Review of Pre-Ignition Research in Methanol Engines

Abstract

Methanol can be synthesized using green electricity and carbon dioxide, making it a green, carbon-neutral fuel with significant potential for widespread application in engines. However, due to its low ignition energy and high laminar flame speed, methanol is susceptible to hotspot-induced pre-ignition and even knocking under high-temperature, high-load engine conditions, posing challenges to engine performance and reliability. This paper systematically reviews the manifestations and mechanisms of pre-ignition and knocking in methanol engines. Pre-ignition can be sustained or sporadic. Sustained pre-ignition is caused by overheating of structural components, while sporadic pre-ignition is often linked to oil droplets entering the combustion chamber from the piston crevice. Residual exhaust gas trapped within the spark plug can also initiate pre-ignition. Knocking, characterized by pressure oscillations, arises from the auto-ignition of hotspots in the end-gas or, potentially, from deflagration-to-detonation transition, although the latter requires further experimental validation. Factors influencing pre-ignition and knocking, including engine oil, in-cylinder deposits, structural hotspots, and the reactivity of the air–fuel mixture, are also analyzed. Based on these factors, the paper concludes that the primary approach to suppressing pre-ignition and knocking in methanol engines is controlling the formation of pre-ignition sources and reducing the reactivity of the air–fuel mixture. Furthermore, it addresses existing issues and limitations in current research, such as combustion testing techniques, numerical simulation accuracy, and the mechanisms of methanol–oil interaction, and offers related recommendations.

1. Introduction

Internal combustion engines will continue to play a critical role in automotive power systems over the next 30 years [1]. Traditional high-carbon fuels, such as gasoline and diesel, generate significant carbon dioxide emissions during combustion, making it necessary to develop low-carbon fuels to reduce the environmental impact of internal combustion engines. In the current energy structure, the proportion of renewable energy in total energy consumption is steadily increasing. Renewable energy sources, primarily wind and solar power, generate fluctuating electricity, which can strain power systems, overload the grid, and reduce energy utilization efficiency, leading to energy waste. Converting this excess electricity into carbon-neutral fuels can further promote rational energy use and minimize environmental damage.

Methanol is a typical carbon-neutral fuel that can be synthesized from carbon dioxide and hydrogen through electrochemical methods. During combustion, methanol emits significantly less carbon than gasoline or diesel and produces almost no sulfur oxides or particulate matter. Additionally, methanol is a liquid at ambient temperature and pressure, making its storage and transportation compatible with existing liquid fuel systems, which greatly reduces its usage costs. More importantly, with the continuous advancement of green electricity-based methanol production and biomass-derived methanol technologies, methanol’s production cost is decreasing, and the carbon neutrality of its production chain is steadily improving. In the transportation sector, methanol fuel has the potential to become a key option for internal combustion engines to achieve carbon neutrality [2]. However, it is worth noting that, due to methanol’s low minimum ignition energy, it is prone to pre-ignition when hotspots exist in the cylinder, leading to knocking [3], which adversely affects engine performance and reliability. Therefore, addressing pre-ignition and knocking issues is critical in the development of methanol engines.

This paper briefly reviews the progress of current research on methanol engines, with a particular focus on pre-ignition and knocking issues. It analyzes their manifestations and mechanisms, summarizes the challenges faced by existing studies, and provides insights into potential measures for mitigating methanol pre-ignition and knocking.

The rest of this paper is structured as follows: Section 2 provides an overview of methanol’s applications in internal combustion engines. Then, Section 3 discusses the current issues in methanol engines, highlighting pre-ignition and knocking. Following these issues, Section 4 reviews the manifestations and mechanisms of pre-ignition and knocking. Given the complexity of the pre-ignition mechanism, Section 5 examines the factors influencing methanol pre-ignition. Finally, Section 6 summarizes the findings and presents an outlook on future research directions to tackle the remaining challenges in pre-ignition and knocking in methanol engines.

2. Application of Methanol Fuel in Engines

Methanol can be used in both spark-ignition (SI) and compression-ignition (CI) engines. It can function as a standalone fuel or as part of a dual-fuel combustion system with gasoline, diesel, or natural gas [3].

2.1. Spark-Ignition Methanol Engines

In spark-ignition methanol engines, methanol can be injected via the intake port or directly into the cylinder. Xie et al. [4] optimized ignition timing and EGR rates under intake port injection conditions to achieve better performance. Güdden et al. [5] conducted tests on large-bore engines and found that methanol’s thermal efficiency could exceed that of natural gas under similar conditions. However, unburned methanol and formaldehyde emissions in methanol engines did not meet IMO emission standards. In studies on direct-injection SI methanol engines, Li et al. [6] found that methanol injection timing and ignition timing significantly affected engine performance and emissions. Advancing injection timing improved air–fuel mixing, accelerated combustion, reduced CO emissions, and increased NO_x emissions. Liu et al. [7] adjusted injection timing to achieve stratified and homogeneous mixing. Stratified mixing conditions yielded faster combustion and improved fuel economy. Gong et al. [8] found that delaying methanol injection and advancing ignition timing improved engine cold-start performance.

2.2. Dual-Fuel Methanol Combustion Systems

Dual-fuel combustion systems can improve performance and emissions compared to single-fuel systems. Song et al. [9] compared two injection modes in methanol/gasoline engines: methanol direct injection plus gasoline port injection (MDI+GPI) and methanol port injection plus gasoline direct injection (MPI+GDI). The MDI+GPI mode avoided evaporation degradation of methanol in the intake port, resulting in a higher power output and lower cyclic variability. Research showed that in both injection modes, the methanol energy proportion should not exceed 60% to enhance combustion and reduce particle number (PN) emissions. Huang et al. [10] studied combustion and emissions in methanol/gasoline engines under lean-burn conditions. At an excess air ratio of 1.2, fuel economy peaked, while at 1.3, NO_x, CO, and HC emissions reached minimal levels. Mishra et al. [11] suggested adding 5–10% methanol to gasoline to improve performance, reduce knocking tendency, and lower emissions. Despite methanol’s high latent heat of vaporization, Liu et al. [12] reported that methanol–gasoline blends reduced CO and HC emissions during cold starts. Wang et al. [13] found that adding methanol to natural gas engines accelerated combustion, shortened ignition delay, reduced flame propagation time, and increased indicated thermal efficiency. Methanol also enhanced lean-burn capability. Chen et al. [14] observed that increasing the methanol substitution rate in natural gas engines reduced combustion duration, decreased HC emissions, and slightly increased NO_x emissions, while raising cylinder pressure and temperature peaks, which increased knocking tendency.

Using hydrogen in methanol engines increased flame propagation speed. While low hydrogen proportions improved thermal efficiency, high hydrogen proportions significantly reduced it. Research showed that injecting hydrogen into the intake port extended the lean-burn limit of methanol engines. At a 9% hydrogen volume fraction, the maximum stable excess air ratio reached three [15].

