Effects of Methanol Addition on the Combustion Process of the Methanol/Diesel Dual-Fuel Based on an Optical Engine

Abstract

The combustion process of traditional diesel engines is mainly determined by the injection timing of diesel. There is a trade-off relationship between the soot and NOx (nitrogen oxides) during this combustion process, making it difficult to reduce these two emissions simultaneously. The use of methanol can not only solve the above problem, but also replace some fossil fuels. However, the effects of methanol injection into the intake duct on the flame propagation in diesel/methanol dual-fuel engines is not yet clear, and there is relatively little research on it. The effects of methanol addition on the combustion process of diesel/methanol dual fuel (DMDF) were achieved based on a modified optical engine in this paper. One injector is installed on the intake inlet to inject methanol, and the other injector is installed in the cylinder to inject diesel in two stages before the top dead center of compression. There are three tests conducted separately in this paper. Firstly, the effects of the methanol ratio (40%, 50%, 60%, and 70%) on the combustion process are investigated, with the total heat remaining unchanged. Secondly, the effects of the pre-injection mass of diesel (20%, 30%, 40%, and 50%) on the combustion process are investigated, which keeps the total diesel mass unchanged. Finally, the effects of the total mass of diesel on the combustion process are investigated while maintaining the mass of methanol unchanged. The dual-fuel combustion process is recorded by a high-speed camera. A combustion analyzer and other equipment were used to analyze the combustion. The results showed that CA10 is delayed, the pressure and the heat release rate (HRR) are reduced, and the number of pixels of the KL factor (KL) decreases significantly with the increasing methanol ratio. CA10 and CA50 are advanced, the pressure and HRR decrease, and the KL increases when the mass of pre-injected diesel increases. CA10 and CA50 are advanced, respectively, and CA90 is postponed due to the increase in diesel mass. The pressure and HRR increase, and the KL increases when the total mass of diesel increases.

1. Introduction

As an important power system for automobiles and ships, diesel engines are widely used worldwide. However, the pollutants emitted by diesel engines cause greenhouse effects, which cause great harm to the environment [1,2]. Many countries have formulated strict emission regulations to control diesel engine emissions. However, the single fuel of diesel performs poorly in suppressing pollutant emissions due to the trade-off relationship [3]. Some researchers have studied the trade-off relationship based on fuel properties. Jos Reijnders [4] investigated the effects of aromaticity and cetane number on the soot–NOx trade-off on a modified DAF heavy-duty engine. The results showed that there is no discernible benefit of a high CN with respect to the soot–NOx trade-off at equal aromaticity. Some advanced biofuels are also used to investigate the trade-off relationship between soot and NOx [5]. The use of oxygen-containing alternative fuels, mainly ethanol [6], butanol [7], methanol [8], and DMF [9], can effectively solve this problem. The effects of alcohol fuels addition on the emissions (such as NOx, soot, HC, etc.) of diesel engine were comprehensively reviewed in reference [10].

In recent years, many scholars have extensively studied the aforementioned fuels [11,12]. Compared to other oxygen-containing fuels, methanol is easier to extract from fossil fuels such as coal. Meanwhile, methanol is easier to store and has a higher carbon/oxygen ratio. It has been favored by researchers worldwide as an alternative fuel [13,14]. Especially, existing research has shown that methanol has significant advantages in reducing NOx emissions [15,16].

The use of methanol can achieve the reactivity-controlled compression ignition (RCCI) mode in existing diesel engines. The RCCI injects lower-activity fuel (such as methanol) into the intake inlet and higher-activity fuel (such as diesel) directly into the cylinder [17,18]. The mixture of higher-activity fuel ignites first and then ignites the mixture of lower-activity fuel to achieve the combustion of DMDF. Wei [19] conducted unregulated emissions of DMDF. The results show that the unregulated emissions are higher compared with the pure diesel mode. Wang [20] conducted a study of emissions characteristics in DMDF engines. The results showed that NOx and soot emissions are reduced at high loads for the DMDF engine. Meanwhile, methanol spontaneously ignites before diesel is injected at high loads. This research also shows that it is necessary to control the temperature inside the cylinder to prevent the occurrence of auto-ignition. Kumar [21] experimentally studied the effects of intake air temperature on dual-fuel engines, and the study concluded that intake air temperature has a significant impact on the dual-fuel engine.

