Performance and Emissions of a Premixed Combustion Engine Fueled by Methanol–Gasoline Blends

Background: Methanol is abundant, safe, and environmentally friendly and has physicochemical properties similar to those of gasoline. It is a promising alternative fuel in China because it can be directly used in both spark- and compression-ignition internal combustion engines. The current development of spark-ignition engines focuses on the reduction of the fuel volume and increase in the compression ratio (CR), which would benet the engine’s thermal eciency. However, increasing the CR may deteriorate particulate matter (PM) due to the high temperature. Methods: Herein, an experimental study was conducted on methanol–gasoline blends in a spark-ignition engine. We examined the performance and formaldehyde emissions of methanol–gasoline blends by using three volume fractions (M0, M15, and M100). In addition, the effects of the CR on PM emissions were investigated. Results: The following relationships were observed: (1) When methanol was blended with gasoline, the formaldehyde emissions increased signicantly. The formaldehyde emissions of 100% methanol were higher than those of the methanol–gasoline blend with a methanol volume fraction of 15%; both of these emissions were higher than those of pure gasoline; (2) Increasing the CR resulted in increased PM emissions; (3) For a given blending ratio, the PM emissions were positively correlated with the CR; and (4) The PM emissions were negatively correlated with the methanol volume fraction. Conclusions: Methanol reduces the heat loss at the wall surface. As the ratio of methanol in gasoline increases, the PM emissions decrease. On the other hand, the PM emissions are positively correlated with the CR. The addition of lower alcohols dilutes the concentrations of soot precursors, thereby reducing the soot emissions. fuel consumption of an engine with pure diesel. the of a new fuel system.


Background
The internal combustion engine continues to play a vital role in the eld of transportation because of its advantages such as an excellent reliability, wide applicability, and high work e ciency (Huang et al., 2017). Because of the imbalance of the supply and demand for petroleum resources and increasingly strict emission regulations, the development of biofuels as replacements of traditional fuels, such as gasoline and diesel, and solutions to mitigate pollution arising from traditional fuel emissions have become key areas of research worldwide. Prior investigations have relied on the following approaches to reduce emission-related pollution and to further improve the thermal e ciency of internal combustion engines. First, fuel-reforming methods (Tartakovsky and Sheintuch, 2018) can be used to crack hydrogen and free radicals by using waste heat in the form of exhaust gas. The hydrogen in these cracked products enhances the ame velocity, thus improving the fuel e ciency and reducing emissions. Second, the use of oxygen-containing fuels as additives can signi cantly reduce soot emissions when the oxygen content of the fuel mixture exceeds 30% (Liu et al., 2017). In addition, emissions of nitrogen oxides and greenhouse gases (GHGs), such as carbon dioxide, can be reduced by using fuels based on alcohols because of the large latent heat of vaporization associated with alcohols as well as the low carbon contents relative to gasoline and diesel (Tucki et al., 2019). Third, in addition to the above-mentioned two methods, a dual-fuel approach can also be employed. Both spark-and compression-ignition engines can use dual fuels to increase the thermal e ciency and reduce emissions (Cheng et  Among these measures, the use of oxygen-containing fuel additives has the following advantages: technological feasibility, low cost, and pronounced emissions reduction effects. Because of the advantages introduced by the regeneration of conventional fuels, considerable efforts have been made to save carbon and improve emission pro les; consequently, lower alcohols, that is, mainly methanol and ethanol, have recently received signi cant attention. In China and globally, methanol and ethanol are widely commercially used as vehicle fuels. Methanol and ethanol are linearly structured lower alcohols that yield low carbon emissions. Tucki et al. (2019) studied the carbon dioxide emissions of biofuels by using the New European Driving Cycle (NEDC). Their results suggested that adding biofuels to gasoline can signi cantly reduce carbon dioxide emissions, but, at the same time, can lead to the increase in the consumption of biofuels. Ethanol is highly miscible with water, which causes an inevitable problem when ethanol is used as a vehicle fuel; under both dry and wet conditions, ethanol can cause the severe corrosion of metallic circuits and nonmetallic rubber materials (Jin et al., 2011). On the other hand, coal reserves account for a large proportion of the energy structure in China, whereas methanol can be obtained by employing multiple techniques (Awad et al., 2018). The four standard processes used to synthesize methanol are coal-to-methanol synthesis, coke oven gas reform, carbon dioxide-to-methanol synthesis (Samimi et al., 2018), and natural gas-to-methanol synthesis. The cost of synthesizing methanol from coal is lower than the costs of synthesizing methanol from natural gas and coke oven gas. The global methanol production capacity has increased since 2011 and the production capacity reached 84.317 million tons in 2018, which represents an annual increase of 1%. Researchers are currently drawn toward renewable resources and methanol extracted via black liquor cogasi cation has become a popular choice (Carvalho et al. 2018).