2.3. Compression-Ignition Methanol Engines

Due to methanol’s low cetane number, achieving pure methanol compression ignition is challenging. In CI engines, methanol primarily serves to replace part of the diesel fuel to reduce carbon emissions, with diesel acting as the ignition source. Methanol in CI engines can be injected via the intake port or directly into the cylinder. To address cold-start difficulties with a methanol port injection, Liu et al. [16] proposed a strategy of operating with pure diesel at idle and low loads and switching to methanol/diesel operation under high loads and high coolant and oil temperatures. Methanol’s longer ignition delay compared to diesel delayed the start of combustion. Methanol/diesel operation resulted in lower NO_x and CO₂ emissions but higher CO and HC emissions compared to pure diesel. Wei et al. [17] studied CI engines with high methanol premixed ratios and found that increasing methanol proportion shifted the load corresponding to maximum cylinder pressure from medium to high load, increased ignition delay, and shortened overall combustion duration, while significantly increasing HC, CO, and formaldehyde emissions. Jia et al. [18] compared methanol port injection, intake stroke direct injection, and compression stroke direct injection, finding no significant thermal efficiency advantage for direct injection, with further optimization of injection timing and pulse width required. Ning et al. [19] increased methanol substitution ratios and delayed injection timing to improve thermal efficiency, albeit with increased knocking tendency.

3. Issues with Methanol as an Engine Fuel

Table 1. Physicochemical properties of some fuels for spark-ignition engines.

Parameter	Methanol (CH3OH)	Gasoline (C5-12) 1	Methane (CH4)	Ethanol (C2H5OH)	Hydrogen (H2)	Ammonia (NH3)
Liquid Density (kg/m³)	791	740	423	789	39	680
Boiling Point (°C)	64.7	30–225	−161.5	78.4	−252.9	−33.3
Lower Heating Value (MJ/kg)	19.9	44.5	50	26.7	120	18.6
Latent Heat of Vaporization (kJ/kg)	1169	290–315	510	846	446	1370
Laminar Flame Speed (m/s)	0.43	0.35	0.38	0.4	1.6	0.07
Auto-Ignition Temperature (°C)	385	247–280	537	365	585	651
Octane Number	109	92–97	120	108.6	93.7	>130
Air–Fuel Ratio	6.5	14.7	17.2	9	34.6	6.1
Flammability Limits (Volume %)	6–36.5	1.4–7.6	4.4–17	3–19	4–75	15–28
Minimum Ignition Energy (mJ)	0.14	0.8	0.28	0.23	0.02	8

¹ Some properties are those of iso-octane.

presents the physical and chemical properties of common fuels used in spark-ignition internal combustion engines. Methanol’s lower heating value is less than half that of gasoline. To maintain equivalent engine power density, methanol injection volumes must be more than double those of gasoline. This also means that, assuming constant engine efficiency, vehicles would require larger quantities of methanol for the same driving range, taking up additional vehicle space.

Additionally, methanol’s high latent heat of vaporization (1169 kJ/kg, over three times that of gasoline) helps reduce intake and combustion temperatures, improving engine power density and reducing nitrogen oxide (NOx) emissions. However, this high latent heat also impairs methanol evaporation, adversely affecting cold-start performance. Under high injection volumes, incomplete evaporation of methanol can lead to wall impingement by methanol droplets, which in turn wets the spark plugs and dilutes/emulsifies cylinder wall oil. This results in unstable ignition and degraded lubrication. Studies on gasoline engines have shown that oil dilution caused by fuel spray impingement on the wall is a major factor leading to pre-ignition [20]. Similarly, methanol’s high latent heat of vaporization increases its tendency to cause pre-ignition [3].

As shown in Table 1, methanol’s minimum ignition energy is only half that of gasoline, and its laminar flame speed is slightly higher. As a result, methanol is easier to ignite and promotes flame propagation. This means that methanol is more likely to experience pre-ignition in the presence of hotspots in the cylinder [21]. However, methanol’s higher auto-ignition temperature and octane number compared to gasoline reduce the likelihood of end-gas auto-ignition, even if pre-ignition tendencies increase. Thus, methanol engines exhibit a lower probability of transitioning from pre-ignition to knocking than gasoline engines [22].

Despite methanol’s high octane number and strong anti-knock properties, extreme conditions or large bore sizes can still cause pre-ignition to lead to knocking [23,24,25]. Figure 1 presents cylinder pressure and heat release rate curves from typical combustion processes in a methanol engine. Under large-bore conditions, early pre-ignition can cause knocking pressures to exceed 35 MPa, far beyond engine-design limits. This “super-knock” can severely compromise engine reliability and performance [26].

Another issue with methanol engines is the corrosiveness of methanol [3]. Although this is not directly related to the topic of pre-ignition in this paper, it is worth mentioning. Methanol corrosion of metals occurs mainly in two ways: (1) During production, transportation, and combustion, methanol undergoes free radical reactions, and the resulting oxidation products, such as formic acid and other organic acids, can corrode the metal surface. (2) The polar nature of methanol allows it to absorb a small amount of water during storage. The presence of water in methanol fuel activates acid corrosion and electrochemical corrosion of metals, exacerbating the acid corrosion of reactive metals. Methanol also has a swelling effect on rubber products, which can cause cracking and affect the performance of rubber seals. Therefore, for methanol engines, the fuel supply system, combustion chamber, exhaust system, and other components need to use materials compatible with methanol or undergo surface treatments. Specific measures can be found in professional materials such as corrosion-related handbooks [27].

4. Pre-Ignition and Knocking in Methanol Engines

4.1. Manifestations of Pre-Ignition in Methanol Engines

Pre-ignition refers to abnormal ignition occurring before the designated ignition timing of the engine, usually before the top dead center (TDC). It can manifest in two forms: sustained and sporadic. Sporadic pre-ignition typically occurs at lower engine speeds [28]. Due to its random timing [26], sporadic pre-ignition does not necessarily result in knocking. As shown in the black pressure curve in Figure 1, when the pre-ignition timing is late, although the ignition timing is earlier than the designated spark timing, the cylinder pressure curve remains smooth without pressure oscillations characteristic of knocking. When the pre-ignition timing is early, as shown in the red pressure curve in Figure 1, the ignition timing has already advanced past the knocking boundary ignition timing. Excessively early flame propagation accelerates the chemical reactions in the end-gas, promoting auto-ignition in the end-gas and leading to knocking.

Sustained pre-ignition typically occurs under high-speed operating conditions [29]. When the in-cylinder thermal load is excessive, certain protrusions in the cylinder may overheat due to inadequate heat transfer, forming structural hot spots that ignite the air–fuel mixture before spark ignition. Sustained pre-ignition exhibits self-reinforcing characteristics [28], causing the pre-ignition timing to advance further, eventually leading to knocking. Knocking-induced pressure waves enhance heat transfer to the walls, further increasing the temperature of the structural hot spots. The mutual reinforcement between pre-ignition and knocking creates a vicious cycle that severely damages the engine.

