Mixing-controlled Combustion Research Articles

Using ducted fuel injection to attenuate or prevent soot formation in mixing-controlled combustion strategies for engine applications

Ducted fuel injection is a strategy that can be used to enhance the fuel/charge-gas mixing within the combustion chamber of a direct-injection compression-ignition engine. The concept involves injecting the fuel through a small tube within the combustion chamber to make the most fuel-rich regions of the micture in the autoignition zone leaner relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). This study is a follow-on to initial proof-of-concept experiments that also were conducted in a constant-volume combustion vessel. While the initial natural luminosity imaging experiments demonstrated that ducted fuel injection lowers soot incandescence dramatically, this study adds a more quantitative diffuse back-illumination diagnostic to measure soot mass, as well as investigates the effects on performance of varying duct geometry (axial gap, length, diameter, and inlet and outlet shapes), ambient density, and charge-gas dilution level. The result is that ducted fuel injection is further proven to be effective at lowering soot by 35–100% across a wide range of operating conditions and geometries, and guidance is offered on geometric parameters that are most important for improving performance and facilitating packaging for engine applications.

A Computational Investigation of Fuel Chemical and Physical Properties Effects on Gasoline Compression Ignition in a Heavy-Duty Diesel Engine

Gasoline compression ignition (GCI) offers the potential to reduce criteria pollutants while achieving high fuel efficiency. This study aims to investigate the fuel chemical and physical properties effects on GCI operation in a heavy-duty diesel engine through closed-cycle, three-dimensional (3D) computational fluid dynamic (CFD) combustion simulations, investigating both mixing-controlled combustion (MCC) at 18.9 compression ratio (CR) and partially premixed combustion (PPC) at 17.3 CR. For this work, fuel chemical properties were studied in terms of the primary reference fuel (PRF) number (0–91) and the octane sensitivity (0–6) while using a fixed fuel physical surrogate. For the fuel physical properties effects investigation, six physical properties were individually perturbed, varying from the gasoline to the diesel range. Combustion simulations were carried out at 1375 RPM and 10 bar brake specific mean pressure (BMEP). Reducing fuel reactivity was found to influence ignition delay time (IDT) more significantly for PPC than for MCC. 0D IDT calculations suggested that the fuel reactivity impact on IDT diminished with an increase in temperature. Moreover, higher reactivity gasolines exhibited stronger negative coefficient (NTC) behavior and their IDTs showed less sensitivity to temperature change. In addition, increasing octane sensitivity was observed to result in higher fuel reactivity and shorter IDT. Under both MCC and PPC, all six physical properties showed little impact on global combustion behavior, NOx, and fuel efficiency. Among the physical properties investigated, only density showed a notable effect on soot emissions. Increasing density led to higher soot due to deteriorated air entrainment into the spray and the slower fuel-air mixing process.

Performance and emissions of a compression-ignition direct-injected natural gas engine with shielded glow plug ignition assist

Performance and emissions experiments were conducted on a compression-ignition direct-injected natural gas engine (DING) equipped with a shielded glow plug ignition assist system. Tests were conducted at three different intake pressures (34.5 kPag, 68.9 kPag, 103.4 kPag), and four nominal (targeted) equivalence ratios (0.2, 0.3, 0.4, 0.5). CH4, NOx, CO and PM emissions were measured and analyzed, showing that emissions levels are influenced by the DING engine’s combustion modes and intake pressure. Premixed combustion which dominates at low equivalence ratios resulted in higher levels of CH4 and NOx emissions, while mixing-controlled combustion, which develops at higher equivalence ratios, resulted in elevated CO and PM levels. Higher intake pressure was found to improve all emissions levels. The most significant effect was the reduction of PM and CO emissions due to improved fuel charge mixing and air entrainment that results from a pressure-driven momentum increase of the engine’s air swirl field. Brake specific emissions and fuel consumption were estimated and compared against levels reported in the literature for dual-fuel port-injection, and High-Pressure Direct-Injection (HPDI) natural gas engines. The most significant finding was that the DING engine exhibits lower fuel consumption and PM emissions levels when compared to values reported in the literature for HPDI engines. The PM emissions advantage was attributed to a higher proportion of premixed combustion and the absence of a diesel pilot in DING engine operation. Lastly, PM size distributions were analyzed, showing that the DING engine produces PM that is smaller than PM of a conventional diesel engine, but similar to the PM reported in the literature for HPDI engines.

