Experimental investigation on n-butanol/methyl oleate dual fuel RCCI combustion in a single cylinder engine at high-load condition

Reactivity controlled compression ignition (RCCI) engines have a high thermal efficiency as well as low emissions of soot and nitrogen oxides (NOx). However, there is a conflict between combustion stability and harmful emissions at high engine load. Therefore, this work presented a novel approach for regulating n-butanol/methyl oleate dual fuel RCCI at high engine load in attaining lower pollutant emissions while maintaining stable combustion and avoiding excessive in-cylinder pressure. The tests were conducted on a single cylinder engine under rated speed and 90% full load. In this study, n-butanol was selected as a low-reactivity fuel for port injection, and n-butanol/methyl oleate blended fuel was used for in-cylinder direct injection. Combustion and emission characteristics of the engine were first investigated with varied ratios of n-butanol port injection (PFI) and direct injection (DI). Results showed that as the ratio of n-butanol PFI and DI rose, the peak cylinder pressure and heat release rate increased, while NOx and soot emissions reduced, and carbon monoxide (CO) and hydrocarbon (HC) emissions increased under most test conditions. When RNBPI = 40% and RNBDI = 20%, the soot and NOx emissions of the engine were near the lowest values of all test conditions, yet the peak in-cylinder pressure and fuel consumption could not increase significantly. Therefore, the possibility of optimizing the combustion process and lowering emissions by adjusting the pilot injection strategy was investigated utilizing these fuel injection ratios. The results revealed that with an appropriate pilot injection ratio and interval, the peak in-cylinder pressure and NOx emission were definitely reduced, while soot, CO, and HC emissions did not significantly increase.


Scientific Reports
| (2021) 11:24211 | https://doi.org/10.1038/s41598-021-03693-y www.nature.com/scientificreports/ Therefore, this work presented a novel approach for regulating n-butanol/methyl oleate dual fuel RCCI at a high engine load. That is, a specified quantity of n-butanol was used for the port injection, the blended fuel of n-butanol and methyl oleate is injected directly into the cylinder. This fuel injection approach could effectively promote the evaporation of in-cylinder direct injection fuel by utilizing the micro-explosion effect of n-butanol 44,45 . It was feasible to lower pollutant emissions while maintaining stable combustion and avoiding excessive in-cylinder pressure by optimizing the fuel injection strategy.

Experimental setup and methodology
Experimental setup. The n-butanol/methyl oleate dual-fuel RCCI combustion experiment was carried out on a single-cylinder four-stroke compression ignition engine. The engine was a single-cylinder version of a multi-cylinder marine diesel engine designed mainly for scientific research. The technical specifications of the test engine are shown in Table 1, and the schematic of the experimental setup is depicted in Fig. 1. The compressor boosted the intake pressure to match that of a multi-cylinder marine diesel engine. The backpressure valve www.nature.com/scientificreports/ throttled the exhaust flow and was intended to replicate the exhaust backpressure. Due of the high intake pressure of the single-cylinder engine, it was difficult to feed EGR gas into the intake pipe. Therefore, the compressor fed high-pressure air to a turbocharger, which increased the pressure of the EGR gas before it entered the intake pipe. The target EGR ratio was achieved by altering the opening of the EGR valve. Fresh intake air and EGR gas were thoroughly mixed in the surge tank. An n-butanol injection system assembly was mounted on the intake pipe. The n-butanol/methyl oleate blend fuel was injected via the common rail system of the single-cylinder engine. The cylinder pressure and heat release rate were determined using a comprehensive set of combustion measuring equipment from the AVL company, including pressure sensors, charge amplifiers, data acquisition, and indicating software. The AVL AMA i60 exhaust measuring system was used to determine gas emissions, and the AVL 415S smoke meter was used to measure smoke emissions. Table 2 shows detailed information about the measuring equipment, including their measuring range, resolution, and uncertainty.