Duan et al. [30] conducted durability tests on a methanol engine with a cylinder bore of 75 mm. At a speed of 5500 r/min, the average cylinder pressure over 100 cycles during a specific period is shown in Figure 2. It can be observed that even the average cylinder pressure across 100 cycles exhibits clear pre-ignition characteristics, indicating that the third cylinder experienced sustained pre-ignition caused by surface ignition. The figure also shows that the maximum pressure in the pre-ignition cylinder is significantly higher than that in other cylinders, but no apparent pressure oscillations indicative of knocking are observed.

The occurrence of such non-knocking pre-ignition is due to methanol’s high octane number and relatively high laminar flame speed, which prevent the end-gas from auto-igniting before the flame propagation completes. Although non-knocking pre-ignition is less destructive to the engine, the absence of knocking prevents knock sensors from detecting abnormal combustion, making it impossible to intervene in the combustion process. If the engine operates under this combustion mode for extended periods, excessive in-cylinder thermal load may cause the erosion of pistons or spark plugs.

In large-bore methanol engines, the end-gas has sufficient reaction time, making knocking more likely to occur when pre-ignition happens, as shown in Figure 1.

Qi et al. [31] and Wang et al. [21] observed sporadic pre-ignition phenomena in methanol during ignition delay tests conducted using a rapid compression machine. Figure 3a illustrates the development of a pre-ignition flame. It can be seen that, when pre-ignition occurs, the flame propagates from a very small kernel, with a clearly defined flame front and a propagation speed between 5 and 15 m/s. As the pre-ignition flame develops further, a distinct yellow spot becomes visible within the flame (Figure 3a(d–f)). This spot represents impurities or hotspots within the combustion chamber, but its position is significantly different from the origin of the pre-ignition flame, making it difficult to determine whether it is the source of pre-ignition.

Figure 3b shows the process of pure compression ignition without pre-ignition. It can be observed that the ignition region lacks a distinct flame front, and its propagation speed exceeds 48 m/s, much faster than in pre-ignition cases. This behavior more closely resembles subsonic autoignition combustion caused by a large ignition delay gradient (i.e., temperature non-uniformity in the mixture) [32].

Figure 3c further illustrates the pre-ignition processes in different test groups. In the first group, a weak yellow spot can be observed in the images, but its location is clearly distant from the initial ignition point. In the second group, no yellow spot is visible, and the ignition is more likely influenced by temperature non-uniformity within the mixture. In the third group, a yellow spot is distinctly observed at the center of the pre-ignition flame’s origin, indicating that pre-ignition was initiated by this hotspot.

Gu et al. [32] classified the combustion modes following hotspot autoignition into four categories based on the ignition delay gradient of the hotspot and the coupling of the resulting pressure wavefront and reaction wavefront. When the ignition delay gradient of the hotspot is extremely small, the combustion mode after hotspot autoignition is supersonic autoignition deflagration. In this mode, the propagation of the reaction front is controlled by chemical reaction kinetics and is independent of the propagation of the pressure wave, with flame propagation speeds far exceeding the speed of sound. Due to the presence of strong turbulence, this mode has difficulty occurring in real engines. When the ignition delay gradient of the hotspot increases moderately, the combustion mode transitions to detonation or developing detonation. In this case, the reaction wavefront couples with the pressure wavefront, and the flame propagation speed approaches the Chapman–Jouguet (C–J) detonation velocity. Super-knock [26] falls under this category. Further increasing the ignition delay gradient of the hotspot leads to subsonic autoignition deflagration, where the resulting flame propagates at subsonic speeds. In this mode, the propagation of the pressure wavefront has little influence on the reaction wavefront. The compression ignition process shown in Figure 3b can be categorized into this mode. When the ignition delay gradient of the hotspot becomes extremely large, the flame propagation speed after hotspot autoignition is at the level of laminar flame speed, resembling normal flame propagation after spark ignition. In this case, the propagation speed of the pressure wavefront far exceeds that of the reaction wavefront, exerting minimal influence on it. The propagation of the pre-ignition flame shown in Figure 3 belongs to this mode. It is worth noting that flame propagation at laminar flame speed is also a form of subsonic deflagration. However, its flame propagation speed is primarily governed by heat and mass transfer process, unlike subsonic autoignition deflagration, which is driven by chemical reaction kinetics. Based on this classification, the process from pre-ignition to knocking can be understood as deflagration induced by hotspot autoignition, which promotes hotspot autoignition in the end-gas, ultimately resulting in detonation [33].

4.2. Mechanisms and Sources of Pre-Ignition

4.2.1. Sustained Pre-Ignition

As mentioned earlier, sustained pre-ignition typically occurs under high-speed and high-load conditions [29]. Its mechanism is relatively well-understood and is caused by surface ignition due to excessive in-cylinder thermal load. The ignition sources for surface ignition are typically protrusions within the cylinder, such as spark plugs or carbon deposits. When the combustion chamber walls have poor heat transfer, these protrusions can overheat under high-speed and high-load conditions, forming hotspots that ignite the air–fuel mixture before the normal spark ignition.

Figure 4 shows the process of sustained pre-ignition caused by a spark plug, as captured by Iwatsuka et al. [29]. It can be observed that the center electrode of the spark plug becomes significantly overheated, appearing dark red. It ignites the air–fuel mixture approximately 7° crank angle (CA) before the designated spark timing, and the flame propagates outward from this point.

Given that the mechanism of sustained pre-ignition is relatively well-understood, the following sections will focus on sporadic pre-ignition. Unless otherwise specified, pre-ignition mentioned hereafter refers to sporadic pre-ignition.

4.2.2. Sporadic Pre-Ignition

Compared to sustained pre-ignition, the occurrence mechanism of sporadic pre-ignition is more complex. Numerous research institutions have conducted extensive research on sporadic pre-ignition, primarily focusing on gasoline engines. Relatively fewer studies have been conducted on methanol engines. Given that both methanol and gasoline engines employ premixed combustion, the research findings on gasoline engine pre-ignition can serve as a guiding reference for understanding the pre-ignition mechanism in methanol engines.

Willand et al. [34] indicated that the pre-ignition sources can be broadly categorized into three types: oil and deposits, structural hot spots, and residual exhaust gas. The base oil in engine oil has a longer carbon chain and thus a higher cetane number, making it susceptible to auto-ignition under high temperature and high-pressure conditions [35]. Additionally, certain metal-based additives [36] in engine oil and metal wear particles [37] contained therein after prolonged use may have a catalytic effect, further promoting the auto-ignition of the oil. Willand et al. [34] suggested that, after entering the combustion chamber via the crankcase, the engine oil auto-ignites during the compression stroke, thereby causing pre-ignition. Deposits, containing oil, may also initiate pre-ignition. Furthermore, detached deposits can form hot spots within the combustion chamber, maintaining a high temperature and leading to pre-ignition during the compression stroke. Structural hot spots are formed when certain areas of the engine combustion chamber surface become overheated. Residual exhaust gas can elevate the charge temperature, making the fuel more prone to auto-ignition. The active components contained within the residual exhaust gas can also accelerate chemical reactions, thereby inducing pre-ignition.