However, the investigations above were mainly focused on the output results and emissions of DMDF engines; the process of flame propagation in the cylinder is less researched. Optical measurement methods have been applied to research the flame propagation process in dual-fuel engines to gain a deeper understanding of this process in recent years. High-speed cameras are used to capture and observe the propagation of flames in engine cylinders with optical windows. Liu [22] studied the effects of different low-reactivity fuels and direct injection timing on combustion based on optical diagnostic technology. The results indicate that the combustion process becomes more uniform as the ethanol increases. Flame propagation speed inside the cylinder slows down with the increase in ethanol. It can be clearly seen that delayed injection time significantly affects the formation and propagation characteristics of flame nuclei by the combustion images. Optical measurement methods are not only used in internal combustion engines, but also have important applications in other burners [23]. By observing the image, the propagation process of flames can be seen more clearly.

Kim [24] studied the effect of pre-injection diesel on the performance of a natural gas/diesel engine. The result shows that the pre-injection can effectively change the location of the early flames in the cylinder, which has an impact on engine emissions. Silvana [25] investigated the effects of methane on NOx and PM based on a three-cylinder engine, which modified an endoscope based on the first cylinder. A CCD (charge-coupled device) camera was applied to capture the image of combustion through the endoscope. The results showed that the addition of methane reduced the temperature in the combustion chamber, so there were fewer NOx emissions. Alexios [26] investigated the ignition and combustion propagation of premixed combustion and spray-driven combustion in a six-cylinder compression ignition engine based on laser-induced fluorescence. The results indicate that there is a lower temperature inside the cylinder, caused by the heat absorption of methanol by evaporation. Cheng [27] evaluated the cycle-to-cycle variations (CCVs) based on an engine. The results indicate that methane concentration has an important impact on CCVs, and CCVs have an inhibitory effect when the concentration is high.

Since the two-stage injection of diesel can initially form a certain amount of premixed combustible mixture, many researchers have studied the combustion process of DMDF under the two-stage injection of diesel. An early pilot injection of diesel can effectively improve the activity in the cylinder. The experiment also showed that NOx decreased as the amount of methanol increased in the pilot injection strategies with the lower oxygen concentration [28].

Tao [29] experimentally investigated the effects of diesel pre-injection timing on combustion. The results indicate that the ignition is delayed and that the thermal efficiency decreases as the pre-injection time advances. The emissions in the cylinder can be effectively improved with an increase in the methanol substitution rate. Zhou [30] also conducted a similar study on a modified engine. The results show that combustion noise is reduced with advancement in pre-injection timing.

Due to the use of two fuels, the combustion process of DMDF engines becomes relatively complex. Although some of the literature has studied DMDF, the influence of key factors on its combustion process is still unclear, such as diesel injection timing, diesel injection quantity, etc. There is limited literature on the study of flame propagation in DMDF engine cylinders using optical measurement methods. The paper attempts to optically investigate the evolution of the flame in the cylinder of diesel/methanol dual fuel (DMDF), which provides a reference for clean and efficient combustion in engines in the future.

2. Experiment Setup

2.1. Engine

The experiment is based on a modified visual single-cylinder optical engine test bench, which mainly includes a dynamometer, optical engine, combustion analyzer, high-speed camera, cylinder pressure sensor, etc. The dynamometer is used to drive the engine and bring it to the set speed. The control board mainly supplies the fuel needed by the engine and controls the fuel injection pressure. Another function is to regulate and control the oil temperature and coolant temperature to make the engine reach the required working condition. The engine uses a transparent piston of quartz glass with a 45° reflector directly below the cylinder. The combustion process can be projected onto a 45° reflector through a quartz glass piston, and then the combustion process can be recorded by a high-speed camera. The combustion analyzer mainly measures and analyzes the data collected by the cylinder pressure sensor and crankshaft position sensor. The experimental setup diagram is shown in Figure 1, and the main parameters of the experimental engine are shown in

Table 1. The main parameters of the engine.

