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

By combining a clean fuel such as gasoline with a high efficiency thermodynamic cycle (compression ignition), it is possible to demonstrate a powertrain that is clean and efficient, thus breaking the historical trade-off between decreasing CO2 and reducing criteria pollutants. The gasoline compression ignition (GCI) engine is a promising technology that can be used to improve thermal efficiency while reducing emissions. Its low temperature combustion does however lead to several problems that need to be overcome. The present study relates to a method and system for combining the advantages of GCI engine technology into a hybrid electric vehicle (HEV) to maximize the benefits. A plausible path is to operate the GCI engine at conditions where the benefits of a GCI engine could be maximized and where an electric motor can supplement the conditions where the GCI is less beneficial. In this study, GCI engines with different cetane number (CN) fuels were selected, and a hybrid simulation tool was used to address the potential of the GCI engines into hybrid electric vehicles. Co-developments can demonstrate efficiency and emission solutions through the achievements of the study, which will address examples of the competitive powertrain and will introduce more than 30% of CO2 reduction vehicle by 2030. [ABSTRACT FROM AUTHOR]

Partially premixed combustion (PPC) often adopts the early fuel-injection strategy that could result in spray-wall interaction involved with piston top-land crevice. This interaction may produce a significant impact on engine combustion and unburned hydrocarbons (UHC) emission, which is still not well understood. In this study, we investigated the detailed spray-wall interaction and its effects on the two-stage ignition, i.e. low- and high-temperature heat release (LTHR and HTHR), and the in-cylinder spatial UHC distribution of PPC in a full-view optical engine at low engine load. The PRF 70 fuel was used as the gasoline surrogate. The high-speed imaging of the natural flame luminosity was acquired to quantify the flame probability distribution. The qualitative fuel-tracer, formaldehyde, and UHC planar laser-induced fluorescence (PLIF) imaging techniques were employed to reveal the fuel, LTHR and UHC distribution characteristics, respectively. The LTHR, HTHR and UHC distribution formed by the fuel trapped in the piston top-land crevice were visualized by PLIF imaging techniques for the first time. The PLIF results indicate that the main UHC formed in the PPC engine comes from the central part of the cylinder close to the injector nozzle, where the overall equivalence ratio is low and the injector dribbling is an important source of UHC. The UHC formed in the piston crevice of the PPC engine depends on the local equivalence ratio of the fuel trapped in the crevice. When the overall equivalence ratio of the charge in the crevice is relatively high, the trapped fuel undergoes both LTHR and HTHR and produces negligible UHC. However, the UHC from the piston crevice becomes considerable when the fuel injection timing is too early so that an overly lean mixture is generated. Based on the above findings, three implications of the PPC operation at low engine load for low UHC emission and high engine efficiency are proposed. [ABSTRACT FROM AUTHOR]

Partially premixed combustion (PPC) is an advanced combustion concept and powertrain technology, which has a great potential to improve the fuel economy of vehicles. The process of PPC is driven by both chemical kinetics and mixing process, and is, therefore, sensitive to inlet conditions and injection process. This article presents a control-oriented combustion and air-path model of a PPC multicylinder engine, and also proposes a model predictive control framework for its transient control. The control system was validated in a transient scenario and its capability was demonstrated through a large range of load transient operation. [ABSTRACT FROM AUTHOR]

By combining a clean fuel such as gasoline with a high efficiency thermodynamic cycle (compression ignition), it is possible to demonstrate a powertrain that is clean and efficient, thus breaking the historical trade-off between decreasing CO2 and reducing criteria pollutants. The gasoline compression ignition (GCI) engine is a promising technology that can be used to improve thermal efficiency while reducing emissions. Its low temperature combustion does however lead to several problems that need to be overcome. The present study relates to a method and system for combining the advantages of GCI engine technology into a hybrid electric vehicle (HEV) to maximize the benefits. A plausible path is to operate the GCI engine at conditions where the benefits of a GCI engine could be maximized and where an electric motor can supplement the conditions where the GCI is less beneficial. In this study, GCI engines with different cetane number (CN) fuels were selected, and a hybrid simulation tool was used to address the potential of the GCI engines into hybrid electric vehicles. Co-developments can demonstrate efficiency and emission solutions through the achievements of the study, which will address examples of the competitive powertrain and will introduce more than 30% of CO2 reduction vehicle by 2030.

