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• A numerical study of gasoline/diesel DDFS combustion at high loads is conducted. • DDFS can control HRR while maintain comparable thermal efficiency at high loads. • The near-TDC injection of fuel with low reactivity is the key parameter for DDFS. • The effect of injection strategy on DDFS combustion at high loads is highlighted. High-load extension is one of the biggest challenges for the low-temperature premixed combustion strategies such as partially premixed combustion (PPC) and reactivity-controlled compression ignition (RCCI). Alternatively, a different dual-fuel combustion strategy, direct dual fuel stratification (DDFS) mode, which combines the benefits of PPC and RCCI by injecting gasoline and diesel into cylinder directly, is investigated in the present study. A comprehensive numerical comparison of PPC, RCCI and DDFS was conducted, and the effects of fuel property of the near top dead center (TDC) injection as well as different injection parameters on the combustion and emission characteristics of DDFS at a typical high load were revealed. The results indicated that DDFS can effectively control the combustion rate while maintain the thermal efficiency comparable to PPC and RCCI at high loads. For DDFS combustion, the near-TDC injection is the key parameter to control the heat release rate. However, soot emissions remarkably increase owing to the diffusion-combustion nature of the near-TDC injection, and adopting lower reactivity fuel such as methanol is a good approach to further reduce the peak pressure rise rate (PPRR) and suppress soot emissions. In addition, the diesel fraction plays a dominant role in controlling combustion phasing while the gasoline fraction serves as a more important role in combustion duration, and the effects of the start of injection of gasoline and diesel on the DDFS performance become more obvious as the gasoline and diesel fractions increase. From the perspective of load extension, a lower diesel fraction should be employed to avoid excessive PPRR, however, the diesel fraction and the start of injection should be coupled properly to avoid misfire. [ABSTRACT FROM AUTHOR]

• Effect of fuel concentration stratification on engine performance is systematically explored. • The PIDI strategy could achieve much better fuel economy and lower combustion noise compared to the 2DI strategy. • As load increases, the effects of different injection strategies on the optimal cases are weakened. • The PIDI strategy shows more dependence on the initial temperature expect for the NO x emissions. In the present study, a numerical investigation of the effect of fuel concentration stratification on engine performance for the combined port- and direct-injection (PIDI) strategy and two direct-injection (2DI) strategy under low-to-medium load conditions was performed by coupling the KIVA-3V code with the genetic algorithm. In addition, the sensitivity of operating parameters on combustion characteristics for different injection strategies was also explored. The results indicated that the PIDI strategy could achieve much better fuel economy and lower combustion noise while with a stronger trade-off relationship between them compared to the 2DI strategy. At low loads, the required initial temperature and EGR rate for the optimal cases of the PIDI strategy are overall lower than those of the 2DI strategy. Moreover, the ranges of the required initial pressure and injection pressure for the PIDI strategy are wider than those of the 2DI strategy. As load increases, the effects of different injection strategies on the optimal cases are weakened. For both strategies, initial temperature plays a dominant role in determining the combustion phasing and engine performance. However, the effects of injection parameters, i.e., injection timing, pilot fraction (2DI strategy) and premixed ratio (PIDI strategy), are dependent on the load. In addition, compared to the 2DI strategy, the combustion phasing, fuel economy and combustion noise of the PIDI strategy show more dependence on the initial temperature, while the NO x emissions are less sensitive to the initial temperature. [ABSTRACT FROM AUTHOR]

Gasoline compression ignition (GCI) engines have received more and more attention owing to their high thermal efficiency and low harmful emissions. However, GCI engines fueled by pure gasoline with low reactivity are limited to poor combustion stability at low loads and high pressure rise rate at high loads. To this end, a kind of second-generation hydrogenated catalytic biodiesel (HCB) from waste cooking oil with high reactivity is blended into the China 95#gasoline with different volume ratios, and the effect of blended ratio on the combustion and emission characteristics of a heavy-duty diesel engine was explored in the present study. The results indicate that ignition performance is significantly improved as the increase in HCB proportion, maximum combustion pressure can be effectively suppressed, and the combustion stability under low load conditions is much enhanced. Furthermore, HCB blended ratio should match combustion phasing controlled by the start of injection (SOI) to obtain better engine performance and wider operation range. In terms of emissions, the gas emissions of nitrogen oxides, carbon monoxide, and unburned hydrocarbon can be significantly reduced with increasing HCB fraction, however, the particulate matter emissions are increased slightly as a penalty. • The performances of GCI fueled by gasoline-hydrogenated catalytic biodiesel are explored experimentally. • Combustion stability at low load is significantly improved as the HCB fraction increases. • HCB blended ratio should match SOI to obtain better engine performance and wider operation range. • NOx, CO and HC emissions are remarkably reduced with the HCB proportion increasing [ABSTRACT FROM AUTHOR]