Items	Specifications
Engine type	1-cylinder
Engine speed/rpm	1000
Bore/mm	95
Stroke/mm	115
Connecting rod length/mm	210
Compression ratio	15
Nozzle diameter/mm	0.15
Spray angle/(°)	150
Swirl rate	1.2
IVC	−133
EVO	125
Intake temperature/(K)	333
Fuel injection pressure/(MPa)	60
Ambient temperature/(K)	300
Ambient pressure/(MPa)	0.101
Fuel temperature/(K)	305

. IVC represents the intake valve closed and EVC represents the exhaust valve open. The list of uncertainty of the equipment is given in

Table 2. List of uncertainty of equipment.

Instruments	Accuracy	Uncertainty (%)
Engine speed	±10 rpm	±0.1
Intake gas temperature	±1 °C	±0.1
Pressure pick up	±0.1 bar	±0.05
Crank angle encoder	±10	±0.05

.

2.2. Fuel Properties and Hardware

Methanol has an important role in reducing emissions for engines due to its lower carbon and higher oxygen content. Methanol is used as a lower-activity fuel in this paper, and diesel is used as a higher-activity fuel for testing. The specific properties of DMDF are shown in

Table 3. Fuel properties [31].

Property	Diesel	Methanol
Molecular formula	-	CH3OH
Density at 20 °C (kg/m3)	840	790
Low heating value (MJ/kg)	42.5	19.7
Auto-ignition temperature (°C)	250	450
Content of C (%)	86	38
Content of H (%)	13	12
Content of O (%)	-	50
Cetane number	51	3–5
Latent heat of evaporation (kJ/kg)	250	1110

.

Cylinder pressure was measured with a pressure sensor, which is embedded in the cylinder head. The engine’s intake heater can quickly heat the intake air to the set temperature and keep the temperature almost unchanged. A high-speed camera of the Photon FASTCAM Mini-AX series was used to record the fuel atomization and combustion processes. The Nikon macro lens of the AF-S VR lens is used, and the image resolution ratio is set to 512 × 512. The frame rate and exposure time were set to 10,000 frames per second and 20 μs, respectively. The diesel injector is equipped in the cylinder with a six-hole injector, and methanol is injected by the injector mounted in the intake inlet.

2.3. Injection Setup

Two fuels are used for the experiment test in this paper. Methanol and diesel were injected into the intake inlet and the cylinder, respectively, as shown in Figure 2. The methanol injection ratio, diesel pre-injection ratio, and total mass of diesel injection are researched in this paper. Three experimental schemes are listed in

Table 4. Experimental schemes.

Parameters		Test 1	Test 2	Test 3
Engine speed (rpm)		1000	1000	1000
Diesel total mass (mg)		5.6	5.6	2.4, 4, 7.2, 8.8
MSR%		40%, 50%, 60%, 70%	30%
Diesel pre-injection ratio %		30%	10%, 30%, 40%, 50%	30%
Diesel injection time °CA	−30	−30	−30	−30
	−6	−6	−6	−6
Methanol injection time °CA	−300	−300	−300	−300
PI fuel	Methanol	Methanol	Methanol	Methanol
DI fuel	Diesel	Diesel	Diesel	Diesel

.

The injection strategy for the experiments is shown in Figure 3. SOI_D1 (start of injection) represents the pre-injection timing of the diesel and SOI_D2 represents the main injection timing of the diesel, respectively. SOI_M represents the injection timing of methanol, which was injected into the intake stroke. TDC and BDC represent the top dead center and bottom dead center, respectively. CA10, CA50, and CA90 represent the crank angle at which 10%, 50%, and 90% of the fuel is burned, respectively. Combustion duration is the difference of crankshaft angle corresponding to A10 and CA90. Ignition delay is defined as the difference between CA10 and the timing of the start of injection. The compression top dead center is defined as 0 °CA, previously counted as ‘-’.