Building upon the research of Willand et al. [34], Dahnz et al. [28] conducted a more detailed classification of pre-ignition sources in gasoline engines and proposed three pathways for engine oil to enter the combustion chamber: turbocharger leakage, valve stem seal leakage, and piston clearance splashing, with piston’s top land crevice splashing being the primary pathway. Gschiel et al. [38] investigated pre-ignition in hydrogen engines and suggested that, in addition to the aforementioned three pathways, engine oil droplets introduced into the intake manifold through the crankcase ventilation system can also serve as a potential pre-ignition source for fuels with lower minimum ignition energy. Combining the research findings of Dahnz et al. [28], Gschiel et al. [38] summarized the pre-ignition sources in engines, as illustrated in Figure 5.

Oil splashing from the top land crevice is widely recognized as the primary pathway leading to sporadic pre-ignition [20]. When fuel spray or droplets impinge on the cylinder walls, the resulting dilution of the oil film reduces the oil’s viscosity and surface tension, causing it to accumulate in the crevice between the piston and the cylinder wall (Figure 6a). The greater the extent of oil dilution, the more significant the reduction in oil viscosity and surface tension. During the compression stroke, as the piston approaches the TDC, it decelerates, with its velocity and acceleration in opposite directions (Figure 6b). At this point, the oil accumulated in the piston crevice is prone to being ejected into the combustion chamber as oil droplets due to inertial forces (Figure 6c). Some flammable components within the oil droplets evaporate from the oil’s surface, forming combustible gases with a short ignition delay. Under specific temperature and pressure conditions, these combustible gases can auto-ignite and ignite the surrounding air–fuel mixture, ultimately causing pre-ignition (Figure 7).

For oil diluted by fuel to splash into the combustion chamber, it must first accumulate to a sufficient level within the piston crevice. The process of re-accumulation takes a certain amount of time, explaining the sporadic and random nature of sporadic pre-ignition.

After oil-induced pre-ignition leads to knocking, the resulting pressure wave oscillations may detach deposits adhered to the combustion chamber walls, causing them to float within the chamber. If these detached deposits persist into the next cycle, their high temperatures could ignite the air–fuel mixture and cause pre-ignition. However, it should be noted that in a series of closely spaced pre-ignition cycles, pre-ignition caused by deposit particles typically occurs following pre-ignition initiated by oil droplets [41], as shown in Figure 8. In such cases, the first pre-ignition and subsequent knocking are induced by oil droplets. The pressure oscillations from knocking not only detach wall deposits but also eject diluted oil accumulated in the piston crevices into the combustion chamber [42], as illustrated in Figure 9. Most of these oil droplets are expelled during the exhaust process, but a small portion may remain in the cylinder and cause pre-ignition in subsequent cycles [43].

In addition to oil and detached deposit particles, residual exhaust gases in the cylinder can also cause sporadic pre-ignition under the influence of structural hotspots. Iwatsuka et al. [29] observed sporadic pre-ignition occurring at the spark plug location. As shown in Figure 10a, visible light appeared in the spark plug cavity approximately 4° CA before the normal spark ignition, indicating ignition inside the spark plug. They conducted numerical simulations of the temperature inside the spark plug, which revealed that at 10° CA before top dead center (TDC), the gas temperature near the side electrode and the root of ceramic insulator exceeded 850 K (Figure 10b). Flow field analysis indicated the presence of a gas recirculation zone near the root of the ceramic insulator (Figure 10c), suggesting inadequate scavenging in this region.

Iwatsuka et al. [29] also measured the ion current near the ceramic insulator, further confirming that flames were generated near the ceramic insulator before the normal spark ignition. They identified the location of the flame generation (Figure 10d). Based on the experimental and numerical simulation results, Iwatsuka et al. [29] concluded that autoignition of high-temperature gases near the root of the ceramic insulator inside the spark plug was the cause of pre-ignition initiated by the spark plug.

Regarding the role of spark plugs in causing sporadic pre-ignition, Hülser et al. [44] used visualization methods to observe that a significant number of pre-ignition points were located at the spark plug electrodes. The distribution of pre-ignition points was found to be related to fuel properties: for gasoline and tetrahydrodimethylfuran, most pre-ignition points were distributed at the electrodes; for dimethylfuran, fewer pre-ignition points were located at the electrodes; and for ethanol, no pre-ignition points were observed at the electrodes. Inoue et al. [45] investigated various spark plug parameters, including heat range, ignition position, center electrode diameter, presence of a copper core in the ground electrode, ground electrode orientation, electrode gap, and the protrusion of the metal shell. The results showed that the impact of spark plugs on pre-ignition was not significant.

4.3. Knocking Behavior of Methanol

4.3.1. End-Gas Auto-Ignition-Induced Knocking

The most direct manifestation of knocking is pressure oscillations, which are typically caused by auto-ignition of hotspots in the end-gas. As previously mentioned, auto-ignition of hotspots can exhibit different combustion modes depending on the ignition delay gradient [32]. Hotspots in the end-gas follow the same rule. These hotspots are generated due to temperature non-uniformities caused by turbulence. Under the compression effects of the upward piston motion and the expansion of the burned zone, the ignition delay gradient within the hotspot gradually decreases, while chemical reactions accelerate. If the flame has not yet propagated to the hotspot when the chemical reaction is complete, the hotspot will auto-ignite, leading to rapid heat release and local pressure increases. This generates shockwaves that oscillate within the combustion chamber, resulting in knocking.

In addition to knocking caused by end-gas auto-ignition, Xu et al. [23] identified another form of knocking in methanol engines through numerical simulations: deflagration-to-detonation transition (DDT). The simulation results are shown in Figure 11. As depicted in Figure 11f, apart from two auto-ignition points of the end-gas near the combustion chamber walls, a detonation was also observed at the flame front propagating from spark ignition. This detonation, originating from normal flame propagation transitioning into detonation, is referred to as deflagration-to-detonation transition. They suggested that the compression wave generated by normal flame propagation accelerates low-temperature reactions in the unburned zone and causes the accumulation of a large amount of intermediate species. These intermediate species facilitate auto-ignition at the two wall regions and, simultaneously, accelerate the propagation of the normal flame front, promoting the transition from deflagration to detonation. The numerical simulation by Xu et al. [23] provides a new perspective for understanding knocking in methanol engines. However, it is worth noting that direct evidence of deflagration-to-detonation transition has not yet been observed in visualization studies on engines or rapid compression machines using methanol or similar high-octane fuels.

Duan et al. [46] observed mild to moderate knocking combustion in a PFI methanol engine under a load of 0.6 MPa. At higher loads, the engine experienced extremely severe knocking in isolated cycles, resembling super-knock in direct-injection gasoline engines. Using methanol direct injection, optimizing injection timing, and adopting split injection strategies effectively reduced knocking tendencies under high loads. Duan et al. also found that in the methanol direct-injection mode, late injection resulted in a more non-uniform mixture due to a shorter evaporation time and weaker in-cylinder turbulence. This reduced combustion speed and decreased knocking intensity. Inoue et al. [47] found that although pre-ignition occurred in overheated combustion chambers with methanol, severe knocking did not develop. Uddeen et al. [48] used a single-cylinder, four-stroke optical engine and observed that methanol produced only mild knocking compared to gasoline.

