Influence of Glycerol on Methanol Fuel Characteristics and Engine Combustion Performance

Abstract

Methanol derived from solar energy is a carbon-neutral alternative fuel for engines. The low viscosity of methanol is one of the problems that restrict its direct compression ignition application in engines. Glycerol is a renewable resource derived from biomass, and its viscosity is more than 1700 times that of methanol. In this study, glycerol was mixed with methanol in different volume fractions (1–50%), and a methanol-glycerol mixture with similar viscosity to diesel was prepared. Then, the particle size, electrical conductivity, viscosity, swelling and corrosion characteristics of the mixed fuel were measured. Finally, the combustion and emission tests of methanol-glycerol mixed fuel were carried out on a heavy-duty multi-cylinder diesel engine. The results show that glycerol can effectively adjust the viscosity of the mixed fuel. The viscosity of the mixed fuel can reach 3.19 mm²/s at 20 °C when blended with 30% glycerol by volume, which meets the requirements of the national standard for diesel fuel. The addition of glycerol can alleviate the corrosion of methanol to the polymer. The test of the mixed fuel in the direct compression ignition engine shows that the thermal efficiency of methanol mixed with 5% glycerol was further improved than that of pure methanol, both of which were significantly higher than the thermal efficiency of diesel compression ignition engines. Methanol and 5% glycerol by volume blends can reduce soot and nitrogen oxide emissions while maintaining low HC and CO emissions. Therefore, proper blending of glycerol in methanol fuel can optimize the fuel properties of methanol and achieve higher thermal efficiency and lower pollutant emissions than pure methanol direct compression ignition.

1. Introduction

The application of fossil energy has promoted the development of human modernization, but it has also made human life highly dependent on non-renewable resources such as petroleum [1]. The environmental pollution caused by the burning of fossil fuels has also become the focus of attention, so it is imperative to find clean alternative fuel [2,3,4]. Methanol is a liquid organic matter with a wide range of sources, which can be obtained from biomass degradation or chemically synthesized from coal. They are the classical ways of the production of methanol [5,6]. More importantly, the production of hydrogen from renewable energy and then synthesis of methanol with carbon dioxide is one of the important ways to achieve carbon-neutral fuel production [7,8]. The high oxygen content of methanol can effectively reduce soot emissions, and at the same time, the better volatile mixing characteristics lead to a uniform gas mixture distribution, which can effectively reduce nitrogen oxide emissions [9,10].

There are three main ways to apply methanol in engines. First, methanol is injected at low pressure to form a homogeneous mixture and then ignited by a spark plug. The main advantage is that the fuel supply system is simple, and the spraying and combustion process are similar to those of gasoline engines. The main disadvantages are low thermal efficiency and high emissions of incompletely combusted methanol and formaldehyde [11]. The second is methanol-diesel dual-fuel compression ignition. Two sets of fuel supply systems are required considering the instability of the methanol-diesel mix fuel. Usually, diesel is injected with high pressure into the cylinder to ignite methanol as a highly active fuel in this combustion mode. The compression ratio of dual-fuel combustion mode is higher than the spark-ignition engine, and there is no intake throttling loss, so the thermal efficiency under different loads is better than the spark-ignition [12,13,14]. But the structure of two sets of fuel supply systems is complex and the dual high-pressure injection system is more likely to be applied to heavy-duty engines with larger bores or marine engines due to space structure and cost constraints [5]. The third way of methanol application is direct compression ignition of pure methanol. Only one fuel supply system is required, and the high compression ratio ensures high thermal efficiency. However, there are two challenges. On one hand, from the perspective of engine technology, how to ensure that methanol fuel is compressed and ignited and burns stably; on the other hand, from the perspective of fuel technology, how to make methanol fuel meet the operating requirements of the high-pressure direct injection fuel supply system [15].