Both numerical simulations and experiments were conducted in a heavy-duty DICI engine, with PRF81 as a gasoline surrogate, to investigate how the fuel stratification, auto-ignition and combustion are affected by the start of injection (SOI). The intake air temperature was adjusted to keep the combustion phasing constant when the SOI was swept from − 100 to − 20 o CA ATDC, covering different regimes of combustion, from HCCI to PPC. It is found that in the HCCI regime the combustion process is less sensitive to the variation of SOI since the fuel/air mixture is fairly homogeneous. The fuel/air mixture is under fuel-lean condition and the required intake temperature for a constant CA50 is the highest. In the PPC regime there is an optimal SOI window, within which the required intake temperature is the lowest to maintain a constant CA50 and the engine thermal efficiency is the highest. The optimal operation window starts at the SOI when all fuel is injected into the piston bowl and ends when the fuel injection is towards the bottom wall of the piston bowl, which results in a high heat transfer losses. The SOI window for optimal engine operation is expected to be fuel injector and piston bowl geometry dependent. During the transition regime, the fuel is injected towards the piston head in the squish region. The combustion process is highly sensitive to SOI due to the high sensitivity of fuel distribution in the cylinder to SOI. The engine thermal efficiency is the lowest due to the incomplete oxidation of fuel in the squish region. • Fuel stratification and combustion characteristics affected by the SOI and engine geometry were systematically studied. • The transition regime from HCCI to PPC and fuel stratification level during the SOI swept were defined. • The mechanisms behind the non-monotonic dependence of intake temperature on SOI were investigated. • The sensitivity of the in-cylinder wall temperature for the different combustion regime was evaluated. • For a given constant combustion phasing, an optimal SOI window was found. [ABSTRACT FROM AUTHOR]

Abstract Partially premixed combustion (PPC) and reactivity controlled compression ignition (RCCI) are two new combustion modes in compression-ignition (CI) engines. However, the detailed in-cylinder ignition and flame development process in these two CI modes were not clearly understood. In the present study, firstly, the fuel stratification, ignition and flame development in PPC and RCCI were comparatively studied on a light-duty optical engine using multiple optical diagnostic techniques. The overall fuel reactivity (PRF number) and concentration (fuel-air equivalence ratio) were kept at 70 and 0.77 for both modes, respectively. Iso-octane and n-heptane were separately used in the port-injection (PI) and direct-injection (DI) for RCCI, while PRF70 fuel was introduced through direct-injection (DI) for PPC. The DI timing for both modes was fixed at –25°CA ATDC. Secondly, the combustion characteristics of PPC and RCCI with more premixed charge were explored by increasing the PI mass fraction for RCCI and using the split DI strategy for PPC. In the first part, results show that RCCI has shorter ignition delay than PPC due to the fuel reactivity stratification. The natural flame luminosity, formaldehyde and OH PLIF images prove that the flame front propagation in the early stage of PPC can be seen, while there is no distinct flame front propagation in RCCI. In the second part, the higher premixed ratio results in more auto-ignition sites and faster combustion rate for PPC. However, the higher premixed ratio reduces the combustion rate in RCCI mode and the flame front propagation can be clearly seen, the flame speed of which is similar to that in spark ignition engines but lower than that in PPC. It can be concluded that the ratio of flame front propagation and auto-ignition in RCCI and PPC can be modulated by the control over the fuel stratification degree through different fuel-injection strategies. [ABSTRACT FROM AUTHOR]