4.3.2. Deflagration-Based Knocking

While pressure oscillations in engine combustion processes are primarily caused by auto-ignition of the end-gas, another type of pressure oscillation can occur under specific conditions. This pressure oscillation arises immediately after ignition and increases in amplitude as pressure rises. Such oscillations are more common in large-bore spark-ignition engines, such as marine natural gas engines [49]. Singh et al. [50] also observed this type of pressure oscillation in methanol engines and considered it a form of knocking.

Since this knocking occurs alongside flame propagation, Singh et al. [50] referred to it as deflagration-based knock. The phenomenon shares similarities with combustion instability observed in rocket engines, where structural responses to spatial acoustic disturbances cause resonances [51]. Deflagration-based knock arises from the coupling of heat release at the flame front with weak acoustic waves in the combustion chamber, leading to gradual amplification of weak pressure fluctuations.

Typically, the pressure oscillation amplitude of deflagration-based knock is lower than that of end-gas auto-ignition knock but tends to intensify with increasing engine speed [50]. Because deflagration-based knock is unrelated to the thermodynamic state of the end-gas, retarding ignition timing is not effective in suppressing it, and it is also insensitive to load variations. Singh et al. [50] suggested that deflagration-based knock is more likely to occur in mixtures with low ignition energy and high laminar flame speed. The reasons are as follows: (1) when the minimum ignition energy of the mixture is low, the flame kernel formed by spark ignition is larger, creating a significant pressure gradient that grows over time; and (2) when the laminar flame speed of the mixture is high, the heat release at the flame front is stronger, which facilitates coupling with acoustic wave oscillations [52].

5. Factors Influencing Methanol Pre-Ignition and Mitigation Methods

5.1. Sustained Pre-Ignition

As discussed earlier, sustained pre-ignition is typically caused by surface ignition due to excessive in-cylinder thermal load. Therefore, reducing in-cylinder thermal load or enhancing heat transfer can effectively suppress sustained pre-ignition. Duan et al. [30] mitigated sustained pre-ignition in methanol engines by using a cold-type spark plug to improve spark plug heat transfer, increasing a piston cooling nozzle to lower piston surface temperatures, and employing low-ash engine oil to reduce the increase in combustion chamber surface thermal resistance caused by deposits.

5.2. Sporadic Pre-Ignition

As mentioned earlier, sporadic pre-ignition can be triggered by various sources, each with complex formation processes and mechanisms. Unfortunately, studies on sporadic pre-ignition in methanol engines are relatively limited. Since both methanol and gasoline engines involve premixed combustion, research findings on pre-ignition in gasoline engines can provide guidance for understanding factors influencing methanol pre-ignition and its mitigation methods. The following sections discuss these factors and methods based on insights from gasoline engine research, focusing on engine oil, particulates, engine design, and operating conditions.

5.2.1. Influence of Engine Oil

• Base Oil

The influence of engine oil on pre-ignition mainly stems from its propensity for auto-ignition. Amann and Alger [35] showed that the cetane number of commonly used automotive engine oils is significantly higher than that of n-heptane, making them highly prone to auto-ignition. Takeuchi et al. [36] found that among different categories of base oils, Type I base oils exhibited the highest pre-ignition frequency, followed by Type II and Type III. PAO (Poly-Alpha-Olefin) base oils had a pre-ignition frequency comparable to Type III oils. Welling et al. [53] conducted extensive testing on base oils and found that pre-ignition frequency increased with the Calculated Ignition Index (CII), as shown in Figure 12. Since CII correlates with viscosity, it can be expected that higher viscosity base oils result in higher pre-ignition frequencies. Andrews et al. [54] confirmed this expectation, showing that viscosity had the highest correlation with pre-ignition frequency among the various physical and chemical properties of engine oil. In addition to CII, ignition characteristics of base oils can also be measured by ignition temperature. Takeuchi et al. [36] found that lower ignition temperatures corresponded to higher pre-ignition frequencies.

In addition to viscosity, the volatility of base oil also significantly impacts pre-ignition frequency. Morikawa et al. [55] found that oil samples with higher distillation ranges exhibited higher pre-ignition frequencies. They noted that this result contradicts expectations, as base oils with lower distillation ranges should have higher pre-ignition frequencies due to their higher volatility, which facilitates entry into the combustion chamber. This unexpected finding suggests that other mechanisms may be involved in how oil volatility affects pre-ignition.

• Additives

Extensive research has shown that metal-based additives significantly influence pre-ignition frequency. Among these, calcium-based additives (used as engine oil detergents) are widely considered pre-ignition promoters [37,56,57,58,59,60,61,62,63,64,65,66]. Ritchie et al. [66] found that the effect of calcium-based additives does not depend on the type of calcium compound (e.g., salicylate, phenate, or sulfonate) or the alkyl chain structure in calcium sulfonate detergents but is solely related to calcium concentration. Fletcher et al. [56] also concluded that, while the form of calcium compound has negligible impact on pre-ignition frequency, calcium concentration shows a linear correlation with pre-ignition frequency. Although pre-ignition frequency is independent of calcium additive type, it may depend on the fuel atmosphere. Kassai et al. [58] showed that increasing calcium additive levels significantly shortened the ignition delay of PRF fuel–air mixtures, particularly under low-octane conditions, but had minimal impact on methane–air mixtures.

The pathways and mechanisms by which calcium-based additives influence pre-ignition remain unclear. Moriyoshi et al. [65] proposed a hypothesis involving the interconversion of CaCO₃ and CaO. When engine oil droplets enter the combustion chamber, high temperatures decompose CaCO₃ into CaO particles. These particles can persist into the next cycle and react with CO₂ in the exhaust gases to regenerate CaCO₃. This process releases heat, heating the particles to 1000 K and creating hotspots that ignite surrounding mixtures.

In contrast to calcium, zinc- and molybdenum-based additives, such as ZnDTP (zinc dialkyldithiophosphate) and MoDTC (molybdenum dithiocarbamate), widely used as antioxidants, are considered pre-ignition inhibitors [36,37,56,57,58,59,60,66,67]. These additives significantly reduce pre-ignition frequency. However, some studies have shown that the inhibitory effects of ZnDTP and MoDTC on pre-ignition may be minimal. Hayakawa et al. [68] conducted HCCI experiments by adding 800 ppm of ZnDTP and 980 ppm of MoDTC to PRF50 fuel and found that ignition delay remained nearly unchanged compared to fuel without additives. ZnDTP also impacts pre-ignition through deposit formation. Swain et al. [22] compared deposits generated by ZnDTP, calcium-based, and magnesium-based additives on spark plug surfaces. At 315 °C, deposits from ZnDTP exhibited the lowest pre-ignition frequency, but at 560 °C, ZnDTP deposits caused the highest pre-ignition frequency.

Other metal-based additives, such as magnesium (Mg) and sodium (Na), are generally believed to have a minimal impact on pre-ignition [61,62,66]. However, sodium-based additives may promote pre-ignition in the presence of calcium-based additives [66].