Until now, the challenges on direct compression ignition of pure methanol have been paid more attention to effectively overcome them by improving methanol fuel characteristics or controlling good combustion conditions. Gong et al. [16] and Cui et al. [17] found that increasing the temperature in the engine cylinder is the dominant factor to ensure the stable ignition of the direct injection methanol fuel in the cylinder. Wang et al. [18] studied different types of preheating systems to enable the methanol ignition engine to start smoothly at the ambient temperature of −10 °C. Liu et al. [19] found that increasing the intake air heating temperature to 120 °C can ensure the smooth start of a heavy-duty multi-cylinder diesel engine fuelled by methanol at a cooling water temperature of 30 °C.

In the aspect of fuel modification, the low viscosity of methanol will cause wear of the high-pressure fuel pump, affecting the reliability of the fuel pump and the mechanical efficiency of the engine. Meanwhile the low viscosity of the fuel will make it difficult to establish a high fuel injection pressure during the fuel supply process [20]. The prolonged fuel injection duration affects the subsequent combustion process, which is one of the important challenges faced by methanol [21]. Due to the high polarity of methanol, and the weak polarity of macromolecular viscosity index improvers, and the large difference in molecular weight between the two, it is difficult to dissolve both. This makes it difficult for general viscosity index improvers to work. Therefore, it is necessary to find methanol-soluble additives with improved viscosity to solve the problem of low viscosity. In addition, the polarity of methanol is similar to that of rubber products, which leads to swelling failure of rubber, which is potentially corrosive to engine components and fuel storage and transportation [22]. The corrosion characteristics of the fuel after the addition of the new viscosity improver also needs to be further explored.

Glycerol, as a liquid organic substance with very high viscosity, can effectively adjust the viscosity of methanol fuel, and with the increase of biodiesel production, the supply of glycerol as a by-product of biodiesel industry has risen sharply [23,24,25]. It belongs to the excess state and adding it to the fuel can utilize the excess glycerol as a resource [26,27]. At present, most of the glycerol is to be used as a fuel, and it is mostly used as a raw material to produce biodiesel, and there is no precedent for direct combustion of glycerol [28].

The purpose of this paper is to investigate the use of glycerol as a performance improver for methanol fuel, to study the structure and stability of the mixed fuel, and to test the effect of glycerol on the viscosity of methanol fuel. At the same time, due to the characteristics of methanol itself, the formation of formic acid after the reaction of free radicals, and water absorption, glycerol will cause different degrees of corrosion to metals [10]. Therefore, this paper will also analyse the swelling resistance and corrosion of various polymers and metal materials in the fuel. Finally, a bench test is carried out on the engine to analyse its pollutant emission characteristics, which provides theoretical and technical reference for the application of the fuel in the heavy-duty diesel engine.

2. Materials and Methods

2.1. Materials

Methanol used in the research was purchased from Tianjin Yuanli Chemical Co., Ltd. (Tianjin, China) with analytical purity (AR, ≥99.5%); glycerol was purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. (Tianjin, China) with purity of AR (≥99.5%).

Table 1. Properties of methanol and glycerol [29,30,31,32].

Parameters	Unit	Methanol	Glycerol
Molecular formula	-	CH3OH	C3H8O3
Molecular weight	g/mol	32.02	92.09
Oxygen content	wt.%	49.97	52.12
Cetane number	-	5	-
Stoichiometric air-fuel ratio	-	6.46	6.83
Latent heat of vaporization	kJ/kg	1162.64	663.75
Flash point	°C	11~12	176
Viscosity (mm2/s)	mm2/s	0.695	1190
Boiling point	°C	64.8	290
Density	g/cm3	0.791	1.261
Low calorific value	MJ/kg	19.93	20.30

(Latent heat of vaporization is the value at 25 °C, Viscosity and Density are the values at 20 °C).

lists some physical and chemical parameters of these substances. The viscosity of glycerol is very large, more than 1700 times that of methanol. At the same time, the density of glycerol is significantly higher than that of methanol. The lower calorific value per unit mass of glycerol and methanol are similar. Mixed fuels with different glycerol concentrations were prepared by adding different amounts of glycerol to methanol. The volume ratios of glycerol were 1%, 3%, 5%, 10%, 20%, 30%, 40%, and 50%, respectively.