The aim of this study is to analyze the relationship between the flame thicknesses and velocity based on thermodynamic three kernels, including spark, arc and jet plasma discharges in a constant volume combustion chamber (CVCC). As the research method, a CVCC designed to ensure 400 cm3 of internal volume were developed by the authors to analyze the combustion characteristics. The compositions of combustion system include the spark plugs (conventional spark, arc and jet types), pressure sensor, temperature sensor, oxygen sensor, control circuit, hardware and high-speed camera. A pressure sensor for measuring the progressive combustion uses a type of Kistler 601CBA00250. It is verified that the discharge level of a jet plasma model is higher by about 3.8 kV then an arc plasma model and the combustion time is also faster than other plasma types. Moreover, in the signal analysis, the charging time in a jet plasma model starts to detect the signal from −0.11 ms to 0 ms and the charging voltage height is raised up to 40 kV due to being affected by stored energy of a capacitor. Consequently, the combustion performances of the arc and jet plasma discharges are noticeable in that flame thickness and velocity characteristics could be affected by ionized kernels in a CVCC. As the air-C3H8 equivalence rates, the progressive combustion and reaction by the arc and jet plasma discharges are more improved than the conventional spark discharge in a rarefied condition. [ABSTRACT FROM AUTHOR]

The need for decarbonization has inspired the utilization of methanol in transportation industry. Via both experimental and modeling frameworks, this study, for the first time, reveals the ignition, heat release and flame development characteristics of methanol mixed with different cetane improvers, i.e., 2-ethylhexyl nitrate (EHN) and di-tertiary‑butyl peroxide (DTBP), under practical, advanced engine conditions. Particularly, a detailed chemical kinetic model is newly developed for methanol/EHN/DTBP mixtures, incorporating the most updated sub-chemistries for relevant fuel components and critical intermediates, to assist in further analysis on the effects of EHN and DTBP on ignition and emission formation under engine thermodynamic environments derived from recorded in-cylinder pressure histories. Results show that stable combustion of methanol/EHN mixtures can be realized under low direct injection (DI) pressure, high in-cylinder thermodynamic condition, featuring the lowest coefficient of variation of indicated mean effective pressure (COV IMEP) of 1.46 %. Throughout the combustion process, the yellow and white flame dominates. At the same DI timing, methanol/EHN mixtures present shorter ignition delay, faster heat release rate (HRR) and more advanced combustion phasing compared with methanol/DTBP mixtures. At similar combustion phasing, the peak flame/OH* natural luminosity intensity of methanol/EHN mixtures is higher. KL factor, which is utilized to evaluate soot concentration, also shows similar trend, indicating higher sooting tendency of methanol/EHN mixtures. Flux analysis further highlights higher production of soot precursor (i.e., C 2 H 2) with EHN than DTBP, primarily via the pathway of C 2 H 4 → C 2 H 3 → C 2 H 2. The active atmosphere created by the decomposition of EHN or DTBP has significant effects on decreasing methanol ignition delay in the early stage of their exothermic process, while the active atmosphere of EHN remains impactful whereas that of DTBP diminishes at later stages. The thermal atmosphere shows greater effects on methanol ignition at later stages of the exothermic process of EHN or DTBP decomposition than the early stages. [ABSTRACT FROM AUTHOR]

Highlights • Partially premixed combustion was applied on a multi-cylinder engine. • Different injection strategies should be adapted at different engine loads. • During stable operations, a peak gross indicated efficiency of 51% was achieved. • During transient operations, an average net indicated efficiency of 47.5% was demonstrated. Abstract Modern transportation requires advanced powertrain systems to reduce the production of greenhouse gas CO 2. Partially premixed combustion (PPC) is one of the most promising methods to achieve low emission and low fuel consumption of internal combustion engines. The present paper evaluated the effects of the calibration parameters on the efficiency and emissions of a multi-cylinder engine using PPC during stable operations, and the performance of the engine during transient operations. The peak gross indicated efficiency of 51.5% and the peak net indicated efficiency of 48.7% were achieved under stable operating conditions. The transient results also demonstrate an average net indicated efficiency of 47.5%. The nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbon (HC) conform with the Euro VI emission legislation in the case of most transient operations except some low load points. [ABSTRACT FROM AUTHOR]