In addition to intentionally added additives, metallic wear particles mixed into engine oil during operation also affect pre-ignition. Hirano et al. [37] found that the presence of copper (Cu) and iron (Fe) in engine oil promotes pre-ignition, likely due to their catalytic effects.

Fujimoto et al. [57] suggested that to maintain the high-temperature cleaning capability of engine oil, a certain amount of calcium-based detergents should be included. To prevent poisoning of three-way catalysts, the phosphorus (P) content in engine oil should be controlled below a specific limit, as shown in Figure 13. Based on this principle, reducing calcium-based detergent content while increasing ZnDTP levels can lower pre-ignition frequency to one-tenth that of conventional gasoline engine oil.

• Engine Oil Droplet Size

Recent studies [69] have shown that during each engine cycle, engine oil can be ejected into the combustion chamber through piston crevices. However, pre-ignition does not occur in every cycle under these conditions. The study suggests that whether pre-ignition occurs depends on multiple factors beyond the composition of the oil and the thermodynamic state inside the cylinder.

Ohmoto et al. [70], Fei et al. [71], and Bhoite et al. [40] investigated the effect of oil droplet size on ignition behavior and reached the same conclusion: the size of oil droplets significantly influences their ignition delay, as shown in Figure 14. When droplet size is too large, evaporation is slow, and the ignition delay required for auto-ignition is longer, making it difficult for ignition to occur before normal spark timing, thus preventing pre-ignition. Conversely, when droplet size is too small, the droplets evaporate and diffuse rapidly, failing to establish the equivalence ratio required for auto-ignition, and thus pre-ignition does not occur. Only under conditions of moderate droplet size can pre-ignition occur. Figure 14 also shows that the initial temperature of the droplets is another critical factor affecting their ability to cause pre-ignition. In addition to this, environmental temperature and pressure are key factors influencing droplet auto-ignition. Environmental temperature elevation promotes droplet evaporation and, within the non-NTC (Negative Temperature Coefficient) region of engine oil, also facilitates chemical reactions in the oil. Although higher environmental pressure hinders droplet evaporation [72], it significantly shortens ignition delay.

Additionally, considering that engine oil in real engines is typically diluted with fuel, the dilution rate of the oil also impacts pre-ignition. Qi et al. [73] tested the effects of four different dilution rates (0%, 25%, 50%, and 75%) on pre-ignition and knocking caused by engine oil. The results showed that at a 25% dilution rate, pre-ignition occurred earliest, and knocking intensity was highest, whereas at a 75% dilution rate, pre-ignition was almost entirely eliminated.

Engine oil can also enter the combustion chamber through crankcase ventilation. Although the crankcase ventilation system is equipped with a filter, gases from the crankcase that pass through the filter may still allow small-diameter oil droplets to escape. These droplets may subsequently collide and aggregate into larger droplets, which then enter the combustion chamber with the intake air. Inoue et al. [45] found in gasoline engines that the higher the crankcase ventilation flow rate, the higher the frequency of pre-ignition events. This suggests that oil droplets from crankcase ventilation may also be another important source of pre-ignition. Compared to gasoline, methanol has a higher laminar flame speed. According to combustion theory, laminar flame thickness is inversely proportional to laminar flame speed. Therefore, methanol flames have a thinner laminar thickness, making them more prone to ignition by smaller hotspots [74]. Additionally, compared to gasoline, methanol has a lower minimum ignition energy, which further exacerbates its tendency for pre-ignition.

5.2.2. Influence of Floating Particles in the Combustion Chamber

Deposits generated within the cylinder can completely detach from the walls and form floating particles in the combustion chamber, acting as spatial hotspots. Due to their high temperature and thermal capacity, detached deposits can retain significant heat during the scavenging process and induce combustion during the subsequent compression stroke. Moreover, if deposits are only partially detached and remain adhered to the combustion chamber walls, they act as structural hotspots. Haenel et al. [75] compared the effects of two different combustion chamber cleanliness levels on pre-ignition. Their study showed that deposits significantly reduced the thermal conditions required for pre-ignition. Combustion chambers with more deposits exhibited effective mean pressures approximately 0.4 MPa lower during the first pre-ignition event compared to clean chambers. Okada et al. [76] directly observed pre-ignition triggered by solid particles in the combustion chamber. After pre-ignition occurred, a large number of floating particles appeared in the cylinder, which subsequently triggered further pre-ignition events. They suggested that detached deposits form floating particles, which continue chemical reactions on their surfaces during the scavenging process, maintaining a certain temperature. During the compression stroke, as the surrounding mixture’s temperature and pressure increase, chemical reactions on the particle surfaces accelerate, igniting the surrounding mixture and causing pre-ignition. This process is illustrated in Figure 15. Lauer et al. [41] used visualization methods to observe pre-ignition caused by floating deposits. However, they noted that the first pre-ignition in such cases was triggered by engine oil. They concluded that in instances of consecutive pre-ignition events, the initial pre-ignition is induced by engine oil, with the resulting knocking and pressure oscillations causing deposits in the cylinder to detach. In subsequent cycles, these detached deposits act as hotspots, leading to further pre-ignition events.

As with the influence of engine oil droplet temperature and size on pre-ignition, the temperature and size of particles are also critical factors affecting their ability to ignite the air–fuel mixture. Wang et al. [77] tested the ability of solid hot particles of various temperatures and sizes to induce pre-ignition by directly introducing them into the intake manifold. The results showed that hot particles must reach a sufficiently large size to trigger pre-ignition. Overall, as particle temperature and size increased, the timing of pre-ignition and the onset of knocking advanced, while peak knocking pressure and the rate of pressure rise increased progressively.

5.2.3. Influence of Engine Design Parameters and Operating Conditions

• Compression Ratio

Zahdeh et al. [78] found that increasing the compression ratio from 9.2 to 10.3 resulted in approximately a twofold increase in pre-ignition frequency. However, reducing the compression ratio from 9.2 to 8.4 had little effect on pre-ignition frequency. This indicates that compression ratio only has a significant impact on pre-ignition when it exceeds a critical threshold. This can be attributed to the shorter ignition delay caused by a higher compression ratio.

• Piston Design

The influence of pistons on pre-ignition is reflected in factors such as piston crown geometry, oil ring tension, and top land dimensions, with oil ring tension having the greatest impact. Higher ring tension allows the piston ring to fit more tightly against the cylinder wall, reducing the amount of engine oil entering the combustion chamber. Zahdeh et al. [78] conducted engine tests and found that increasing oil ring tension from 30 N to 40 N reduced pre-ignition frequency by 76%, while reducing it from 30 N to 20 N increased pre-ignition frequency by 24%. Their study also showed that under the same compression ratio, flat-top pistons exhibited lower pre-ignition frequencies compared to bowl-shaped pistons. Even when forced oil cooling for flat-top pistons was stopped, pre-ignition frequency remained lower than that of bowl-shaped pistons. Additionally, low piston ring tension allows more oil to escape through the top land crevice, resulting in a higher pre-ignition frequency. Smaller top land crevice volumes cause oil to accumulate more easily in the crevice, making it more likely to be ejected into the combustion chamber and leading to higher pre-ignition frequency. Amann et al. [79] found that increasing the chamfer angle of the piston crown to reduce crevice volume (as shown in Figure 16) lowered pre-ignition frequency compared to the original piston design. While the reduction in compression ratio (approximately 0.25) contributed to this effect, Amann et al. concluded that the primary reason was that the chamfer facilitated flame propagation into the crevice, preventing diluted oil from accumulating within it.