Experimental methodology. When the engine was under heavy load, a high ratio of n-butanol PFI resulted in excessive peak pressure in the engine cylinder. Thus, the ratio of n-butanol PFI (represented by R NBPI ) was 20-60% of the total heating value of fuel injection, while the ratio of n-butanol DI (represented by R NBDI ) was between 0-30%. R NBPI and R NBDI are expressed using the following equations: where m NBPI is the mass of n-butanol PFI, m NBDI is the mass of n-butanol DI, m MO is the mass of methyl oleate DI. The engine was operated at its rated speed (1800 r/min) with a load of 103.5 kW (corresponding to 90% of maximum engine out power). The engine was not tested at full load to avoid exceeding the engine's permitted value for cylinder pressure when the n-butanol PFI ratio is too high. The EGR rate was 20%. To maintain high thermal efficiency for the diesel engine, the injection timing of the direct injection fuel was adjusted in real time in each test case to maintain the CA50 (crankshaft angle corresponding to 50% accumulative heat release rate) at 10° CA ATDC. The heating value of the fuel delivered to the engine stayed constant during each cycle. To begin, the engine ran at 1800 rpm and 103.5 kW when fueled with pure methyl oleate in-cylinder direct injection. The    (2). The experiment was conducted in two phases. To begin, the combustion and emission characteristics of the engine fueled with various ratios of n-butanol PFI and DI were investigated in order to determine the ideal injection ratios with superior engine performance and emissions. Following that, various pilot-main injection strategies were investigated based on these optimum injection ratios to further optimize combustion and reduce emissions.

Results and discussions
Combustion and emission characteristics of the engine with varied ratios of n-butanol PFI and DI. The performance of the engine with R NBPI = 20%, 40%, 60% and R NBPI = 0, 10%, 20%, 30% was initially investigated. The operating settings of the engine are shown in Table 4.
The comparison of cylinder pressures and heat release rates for various fuel injection strategies is shown in Fig. 2. In general, as the ratio of n-butanol PFI and DI rose, the peak cylinder pressure increased progressively. When R NBPI = 20% and R NBDI = 0, the peak cylinder pressure was 16.02 MPa, as R NBDI grew to 30%, the peak cylinder pressure rose to 16.41 MPa. For R NBDI = 40%, the peak cylinder pressure rose from 16.29 to 17.17 MPa as the R NBDI increased from 0 to 30%. For R NBDI = 60% and R NBDI = 0, the peak cylinder pressure was 16.57 MPa. The highest peak cylinder pressure (17.57 MPa) occurred when R NBPI = 60% and R NBDI = 30%, which elevated where it atomized and evaporated. When the working medium in the cylinder reached a specified temperature and pressure, the methyl oleate induced spontaneous combustion and then ignited the n-butanol. The ignition delay time was prolonged as the ratio of n-butanol PFI and DI increased. During the ignition delay period, a larger combustible mixture was generated, resulting in a greater heat release of premixed combustion. It can also be noticed that, as the ratio of n-butanol PFI and DI rose, the heat release rate increased progressively. In proportion to the rise in R NBDI , the peak of premixed exotherm gradually increased, as did the amount of premixed heat released. For R NBPI = 60% and R NBDI = 30%, the heat release rate exhibited a single peak. The reason for this was that the quantity of methyl oleate injected was lowered, the amount of active free radicals produced by the low-temperature reactions of methyl oleate dropped, and the activity of the in-cylinder medium decreased, resulting in a prolonged ignition delay time. Additionally, the n-butanol injected into the cylinder promoted the atomization of methyl oleate, resulting in the complete mixing of methyl oleate with the n-butanol in the cylinder. When methyl oleate was self-ignited, it simultaneously ignited the n-butanol. As a consequence, only one heat release process with a high peak heat release rate existed. Figure 3 compares the combustion durations for various fuel injection strategies. As can be observed, when R NBPI = 40% and 60%, the combustion duration was considerably shorter than when R NBPI = 20%. This was because when the ratio of n-butanol PFI rose, the quantity of methyl oleate DI dropped, resulting in a greater volume of combustible mixture with a longer ignition delay time. When methyl oleate was self-ignited, it accelerated the combustion of a large number of premixed fuels, which led to a shorter combustion duration. The combustion duration gradually decreased with increasing R NBPI for the same R NBDI . This was due to the fact that as the amount of n-butanol DI increased, the cetane number of the blended fuel decreased, causing the ignition delay time to lengthen, which increased the proportion of premixed combustion. The premixed combustion rate was faster than diffusion combustion, resulting in a shorter combustion duration. When RNBPI = 60%, the combustion duration with R NBDI = 30% is significantly lower than that with RNBDI = 20%. This was owing to the lengthy ignition delay time with a modest amount of methyl oleate, which enabled the methyl oleate to adequately atomize and evaporate in the cylinder. As demonstrated in Fig. 2c, the consumption of methyl oleate was predominantly premixed combustion with a fast combustion rate, resulting in a very short combustion duration.