The main driver to investigate low temperature combustion concepts, such as partially premixed combustion (PPC), is the promise of low particulate matter (PM) and nitric oxide (NOx) emissions. A critical prerequisite for PPC is to temporally isolate the fuel injection and combustion events. In practice, exhaust gas recirculation (EGR) is applied in order to sufficiently extend the ignition delay to that effect. Hereby, in general, higher EGR rates are necessary for fuels with higher cetane numbers (CN). Against this background, the objective of this paper is to investigate the efficacy, with respect to PM-NOx emissions and engine efficiency, of gasoline as a potential gas-to-liquid (GTL) CN suppressant in various dosages. The performance of the resulting GTLine blend will be evaluated under PPC operating conditions in a heavy-duty direct-injected diesel engine. Setting aside for a moment any potential practical issues (e.g., flash point, vapor pressure) that fall outside the scope of this study, our data suggest that blending gasoline to otherwise high CN GTL appears to be a promising route to improve not only the efficiency, but also PM and NOx emissions, particularly when operating in PPC mode. Interestingly, this benefit is notwithstanding the high aromaticity of the gasoline compared to GTL. Given the ongoing dieselization trend and associated surplus of gasoline in many regions, notably Europe, along with the fact that the cost price of gasoline is significantly lower than that of GTL, the proposed GTLine approach promises to be a cost effective way to accommodate GTL in a world wherein low temperature combustion concepts, such as PPC, appear to be really taking off. [ABSTRACT FROM AUTHOR]

Based on a single-cylinder engine modified from a heavy-duty compression ignition engine with gasoline compression ignition (GCI), the influences of (i) delaying the intake valve closing timing (IVCT), (ii) a two-pulse fuel injection strategy and (iii) the injection pressure on combustion and emissions under high-load conditions are studied. By delaying the IVCT, the effective compression ratio and cylinder temperature during the compression stroke can be reduced, which can increase the fuel ignition delay and reduce soot emission and the heat-transfer loss rate, contributing to expand the engine’s high-load limit for high efficiency and clean combustion. With IVCT delay at an inlet pressure of 3.0 bar and a gross indicated mean effective pressure (IMEPg) of 16.5 bar, the gross indicated thermal efficiency (ITEg) is 52.3%, which increases by 2.4% compared to the case without IVCT delay. Combining IVCT delay, two-pulse injection, and an appropriate injection pressure can reduce the fuel mixing time, which can decrease the ringing intensity (RI) and achieve clean combustion at high engine load. Using IVCT delay, an intake pressure of 3.4 bar, an exhaust-gas recirculation (EGR) rate of 47%, a pilot injection of 10% at (−70, −7), an injection pressure of 80 MPa, and an IMEPg of 15 bar, the ITEg is 53.1% and the emissions of soot and NOx are 0.0068 g/kWh and 0.21 g/kWh, respectively. [ABSTRACT FROM AUTHOR]

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

Gasoline partially premixed combustion (PPC) has been reported as an advancing concept for high efficient and clean combustion, while there still remains some obstacles. In this study, polyoxymethylene dimethyl ethers (PODE), which has superior properties of high oxygen content, high cetane number and no C C bond, is employed as an additive to optimize the properties of gasoline and solve the problems faced in gasoline PPC. The effects of gasoline/PODE blends with PODE volume blending ratio of 0, 10% and 20% on combustion and emission have been investigated from low to high loads in a multi-cylinder heavy-duty diesel engine. The experimental results show that the soot emission can be maximally reduced by 79% and 94% at high load when fueling PODE 10 and PODE20, respectively. The NOx-soot trade off relationship can be dramatically improved without penalty of fuel economy by fueling gasoline/PODE blends. The raw NOx and soot emissions can meet Euro V standards at high load and Euro VI standards at medium load by fueling both PODE10 and PODE20. Furthermore, the pressure rise rate can be maximally reduced by 24% and 47% when fueling PODE10 and PODE20. At medium load, the combustion controllability and sensitivity can be improved by gasoline/PODE blends. At low load, the COV IMEP can be reduced to 2.7% by fueling PODE20 which means a stable combustion can be obtained. Furthermore, the combustion efficiency can be dramatically improved from 85.1% to 99.6% by fueling PODE20, which leads to lower HC and CO emissions. Overall, PODE shows great advantage in solving the obstacles faced in gasoline PPC. [ABSTRACT FROM AUTHOR]