As a simple alcohol-based fuel, methanol is a colorless, transparent, and volatile water-soluble liquid with a slight alcoholic taste. Table 1 lists the physicochemical properties of methanol. The low heating value (LHV), high octane number, and low cetane number render methanol suitable for the application as fuel. dual-fuel mode, however, requires the addition of a new fuel supply system. Methanol has a higher octane number than gasoline, which implies that methanol is more suitable for spark-ignition engines and does not require the change of the structural parameters of the internal combustion engine. Furthermore, the blended combustion of methanol and gasoline has been thoroughly investigated in previous studies. When methanol and gasoline are mixed with a volume fraction above 25% but below 75%, phase separation occurs. Therefore, the addition of a co-solvent is required when using a blended  (2018) conducted life cycle assessments on ten types of present-day vehicles including hydrogen, compressed natural gas, diesel, gasoline, lique ed petroleum gas, methanol, ammonia, hybrid electric, renewable mix, and electric vehicles. Figure 1 shows the life cycle assessments of the vehicles in terms of the human toxicity and ozone layer.
The results in Fig. 1 provide evidence that electric and plug-in hybrid electric vehicles yield a higher human toxicity than MV during the manufacturing and maintenance phases. The value of the human toxicity for electric vehicles reached 0.25 kg 1,4-DB eq km − 1 , whereas the value for vehicles powered by methanol was less than 0.05 kg 1,4-DB eq km − 1 . Hence, MV are less harmful to the environment than traditional gasoline and diesel vehicles. According to previous studies, methanol-gasoline blends with methanol volume fractions above 25% tend to phase-separate. Thus, the volume fractions of methanol used in this study were 15% and 100%. Experiments on the engine power, fuel economy, and unregulated emissions were performed in a naturally aspirated four-cylinder spark-ignition engine. In this study, we also measured the physicochemical properties of methanol based on the test conditions listed in Table 1. Leach et al. (2018) studied the effect of E85 on the particulate number (PN) concentration by varying the EGR, exhaust backpressure, and excess air ratio in a small highly boosted gasoline engine. The results showed that E85 has lower PN emissions across the operating range. However, the gasoline-ethanolmethanol (GEM) blended fuel exhibits entirely different particulate matter (PM) concentrations depending on the operating conditions. Furthermore, due to the lower heating value of methanol than gasoline, a higher compression ratio (CR) may be a solution. The CR directly affects the indicated e ciency of the engine. At a constant mechanical e ciency, increasing the indicated e ciency can improve the effective e ciency of an internal combustion engine. In this study, we analyze the effects of different CRs on the unregulated emissions of methanol by a small single-cylinder gasoline engine.

Experimental Con guration
Methanol (CH 3 OH) is a next-generation biofuel, which belongs to the alcohol family and consists of one carbon atom (single bond). Table 1 lists the physicochemical properties of methanol. Previous studies have reported that methanol-gasoline blends with methanol volume fractions exceeding 25% tend to phase-separate.