The BSFCs for various fuel injection strategies are shown in Fig. 4. It was obvious that as the increment of R NBPI , the BSFC gradually increased. Because n-butanol has a lower heating value than methyl oleate, a higher R NBPI needed more fuel injection to achieve the same power output. When R NBPI = 20% and R NBDI = 10%, the BSFC dropped slightly. This was because the n-butanol DI caused "micro explosion" due to its low boiling point, promoting the atomization of methyl oleate. Thereby the combustion efficiency was improved. However, with a further increment of R NBDI , BSFC progressively rose. When R NBPI = 40% and 60%, the BSFC rose as the R NBDI increased. This was also due to the fact that a higher R NBDI resulted in a lower heating value per unit mass of fuel for direct in-cylinder injection, which led to a higher rate of fuel consumption. Figure 5 illustrates the variation in emissions caused by different fuel injection strategies. As shown in Fig. 5a, soot emissions decreased as the ratio of n-butanol PFI rose. The reason was that with the increment of the n-butanol PFI ratio, the quantity of methyl oleate DI in the cylinder dropped. The area of the excessively rich mixture shrunk, resulting in a reduction in soot precursor production. When R NBPI = 40% and 60%, soot emissions were considerably lower than R NBPI = 20%. At the same R NBPI , soot emissions decreased as the ratio of n-butanol DI rose. This was because, on the one hand, the injection quantity of methyl oleate was reduced. On the other hand, the "micro explosion" action of n-butanol DI promoted the atomization of methyl oleate. When R NBPI = 40% and 60%, and R NBDI = 20%, the soot emissions were already relatively low. As R NBDI increased from 20 to 30%, the reduction in soot emission became less noteworthy. This occurred because when the R NBDI   www.nature.com/scientificreports/ reached 20%, the methyl oleate may already be atomized more completely owing to the micro-explosion action of n-butanol DI, and its diffusion combustion was substantially decreased, resulting in extremely low soot emission. Even though the R NBDI was increased to 30%, the atomization of methyl oleate remained adequate, just as it had been when the R NBDI was 20%. As a consequence, the potential of further lowering soot emissions was not especially apparent. In addition, it can be observed in the figure that the test cases with extremely low soot emissions were R NBPI = 40%, R NBDI = 20% and 30%, and R NBPI = 60%, R NBDI = 20% and 30%. Compared to R NBPI = 20% and R NBDI = 0, soot emissions were decreased by 84.7% when R NBPI = 40% and R NBDI = 20%, and by 93.7% when R NBPI = 60% and R NBDI = 30%. As shown in Fig. 5b, NOx emissions decreased as the ratio of n-butanol PFI rose. This was due to the fact that as the methyl oleate DI ratio decreased, the region of the overly rich methyl oleate mixture diminished, thus reducing high temperature area in the cylinder and resulting in lower NOx emissions. For R NBPI = 40% and 60%, NOx emissions were much lower than R NBPI = 20%. A reduction in methyl oleate injection into the combustion chamber was the primary reason for this. On the other hand, a longer ignition delay period allowed the methyl oleate to fully atomize and evaporate, thereby reducing the high temperature area in the cylinder. When R NBPI = 20%, NOx emissions decreased as the ratio of n-butanol DI rose. This was because n-butanol DI has a high latent heat of evaporation, which was beneficial to reducing the temperature in the cylinder. When R NBPI = 40% and 60%, there was no discernible change in NOx emissions. The n-butanol DI lowered the temperature in the cylinder. It did, however, simultaneously introduce a specific quantity of O atoms. The final NOx emissions were the consequence of the combined effects of temperature, the duration of the combustion heat release, and the quantity of O atoms.