• Spark Plugs

Inoue et al. [45] suggested that spark plug design has minimal impact on pre-ignition frequency. However, Zahdeh et al. [78] found that the orientation of the weld point of the spark plug ground electrode significantly influences pre-ignition frequency. When the weld point faces the exhaust valve, pre-ignition frequency increases, and the knocking intensity becomes higher. Suga et al. [80] studied the effect of different spark plug electrode materials on the pre-ignition frequency of M85 fuel and found that platinum electrodes are more prone to surface ignition, leading to pre-ignition. This is attributed to methanol’s higher tendency to decompose on high-temperature metal surfaces. When high-electronegativity noble metals like platinum are used, the decomposition rate of methanol increases further [80,81]. To suppress methanol decomposition on high-temperature metal surfaces, spark plugs made of platinum should be avoided [5]. Nickel and tungsten carbide materials can replace platinum [80,82], or the electrode surface can be coated with low-electronegativity compounds, such as CrC [81].

• Thermodynamic Conditions

Thermodynamic conditions refer to the pressure, temperature, and composition within the combustion chamber. Regarding pressure, once the engine compression ratio is fixed, intake pressure becomes the primary factor affecting in-cylinder pressure. Since pre-ignition and knocking typically occur only after significant boosting, increasing the intake pressure further raises the pre-ignition frequency [45,78].

For temperature, two main factors influence pre-ignition: the intake air temperature and coolant temperature. Zahdeh et al. [78] and Li et al. [83] showed that reducing intake air temperature from 50 °C to approximately 30 °C can reduce pre-ignition frequency to some extent. However, Amann et al. [84] and Inoue et al. [45] found that within the ranges of 26–42 °C and 40–65 °C, respectively, intake air temperature had no significant impact on pre-ignition frequency, and in some cases, lowering intake air temperature slightly increased pre-ignition frequency. Zaccardi and Escudié [85] suggested that while lower temperatures extend the ignition delay for gas-phase reactions and inhibit auto-ignition, they increase the likelihood of fuel impingement and oil dilution, forming localized rich zones that shorten ignition delay and promote pre-ignition.

For coolant temperature, the effect is similar to intake air temperature but varies across different engines. Dahnz et al. [86], Zahdeh et al. [78], and Li et al. [83] found that higher coolant temperatures reduce pre-ignition frequency because elevated wall temperatures promote fuel evaporation, lowering the likelihood of oil dilution. Conversely, Amann et al. [84] and Zhang et al. [87] found that pre-ignition frequency increases with higher coolant temperatures. This is because higher coolant temperatures elevate the temperature of the mixture near the end of the compression stroke, promoting pre-ignition. The effect of coolant temperature depends on whether its influence on mixture temperature or fuel evaporation is more dominant.

For mixture composition, adjustments such as enrichment, leaning, or exhaust gas recirculation (EGR) can influence pre-ignition. Enriching the mixture lowers in-cylinder temperature and reduces reaction activity, effectively suppressing pre-ignition [45,78,88,89,90], but enrichment also increases fuel consumption and worsens emissions, making it generally undesirable. Leaning the mixture is less effective at suppressing pre-ignition [83,91]. Zahdeh et al. [78] and Inoue et al. [45] even found that leaning increased pre-ignition frequency until the excess air ratio exceeded 1.1, at which point pre-ignition frequency decreased, but engine performance also declined. EGR introduces multi-atomic gases like CO₂ and H₂O, which dilute the mixture and lower in-cylinder temperature, effectively suppressing pre-ignition and super-knock. Amann et al. [92] used 10% EGR to increase BMEP from 1.24 MPa to 1.45 MPa while reducing pre-ignition frequency by 80% and knocking intensity by 30%. Zaccardi and Escudié [85] found that increasing EGR to 10% under constant engine load reduced pre-ignition frequency by over 50%.

Most of the EGR approaches mentioned above involve external EGR, where the exhaust gases are cooled before being reintroduced. When using internal EGR, achieved by adjusting valve timing or exhaust backpressure to retain exhaust gases in the cylinder, EGR may promote pre-ignition. Inoue et al. [47] found that increasing exhaust backpressure from 1.7 kPa to 6.1 kPa lowered the pre-ignition temperature caused by spark plugs by 40 K. This is attributed to the enhanced reactivity of methanol–air mixtures mixed with high-temperature residual gases, thereby promoting pre-ignition.

• Spray Wall Impingement and Injection Strategies

From the perspective of engine oil, the primary direct cause of pre-ignition is the dilution of cylinder wall oil films by fuel droplets. The most direct measure to mitigate pre-ignition is to reduce the probability of fuel droplets impinging on walls. For port injection engines, it is necessary to ensure that the injected fuel evaporates within the intake manifold, reducing the likelihood of liquid fuel adhering to walls and entering the combustion chamber. For direct-injection engines, it is essential to avoid direct spray impingement on cylinder walls. Zahdeh et al. [78] adjusted the orientation of the injector in a direct-injection gasoline engine so that the spray was directed toward the piston crown instead of the cylinder wall, significantly reducing pre-ignition frequency. When using flat-top pistons in conjunction with this orientation, no pre-ignition was observed in 14 consecutive tests.

Increasing in-cylinder airflow can also reduce pre-ignition frequency. Palaveev et al. [93] used tumble control valves in the intake manifold. Their experiments showed that when the valves were closed (resulting in stronger airflow), pre-ignition frequency was lower than when the valves were open (resulting in weaker airflow). Stronger tumble airflow promotes fuel droplet evaporation, reducing spray penetration and carrying droplets away from the walls, lowering wall impingement probability.

After the combustion chamber structure is fixed, the most convenient method to control spray penetration is split injection. Studies by Zahdeh et al. [78], Palaveev et al. [93], and Mayer et al. [94] showed that using split injection significantly reduced pre-ignition frequency by lowering the probability of spray impingement. Wang et al. [89,90] optimized the timing and ratio of split injection to achieve effective pre-ignition suppression while maintaining fuel economy, exhaust temperature, and emissions.

6. Current Challenges in Methanol Pre-Ignition Research

6.1. Limitations in In-Cylinder Combustion Testing Techniques

Current research on pre-ignition primarily relies on cylinder pressure data to analyze combustion conditions. However, cylinder pressure data only reflect the combustion state at a single point, making it difficult to evaluate the overall combustion conditions within the cylinder. Methanol, with its low minimum ignition energy, is more prone to pre-ignition and exhibits a wider range of operating conditions and diverse types of pre-ignition sources compared to gasoline fuels. Therefore, optical diagnostic methods are needed to directly detect ignition source types, ignition locations, and flame propagation processes, enabling a more comprehensive understanding of methanol pre-ignition processes. Due to the sporadic and random nature of pre-ignition, long-term continuous testing is necessary to collect sufficient data to comprehensively analyze pre-ignition source types and their pathways into the cylinder. Optical diagnostics are typically conducted on optical engines, but for pre-ignition studies, optical engines often fail to reach the operating range where pre-ignition occurs and cannot sustain long-term operation under pre-ignition or knock conditions. In such cases, using endoscopes combined with high-speed imaging directly on the engine is an effective method for long-term visualization of pre-ignition combustion. While endoscopes can enable prolonged observation of combustion processes, prolonged exposure to high temperatures and strong pressure wave impacts within the combustion chamber poses challenges to the safety and longevity of the optical components at the front end of the endoscope.