Engine bench tests were performed on an inline 6-cylinder 24-valve heavy duty diesel engine equipped with a high-pressure common rail fuel injection system. The detailed parameters of the engine are listed in

Table 2. Test engine specifications.

Parameters	Unit	Value
Engine displacement	L	7.7
Compression ratio	-	17.5
Bore size × stroke	mm	110 × 135
Max torque/speed	N·m/rpm	1450/1100~1700
Rated power/speed	Kw/rpm	257/2200

. The engine meets Euro VI emissions regulations by using high pressure Exhaust Gas Re-circulation (EGR), diesel oxidation catalyst (DOC), diesel particulate filter (DPF) and selective catalytic reduction (SCR) aftertreatment technologies. In this experiment, EGR, DOC, DPF and SCR are removed, and all emission data are the original emissions in the cylinder.

2.2. Methods

2.2.1. Fuel Characteristic Test Methods

The instrument for measuring the particle size of the fuel was a nanoparticle size and Zeta potential analyser produced by Anton Paar, Graz, Austria, the model was Litesizer 500, and the particle size measurement range was 0.3 nm–10 μm. The instrument works on the principle of dynamic light scattering (DLS). Using a disposable sample cell, add 1 mL of sample to the instrument. At a constant temperature of 20 °C, a semiconductor laser emits laser light (40 mW, 658 nm) and is incident on a rectangular sample cell containing a sample. When the laser light passes through the colloid, the particles scatter the laser light, and the detector can detect the scattering at the set scattered light in the angular direction. The scattered light received by the detector is processed by the correlator, and the obtained data is sent to the computer for data analysis and calculation and a sample was taken and measured three times

The conductivity meter was the model ORION 3 STAR, available from Thermo Company (Waltham, MA, USA). When measuring conductivity, place the agent to be tested in a constant temperature water bath, and when the temperature remains stable, fully immerse the conductivity meter probe under the liquid surface for reading. After measuring a group of samples, rinse the probe with pure methanol, wipe it with filter paper, and dry it before testing the next group of samples. Each sample was taken and measured three times. Volumes of 0.2 mL, 0.5 mL and 1.0 mL of distilled water were also added to the mixed fuels with different glycerol volume ratios, respectively, to explore the influence of moisture on the structure of the mixed fuels.

The model of the kinematic viscometer was DSY-019A, supplied by Anton Paar. During the entire measurement, we used an ethanol bath when the temperature was below 20 °C and an oil bath when the temperature was above 20 °C. It is worth noting that when placing the sample, the glass viscometer needs to be adjusted to the vertical state. Repeat the measurement three times to get the average value and multiply the time for the liquid level to drop by the coefficient corresponding to the viscometer to obtain the viscosity of the sample.

2.2.2. Swell Ability and Corrosion Test Methods

To study the corrosion and swelling characteristics of methanol and mixed fuel mixed with glycerol on typical metals, rubbers and plastics, the selected polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) plastic pipes and nitrile rubber were purchased from Tianjin Bailixin Rubber and Plastic Products Sales Co., Ltd. (Tianjin, China). The ABS resin was purchased from Shenzhen Huibaijia Plastic Co., Ltd. (Shenzhen, China), and the stainless-steel sheet, aluminium alloy sheet and copper sheet were purchased from Tianjin Yicheng Metal Technology (Tianjin, China).

