel methanol dual fuel engine

Methanol is regarded as an alternative fuel of diesel for reducing the particulate matter emissions on diesel engines. However, recent studies reveal that methanol has the different effects on particulate matter characteristics at high loads. In order to reduce particulate matter emissions further within the full operating range of the engine, the characteristics of particulate matter and the correlation between soot and smoke opacity were investigated on a turbocharged and intercooled diesel engine under diesel/methanol dual fuel mode. During the experiment, particulate characteristics are primarily exhibited by the characteristics of soot and particulate number. Results show that the engine-out soot and particulate number emissions increase with the increase of methanol substitution percent under high intake temperature at high loads. On the contrary, the engine-out soot and particulate number decrease with the increase of methanol substitution percent under low intake temperature at high loads. There is an applicable scope of methanol substitution percent that effectively reduces soot and particulate number emissions. Methanol substitution percent which is larger than 20% has a significant effect on the decrease of soot and particulate number. At low and medium loads, the effect of methanol substitution percent on soot and particulate number emissions is dependent on the start of injection. The effect of methanol substitution percent on the decrease of particles strengthens as the start of injection is away from the top dead center. The inflection point of particulate number and soot emissions moves forward with the increased methanol substitution percent. Compared to pure diesel mode, the strong correlation between soot and smoke opacity does not exist on diesel/methanol dual fuel mode. However, the correlation between soot and smoke opacity comes back in usage of the diesel oxidation catalyst or exhaust gas recirculation on diesel/methanol dual fuel mode. Most of nitrogen dioxide and unburned hydrocarbons are reduced by the diesel oxidation catalyst on diesel/methanol dual fuel mode.

Exploitation of injection fusion strategies in diesel engines equipped with solenoid injectors

Pilot–main injection strategies are investigated in the short dwell-time range at medium load and speed conditions for a low-temperature combustion diesel engine endowed with indirect-acting solenoid injectors. Dwell-time sweeps have been carried out for pilot injections with different energizing times under steady-state working conditions and constant combustion phasing. The experimental results on the engine are combined with in-cylinder analyses of pressure, heat release rate, temperature and emission time histories. As the electric dwell time is reduced toward zero, combustion noise reaches a minimum at a dwell time of 120–140 µs, and a reduction of 1–4.5 dB is possible compared to levels at dwell time = 500 µs. This decrease in combustion noise is obtained in conjunction with a local reduction in engine-out nitrogen oxide emissions, but penalties occur for engine-out soot emissions and brake-specific fuel consumption. The pilot and main injection shots feature fusion phenomena at the shortest dwell-time values, thus giving rise to rate-shaped single injections, which resemble the boot-shaped profiles in direct-acting piezoelectric injectors. The increase in the velocity of the needle, which occurs during the nozzle opening phase pertaining to the main injection for closely coupled injections as dwell time is reduced, is shown to be not the primary cause responsible for the observed trends in combustion noise and soot emissions as dwell-time changes. The combination of simulation and experimental data supports the theory that the main injection interacts with the pilot mixture field: the relative phasing between pilot and main combustion and the in-cylinder equivalent ratio distribution are the dominant aspects in the definition of the engine performance in the short dwell-time range.

Swirl ratio and post injection strategies to improve late cycle diffusion combustion in a light-duty diesel engine

Nitrogen oxides (NOx) and soot emissions are the most important pollutants from direct-injection diesel engines. In particular, soot formation and oxidation determine the net engine-out soot emissions. These phenomena are complex and competing processes during diesel combustion. Despite many researches implicate the mechanisms of soot formation with soot emissions, the enhancement of the late cycle soot oxidation is the dominant mechanism for a reduction of engine-out soot emissions. The mixing process and the in-cylinder bulk temperature are two important parameters in the development of soot oxidation process. The current research compares different engine strategies to enhance the late cycle mixing controlled combustion process and therefore enhance soot oxidation while maintaining similar gross indicated efficiency in a light-duty engine. For this purpose, a simplified methodology has been used, which analyzes the effect of mixing process and in-cylinder bulk gas temperature on soot oxidation during the late cycle combustion. For carrying out this research, theoretical and experimental tools were used. In particular, the experimental measurements were made in a single-cylinder direct-injection light-duty diesel engine varying the swirl ratio and the injection pattern as injection pressure, Start of Energizing (SoE), Energizing Time (ET) and number of injections events. To analyze soot emissions, the combustion luminosity was measured by an optoelectronic probe and the optical thickness parameter (KL) was evaluated by the two-color pyrometry method. The apparent combustion time (ACT-1) was used as mixing time tracer. Results show that an increase in swirl ratio implies an improvement on the mixing process and higher values of average bulk temperature during the late-cycle diffusion combustion. Both phenomena produce an enhancement in the soot oxidation process. In the lowest swirl ratio case, a suitable injection strategy based on multiple injections, provides similar results of soot oxidation process (and therefore, the emissions) as high swirl ratio case.

A Comparison of Methyl Decanoate and Tripropylene Glycol Monomethyl Ether for Soot-Free Combustion in an Optical Direct-Injection Diesel Engine

Oxygenated fuels have beneficial effects for leaner lifted-flame combustion (LLFC), a nonsooting mode of mixing-controlled combustion associated with lift-off length equivalence ratios below approximately 2. A single-cylinder heavy-duty optical compression-ignition engine was used to compare neat methyl decanoate (MD) and T50, a 50/50 blend by volume of tripropylene glycol monomethyl ether (TPGME) and #2 ultralow sulfur emissions-certification diesel fuel (CF). High-speed, simultaneous imaging of natural luminosity (NL) and chemiluminescence (CL) were employed to investigate the ignition, combustion, and soot formation/oxidation processes at two injection pressures and three dilution levels. Additional Mie scattering measurements observed fuel-property effects on the liquid length of the injected spray. Results indicate that both MD and T50 effectively eliminated engine-out smoke emissions by decreasing soot formation and increasing soot oxidation during and after the end of fuel injection. MD further reduced soot emissions by 50–90% compared with T50, because TPGME could not completely compensate for the aromatics in the CF. Despite the low engine-out soot emissions, both fuels produced in-cylinder soot because the equivalence ratio at the lift-off length never reached the nonsooting limit. With respect to the other engine-out emissions, T50 had up to 16% higher nitrogen oxides (NOx) emissions compared with MD, but neither fuel showed the traditional soot-NOx trade-off associated with conventional mixing-controlled combustion. In addition, T50 had up to 15% and 26% lower unburned hydrocarbons (HC) and CO emissions, respectively, compared with MD.