The mass of diesel substituted by methanol was qualified as the methanol energy ratio (MER), which refers to the ratio of the energy released by the complete combustion of methanol to the total energy released by the diesel/methanol dual fuel.

$$\mathrm{M}\mathrm{E}\mathrm{R}=\frac{{\mathrm{m}}_{\mathrm{M}}\times {\mathrm{L}\mathrm{H}\mathrm{V}}_{\mathrm{M}}}{{\mathrm{m}}_{\mathrm{D}}\times {\mathrm{L}\mathrm{H}\mathrm{V}}_{\mathrm{D}}+{\mathrm{m}}_{\mathrm{M}}\times {\mathrm{L}\mathrm{H}\mathrm{V}}_{\mathrm{M}}}$$

where ${\mathrm{m}}_{\mathrm{M}}$ is the methanol mass in mg, LHV_M is the methanol low calorific value in J/mg, ${\mathrm{m}}_{\mathrm{D}}$ is the diesel mass in mg, and LHV_D is the diesel low calorific value in J/mg.

2.4. Method

The in-cylinder HRR is calculated according to the first law of thermodynamics, which includes the rate of change in external work during the combustion of working fluids, the rate of heat loss, and the rate of internal energy.

$$\frac{\mathrm{d}\mathrm{Q}}{\mathrm{d}\mathsf{\phi }}=\frac{\mathrm{d}\mathrm{W}}{\mathrm{d}\mathsf{\phi }}+\frac{\mathrm{d}{\mathrm{Q}}_{\mathsf{\omega }}}{\mathrm{d}\mathsf{\phi }}+\frac{\mathrm{d}\mathrm{U}}{\mathrm{d}\mathsf{\phi }}$$

Q represents the total energy released by combustion; W represents work done externally; U represents the internal energy of working fluids; Q$\mathsf{\omega }$ represents the heat lost; and φ represents the crank angle.

The blackbody furnace is used to calibrate the temperature, and the radiation coefficient is almost unchanged with temperature. The blackbody furnace heats the furnace chamber through a closed tubular heating source inside the furnace body. A thermocouple is embedded in the high-temperature chamber, and the true temperature of the chamber can be directly obtained during the normal operation of the blackbody furnace. A high-speed camera is used to record images at each temperature. The distribution of KL was obtained according to the two-color method. The blackbody furnace used in this test is shown in Figure 4.

The two-color method is based on the thermal radiation theory, which can simultaneously obtain the soot distribution and temperature distribution of flames. It has been widely used in the study of engine combustion processes. Researchers believe that the bright radiation of diesel flames is generated by soot particles during diesel combustion. The temperature of the flame is represented by the temperature of the soot particles, and the KL factor is used to characterize the soot particles. The intensity of the radiant light of a flame at two wavelengths is measured. The relationship between radiant intensity and temperature is established by radiometry. The solution process is described in detail in the reference [32].

3. Results and Discussions

3.1. Methanol Ratio

To investigate the effect of the methanol ratio on engine performance, the total heat released by the complete combustion of the fuel remained unchanged (340 J) in Test 1. The proportion of methanol is increased sequentially, and the heat released by methanol to the total heat is 40%, 50%, 60%, and 70%, respectively. Diesel is injected in two stages, with pre-injection accounting for 30% and main injection accounting for 70%. SOI_D1 is set to −30 °CA, SOI_D2 is set to −9 °CA, and SOI_M is set to −300 °CA.