Gasoline compression ignition (GCI) engines have a great potential to achieve the simultaneous reduction of smoke and nitrogen oxides (NO x ) emissions via partially premixed combustion (PPC) using low cetane number fuel for the extended pre-combustion mixing time. The premixed combustion realised in high compression ratio engines also improves the engine efficiency with previous studies often reporting 50% brake efficiency or higher. The ability to control the combustion phasing by the fuel injection timing differentiates this new regime from widely investigated homogenous charge compression ignition (HCCI) or its variants making it a practical alternative to conventional gasoline or diesel combustion. In this study, ethanol produced from biomass has been selected as a GCI fuel, considering its higher octane number (i.e. lower cetane number), evaporative cooling and oxygen contents than gasoline, all of which could further improve the GCI combustion. The ethanol-fuelled GCI, or in short ECI, was investigated in a single-cylinder automotive-size diesel engine connected to an Eddy Current (EC) dynamometer. The focus is the engine start-up conditions and the influence of intake air temperature and common-rail pressure on ECI combustion. From the experiments, it is found that the engine can be successfully started by ECI combustion using a conventional start motor at low engine speed of 1000 rpm when the intake air temperature is higher than 60 °C. For higher engine speed of 2000 rpm and stable operations, however, a double injection strategy and increased intake air temperature of 80 °C are required suggesting the important role of wall wetting on ECI combustion. From the intake air temperature variations up to 100 °C, it is observed that both the peak in-cylinder pressure and heat release rate increase, leading to the improved engine efficiency. The measured engine-out emissions of unburnt hydrocarbon and carbon monoxide also show a decreasing trend with increasing intake air temperature, likely due to the reduced wall wetting. The smoke and NO x emissions of ECI combustion are much lower than those of a conventional diesel, regardless of the intake air temperature. The common-rail pressure variations at fixed brake power conditions show that the friction loss increases with increasing common-rail pressure, leading to the increased brake specific fuel consumption. This suggests that the common-rail pressure for ECI applications should remain low for the efficiency gain. The engine-out emissions also exhibit an increasing trend with increasing common-rail pressure although the smoke and NO x levels are always lower than that of a conventional diesel. Compared to the diesel reference case, the optimised engine operating conditions of this study achieves 50% higher fuel conversion efficiency, 5% lower brake specific fuel consumption and 27% lower NO x emissions while smoke emissions are kept at a negligible level. [ABSTRACT FROM AUTHOR]

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

Dual fuel pilot-ignited natural gas engines are identified as an efficient and viable alternative to conventional diesel engines. This paper examines cyclic combustion fluctuations in conventional dual fuel and in dual fuel partially premixed combustion (PPC). Conventional dual fueling with 95% (energy basis) natural gas (NG) substitution reduces NOx emissions by almost 90% relative to neat diesel operation; however, this is accompanied by 98% increase in HC emissions, 10 percentage points reduction in fuel conversion efficiency (FCE) and 12 percentage points increase in COVimep. Dual fuel PPC is achieved by appropriately timed injection of a small amount of diesel fuel (2-3% on an energy basis) to ignite a premixed natural gas-air mixture to attain very low NOx emissions (less than 0.2 g/kWh). Cyclic variations in both combustion modes were analyzed by observing the cyclic fluctuations in start of combustion (SOC), peak cylinder pressures (Pmax), combustion phasing (Ca50), and the separation between the diesel injection event and Ca50 (termed "relative combustion phasing"). For conventional dual fueling, as NG substitution increases, Pmax decreases, SOC and Ca50 are delayed, and cyclic variations increase. For dual fuel PPC, as diesel injection timing is advanced from 20 deg to 60 deg BTDC, Pmax is observed to increase and reach a maximum at 40 deg BTDC and then decrease with further pilot injection advance to 60 deg BTDC, the Ca50 is progressively phased closer to TDC with injection advance from 20 deg to 40 deg BTDC, and is then retarded away from TDC with further injection advance to 60 deg BTDC. For both combustion modes, cyclic variations were characterized by alternating slow and fast burn cycles, especially at high NG substitutions and advanced injection timings. Finally, heat release return maps were analyzed to demonstrate thermal management strategies as an effective tool to mitigate cyclic combustion variations, especially in dual fuel PPC. [ABSTRACT FROM AUTHOR]