6.2. Limitations in Numerical Simulation Accuracy

Numerical simulation is a crucial tool in engine development, significantly reducing experimental costs. However, it demands substantial computational resources, particularly for large geometries, fine mesh sizes, and large-scale reaction kinetics models. Currently, three-dimensional numerical simulations of engines are mostly limited to single-cylinder combustion processes, and the commonly used RANS (Reynolds-Averaged Navier-Stokes) methods are tailored for relatively large mesh sizes. While these methods achieve satisfactory accuracy for predicting engine combustion performance in engineering applications, they remain inadequate for predicting pre-ignition. For known pre-ignition sources, appropriate computational models combined with detailed chemical reaction kinetics and advanced methods, such as Large Eddy Simulation (LES), can provide detailed insights into how pre-ignition sources trigger pre-ignition and subsequent knock within the combustion chamber. However, predicting the formation of pre-ignition sources and the probability of pre-ignition under given conditions is highly challenging. Methanol pre-ignition and knock involve the entire engine operating cycle, including processes such as liquid fuel atomization and evaporation, gas-phase chemical reactions, multiphase gas–liquid–solid flow interactions, and heat transfer. Additionally, factors such as the generation of pre-ignition sources, their pathways into the combustion chamber, cycle-to-cycle variations, cylinder-to-cylinder variations, and numerous engine design details introduce significant uncertainties to the application of numerical simulation tools for predicting pre-ignition. Under current conditions, the occurrence of pre-ignition must still primarily rely on test bench experiments and optical diagnostics to analyze pre-ignition sources effectively and mitigate their occurrence.

6.3. Unclear Mechanisms of Methanol–Oil Interaction

Engine oil is a primary factor contributing to pre-ignition in methanol engines. However, the mechanisms underlying the complex interactions between methanol and engine oil under high-temperature and high-pressure conditions in the cylinder remain unclear. Commercial engine oils contain various components, including base oils and functional additives. On the one hand, base oils are not single components but mixtures containing various monomers and polymers with a wide range of carbon chain lengths, leading to complex chemical reactions. Currently, no reaction kinetics model can accurately describe the combustion chemistry of engine oil. Simplified substitutes, like n-heptane or cetane, are commonly used instead. When methanol is introduced, engine oil ignites in a methanol environment, further complicating the chemical reaction processes. On the other hand, engine oil additives contain metal components, some of which promote pre-ignition while others inhibit it, but their respective mechanisms remain unclear. Current understanding of engine oil-induced pre-ignition is still limited to macroscopic observations. To further elucidate the chemical mechanisms underlying methanol–oil interactions, it is necessary to combine molecular chemistry theory and experimental methods to analyze the reaction pathways and products of methanol and various oil components under different operating conditions. This would provide deeper insights into the reaction dynamics and environmental dependencies, enabling the development of methanol-compatible oil formulations with enhanced resistance to pre-ignition and offering more reliable technical support for the application of methanol fuels in engines.

7. Summary and Outlook

Methanol engines hold significant potential for promoting low-carbon fuels and reducing emissions, but pre-ignition and knock remain the primary challenges limiting their widespread application. Methanol engines’ tendency for pre-ignition is closely related to the low minimum ignition energy and high laminar flame speed of methanol. Depending on operating conditions, pre-ignition may manifest as either sustained or sporadic. Sustained pre-ignition is primarily caused by excessive in-cylinder thermal loads, resulting in overheated protrusions within the combustion chamber acting as hotspots. Sporadic pre-ignition is more complex and can originate from various sources, including oil droplets, deposits, structural hotspots, and residual gases. Among these, oil droplets entering the combustion chamber via piston top land crevices are the primary pathway. Fuel spray or droplets dilute the oil film on the cylinder wall, and the diluted oil accumulates in the piston ring gaps. Under the influence of inertia, the diluted oil is ejected into the combustion chamber, where it auto-ignites under high-temperature and high-pressure conditions, triggering pre-ignition.

Methanol’s high octane rating makes pre-ignition less likely to cause end-gas auto-ignition and knock in small-bore engines. However, methanol’s high laminar flame speed may cause pressure waves generated during flame propagation to couple with heat release, leading to low-amplitude pressure oscillations. In large-bore engines, methanol pre-ignition can still result in high-intensity knock.

For sustained pre-ignition, measures such as using cold spark plugs and enhancing cooling for pistons and cylinder walls can reduce in-cylinder thermal loads and effectively suppress pre-ignition. For sporadic pre-ignition, controlling the formation of pre-ignition sources and reducing the reactivity of the air–fuel mixture are key strategies. Effective methods include optimizing oil formulations to reduce calcium additive levels, using multiple injections to minimize spray impingement, enhancing cooling and employing EGR to suppress hotspot reactivity, and optimizing piston design to reduce oil accumulation and ejection. Methanol engines must also address oil droplet intrusion into the combustion chamber via other pathways such as crankcase ventilation, turbocharger leaking and valve stem leaking.

Current research on methanol pre-ignition and knock is still limited, and further studies on mechanisms and mitigation strategies should focus on:

• Enhancing optical diagnostics for methanol engine combustion, enabling comprehensive analysis of ignition source types, locations, and flame propagation to deepen understanding of methanol pre-ignition processes;
• Incorporating Large Eddy Simulation (LES) and more detailed chemical kinetic models into numerical simulations to improve predictions of pre-ignition and knock with greater precision;
• Advancing research on the chemical reaction dynamics of engine oil in methanol environments to understand the theoretical basis for oil-induced pre-ignition and guide the development of pre-ignition-resistant oil formulations;
• Leveraging artificial intelligence to analyze pre-ignition patterns by extracting key features from extensive experimental data, enabling real-time monitoring and intelligent predictions of engine operating states;
• Employing novel methods to suppress mixture reactivity, such as water injection, which has proven effective in reducing cylinder temperatures and minimizing hotspots. Precise control of water injection timing, quantity, and location can mitigate rapid flame propagation, reducing pre-ignition and knocking risks. Additionally, ammonia, a low-carbon fuel with a high octane rating, shows promise for reducing methanol reactivity due to its high solubility in methanol;
• Utilizing in-cylinder direct injection and diffusion combustion techniques. Direct injection combined with multiple injections can enhance methanol atomization, reduce wall impingement, and lower the likelihood of oil dilution. Diffusion combustion, avoiding premixed flames altogether, is one of the most effective strategies for pre-ignition suppression.