In this study, the room temperature impregnation method was used to study the swelling effect of mixed fuels on polymers. Cut plastic tube samples with an outer diameter of 2 cm and a thickness of 2 mm into 1 cm lengths so that they have the same surface area in contact with the fuel. After being sealed and protected from light for 28 days at a temperature of 30 ± 2 °C, the polymer material will suffer from obvious swelling and erosion [33]. The rate of change in mass was calculated based on the mass of the polymer before soaking. The relationship between mass change rate and swelling resistance is shown in

Table 3. Evaluation index of swelling resistance of polymer [33].

Grade	Rubber	Plastic
A	Both Δm and ΔV do not exceed 5%	Both Δm and ΔV do not exceed 5%
B	Both Δm and ΔV do not exceed 10%	Both Δm and ΔV do not exceed 10%
C	Both Δm and ΔV do not exceed 20%	Both Δm and ΔV do not exceed 15%
D	Both Δm and ΔV do not exceed 30%	Both Δm and ΔV do not exceed 20%
E	Others	Others

(A: Permanently resistant to swelling; B: Long-term resistant to swelling; C: Medium-term resistant to swelling; D: Short-term resistant to swelling; E: Not resistant to swelling).

.

The corrosion test in this paper was carried out according to the standard GB/T 5096-2017. A metal sheet of 75 mm × 12 mm × 2 mm was polished and immersed completely in mixed fuels of different proportions and immersed at 50 °C for 3 h. After immersion, the changes in colour and texture of the metal sheets were compared, and the corrosion conditions were evaluated [34].

2.2.3. Engine Test Methods

The in-cylinder pressure test of the engine relates to the corresponding charge amplifier (Kistler 5011B10, Winterthur, Switzerland) and data acquisition system through a pressure sensor (Kistler 6125C), and 100 engine cylinders were continuously collected by an optical crank angle encoder (Kistler 2614A4) with a resolution of 0.5 °CA pressure data. Gaseous emissions, including nitrogen oxides (NOx), formaldehyde (HCHO), methanol (MEOH), hydrocarbons (HC) and carbon monoxide (CO), were measured by an exhaust emission analyser (HORIBA MEXA7100DECR, Kyoto, Japan) and exhaust soot was measured with a filter paper smoke meter (AVL 415S). The combustion experiment conditions were as follows: the rotational speed was 1200 rpm, the load rate was 10%, the single injection, the injection pressure was 40 MPa, and the main injection timing was −12 °CA ATDC. During the experiment, the intake air temperature and cooling water temperature were controlled at 50 ± 2 °C and 85 ± 2 °C respectively when using diesel fuel, and the intake air temperature required to maintain stable ignition when using methanol fuel was 120 °C. For comparative study, diesel fuel tests at an intake air temperature of 120 °C were also carried out. To explore the effect of blending glycerol, the test initially studied the measurement of 5% glycerol. At the same time, Afton 4140F anti-wear agent was added to reduce the wear of the fuel on the fuel supply system when testing methanol and methanol/glycerol mixed fuel. The diameter of the fuel wear scar can be less than 460 microns, which is the standard of diesel fuel wear scar and can well protect the fuel supply system [35,36]. At each test point, the engine was run for several minutes until the controlled measurement parameters stabilized. Combustion pressure, performance and emissions are then measured and recorded.

3. Results and Discussion

3.1. Structural Analysis of Designed Fuel

Methanol and glycerol are polar organic substances with strong mutual solubility and can be dissolved in any ratio at the temperature involved in the experiment (−20~60 °C). The mixed fuel was homogenized by a vortex for one minute, and the particle size and conductivity measurements were completed within one day after mixing. The rotational speed of the vortexer was 10,000 rpm. As high concentration alcohols have strong hygroscopicity and can absorb moisture in the air, the alternative fuel is inevitably polluted by water during storage and transportation. Moreover, the presence of moisture in methanol fuel will accelerate acid corrosion and electrochemical corrosion of metals [33]. To study the effect of trace amounts of water on alternative fuels, 0.2 mL (2%), 0.5 mL (5%), and 1.0 mL (10%) of distilled water were added to the mixed fuels in the same proportions (10 mL of mixed fuels in total). The conductivity of the distilled water used in the experiment was measured to be about 0.4 μS/cm. Their particle sizes and electrical conductivity were measured and compared with the water-free group. Under different glycerol ratios, the addition of water only shifts the conductivity of the mixture to that of water. The results are shown in Figure 1 and Figure 2. It should be noted that even if 1.0 mL of distilled water is included, the fuel mixture is still non-stratified.