In the present study, various low temperature combustion strategies were investigated using single cylinder engine experiments. The combustion strategies that were investigated premix the majority of the fuel and do not require exhaust gas recirculation (EGR) to achieve ultra-low NOx and soot emissions for low- to mid-load engine operation. These types of advanced compression ignition combustion strategies have been shown to have challenges with combustion phasing control. The focus of the study was to compare engine performance and emissions, combustion sensitivity to intake conditions, and the ability to control any observed sensitivity through the fuel injection strategy. Even though these are steady state engine experiments, this will demonstrate a given combustion strategies controllability on a cycle-to-cycle basis. The combustion strategies that were investigated are fully premixed dual-fuel homogeneous charge compression ignition (HCCI), dual-fuel reactivity controlled compression ignition (RCCI), and single-fuel partially premixed combustion (PPC). The baseline operating condition was an engine load representative of a light-duty engine: 5.5 bar gross indicated mean effective pressure (IMEP) and 1500 rev/min. At the baseline operating condition, in which the boundary conditions were chosen to yield near optimal engine performance, all three combustion strategies demonstrated high gross indicated efficiency (∼47%) and ultra-low NOx and soot emissions. By perturbing the intake conditions, it was found that all three combustion strategies display similar combustion phasing sensitivities. Both dual-fuel HCCI and RCCI were able to readily correct the observed sensitivities through the global fuel reactivity with no negative implications on the NOx emissions. However, single-fuel PPC was unable to correct for the observed combustion phasing sensitivity and, in some cases, had negative implications on the NOx emissions. [ABSTRACT FROM AUTHOR]

Highlights: [•] Gasoline/diesel can form desired blends properties to fit operating conditions. [•] Mixture reactivity and mixture stratification joint control the combustion. [•] The earlier diesel injection tends to increase the mixture reactivity in-cylinder. [•] The later diesel injection tends to increase the mixture stratification in-cylinder. [•] Gasoline/diesel stratification can expand high load range significantly. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of a compression愦灭;ndash;ignition 愦灭;lpar;CI愦灭;rpar; heavy-duty engine fuelled with diesel and gasoline-like fuels. A state-of-the-art engine computational fluid dynamics 愦灭;lpar;CFD愦灭;rpar; tool was used to explore the influences of the physical and chemical properties of diesel and gasoline-like fuels 愦灭;lpar;no.愦灭;hairsp;91 gasoline and E10愦灭;rpar; on spray development, auto-ignition and combustion processes, and pollutant formation. The CFD simulation results were found to be consistent with available experimental measurements, both qualitatively and quantitatively. The results indicate that gasoline-like fuels are able to achieve better premixed charge in CI engines owing to their higher volatility and lower ignitability compared to diesel fuel, which promotes efficient, clean, and low temperature combustion. However, the high combustion pressure rise rate becomes problematic under some circumstances, especially for high octane number fuels, such as E10. Both previous experimental measurements and the present numerical results show that gasoline-like fuels have great potential to be used in future CI engines, but the injection strategies and injection system have to be optimized based on the fuel properties. [ABSTRACT FROM AUTHOR]