Effect of Fuel Reactivity on Ignitability and Combustion Phasing in a Heavy-Duty Engine Simulation for Mixing-Controlled and Partially Premixed Combustion

Light-end fuels have recently garnered interest as potential fuel for advanced compression ignition (CI) engines. This next generation of engines, which aim to combine the high efficiency of diesel engines with the relative simplicity of gasoline engines, may allow engine manufacturers to continue improving efficiency and reducing emissions without a large increase in engine and aftertreatment system complexity. In this work, a 1D heavy-duty engine model was validated with measured data and then used to generate boundary conditions for the detailed chemical kinetic simulation corresponding to various combustion modes and operating points. Using these boundary conditions, homogeneous simulations were conducted for 242 fuels with research octane number (RON) from 40 to 100 and sensitivity (S) from 0 to 12. Combustion phasing (CA50) was most dependent on RON and less dependent on S under all conditions. Both RON and S had a greater effect on combustion phasing under partially premixed compression ignition (PPCI) conditions (19.3 deg) than under mixing-controlled combustion (MCC) conditions (5.8 deg). The effect of RON and S were also greatest for the lowest reactivity (RON > 90) fuels and under low-load conditions. The results for CA50 reflect the relative ignition delay for the various fuels at the start-of-injection (SOI) temperature. At higher SOI temperatures (>950K), CA50 was found to be less dependent on fuel sensitivity due to the convergence of ignition delay behavior of different fuels in the high-temperature region. Combustion of light-end fuels in CI engines can be an important opportunity for regulators, consumers, and engine-makers alike. However, selection of the right fuel specifications will be critical in development of the combustion strategy. This work, therefore, provides a first look at quantifying the effect of light-end fuel chemistry on advanced CI engine combustion across the entire light-end fuel reactivity space and provides a comparison of the trends for different combustion modes.

Modeling flame extinction and reignition in large eddy simulations with fast chemistry

This work seeks to support the validation of large eddy simulation models used to simulate fire suppression. The emphasis in the present study is on the prediction of flame extinction and the prevention of spurious reignition using a fast chemistry, mixing-controlled combustion model applicable to realistic fire scenarios of engineering interest. The configuration provides a buoyant, turbulent methane diffusion flame within a controlled co-flowing oxidizer. The oxidizer allows for the supply of a mixture of air and nitrogen, including conditions for which oxygen-dilution in the oxidizer leads to flame extinction. Measurements to support model validation include local profiles of thermocouple temperature and oxygen mole fraction, global combustion efficiency, and the limiting oxygen index. The present study evaluates the performance of critical flame temperature based extinction and reignition models using the Fire Dynamics Simulator, an open-source fire dynamics solver. Alternate model cases are explored, each offering a unique treatment of extinction and reignition. Comparisons between simulated results and experimental measurements are used to evaluate the capability of these models to accurately describe flame extinction. Of the considered cases, those that include provisions to prevent spurious reignition show excellent agreement with measured data, whereas a baseline case lacking explicit reignition treatment fails to predict extinction.

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.

Development and Validation of a Quasi-Dimensional Dual Fuel (Diesel – Natural Gas) Combustion Model