The actual measurement results in Figure 1 show that the particle size of mixed fuels was in the range of 1–5 nm without the water addition, which is significantly positively correlated with the proportion of glycerol, and the particle size of mixed fuels was significantly larger than the molecular size of glycerol (about 0.4 nm). It is generally believed that the mixed solution with a particle size between 5 and 100 nm is a microemulsion. It does not belong to the miscible state, but it is thermodynamically stable, and as a rule of thumb, microemulsions do not separate after five minutes of centrifugation [37]. The structure of microemulsions has been extensively studied and can be divided into three types: oil-in-water, water-in-oil, and double-continuous structures, which are affected by the ratio of different polar components [38]. The main component of the alternative fuel in this study was methanol; methanol constitutes the continuous phase, and glycerol is used as the dispersed phase. During the mixing process, as methanol and glycerol are both strong polar substances, they are both prone to form hydrogen bonds, so there is a strong tendency to disperse each other [39]. The glycerol droplets were gradually broken up by methanol resulting in a progressively smaller particle size. The difficulty of being continuously dispersed also increases, and finally a dynamic equilibrium is reached, so that glycerol maintains a relatively stable dispersion state. Therefore, methanol and glycerol can be considered to be in a microemulsion state after being dissolved in each other.

The electrical conductivity can reflect the structure of the microemulsion. Figure 2a shows the electrical conductivity under different volume ratios of glycerol without water addition. Because the conductivity of pure glycerol is lower than that of methanol, the conductivity of the mixed fuel decreases with the increase of the proportion of glycerol. The figure shows that the rate of change of electrical conductivity is very obvious. The average rate of change within the interval was calculated from the endpoint values of each interval. In the interval of increasing the volume ratio of glycerol from 1% to 3%, the conductivity decreases by an average of 19.57% for every 1% increase of glycerol. In the range from 30% to 40%, the conductivity of glycerol decreases by an average of 2.55% for every 1% increase in glycerol. This may be because when the proportion of glycerol is low, the system is an oil-in-water structure (methanol is regarded as the water phase, and glycerol is regarded as the oil phase), and the newly added glycerol directly increases the difficulty in conducting electricity in the microstructure. When the proportion of glycerol is greater than 10%, the system becomes a double-continuous structure, and the effect of the newly added glycerol on the original conductive path is weakened. By analysing the change in electrical conductivity, it can be speculated that the proportion of microemulsion from “oil-in-water” structure to “double-continuous” structure is likely to be between 15% and 25%. Figure 2b shows that the addition of water shifts the value of the mixed conductivity towards that of water. The amount of water added in the experiment is large, and the conductivity does not fluctuate drastically in this case. Therefore, when the fuel is actually used, the effect of the absorbed moisture will be small.

Viscosity is one of the important physical parameters of the fuel, which is used to measure the fluidity and atomization performance of the fuel. The viscosity has a significant impact on the fuel injection system [40]. The kinematic viscosity of diesel fuel should be between 3 and 8 mm²/s at 20 °C. As can be seen in Figure 3, at 20 °C, when the volume ratio of glycerol is between 30% and 45%, the viscosity is within this range, which is suitable for direct application in the fuel system and combustion process of diesel engines. However, since the fuel injection process will be affected by the higher temperature around the engine, the actual injection situation still needs to be verified.