The results of pressure and HRR under different methanol ratios are shown in Figure 5. The cylinder pressure and HRR both decrease with the methanol ratio increase. Figure 6 shows the effects of methanol ratios on the combustion phase. CA10, CA50, and CA90 are correspondingly delayed as the methanol ratio increases, but the changes are not significant. Therefore, it can be summarized that the increase in methanol substitution rate has a relatively small impact on the combustion phase under certain conditions. The heat absorbed by methanol vaporization increases as the proportion of methanol increases, leading to a temperature decrease inside the cylinder. The higher methanol concentration and the lower diesel concentration have an important influence on the activity of the cylinder fuel. The reduced activity of the mixture leads to a combustion delay. The combustion of fuel in the cylinder occurs during the downward movement of the piston, where the volume in the cylinder is larger and the cylinder pressure is lower. The temperature is also lower inside the cylinder as the methanol ratio increases. Therefore, the peak cylinder pressure and HRR decrease.

Figure 7 is the flame image in the cylinder under different methanol ratios. The timing of ignition is delayed, and the flame brightness in the cylinder decreases as the methanol ratio increases. The entire combustion process is longer, and the flame brightness is also brighter at 40%MER. The combustion of the fuel spray can be seen in the picture. The brightness of the flame has decreased, and the entire combustion process is very weak at 70%MER. The decrease in the flame brightness means that the radiant intensity in soot particles is reduced based on the two-color method principle above, indicating that soot particles are fewer. Therefore, soot particles are decreasing with the increase in methanol ratios. This is because the concentration of diesel increases when the proportion of methanol decreases, which indicates the ratio of diffusion combustion becomes larger. An increase in flame brightness indicates more soot formation. The combustible mixture is more uniform because methanol is injected into the intake inlet; thus, the ratio of premixed combustion increases when the methanol ratio increases, resulting in a reduction in soot formation.

The temperature and soot distribution are shown in Figure 8, taking 50%MER and 60%MER for examples. The combustion flame appears near the cylinder wall at the beginning, and then the front flame propagates towards the center of the cylinder, which can be seen in Figure 7. The distribution of temperature and KL factor also shows this evolutionary trend. It can be seen that the temperature of 50%MER is higher than that of 60%MER under the same crank angle, and the high-temperature area is also larger. It can be seen from the distribution of the KL factor that the areas with a higher KL factor are focused on the central area of the flame (soot is mainly distributed in the central part of the flame). And the soot around the flame is lower. This is mainly because the fuel concentration in the central region of the flame is higher and cannot easily mix with air, resulting in incomplete combustion in the central region. The fuel on the periphery of the flame has a larger area in contact with the air, and the concentration is relatively lower, so the combustion is relatively complete, and the soot formation is lower.

Figure 9 shows the KL, taking 50%MER and 60%MER as examples. It can be clearly seen that the KL in each range at 50%MER is greater than that at 60%MER. The peak number of pixels of the KL factor with 50%MER is about 3.5 times that of 60%MER. The timing of the KL factor occurring at 50%MER is earlier than that of 60%MER, but the change is not significant. The end timing of the KL factor at 50%MER is later than that of 60%MER. It was found that the proportion of methanol has a significant impact on the KL. The increase in the proportion of methanol leads to an increase in the concentration of low activity fuel in the cylinder. The concentration of oxygen atoms in the cylinder increases due to methanol being an oxygen-containing fuel, accelerating the oxidation rate of soot. Soot formation can be reduced by increasing the proportion of methanol in special engine conditions. Therefore, it is feasible to reduce soot formation by increasing the proportion of methanol appropriately.

3.2. Pre-Injection Mass of Diesel

Two stages of diesel injection were used in this test. The pre-injected diesel cannot ignite the main fuel when the pre-injected diesel is low. The main fuel will be ignited when the amount of pre-injected diesel is larger. SOI_D2 is set to −9 °CA and SOI_D1 is set to −30 °CA. The total mass of diesel injected in two stages is 5.6 mg. The pre-injection proportion of diesel is set at 10% (PRE-10%), 30% (PRE-30%), 40% (PRE-40%), and 50% (PRE-50%), respectively. The methanol injection mass is 5.19 mg, while the methanol injection timing is set to −300 °CA.