This paper presents a newly developed quasi- dimensional multi-zone dual fuel combustion model, which has been integrated within the commercial engine system simulation framework. Model is based on the modified Multi-Zone Combustion Model and Fractal Combustion Model. Modified Multi-Zone Combustion Model handles the part of the combustion process that is governed by the mixing-controlled combustion, while the modified Fractal Combustion Model handles the part that is governed by the flame propagation through the combustion chamber. The developed quasi-dimensional dual fuel combustion model features phenomenological description of spray processes, i.e. liquid spray break-up, fresh charge entrainment, droplet heat-up and evaporation process. In order to capture the chemical effects on the ignition delay, special ignition delay table has been made. Additionally, to capture the effect of entrained methane on the chemical reaction rate, special table that features chemical reaction time scale has also been made. The start of flame propagation is calculated through the newly developed sub- model based on knock integral calculation. The existing k-e turbulence model has been extended to account for the effect of diesel pilot injection on the increase of in-cylinder turbulence level. Since in the conventional dual fuel combustion process multiple flames propagate through the combustion chamber, multiple flame propagation model has been developed. This model can describe arbitrary number of flames that propagate through the combustion chamber. The validation data for the developed quasi-dimensional dual fuel combustion model have been acquired on a 2.0 liter Diesel engine, which has been modified to operate in the conventional dual fuel combustion mode. The presented model has been validated at different loads and diesel substitution ratios, and there is a good fit between the measured and cycle-simulation data.

CFD-Guided Heavy Duty Mixing-Controlled Combustion System Optimization with a Gasoline-Like Fuel

CFD-Guided Heavy Duty Mixing-Controlled Combustion System Optimization with a Gasoline-Like Fuel

Numerical investigation on the combined effects of varying piston bowl geometries and ramp injection rate-shapes on the combustion characteristics of a kerosene-diesel fueled direct injection compression ignition engine

In this work, the combustion characteristics of a direct injection compression ignition (DICI) engine fueled with kerosene-diesel blends, using different piston bowl geometries together with varying injection rate-shapes were investigated. A total of three combustion bowl geometries, namely the omega combustion chamber (OCC), the shallow-depth combustion chamber (SCC) and the shallow-depth re-entrant combustion chamber (SRCC), were used together with six different ramp injection rate-shapes and pure diesel, kerosene-diesel and pure kerosene fuels. It is seen that the SRCC geometry, which has the shortest throat length, gives the highest turbulence kinetic energy (TKE) and this resulted in two peak heat-releases, with a primary peak heat-release during the premixed combustion phase and a secondary peak heat-release during the mixing-controlled combustion phase. In addition, the SCC geometry gives rather distinct premixed combustion and mixing-controlled combustion phases due to the fact that combustion is predominantly controlled by the injected fuel spray itself because of less turbulence. Also, when kerosene is used in place of diesel, the heat-release during the premixed combustion phase increases and diminishes during the mixing-controlled and late combustion phases. It is interesting to note that the effect of injection rate-shaping on the heat-release rate is more obvious for bowl geometries that generate less TKE. Moreover, bowl geometries that generate higher TKEs as well as fuels with lower viscosities generally give lower carbon monoxide (CO) emissions and higher nitrogen oxide (NO) emissions. More importantly, it is possible to achieve low NO and CO emissions simultaneously by using the appropriate bowl geometry, injection rate-shape and fuel, although a slight decrease in power is inevitable.

An Evaluation of Combustion and Emissions Performance With Low Cetane Naphtha Fuels in a Multicylinder Heavy-Duty Diesel Engine