Highlights • Present DPCS model can well reproduce the physical and chemical behavior of SME. • Predicted liquid penetrations agree well with measurements under wide conditions. • Single-component model overestimates peak heat release rate more than 50%. • DPCS model reproduces ISFC and emissions better than single-component model. Abstract A decoupling physical–chemical surrogate (DPCS) model was established for simulation of the spray and combustion characteristics of multi-component biodiesel fuels. In the DPCS model, the physical and chemical properties of biodiesel fuels are described separately. For the case study of soybean methyl ester (SME), the physical properties are represented based on the five primary components, i.e., methyl palmitate, methyl stearate, methyl oleate, methyl linoleate, and methyl linoleate. Meanwhile, the chemical kinetics of SME are described by a skeletal reaction mechanism composed of methyl decanoate, methyl 5-decenoate, and n -decane. Furthermore, an improved quasi-dimensional multi-component vaporization model was applied to predict the fuel vaporization process. To validate the DPCS model, the predictions from the present model and the previous models are compared with the experimental data, including the liquid penetration in a constant-volume bomb and the combustion and emission characteristics in a premixed charge compression ignition (PCCI) engine. The results indicate that the predictions of the DPCS model agree better with the measurements than the previous models considering only the single-component physical and/or single-component chemical properties of SME on the spray, ignition, and combustion behaviors. It is found that the ignition delay and heat release rate of PCCI combustion are dominated by the evaporation rate of SME and the fuel-reactivity stratification within the cylinder. By considering the multi-component properties of SME, the combustion and emission characteristics can be satisfactorily reproduced by the DPCS model. Meanwhile, the computational time can be well controlled due to the simplification of the physical and chemical surrogate sub-models. [ABSTRACT FROM AUTHOR]

The flame characteristics of wall-impinging diesel fuel spray at single injection and split injection strategies were investigated in a high-temperature high-pressure constant volume combustion vessel. The 2D piston cavity has been used to form the impingement spray flame and the two-color method has been used to analyze the flame process. Results reveal that 2nd ignition delay can be defined in a specific area surrounded by single and split injections when the 1st injection proportion is from 25% to 75%. The KL factor and distribution area of a single injection are smaller than that of a split injection at EOI + 0.2 ms. The soot oxidation area is far away from the wall surface. However, the soot formation area is mainly concentrated along the cavity groove. The soot oxidation area proportion plays a decisive role in the emission of soot. The split injection interval has a key role in the soot formation dominant process at the initial stage of ignition. The initial mean flame temperature changes slightly, the mean flame temperature decreases earlier, the soot oxidation degree becomes higher, no interaction or small interaction between the 1st and 2nd injection, and the maximum degree of soot formation and oxidation dominant process occur when the 1st injection proportion equals 50%. The mean flame temperature drop time of the single injection is advanced. The total flame area and the soot formation and oxidation dominant process before and after EOI + 0.2 ms were also discussed. The total flame area of split injection is larger than single injection after EOI + 0.2 ms. • The soot oxidation area proportion plays a decisive role in the emission of soot. • The split injection interval has a key role in the soot formation dominant process at the initial stage of ignition. • The soot has a great relationship with the high flame temperature area ratio but not the total flame area. • The 2nd ignition delay can be defined in a specific area surrounded by single and split injections. • There is no/small interaction between the 1st and 2nd injection when the 1st injection proportion equals 50%. [ABSTRACT FROM AUTHOR]