Figure 10 shows the cylinder pressure and HRR under different diesel pre-injection ratios. The peak pressure and HRR decrease and advance as the pre-injection ratio increases, indicating that first-stage injected diesel has a weak ignition and has a certain ignition effect on the main fuel injection. The concentration of the diesel combustion flame center in the main injection stage decreases, and the combustion speed slows down, resulting in a decrease in HRR and pressure.

Figure 11 shows the combustion phase of different diesel pre-injection ratios. The CA10 and CA50 advance, respectively, with the increase in diesel pre-injection ratio. The variation in CA10 is more significant. The crank angle of CA10 is about 0 °CA at PRE-10%, and the crank angle of CA10 is about −10 °CA at PRE-50%. The CA50 and CA90 have little variation within 2 °CA. As the amount of diesel in the first-stage increases, so does the highly reactive fuel. The combustible mixture formed by diesel volatilization moves forward with the timing advances of the pre-injection of diesel, indicating that the combustion process also moves forward. The combustible mixture in the cylinder begins to ignite weakly. The weak ignition is beneficial for improving the overall activity of the fuel in the cylinder, making it easier for the mixture in the cylinder to ignite, which leads to an advance in CA10.

Figure 12 shows the combustion process of the combustible mixture under different pre-injected diesel ratios. The timing of the flame appearance advances when the pre-injection ratio increases from these flame images, and the brightness of the flame also increases. The flame reaches its brightest point at about 2 °CA, and then gradually dims as the combustion process progresses. The flame image of PRE-10% only has a faint light at 7.8 °CA, while the flame image of PRE-50% still has a large area of light, indicating that the combustion time is longer when the proportion of diesel pre-injection increases. Figure 13 shows the distribution of combustion temperature and KL, which take PRE-30% and PRE-40% as examples. The high-temperature areas are relatively scattered at PRE-30%, and the high-temperature areas are more concentrated at PRE-40%. Meanwhile, it was found that the area of KL factor distribution also increased with the increase in the pre-injection ratio. Therefore, the distribution of soot can be reduced by decreasing the pre-injection ratio under certain conditions.

To compare the KL under different diesel pre-injection ratios more intuitively, the KL in each range is obtained by processing the flame image. The KL in each range is obtained in Figure 14, taking PRE-30% and PRE-40% as examples. It can be seen that the timing of the KL factor appearance at PRE-40% is earlier than the appearance timing at PRE-30%. The peak number of pixels of the KL factor is approximately 12,000 at PRE-30%, and the KL is approximately 15,000 at PRE-40%. The peak number of pixels of the KL factor appeared nearly at the same time in the PRE-30% and PRE-40%. The KL in each range of the PRE-40% is greater than that in each range of the PRE-30% before 5 °CA. However, the KL factor disappears earlier at PRE-40%, indicating that the increment of the diesel pre-injection ratio will accelerate the oxidation of soot.

3.3. Total Mass of Diesel

For DMDF engines, an increase in diesel injection will increase the overall activity of the fuel in the cylinder as diesel is a highly active fuel, which can improve the combustion of the cylinder. Diesel is injected in two stages: SOI_D2 is set to −9 °CA and SOI_D1 is set to −30 °CA. The total mass of diesel in the two stages is 2.4 mg (M2.4), 4 mg (M4), 7.2 mg (M7.2), and 8.8 mg (M8.8), with the mass of pre-injection accounting for 30% and the mass of main injection accounting for 70%. The methanol injection time is set to −300 °CA, which is in the intake stroke, and the mass of methanol is 5.19 mg. The effect of the total mass of diesel on the performance of the DMDF engine is investigated.

Figure 15 shows the pressure and HRR under different total masses of diesel. The pressure and HRR increase, respectively, as the total mass of diesel increases. This is because the total heat of the fuel increases. The fuel ignites weakly and releases heat slowly at M2.4. Then, the amount of pre-injected fuel is increased as the total amount of diesel fuel increases, which indicates that the activity of the fuel increases. The combustion in the cylinder becomes more intense in the engine after the main diesel is injected, resulting in a significant increase in pressure and HRR. Figure 16 shows the effect of the total mass of diesel on the combustion phase. CA10 and CA50 are advanced, respectively, and CA90 is postponed due to the increase in diesel mass. This is because there is an increase in highly active fuel as the mass of diesel increases, making it easier to ignite. Therefore, CA10 is advanced.