Figure 3 also shows that the effect of temperature on viscosity is more pronounced at higher proportions of glycerol. When the volume ratio of glycerol is 5%, the temperature increases from 20 °C to 40 °C, and the viscosity decreases by about 24.29%. When the volume ratio of glycerol is 40%, the viscosity decreases by about 44.87% under the same conditions. The viscous force of liquid substances is caused by the intermolecular attraction [41]. The higher the temperature, the more intense the thermal motion of the molecules. For liquids, the molecular spacing will increase and the viscosity will decrease. As the measured equivalent particle size of the component with the higher volume of glycerol is larger, the change of molecular spacing is more affected by the thermal motion of molecules when the temperature increases, resulting in a faster decrease in viscosity. In conclusion, it is speculated that the smaller aggregates of glycerol molecules reduce the resistance during movement. The hydrogen bonds between alcohols, especially glycerol molecules, will be destroyed by high temperature, resulting in the reduction of their viscosity.

3.2. Swelling and Corrosion Analysis of Designed Fuel

The corrosion of mixed fuels to substances can be divided into static corrosion and dynamic corrosion. In oil storage equipment and low-flow oil pipelines, the corrosion caused by the direct contact of the equipment material and the chemical reaction of the fuel is regarded as static corrosion [42]. In the engine, the components in contact with the fuel move at high speed, and the apparent change caused by friction is called dynamic corrosion. This paper only evaluates the static corrosion through the swelling of the polymer and the oxidative corrosion of the metal. As the engine running time is not long enough, no dynamic corrosion problem is found.

Different fuels were evaluated according to the classification standard of polymer swelling resistance, and the results are shown in Figure 4. This shows that the corrosive effect of different mixed fuels on plastic products is very weak, and the corrosion on nitrile rubber is strong. Nitrile rubber becomes hard after soaking, loses elasticity, and completely loses its proper performance. When pure glycerol is used to soak the polymer, there is no change in the quality and texture of the polymer before and after soaking, and the corrosiveness of the mixed fuel is also significantly weaker than that of pure methanol. Therefore, the addition of glycerol can effectively alleviate the effect of methanol on the polymer swelling and corrosion. This may be because glycerol molecules are more attractive to methanol than rubber molecules due to their smaller molecular weight and the tendency to form many hydrogen bonds.

Figure 5 shows the corrosion of copper, carbon steel, aluminium alloy, and stainless steel in mixed fuel systems with mixed fuels under different glycerol ratios. In this figure, tube 1 is pure methanol, tube 10 is pure glycerol, and the volume ratio of glycerol in tubes 2–9 were 1%, 3%, 5%, 10%, 20%, 30%, 40%, 50%, respectively. It shows that the four metals are not corroded in the methanol-glycerol mixed system, and the mixed fuel does not change after soaking the metal. Among them, the corrosion resistance of copper has reached the level of Grade 1 according to the national standard GB/T 5096-2017 (the copper sheet after immersion is light orange, almost the same as the newly polished copper sheet). It is worth noting that the current standard soaking time is short, after greatly increasing the soaking time (two weeks), it is found that the carbon steel soaked in pure methanol has obvious rust spots. The whole process remains sealed, which indicates that methanol fuel has certain corrosion to carbon steel, but it has no corrosion to other tested metal materials under the tested conditions.

3.3. Application of Methanol Substitute Fuel in Diesel Engine

The combustion process of methanol fuel in the engine is significantly different from that of diesel fuel, and the addition of glycerol is expected to improve some deficiencies in the characteristics of methanol fuel [43]. Figure 6 depicts the combustion phase of different fuels. It shows that the ignition delay (ID) of pure methanol fuel is longer than that of diesel fuel, and CA10-50 (the crank angle experienced in the range of 10–50% of the cumulative heat release of combustion) is shorter. Adding 5% by volume of glycerol shortens the flame retardation period, but also shortens CA50-90 (the crank angle experienced in the range of 50–90% of the cumulative heat release of combustion).