• The coupling effect of TBC model and spray impingement model is considered. • TBC has potential to increase thermal efficiency by 5.5% for GCI and 5.1% for RCCI. • TBC help accelerate combustion process and improve emission in the near wall zone. Nowadays, improving the thermal efficiency of internal combustion engine (IC Engine) has becoming increasingly essential. In this paper, the influences of thermal barrier coating (TBC) on the near wall combustion and pollutant distribution in Gasoline/Diesel RCCI (Reactivity Controlled Compression Ignition) and GCI (Gasoline Compression Ignition) combustion are investigated. Mathematical model of TBC is applied in Kiva3v CFD code for the numerical investigation, and GCI experiments is conducted for the real engine tests. The result shows that for Gasoline/Diesel RCCI combustion, spray/wall impingement is observed resulting in fuel film which becomes the main source of near wall soot formation. While for GCI combustion, no film is found and most of the pollutants are concentrated in the bowl region. A thermal reorganization effect is found with TBC, which not only keeps higher near wall temperature for the chemical reaction, but also enlarges the bowl-center high temperature area. By applying TBC, the thermal efficiency can be improved for both combustion modes. Higher peak pressure and advanced combustion phase are also produced which accelerate the boundary combustion especially in the squish region, therefore both unburned products and soot can be reduced at the cost of increased NO x emission. [ABSTRACT FROM AUTHOR]

Low temperature combustion (LTC) of high-octane number fuels in compression ignition engines offers an opportunity to simultaneously achieve high engine thermal efficiency and low emissions of NO x and particulate matter without using expensive after-treatment technologies. LTC engines are known to be sensitive to the operation conditions and combustor geometry. It is important to understand the fundamental flow and combustion physics in order to develop the technology further for commercial application. A joint numerical and experimental investigation was conducted in a heavy-duty compression ignition engine using a primary reference fuel with an octane number of 81 to investigate the effects of injection timing, piston geometry, and compression ratio (CR) on the fuel/air mixing and combustion covering different regimes of LTC engines, homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and the transition regime from HCCI to PPC. The results show that with the same combustion timing, a higher CR leads to a lower NO x , but a higher emission of UHC and CO. The piston geometry shows a significant impact on the combustion and emission process in the transition regime while it has minor influence in the HCCI and PPC regimes. It is found that high engine efficiency and low emissions of NO x , CO and UHC can be achieved in the earlier PPC regime and later transition regime. The fundamental reason behind this is the stratification of the mixture in composition, temperature and reactivity, which is dictated by the interaction between the spray and the cylinder/piston walls. • Engine performance with different SOI, compression ratio, piston geometry is studied. • Three regimes (homogeneous charge compression ignition, partially premixed combustion and transition) during the SOI variation are investigated. • Piston bowl shows significant impact on the charge stratification in transition regime. • Compression ratio shows a tradeoff of NOx reduction with unburned hydrocarbons and carbon monoxide emissions. • Optimal SOI window range from -50 to -42 oCA with the stepped-lip piston is found. [ABSTRACT FROM AUTHOR]

• PPC engine's performance is evaluated with RON90 gasoline and alkylate gasoline. • The unoptimized PPC engine with RON90 fuel can operate between 5 and 18 bar IMEPg. • Similar brake efficiency (~41%) as in diesel operation but improved soot emissions. • Gasoline with ethanol has better PM emissions compared to fuel with no aromatics. Gasoline Compression Ignition research has shown that it can provide diesel like engine efficiency while maintaining gasoline like exhaust emissions. While research has shown that lower octane fuels might be good for that, they are also not available now. A good compromise for that would be the use of higher octane gasoline which is available in most places. For this study, experiments were performed under Partially Premixed Combustion (PPC) conditions with RON 90 gasoline, in a 2 L multi-cylinder diesel engine which complies with Euro 6 emissions standards. The aim is to evaluate the efficiency, the emissions and the achievable load in terms of minimum and maximum at three different speeds, 1200, 1800 2400 rpm and with two different RON 90 gasolines and the standard diesel engine hardware. The two fuels were a regular pump gasoline and an alkylate gasoline, which was chosen to represent a pathway to a renewable fuel. Results show that the minimum load is around 5 bar IMEPg, limited by high COV (coefficient of variation) values and high air intake temperature, while the maximum load reaches 18 bar IMEPg, limited by lambda value, pressure and mechanical limitations. While efficiency is similar between the two fuels with a brake value of around 40%, at higher loads the alkylate fuel produces higher amounts of soot while the regular gasoline has higher carbon monoxide at low loads. Finally, an energy balance comparison between the two gasolines and diesel is made, showing improved efficiency and soot emissions under PPC. [ABSTRACT FROM AUTHOR]

