Influence of Pilot Injection on Combustion Characteristic of Methanol–Diesel Dual-Fuel Engine

Abstract

An experimental study regarding methanol–diesel dual-fuel (DF) engines was conducted on a modified engine to explore the effects of pilot injection timing and period on the two-stage combustion process caused by the pilot injection strategy. In this study, the two-stage combustion process was determined according to the first two peaks of the second derivative of an in-cylinder pressure (d²p/dφ²) curve. The results show that the peak pressure rise rate (PRR) tended to decrease with advancing pilot injection timing at a high co-combustion ratio (CCR), which reduced combustion noise. The start of the combustion of the main injection diesel (SOC2) could be advanced by increasing the pilot injection period or advancing pilot injection timing at a 42% CCR. At an 18% CCR, the pilot injection timing and period had no significant effect on SOC2. With the advancement of pilot injection timing, the start of the combustion of pilot injection diesel (SOC1) advanced, and generally, the coefficient of variation of the PRR (COV_PRR) of the two-stage combustion process increased first and then decreased. However, with the increase in the pilot injection period, SOC1 almost always remained constant and the COV_PRR of the two-stage combustion process generally increased.

1. Introduction

Diesel engines are widely used in the field of transportation because of their high thermal efficiency, durability, and high torque delivery [1,2]. However, simultaneously reducing nitrogen oxides (NOx) and particulate matter (PM) remains a challenge [3,4]. With the growing global energy crisis and the increasingly stringent emissions regulations, the exploitation of new combustion strategies associated with alternative fuels for diesel engines has become extremely important [5].

Over the past few decades, many alternative fuels have been proposed, such as methanol [6,7,8], biodiesel [9,10], biogas [11,12], natural gas [13], hydrogen [14], butanol [15], etc. Methanol, as the simplest saturated unitary alcohol, is considered one of the most promising clean alternative fuels to diesel due to its wider ignition limit, good knock resistance, and wide range of production sources [16,17,18]. At present, methanol has been applied in electronically controlled diesel engines through DF technology. Additionally, much research has been carried out regarding the combustion and emission characteristics of methanol–diesel DF engines [19,20,21,22,23,24,25].

Wei et al. [19] explored the effect of a high methanol premixed ratio on the combustion and emission characteristics of a DF engine. The results suggested that as the methanol premixed ratio increased, the ignition delay increased, and the soot and NOx emissions were significantly reduced, while hydrocarbon (HC) and carbon monoxide (CO) emissions increased. Chen et al. [20] pointed out that when the tested engine operated with a high intake temperature at high loads, an increased ratio of premixed methanol caused the soot and PM emissions to increase. Wang et al. [21] used several methods to reduce NOx and soot emissions at low loads while maintaining high thermal efficiency. They found that utilizing exhaust gas recirculation (EGR) and retarding diesel injection timing could achieve low NOx and soot emissions. The intake air heating had the benefit of improving brake thermal efficiency and had little effect on soot emissions but decreased the level of NOx emissions. Furthermore, Chen et al. [22] researched the effects of the water port injection strategy on combustion and emission characteristics in a methanol–diesel DF engine. It was highlighted that the trade-off relationship between NOx and PM emissions could be overcome by increasing water port injection quantity, but the level of HC and CO emissions increased, and the ignition delay became prolonged. Li et al. [25] discovered that as the methanol injection quantity increased, the heat release rate (HRR) of rapid combustion increased, leading to a higher roar-combustion level and cycle-to-cycle variation in indicated mean effective pressure (IMEP).

In the DF mode, methanol is injected into the intake port with low injection pressure to form a methanol–air mixture, which is conducive to reducing the in-cylinder peak temperature, resulting in a reduction in thermal NOx. Meanwhile, the premixed methanol–air mixture could improve the diffusion combustion process of diesel and reduce soot emissions [26].