Future projections in global transportation fuel use show a demand shift toward diesel and away from gasoline. At the same time, greenhouse gas regulations will drive higher vehicle fuel efficiency and lower well-to-wheel CO2 production. Naphtha, a contributor to the gasoline stream and requiring less processing at the refinery level, is an attractive candidate to mitigate this demand shift while lowering the overall greenhouse gas impact. This study investigates the combustion and emissions performance of two naphtha fuels (Naphtha 1: RON59 and Naphtha 2: RON69) and one ultra-low sulfur diesel (ULSD) in a model year (MY) 2013, six-cylinder, heavy-duty diesel engine. Engine testing was focused on the heavy-duty supplemental emissions test (SET) “B” speed over a load sweep from 5 to 15 bar BMEP (brake-specific mean pressure). At each operating point, NOx sweeps were conducted over wide ranges. At 10–15 bar BMEP, mixing-controlled combustion dominates the engine combustion process. Under a compression ratio of 18.9, cylinder pressure and temperature at these load conditions are sufficiently high to suppress the reactivity difference between ULSD and the two naphtha fuels. As a result, the three test fuels showed similar ignition delay (ID). Nevertheless, naphtha fuels still exhibited notable soot reduction compared to ULSD. Under mixing-controlled combustion, this is likely due to their lower aromatic content and higher volatility. At 10 bar BMEP, Naphtha 1 generated less soot than Naphtha 2 since it contains less aromatics and is more volatile. When operated at light load, in a less reactive thermal environment, the lower reactivity naphtha fuels lead to longer IDs than ULSD. As a result, the soot benefit of naphtha fuels was enhanced. Utilizing the soot benefit of the naphtha fuels, engine-out NOx was calibrated from the production level of 3–4 g/hp-hr down to 2–2.5 g/hp-hr over the 12 nonidle SET steady-state modes. At this reduced NOx level, naphtha fuels were still able to maintain a soot advantage over ULSD and remain “soot-free” while achieving diesel-equivalent fuel efficiency. Finally, low-temperature combustion (LTC) operation (NOx ≤ 0.2 g/hp-hr and smoke ≤ 0.2 FSN) was achieved with both of the naphtha fuels at 5 bar BMEP through a late injection approach with high injection pressure. Under high exhaust gas recirculation (EGR) dilution, Naphtha 2 showed an appreciably longer ID than Naphtha 1, resulting in a soot reduction benefit.

Leaner Lifted-Flame Combustion Enabled by the Use of an Oxygenated Fuel in an Optical CI Engine

Leaner lifted-flame combustion (LLFC) is a mixing-controlled combustion strategy for compression-ignition engines that does not produce soot because the equivalence ratio at the lift-off length, Φ(H), is less than or equal to approximately two. In addition to preventing soot formation, LLFC can simultaneously control emissions of nitrogen oxides because it is tolerant to the use of exhaust-gas recirculation for lowering in-cylinder temperatures. LLFC can be achieved through the use of oxygenated fuels and enhanced fuel/charge-gas mixing upstream of the lift-off length. Enhanced mixing can be obtained via higher injection pressures, smaller injector orifice diameters, lower intake-manifold and coolant temperatures, and/or more retarded injection timings. This study focuses on the effects of an oxygenated fuel blend (T50) made up of 50% by volume tri-propylene glycol mono-methyl ether with a #2 ULSD emissions-certification fuel (CFA), compared against baseline testing of the CFA fuel without the oxygenate. Experimental measurements include crank-angle resolved natural luminosity (NL) and chemiluminescence (CL) imaging diagnostics. EGR is simulated by adding nitrogen and carbon dioxide to the intake charge to produce a 16% intake-oxygen mole fraction (XO2), and results are compared against cases with no charge dilution (i.e., 21% XO2). Initial experiments with a two-hole tip achieved soot-free LLFCmore » at low loads with the T50 fuel, 240 MPa injection pressure, 50 °C intake-manifold temperature (IMT), 95 °C coolant temperature, and -5 crank-angle degree (CAD) after top-dead-center (ATDC) start of combustion (SOC) timing. The strategy was extended to more moderate loads by employing a 6-hole injector tip, where lowering the IMT to 30 °C, reducing the coolant temperature to 85 °C, and retarding the SOC timing to +5 CAD ATDC resulted in sustained LLFC at both 21% and 16% XO2 at up to 6.2 bar gross indicated mean effective pressure (gIMEP) with T50. The achievement of LLFC was confirmed by independent soot measurements using a smoke meter, spatially integrated natural luminosity from the NL diagnostics, and laser-induced incandescence for measuring soot volume fraction in the exhaust stream. In contrast to the results with T50, LLFC was not achieved under any of the test conditions with CFA. Combustion was stable under LLFC conditions, with a coefficient of variation of gIMEP less than 1.5%. Nitrogen oxides and hydrocarbon emissions were also lowered by up to 25% each for LLFC with T50 relative to using the same operating conditions with CFA. Combustion noise was similarly reduced by 4-6 dBA, and ringing intensity by 60-80%, for LLFC with T50.« less