The injection strategy is commonly manipulated to control the stratification level to simultaneously achieve a low pollutant emission and a high efficiency in internal combustion engines. This paper reports on a joint experimental and numerical study of the emission characteristics and engine performance in a heavy-duty direct-injection compression ignition (DICI) engine operating in the HCCI and PPC regimes, with a primary reference fuel made up of iso-octane (81% by volume) and n-heptane (19%) as a model gasoline fuel. The injection timing was varied to achieve different levels of stratification of the charge in the cylinder while the intake air temperature was adjusted accordingly to keep the same combustion phasing at 3° crank angle (CA) after top dead centre (ATDC). The main results of the present study are: (1) In gasoline DICI engines, the fuel is injected into the cylinder at an earlier SOI. The combustion process can be divided into different regimes, HCCI, PPC, and transition from HCCI to PPC, depending on SOI. Two distinctive classes of in-cylinder combustion temperature distributions could be found from the simulation results for the studied engine: one was for the SOI range from - 100 to - 48 °CA ATDC, which was the HCCI regime and/or the transition from HCCI to PPC regime, where the mean effective in-cylinder temperature was lower than 1700 K. The second class was for the SOI range from - 44 to - 20 °CA ATDC, where the combustion temperature was higher than 1850 K. This corresponded to the PPC regime. (2) The NO x emission was not only affected by the mean temperature but also the distribution of temperature in the cylinder. (3) The main source of unburned hydrocarbon (UHC) emission in the HCCI and the transition regimes was the fuel trapped in the crevice region where the oxidation process could not function properly. (4) Carbon monoxide (CO) emissions showed a non-monotonic variation with the injection timing, with one peak forming in the bowl region and one forming in the squish region during the transition from HCCI to PPC. The main source of CO emission was in the low temperature and fuel-lean region in the cylinder. (5) In the present engine operation, the pressure rise rate (PRR) in the PPC regime was higher than that in the HCCI regime, which is contrary to most results reported in the literature. This is a combined effect of low equivalence ratio in the piston bowl and in the squish region, and the stratification of the ignition delay time in the mixture. (6) Highest thermodynamic efficiency was achieved in the PPC regime with SOI from - 44 to - 31 °CA ATDC. In the transition from HCCI to PPC regime, the thermodynamic efficiency reached its lowest due to the poor combustion efficiency. [ABSTRACT FROM AUTHOR]

• The development and characteristics of low temperature combustion are reviewed. • The various modes of achieving low temperature combustion are analyzed fuel wise. • The effects of various fuels on low temperature combustion are discussed. • Limitations and key factors of low temperature combustion engines are pointed out. • Vast experimental data on low temperature combustion for many fuels are tabulated. The fast rate of depletion of fossil fuel resources due to increasing demands and the adverse environmental impact by the automotive engines forced researchers to develop alternative strategies to meet the stringent emission norms in terms of oxides of nitrogen and particulate matter. In this regard, low temperature combustion is one of the promising advanced in-cylinder combustion strategies for reducing both oxides of nitrogen and particulate matter emissions simultaneously with a beneficial effect on specific fuel consumption. The low temperature combustion is achieved through homogeneous charge compression ignition, premixed charge compression ignition, reactivity controlled compression ignition and gasoline compression ignition. In this paper, an attempt is made to assemble and summarize a listing of important research articles on low-temperature combustion using a wide variety of conventional and alternate renewable fuels. The effect of low-temperature combustion on engine performance and emission characteristics over a wide range of engine test conditions and the challenges faced in these strategies are also described. From the assemblage of articles on low-temperature combustion using conventional and renewable fuels, it is understood that this strategy can help in achieving better performance, lower cylinder pressure and heat release rate, and simultaneous reductions of nitrogen oxides and particulate matter, but typically with an increase in carbon monoxide and hydrocarbon emissions. This literature review is expected to be useful to the researchers for understanding the concept, challenges, and the state-of-the-art of the different modes of low-temperature combustion using conventional and sustainable alternate fuels. [ABSTRACT FROM AUTHOR]