Figure 17 shows the combustion process when the mass of diesel increases and the mass of methanol remains unchanged. The timing of ignition occurs earlier and the combustion duration is prolonged as the mass of diesel increases. The flame brightness increases as the mass of the diesel increases. The brightness of the flame at M2.4 is much lower than that at M8.8 in the entire combustion process. Only a small portion of the cylinder has a bright fire at M2.4, and the six beams of fuel sprayed are obviously on fire at M8.8. The flame area is larger around the cylinder wall, while the flame is relatively smaller towards the center of the cylinder. The penetration distance of the diesel spray becomes longer, and the surroundings of the cylinder walls are filled with higher concentrations of diesel molecules as the mass of diesel increases, resulting in a larger flame area near the cylinder wall.

The temperature and KL distribution are shown in Figure 18, taking M5.6 and M8.8 as examples. The temperature is relatively lower, and the area of the high-temperature area is relatively smaller at 3.6 °CA of M5.6. The temperature is relatively higher at M8.8, and the flame is very bright. Six high-temperature areas can be seen, and the area of the high-temperature area is also relatively larger. Meanwhile, the area of KL factor reaches its maximum near 3.6 °CA, and then decreases as the combustion process develops at M8.8. It can be seen that the flame is relatively bright around the cylinder walls because the concentrations of the combustible mixture are higher. Therefore, there is more soot formation near the cylinder wall.

Figure 19 shows the KL at M5.6 and M8.8. It can be seen that the peak number of pixels of the KL factor is about 70,000 at M8.8, and the peak number of pixels of the KL factor is about 10,000 at M5.6. The KL at M8.8 is much greater than that at M5.6. Meanwhile, the time of KL factor distribution is longer at M8.8. The ignition process of a diesel engine is compression combustion, and the concentration of the combustible mixture in the cylinder is very important for the soot formation. The concentration of combustible mixture in the cylinder increases with the increase in the mass of diesel in the cylinder. The mixture concentration is high in some areas of the cylinder, and some mixtures may not burn completely. The soot formation will be increased.

4. Conclusions

There is relatively little research on the combustion process and soot distribution in the cylinder of DMDF engines. This article is based on an optical engine and high-speed photography technology, and uses a dual-color measurement method to study the combustion process inside the cylinder of a DMDF engine. The effects of methanol ratio, diesel pre-injection ratio, and the total mass of diesel on the combustion process of DMDF are researched based on an optical engine test platform. The conclusions are listed below:

(1)

The peak pressure and HRR decrease with the methanol ratio increase. CA10, CA50, and CA90 are correspondingly delayed as the methanol ratio increases because of the increased heat absorption by methanol vaporization. The number of pixels representing the KL factor of soot significantly decreases.

(2)

The diesel injected in the early stage increases, which indicates that the activity of the fuel in the early stage increases. The higher activity of the fuel has a positive effect on the main injection fuel, leading to a shortened ignition delay. The CA10 and CA50 advance, respectively. The KL increases slightly as the mass of pre-injected diesel increases.

(3)

The concentration of the mixture increases when the total mass of diesel increases, resulting in an increase in heat generation and an increase in cylinder pressure. CA10 and CA50 are advanced, respectively, and CA90 is postponed. The KL increases when the total mass of diesel increases.

(4)

The peak cylinder pressure for 50%MER, 60%MER, and 70%MER decreased by 2.53%, 6.04%, and 9.76%, respectively, compared with 40%MER. The peak cylinder pressure for PRE-30, PRE-40, and PRE-50 decreased by 2.54%, 2.37%, and 5.29%, respectively, compared with PRE-10. The peak cylinder pressure for M4.0, M7.7, and M8.8 decreased by 11.89%, 23.88%, and 31.43%, respectively, compared with M2.4.