These previous studies have confirmed that DF engines fueled with methanol and diesel have great potential to inhibit the emissions of NOx and PM. However, the application of methanol in DF engines results in high HC and CO emissions at low loads and reduces thermal efficiency [27]. Moreover, under high loads and large methanol substitution ratio conditions, the rich premixed methanol–air mixture will accelerate the rise in HRR, leading to a sharp rise in cylinder pressure and a rough running phenomenon [28].

As is known, the ignition and combustion process of a methanol–air mixture mainly depends on the direct injection of diesel, as well as the in-cylinder thermal atmosphere [29,30]. Based on this, in recent years, some research on diesel injection strategies has been conducted. Wei et al. [31] studied the effects of pilot injection on the combustion and emission characteristics of methanol–diesel DF engines. They reported that the fuel economy and combustion stability were improved by pilot injection at a high methanol substitution ratio, and lower CO and HC emissions were achieved, except for NOx emissions. Liu et al. [32] investigated the effects of the pilot injection strategy on performance and emission characteristics in a DF engine at a low load. They found that the two-stage combustion process occurred using the pilot injection method and that the bimodal distribution trend of the HRR curve became more obvious by increasing pilot injection quantity. Nevertheless, with the increased methanol substitution ratio, the two-stage combustion process disappeared. In addition, the advanced pilot injection timing could increase the peak HRR and decrease the cycle-to-cycle variation in IMEP. Hu et al. [33] researched the combustion process of the methanol–diesel DF mode in a constant-volume combustion chamber. They reported that, with pilot injection, the flame surface wrinkles were more complicated, and the area of natural flame luminosity became larger, but the flame lift-off length was shortened.

Aside from pilot injection, Wu et al. [34] investigated the effects of a late-injection strategy on the combustion and emissions of a methanol–diesel DF engine. It was shown that suitable late-injection timing and late-injection quantity helped to simultaneously reduce the NOx and PM emissions. Panda et al. [35] applied pilot and post-injection strategies in a methanol–diesel DF engine and found that by selecting suitable post-injection timing and quantity combined with the pilot and main injections, HC, CO, NOx, and soot levels were all improved, and the combustion stability was increased. In addition, they found that the reduction in soot and NOx through a late-injection strategy was attributed to the increased in-cylinder temperature (T) and improved turbulence.

The investigations mentioned above reveal that the diesel injection strategy has demonstrated great potential to inhibit NOx and PM emissions while attaining low combustion noise levels. These previous studies mainly focused on the influence of diesel injection parameters on the results of the whole combustion process. However, in terms of the pilot injection strategy, two-stage heat release occurs during the combustion process. Few studies on the effects of pilot injection parameters on the two-stage combustion process have been conducted. Therefore, the novelty of this study is that it investigated the impact of pilot injection timing and quantity on the first and second combustion stage to reveal the relationship between the first and second combustion stage, combustion noise, and operation stability. The results of this paper contribute to the development of an optimal diesel injection strategy to obtain higher CCR targets in a methanol–diesel DF engine at high loads.

2. Experimental Apparatus and Procedure

2.1. Experimental Setup

The tests were conducted on a DF engine modified by a four-cylinder, four-stroke, high-pressure common-rail diesel engine. The engine specifications are listed in

Table 1. Parameters of the test engine.

Items	Values
Type	WP4G154E330
Bore (mm)	105
Stroke (mm)	130
Link length (mm)	210
Compression ratio	18.0:1
Displacement (L)	4.5
Rated power (kW)/speed (r/min)	113/2300
Rated torque (Nm)/speed (r/min)	520/1600~1800

. Figure 1 shows the schematic of the engine test system. Each cylinder was modified to operate in the DF combustion mode by adding a pipe injection (PI) methanol injector in the intake manifold. The injection timing and period of both PI and direct injection (DI) were controlled by their respective ECU; thus, mixtures with different fuel–air equivalence ratios could be obtained. In addition, the diesel injection signals of injector #1 were monitored using a current clamp (A622, Tektronix, Johnston, OH, USA), and the consumption of methanol and diesel were measured through an electronic balance (ES100K*1, Longteng electronic, Shenyang, China) and a transient fuel consumption meter (Toceil-CMFG010, Toceil, Shanghai, China), respectively. An eddy current dynamometer was applied to load the engine. The cylinder pressure was measured using a pressure transducer (6052A, Kistler, Winterthur, Switzerland). Before the measured pressure data were inputted to a combustion analyzer (Kibox 2893A, Kistler) to be recorded and analyzed, a charge amplifier (5019B, Kistler) was used due to the tiny signal of the pressure transducer.