Accordingly, the power and fuel economy of methanol-gasoline blends with methanol volume fractions of 15% and 100% as well as pure gasoline were compared in this study.
The formaldehyde emissions of methanol-gasoline blends with methanol volume fractions of 0%, 15%, and 100% were measured under different engine loads. Table 2 lists the parameters of the experimental engine. The combustion process of an internal combustion engine is a complex multi-physics coupled eld of temperature, pressure, and turbulence intensity. The direct solution is complex and costly; thus analyses of the reaction path of unregulated emissions are challenging. Therefore, this discipline has also attracted signi cant attention (Luong et   xtensive simulation studies on the combustion process of internal combustion engines have been conducted. Typical simulation methods include large-eddy simulations on one-eighth of a combustion chamber. More precise simulation models yield more reliable results as well as a higher stiffness with respect to the equations (Wu et al., 2018); Consequently, the calculation cost also increases. Thus, in this study, the CR was varied to adjust the maximum combustion pressure to compare the change in unregulated emissions at different CRs. Table 3 lists the parameters of the adjustable single-cylinder gasoline engine used in the experiment. In the experiment, the maximum CR of M15 (15% volume fraction of methanol and 85% volume fraction of gasoline) was set to 7.8, while the maximum CR of M100 (pure methanol) was set to 10.0 to mitigate the damage to the internal combustion engine from the knock of the methanol-gasoline blends. The speed was controlled at 900 rpm, the intake air temperature was 307 ± 0.5 K, and the ignition advance angle, which signi cantly affects the combustion and emissions according to earlier work , was xed at 13° BTDC. The CR was adjusted to a smaller value such that the knock meter readings of different ratios of methanol-gasoline blends were identical.

Results And Analysis
In this study, both the external and load characteristics of a spark-ignition engine fueled with different volume fractions of methanol were measured. A comparative analysis was performed on the methanolgasoline blends of different volume fractions (M0, M15, and M100) to evaluate the power and fuel economy of the engine when using methanol-gasoline blends. Co-solvents were not added to the methanol-gasoline blends during the experiments and the throttle of the methanol-gasoline engine was kept open. The water and oil temperatures were also kept constant. An AVL 444 gas analyzer was used to measure the excess air ratio of the engine exhaust. When the excess air ratio was equal to 1, the relationships between the three performance indicators of the engine, that is, the effective power (Pe), torque (Me), fuel consumption rate (be), and engine speed (n), were measured. The fuel consumption rate was also calculated to obtain the fuel consumption ratios of M0, M15, and M100, that is, the methanolgasoline blends with methanol volume fractions of 0%, 15%, and 100%, respectively. In addition, the changes in the formaldehyde emissions with the methanol ratio under different engine loads were measured. Figure 2 shows the engine performance at 100% load for different methanol ratios.
The external characteristic curve of gasoline in Fig. 3 indicates that the torque reached a maximum value at a particular speed. Below this speed, the in-cylinder ow weakened, the ame velocity decreased, and the leakage and heat loss increased. Above this speed, the friction loss, accessory consumption, and pumping loss substantially increased with the increase in the speed. As a result, the torque did not increase because of the increasing mechanical losses. As shown in Fig. 2, the power curve of gasoline is proportional to the torque multiplied by the speed. Thus, the gasoline power curve rst increases with the speed.
After reaching a particular speed, the rate of the decrease in the torque is higher than the rate of the increase in the speed, which causes the power to decrease. In Fig. 2, the torque of M15 is lower than that of gasoline. When the speed was varied, the torque of M100 was higher than that of M15 and gasoline.
This is because methanol's latent heat associated with vaporization is higher than that of gasoline. Based on Eq. (1) (Frenklach et al., 2019), the heat loss at the wall surface is the smallest for M100: where T g is the exhaust gas temperature (K) and h C is the heat transfer coe cient (W/m 2 · K) (Natesan et al., 2019).