One-dimensional model of heat-recovery, non-recovery coke ovens. Part III: Upper-oven, down-comers and sole-flues

This paper is Part III of the series (Buczynski et al., 2016) concerning the mathematical modeling of HR/NR coke ovens and it formulates one-dimensional models describing combustion and heat transfer in the space above coking-bed (upper-oven), in down-comers and in sole-flues. The space above a coking-bed is modeled using the well-stirred reactor concept while other oven branches are modeled using the long plug-flow furnace concept. The numerical simulations indicate a slow mixing-controlled combustion of gaseous mixtures in the down-comers and in the sole-flue channels. Such a behavior is typical for industrial coke ovens. Thermal radiation is the main mechanism of heat transfer and it accounts for up to 90% of the overall heat transfer rate. The mathematical model calculates also the refractory temperatures and in some sole-flue branches they approach too high values which may render oven damages.

Modeling study on the effect of piston bowl geometries in a gasoline/biodiesel fueled RCCI engine at high speed

This paper reports the numerical investigation on the effects of three bowl geometries on a gasoline/biodiesel fueled RCCI engine operated at high engine speed. The three bowl geometries are HCC (Hemispherical Combustion Chamber), SCC (Shallow depth Combustion Chamber) and OCC (Omega Combustion Chamber). To simulate the combustion in an RCCI engine, coupled KIVA4–CHEMKIN code was used. One recently developed reaction mechanism, which contains 107 species and 425 reactions, was adopted in this study to mimic the combustion of gasoline and biodiesel. During the simulation, the engine speed was fixed at 3600rpm. The low reactivity fuel gasoline was premixed with air with energy percentages of 20% and 40%; accordingly, to maintain the same energy input, the percentages of biodiesel were 80% and 60% (B80 and B60). In addition, the SOI timing was varied at three levels: −11, −35 and −60degATDC for B80 and B60, respectively. With SOI timing of −11degATDC, the combustion is mixing-controlled; in contrast, advancing SOI timing to −60degATDC, the combustion turns into the reactivity-controlled. Comparing the results on combustion characteristics, engine performance and emissions among different bowl geometries, it is concluded that the original OCC design for Toyota diesel engine is better for mixing-controlled combustion; whereas, SCC is the most suitable piston design for RCCI combustion among the three selected geometries under the investigated operating conditions of the engine. With SCC, better combustion and performance can be achieved while maintaining relatively lower CO, NO and soot emissions.

Investigation of a tripropylene-glycol monomethyl ether and diesel blend for soot-free combustion in an optical direct-injection diesel engine

Natural luminosity and chemiluminescence imaging diagnostics were employed to investigate if a 50/50 blend by volume of tripropylene-glycol monomethyl ether (TPGME) and ultra-low sulfur #2 diesel certification fuel (CF) could enable leaner-lifted flame combustion (LLFC), a non-sooting mode of mixing-controlled combustion associated with equivalence ratios below approximately 2. The experiments were performed in a single-cylinder heavy-duty optical compression–ignition engine at three injection pressures and three dilution levels. Results indicate that TPGME addition effectively eliminated engine-out smoke emissions by curtailing soot production and/or increasing soot oxidation during and after the end of fuel injection. TPGME greatly reduced soot luminosity when compared with neat CF, but did not enable LLFC because the equivalence ratios at the lift-off length, ϕ(H), never reached the non-sooting limit and incandescence from hot soot within the combustion chambered remained visible. Concerning other engine-out emissions, injection pressure influenced the effects of TPGME addition on NOx emissions. HC and CO emissions were higher compared to the baseline fuel, likely due to the lower net heat of combustion of TPGME and the need to limit fuel-injection duration for valid optical measurements.