• Hydrogen enriched gas engines with turbulent jet ignition was investigated. • Turbulent jets play a major role for promoting flame speed during CA10 to CA50. • Hydrogen addition plays a major role for promoting flame speed during CA50 to CA90. • Considering thermal efficiency and emissions, the optimal hydrogen fraction is 10%. The combination of hydrogen enrichment and pre-chamber initiated turbulent jet ignition was proposed to improve the combustion and performance of large bore gas engines. In order to clarify the effects of hydrogen enrichment level on the combustion and emission formation processes of a large bore medium-speed natural gas engine ignited by pre-chamber initiated turbulent jets, a computational fluid dynamics simulation was carried out to calculate the in-cylinder gas flow and chemical reactions, which were calculated with a detailed oxidation reaction mechanism of methane. In the study, the hydrogen volume fraction was varied from 0% to 30% with a step of 5%, and the excess air ratios of the mixture in the pre-chamber and the main chamber were maintained at 1.0 and 2.0, respectively. The numerical results show that the turbulent jets and the chemical dynamics of the hydrogen enriched fuel played a dominant role in the combustion rates at the former and the latter halves stage of the combustion process, respectively. Moreover, as the increase of hydrogen fraction, the emissions of carbon monoxide, total hydrocarbons and non-methane hydrocarbons and the indicated specific fuel consumption decrease, however, the emissions of nitrogen oxides increase. [ABSTRACT FROM AUTHOR]

• Double injection strategies can solve PRRmax problem for G70H30 and G80H20 in a GCI engine. • −50 °C aTDC pilot SOI with 30% pilot ratio is recommended. • Advancing the pilot SOI can get lower NOx and PM emissions simultaneously. • The PM is effected by pilot SOI timing while NOx is influenced by pilot ratio. A new type of fuel named hydrogenated catalytic biodiesel blended with gasoline is proposed to apply in a gasoline compression ignition engine for low-temperature combustion. However, the higher maximum pressure rise rate and retarded combustion phasing under high loads of single injection mode are the main problems. Hence, the double injection strategy is proposed to solve the above problems and to find the optimal injection conditions for this tested fuel. The combustion and emission characteristics are tested at high load by varying different pilot injection ratio from 20 to 30% and pilot injection timing from −60 to −40 °CA aTDC for this test fuel. The results show that increasing pilot injection ratio can advance the combustion phasing, while it increases the maximum pressure rise rate. −50 °CA aTDC of pilot start of injection timing has advanced combustion phasing and higher maximum pressure rise rate than that of −40 and −60 °CA aTDC conditions. Moreover, the NOx emission is mainly effect by pilot ratio, while PM, THC and CO emissions are controlled by pilot start of injection timing. 30% of pilot ratio with −50 °CA aTDC of pilot start of injection timing is an optimal injection strategy for G80H20 and G70H30 blends considering combustion and emission characteristics. [ABSTRACT FROM AUTHOR]

• A new biodiesel is successful running Gasoline Compression Ignition engine. • Increasing proportion of biodiesel in the gasoline decreases the NOx emissions. • The CO and HC emissions are increase with increasing gasoline proportion. • 20% of pilot injection ratio can solve noise and PM emission problems. Gasoline compression ignition mode is one of the low-temperature combustion strategies using gasoline instead of diesel fuel and it is better than the other low-temperature combustion modes in terms of ignition controllability. Difficulty in the ignition at low loads and maximum pressure rise rate during high loads is the main problem in commercializing this engine. In order to solve this problem, hydrogenated catalytic biodiesel is blended with gasoline in different proportions and its combustion and emission characteristics under different working loads are investigated in a single injection mode. Stable combustion is achieved using this gasoline/hydrogenated catalytic biodiesel blends without any combustion assistance and increasing intake temperature. Results show that G70H30 blending fuel illustrates a comprehensively better combustion and emissions performance. However, the high maximum pressure rise rate at high loads and high particulate matter emission are the main problems. Hence, multiple injection strategies are applied for G70H30 to solve the above problem. It reveals that the particulate matter emission for 20% pilot injection ratio is lower than that of single injection mode. The maximum pressure rise rate of 20% or 30% pilot injection ratio can meet the engine limit. While the carbon monoxide and hydrocarbons emissions for double injection mode are higher than that of single injection mode and it increases with increasing pilot injection ratio. [ABSTRACT FROM AUTHOR]