The power, in descending order, is M100 > gasoline > M15, which indicates that, as the speed increases, the torque reduction rate of M100 is the smallest, whereas those of gasoline and M15 are comparable.
The latent heat of vaporization of M100 is higher than that of gasoline. As a result, the exhaust temperature is low, where A represents the heat transfer area, with a minimal wall surface heat loss. The power curves of M100 and M15 are comparable to that of gasoline. At low speeds, the friction losses near the piston and tappet are the largest, resulting in a low mechanical e ciency associated with M100. Figures 3, 4, and 5 show the load characteristics and changes in the formaldehyde emissions due the loading of gasoline, M15, and M100, respectively.
According to Table 1, the LHV of methanol is signi cantly lower than that of gasoline. As a result, the BSFC increases with the increase in the volume fraction of methanol and M100 has the highest BSFC. In addition, the combustion product of methanol is formaldehyde. With an increase in the volume fraction of methanol, the total formaldehyde emissions linearly increase at each power. The results in Figs. 2, 4, and 5 demonstrate that the power of M100 corresponding to the lowest formaldehyde emissions is ~ 19.2 kW.
Compared with M15 and gasoline, the power of the methanol-gasoline blends corresponding to the lowest formaldehyde emissions in descending order is P EM100 > P EM15 > P Egasoline . The selection of a higher power operating condition at 2,000 rpm for the spark-ignition engine fueled with M100 reduces the formaldehyde emissions without affecting the power and fuel economy of the internal combustion engine.
In the experiment, the maximum CR of M15 (15% volume fraction of methanol and 85% volume fraction of gasoline) was set to 7.8, while the maximum CR of M100 (pure methanol) was set to 10.0 to mitigate the damage to the cylinder liner and bearing bush as a result of the knocking of the methanol-gasoline blends. The test speed was controlled at 900 rpm, with an intake air temperature of 307 ± 0.5 K. Figure 6 demonstrates that the PM mass concentration positively correlates with the CR.
The Otto cycle resulting from heating an ideal gas reduces the heat dissipation of the ideal Otto cycle as the CR increases, thereby improving the indicated thermal e ciency. However, an increase in the CR increases the in-cylinder turbulence intensity and the combustion rate further accelerates. The PM mass concentration decreases as the methanol concentration increases. This can be explained by the oxygen content of methanol, which leads to more oxygenated PM. A non-uniform premixed gas forms locally, which increases the mass concentration emission. Figure 6 also shows that the increased methanol ratio reduces the PM concentration of M100 below that of M15. This result is consistent with the observations of Li et al. (2009) and Uslu and Celik (2020) based on the process of soot surface growth who reported that the addition of lower alcohols dilutes the concentration of soot precursors, thereby inhibiting soot emissions.

Conclusions
The external and load characteristics of a spark-ignition engine fueled with different volume fractions of methanol were determined in this study. In addition, unregulated emission tests and PM emission tests with varying CRs were performed on methanol-gasoline blends with different volume fractions (M0, M15, and M100). Based on the experiments, the following conclusions can be drawn: As the speed increases, the torque reduction rate of M100 becomes the smallest, whereas the torque reduction rates of gasoline and M15 are comparable. The power in descending order is M100 > gasoline > M15. Thus, methanol reduces the heat loss at the wall surface. The power of the methanol-gasoline blends corresponding to the lowest formaldehyde emissions in descending order is P EM100 > P EM15 > P Egasoline . The selection of higher power operation at 2,000 rpm for the spark-ignition engine fueled with M100 reduces the formaldehyde emissions without affecting the power and fuel economy of the internal combustion engine.
As the ratio of methanol in gasoline increases, the PM emissions decrease. On the other hand, the PM emissions are positively correlated with the CR. The addition of lower alcohols dilutes the concentrations of soot precursors, thereby reducing the soot emissions.

Declarations
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