Invited Review: Combustion instability in spray-guided stratified-charge engines: A review

This article reviews systematic research on combustion instabilities (principally rare, random misfires and partial burns) in spray-guided stratified-charge (SGSC) engines operated at part load with highly stratified fuel -air -residual mixtures. Results from high-speed optical imaging diagnostics and numerical simulation provide a conceptual framework and quantify the sensitivity of ignition and flame propagation to strong, cyclically varying temporal and spatial gradients in the flow field and in the fuel -air -residual distribution. For SGSC engines using multi-hole injectors, spark stretching and locally rich ignition are beneficial. Combustion instability is dominated by convective flow fluctuations that impede motion of the spark or flame kernel toward the bulk of the fuel, coupled with low flame speeds due to locally lean mixtures surrounding the kernel. In SGSC engines using outwardly opening piezo-electric injectors, ignition and early flame growth are strongly influenced by the spray's characteristic recirculation vortex. For both injection systems, the spray and the intake/compression-generated flow field influence each other. Factors underlying the benefits of multi-pulse injection are identified. Unresolved questions include (1) the extent to which piezo-SGSC misfires are caused by failure to form a flame kernel rather than by flame-kernel extinction (as in multi-hole SGSC engines); (2) the relative contributions of partially premixed flame propagation and mixing-controlled combustion under the exceptionally late-injection conditions that permit SGSC operation on E85-like fuels with very low NOx and soot emissions; and (3) the effects of flow-field variability on later combustion, where fuel-air-residual mixing within the piston bowl becomes important.

A computationally efficient combustion trajectory prediction model developed for real-time diesel combustion control

The heterogeneous nature of diesel combustion adds many complexities that make understanding the combustion process difficult. Many researchers have made great efforts in diagnostics, prediction, and control capabilities. In this work, a computationally efficient thermodynamic-based model (15 ms on 2010 dual core processor) has been created that predicts the combustion trajectory (path through the ϕ–T plane) with the goal of bridging the gap between typical off-line engine prediction simulations and on-line real-time engine control strategies. The ϕ–T plane is often used to help illustrate the soot and NOx formation behavior during diesel combustion. The experimental engine operating conditions shown illustrate how exhaust gas recirculation influences the combustion trajectory at different timings—that is, showing the typical soot–NOx trade-off for diesel engines and the defeat of this trade-off when low-temperature combustion is obtained. The major insight gained is that the low-temperature combustion trajectory looks similar to a conventional one with just subtle differences that keep it from moving into the soot formation region. Additionally, the traditional conceptual explanations for diesel combustion are explored relative to how they are illustrated by the combustion trajectory, especially the transition from premixed to mixing-controlled combustion. Understanding that behavior in this context aids in explaining the different observations for the low-temperature combustion modes. The fact that these observations are made using this simplified modeling approach is promising for future use of this type of thermodynamic-based models in real-time engine control.

Investigation of Methyl Decanoate Combustion in an Optical Direct-Injection Diesel Engine

An optically accessible heavy-duty diesel engine was used to investigate the impact of methyl decanoate (MD) on combustion and emissions. Specific goals of the study were to produce experimental data for validating engine combustion models using MD (a biodiesel surrogate), as well as to determine if MD could enable soot-free leaner-lifted flame combustion (LLFC), a mode of mixing-controlled combustion associated with equivalence ratios below approximately 2. An ultralow sulfur diesel certification fuel (CF) was used as the baseline fuel, and experiments were conducted at two fuel-injection pressures with three levels of charge-gas dilution; start of combustion and duration of fuel injection were held constant. In addition to conventional pressure-based and engine-out emissions measurements, exhaust laser-induced incandescence, in-cylinder natural luminosity, and in-cylinder chemiluminescence diagnostics were used to provide detailed insight into combustion processes. Results indicate that MD effectively eliminated soot emissions but that soot formation still occurred in-cylinder, with equivalence ratios at the flame lift-off length in excess of approximately 3. Nevertheless, the oxygen content of MD sufficiently limited soot formation and promoted soot oxidation such that very little soot remained at exhaust-valve open. Nitrogen oxides (NOx) emissions for MD relative to CF showed different trends depending on fuel-injection pressure, with distinct fuel effects influencing NOx formation depending on engine operating condition. Hydrocarbon (HC) and CO emissions were higher for MD compared to CF and corresponded to lower fuel-conversion and combustion efficiencies. These differences were attributed to the lower-load conditions of MD, resulting from its lower energy density and the need to limit fuel-injection duration to obtain valid lift-off length measurements.