CO emissions increased dramatically as the ratio of n-butanol PFI rose, as seen in Fig. 5c. Compared to R NBPI = 20%, CO emissions increased more than doubled when R NBPI = 40% and three to four times when R NBPI = 60%. This was because as the ratio of n-butanol PFI increased, the combustion duration was shortened, and when the cylinder temperature dropped, the CO generated during the n-butanol combustion process could not be further oxidized to CO 2 . When R NBPI = 20% and 40%, and R NBDI = 10%, CO emissions decreased slightly. This was due to a tiny quantity of n-butanol DI enhanced methyl oleate evaporation, which improved combustion efficiency. CO emissions steadily increased when the ratio of n-butanol PFI was raised further. Because of an increase in the quantity of n-butanol in the cylinder that had not been completely oxidized. Continued growth in R NBDI resulted in a progressive increase in CO emissions. Because of the high R NBDI , the fuel direct injection into the cylinder atomized more completely, leading to a low combustion temperature; on the other hand, the shorter combustion duration resulted in an increase in the amount of CO that could not oxidize to CO 2 . When R NBPI = 60%, CO emissions increased in proportion to the n-butanol DI ratio. Figure 5d illustrates a significant rise in HC emissions when the n-butanol PFI ratio is raised. The reason for this was that as the ratio of n-butanol PFI increased, the n-butanol in the cylinder was quenched during the combustion process, increasing the quantity of n-butanol stored in the slit. When the piston went down, this portion of the n-butanol fails to completely oxidize, resulting in higher HC emissions. Furthermore, the n-butanol concentration at the top surface of the piston and the cylinder wall surface is high due to the increased port injection ratios, and quenching occurs when the flame propagates to the cold wall surface, leading to higher HC emissions. For R NBPI = 20%, the ratio of n-butanol DI had minimal effect on HC emission. While for R NBPI = 40% and 60%, HC emissions increased dramatically with an increase in R NBDI .

Effect of pilot injection strategies on combustion and emission characteristics.
According to the analysis in "Combustion and emission characteristics of the engine with varied ratios of n-butanol PFI and DI", with R NBPI = 40% and R NBDI = 20%, the engine's soot and NOx emissions were near the lowest of all test instances. Simultaneously, the peak in-cylinder pressure did not rise noticeably. Thus, based on these ratios of n-butanol injection, this section investigated the potential of optimizing combustion and lowering emissions by adjusting the pilot injection strategy of direct injection fuel. The engine's operating parameters are listed in Table 5. The main injection timing was fixed at − 12° CA ATDC. The interval between the pilot injection timing and the main injection timing was varied from 15° to 35°, while the pilot injection ratio was set between 0 and 20%. The effects of the pilot injection interval and the pilot injection ratio on the combustion and emission characteristics of the engine were investigated. www.nature.com/scientificreports/ The comparison of peak cylinder pressures for various pilot injection ratios and intervals is shown in Fig. 6. The pilot injection ratio of zero in this illustration represented a single injection. The peak in-cylinder pressure was obviously the lowest with a pilot injection interval of 20° CA and a pilot injection ratio of 10%. With increasing pilot injection ratio and keeping the same pilot injection interval, the peak in-cylinder pressure first declined and subsequently increased. With a 5% pilot injection ratio, the peak in-cylinder pressure rose when the pilot injection interval increased. By comparison, at a 20% pilot injection ratio, the peak in-cylinder pressure reduced somewhat as the pilot injection interval increased. When the pilot injection ratio was 10% and 15%, the peak in-cylinder pressure initially lowered and climbed as the pilot injection interval increased. Many active free radicals were created concurrently with exotherm when the pilot injection fuel was delivered into the cylinder at a specified pressure and temperature. The oxidation of the main injection fuel was promoted when it was injected into the active free radical zone, so the ignition delay time was shortened. The shortened ignition delay period reduced the quantity of main injection fuel for premixed combustion, resulting in a reduction in maximum cylinder pressure. However, due to the extremely short ignition delay period, the ignition timing was significantly advanced, resulting in a further rise in peak cylinder pressure. Thus, a proper pilot injection ratio and interval could limit the quantity of premixed combustion of the main injection fuel. Simultaneously, the ignition timing should not be advanced excessively. As a result, the maximum cylinder pressures could be reduced. Through the use of pilot injection, it was possible to lower the peak cylinder pressure from 16.6 to 16.2 MPa. Figure 7 compares BSFCs with a variety of pilot injection ratios and intervals. It can be noticed that BSFCs increased as the pilot injection ratio and interval rose. A higher pilot injection ratio and earlier pilot injection timing ensured that the fuel was spread uniformly throughout the cylinder, lowering the temperature and efficiency of combustion. Additionally, fuel entered the gap between the piston and the cylinder lining, resulting in incomplete combustion. An extensive pilot injection interval caused fuel to collide with the top surface of the piston or the surface of the cylinder liner wall, which contributed to incomplete combustion as well.  In comparison to single injection, the various pilot injection schemes significantly reduced NOx emissions. The lowest NOx emission occurred when the pilot injection interval was 25° CA and the pilot injection ratio was 10%. For a 5% pilot injection ratio, increasing the pilot injection intervals had no discernible effect on NOx emissions. NOx emissions initially dropped and then rose as the pilot injection interval increased at a 10-20% pilot injection ratio. Utilizing an appropriate pilot injection ratio and interval contributed to the reduction of NOx emissions. Due to the heat dissipation of the pilot injection fuel, if the pilot injection interval was too short, the main injection fuel would be injected into the high temperature zone, raising in-cylinder temperatures and NOx emissions. When the pilot injection interval was excessively lengthy, the ignition delay time was substantially increased, and the heat release during premix combustion increased, resulting in a rise in in-cylinder temperature and higher NOx emissions. The NOx emission could be decreased from 3.15 to 2.80 g/kW h by utilizing the pilot injection.