Figure 7 shows the in-cylinder pressure and thermal efficiency under different crankshaft rotation angles. The cylinder pressures and heat release rates of diesel fuel at two different intake temperatures were similar. Pure methanol fuel has longer ignition delay period, more premixed gas, more concentrated combustion heat release, and higher peak heat release rate. The addition of glycerol shortens the combustion delay period of pure methanol fuel, and the combustion phase is closer to the top dead centre, so the peak heat release and in-cylinder pressure peak are higher, and the combustion duration is shorter than that of pure methanol. Methanol and the mixed fuel mixed with 5% glycerol have a significantly higher indicated thermal efficiency than diesel fuel due to the concentrated combustion heat release, as shown in Figure 8. Moreover, after adding glycerol, the indicated thermal efficiency is further improved, which should be mainly caused by the combustion phase being closer to the top dead centre. It also proves that the thermal efficiency of mixed fuel is higher than that of direct ignition and dual fuel mode at the similar engine loads.

Figure 9 is a comparison diagram of pollutant content in diesel engine exhaust. The HCHO, MEOH, HC and CO emissions of pure methanol fuel were higher than those of diesel fuel, but the addition of glycerol in the system effectively reduced the emissions of HC and CO pollutants. For HCHO and MEOH emissions, 5% glycerol did not cause significant improvement. The ignition delay period of methanol fuel was longer, which caused more mixture to enter the gap area of the in-cylinder combustion chamber and could not be completely oxidized and combusted. This led to an increase in HC and CO emissions, and an increase in unburned methanol and formaldehyde emissions from low-temperature combustion. However, it should be emphasized that the HCHO, MEOH, HC, and CO emissions of the current compression ignition mode were significantly lower than those of the methanol spark ignition mode and the low-pressure dual-fuel mode, and the reduction ratio reached more than 90% [6]. The soot and NOx emitted by methanol fuel were very low, and the addition of glycerol did not change them. It is mainly because the long ignition delay period improves the non-uniformity of the mixture and reduces NOx emissions, and the high oxygen content inhibits soot emissions. Overall, under the test load, the indicated thermal efficiency and pollutant emission of direct methanol compression ignition, especially methanol and formaldehyde, were better than the direct ignition and dual fuel mode. Furthermore, methanol-glycerol blends can achieve better indicated thermal efficiency and emissions than pure methanol for major pollutants.

4. Conclusions

(1)

Methanol and glycerol have good mutual solubility and stratification does not occur when some water is absorbed from the air. Through the analysis of particle size and electrical conductivity, it was proved that the mixed solution of methanol and glycerol exists stably in the form of a microemulsion.

(2)

The viscosity of the fuel with methanol as the main component can be reconciled by glycerol. At 20 °C, when the volume ratio of glycerol is between 30% and 45%, the viscosity of the mixed fuel is comparable to that of diesel. The viscosity of the mixed fuel is greatly affected by the temperature. And the more glycerol added, the more the viscosity is affected by the temperature.

(3)

Pure methanol will seriously corrode rubber products, and the addition of glycerol can effectively alleviate the swelling effect of methanol on the polymer, and the addition of 1% glycerol can effectively prolong the corrosion resistance of nitrile rubber. At the same time, under the test conditions, the mixed fuels of methanol and glycerol with different addition ratios did not corrode common metal materials in a short time, but the carbon steel showed rust spots after being stored for two weeks.

(4)

Engine tests shows that adding 5% glycerol to methanol fuel can reduce the ignition delay period of pure methanol fuel, improve the pressure in the cylinder, and greatly improve the indicated thermal efficiency of diesel compression ignition engine. The indicated thermal efficiency was increased from 38.3% of the original engine as fulling diesel to 43.1% as fulling methanol with 5% glycerol. And the mixed use of methanol and 5% glycerol by volume can reduce the emissions of soot and nitrogen oxides while maintaining the lower HC and CO emissions.