Influence of charge density and oxygen concentration on combustion paths, thermal efficiency and emissions in a heavy-duty diesel engine

Experiments and simulations were conducted to study effects of charge density, temperature, and oxygen concentration on the mixing-controlled engine combustion pathway in heavy-duty diesel engines. Due to the inherent heterogeneity of diesel combustion in high-load operations, the rich and lean mixtures are simultaneous present. The mass and accompanying heat transfers were found to be decisive in determining the combustion path. The chemical transformation from a richer mixture to a leaner mixture is primarily driven by charge density, which activates the combustion process, and reduction in oxygen concentration, which stagnates the mass and heat transfer and chemical transformation, reduces the reactivity of the mixtures. The difference in mass and heat transfer processes causes differences in the mass fractions of mixtures with different equivalence ratio intervals. The different mixtures produce different mass fractions of intermediate combustion products (carbon dioxide, CO), different heat releases, and different mass temperature distributions. It is found that the accumulated CO correlates well with the gross indicated thermal efficiency and soot emission; the mass averaged temperature and the high temperature abidance scale (HTAS) correlate well with NOx emissions. A significant optimization of the overall engine performance could be achieved by simultaneously minimizing the HTAS and accumulated CO.

Evaluation of massive exhaust gas recirculation and Miller cycle strategies for mixing-controlled low temperature combustion in a heavy duty diesel engine

The future of compression ignition engines depends on their ability for keeping their competitiveness in terms of fuel consumption compared to spark-ignition engines. In this competitive framework, the Low Temperature Combustion (LTC) concept is a promising alternative to decrease NOx and soot emissions. Thus, this research focuses on implementing the LTC concept, but keeping the conventional mixing-controlled combustion process to overcome the well-known drawbacks of the highly-premixed combustion concepts, including load limitations and lack of combustion control.Two strategies for implementing the mixing-controlled LTC concept were evaluated. The first strategy relies on decreasing the intake oxygen concentration introducing high rates of cooled EGR (exhaust gas recirculation). The second strategy consists of decreasing the compression temperature by advancing the intake valves closing angle to reduce the effective compression ratio, compensating the air mass losses by increasing boost pressure (Miller cycle). These strategies were tested in a single-cylinder heavy-duty research engine. Additionally, 3D-CFD modeling was used to give insight into local in-cylinder conditions during the injection-combustion process.Results confirm the suitability of both strategies for reducing NOx and soot emissions, while their main drawback is the increment in fuel consumption. However, they present intrinsic differences in terms of local equivalence ratios and temperatures along combustion.

Insights into the hydrocarbon and carbon monoxide emissions in moderately and highly dilute Low Temperature Combustion

Low-temperature combustion in diesel engines offers attractive benefits through simultaneous reduction in the nitrogen oxide emissions and the soot emissions. However, it is known that the in-cylinder conditions typical of low-temperature combustion operation tend to produce high emissions of unburned hydrocarbons and carbon monoxide, reducing the combustion efficiency. The present study develops from the hypothesis that this characteristic poor combustion efficiency is due to the in-cylinder mixture preparation strategies which are non-optimally matched to the requirements of the low-temperature combustion mode. In this work, the effects of three key fuel path parameters, namely the injection fuel quantity ratio, the dwell and the injection timing, on the carbon monoxide and hydrocarbon emissions were examined using a central-composite-design design-of-experiments method. The experiments were performed on a single-cylinder diesel research engine operating in a mixing-controlled low-temperature combustion mode at high and moderate exhaust gas recirculation rates with a split fuel injection for all conditions. The experiments identified the potential of fuel metering control for optimising the hydrocarbon emissions in low-temperature combustion by showing the effects of the fuel control parameters on the fuel mixing quality and the emission formation mechanisms. The response surfaces created from the detailed statistical analysis give a potent visualisation of the constraints on low-temperature combustion operation. This in turn allowed improved prescription of combustion modifications with the potential to moderate the negative effects observed.