Soot emissions with various pilot injection ratios and intervals are shown in Fig. 8b. As the graph indicates, soot emissions rose with increasing pilot injection ratios for the same pilot injection interval. This was because the pilot injection shortened the ignition delay period of the main injection, resulting in increased diffusion combustion and soot emissions. Soot emissions decreased with increasing pilot injection intervals. As the pilot injection interval rose, the mixture became more homogeneous, decreasing soot formation. Reduced soot emissions could be accomplished by utilizing a lower pilot injection ratio and a longer pilot injection interval in comparison to a single injection. Figure 8c shows that, in comparison to a single injection, HC emissions rose as the pilot injection interval and ratio increased. For a 5% pilot injection ratio, the HC emission at a pilot injection interval of 35° rose slightly compared to a pilot injection interval of 15°. When the pilot injection ratio was 15% and 20%, the HC emission increased dramatically as the pilot injection interval increased. With an earlier pilot injection time and a higher pilot injection ratio, the fuel mixture escaped into the gap between the cylinder and the piston, resulting in incomplete combustion and increased HC emissions. Additionally, wall wetness was caused by an early pilot injection or a high pilot injection ratio. The fuel clinging to the cylinder wall would also burn incompletely due to the low wall temperature, increasing HC emissions even more.
As illustrated in Fig. 8d, CO emissions increased with the increment of pilot injection interval and ratio compared to a single injection. The rationale for this was that with a higher pilot injection interval and ratio, the fuel was distributed more uniformly throughout the cylinder, resulting in a lower combustion temperature. Alternatively, due to the large amount of premixed combustion, the combustion speed was enhanced, leading the combustion to finish earlier, and the temperature within the cylinder decreased, resulting in the inability of www.nature.com/scientificreports/ CO to continue being oxidized to CO 2 . It was obvious that utilizing pilot injection would result in incomplete combustion, hence increasing CO emissions.

Conclusions
In order to guarantee that the RCCI combustion process does not produce excessive in-cylinder pressure at high load while generating low pollutant emissions, this study proposed a novel RCCI combustion approach using port injection of n-butanol and in-cylinder injection of n-butanol/methyl oleate blend. The experimental research was conducted to investigate the combustion and emission characteristics of the engine with varied ratios of n-butanol PFI and DI under 90% of full load. The potential of optimizing combustion and lowering emissions by adjusting the pilot injection strategy of direct injection fuel was investigated under test conditions of R NBPI = 40% and R NBDI = 20%. The major conclusions are summarized as follows.
1. As the ratio of n-butanol PFI and DI rose, the peak cylinder pressure and heat release rate increased, the combustion duration decreased, and CO and HC emissions increased. When R NBPI = 40% and 60%, NOx and soot emissions were significantly lower than R NBPI = 20%. 2. Increased n-butanol DI ratio resulted in an increase in the peak cylinder pressure and heat release rate while lowering combustion duration. NOx and soot emissions dropped when the n-butanol DI ratio increased with R NBPI = 20%. There was no discernible change in NOx emissions for R NBPI = 40% and 60%, whereas the reduction in soot emission was modest as R NBDI increases from 20 to 30%. 3. Pilot injection was feasible to lower NOx emissions while avoiding excessive in-cylinder pressure. With R NBPI = 40% and R NBDI = 20%, the peak in-cylinder pressure was obviously the lowest with a pilot injection interval of 20° CA and a pilot injection ratio of 10%. The lowest NOx emission occurred when the pilot injection interval was 25° CA and the pilot injection ratio was 10%. At the same pilot injection interval, soot emissions rose as the pilot injection ratios increased. CO and HC emissions rose with an increment of the pilot injection interval and ratio.