As is known, the uncertainty of measured and calculated parameters is important in the verification of the accuracies of the test results, so each group of fuel consumption data and the corresponding in-cylinder pressure was measured three times in this study. Moreover, all of the instruments were calibrated before the experiment, and the specifications are listed in

Table 2. The specifications of testing instruments.

Measured Parameter	Instrument	Measuring Range	Accuracy
Torque	FST3 CW160B	0~600 Nm	0.22%
Speed	FST3 CW160B	0~10,000 r/min	0.22%
Diesel consumption	Toceil CMFG010	0~60 kg/h	≤0.12%
Methanol consumption	Longteng ES100K*1	0~100 kg	±1 g
In-cylinder pressure	Kistler 6052A	0~25 MPa	0.50%
Crank angle	Kistler 6291	0~720 °CA	0.1 °CA
Diesel injection signal	Tektronix A622	0.5~100 A	±0.01 ms

.

2.2. Experimental Fuel

Methanol and diesel were used in the tests. The main properties of the two fuels are listed in

Table 3. Properties of two fuels [3,25].

Items	Methanol	Diesel
Chemical formula	CH4O	–
Oxygen content (%)	50	−
Stoichiometric air–fuel ratio	6.5	250
Latent heat of vaporization (kJ/kg)	1110	0.835
Density (g/cm3)	0.79	42.5
Lower heating value (MJ/kg)	19.66	52.6
Cetane number	3

.

2.3. Experimental Procedures and Conditions

The engine was always started in diesel mode until the oil and coolant temperature reached normal values (95 °C and 85 °C). Before the engine was switched from diesel mode to DF mode, the engine was revved up and stabilized at 1800 r/min, which is the maximum speed of the rated torque. Subsequently, according to the test condition, the quantity of direct injection diesel was adjusted using the original high-pressure common-rail system through the Inca software. At the same time, methanol was injected into the intake manifold and adjusted through the pilot injection period via methanol’s ECU to compensate for the decrease in torque that arose from the change in diesel quantity. During the experiment, the injection pressures of methanol and diesel were maintained at 0.39 MPa and 103 MPa, respectively. In addition, the measured parameters were recorded when the engine had been running steadily for at least one minute in each test condition. The detailed test conditions are shown in

Table 4. The test conditions.

Engine Speed (r/min)	Engine Load (%)	CCR (%)	Main Injection Timing (°CA BTDC)	Pilot Injection Timing (°CA BTDC)	Main Injection Period (°CA)	Pilot Injection Period (°CA)
1800	66	18/32	9	14.5	9.8/7.6	2
1800	66	18/32	9	19	9.7/7.4	2
1800	66	18/32	9	24	9.5/7.3	2
1800	66	18/32	9	29	9.5/7.4	2
1800	66	18/32	9	19	9.9/7.5	1.4
1800	66	18/32	9	19	9.7/7.4	2
1800	66	18/32	9	19	9.4/7.2	2.5
1800	66	18/32	9	19	9.1/7.0	3

.

2.4. Parameters Definition

2.4.1. Co-Combustion Ratio

CCR is defined as the percentage of methanol energy to total energy as follows:

$$CCR=\frac{{\dot{m}}_M\times LH{V}_M}{{\dot{m}}_D\times LH{V}_D+{\dot{m}}_M\times LH{V}_M}\times 100\%$$

(1)

where ${\dot{m}}_D$ is the mass flow rates (kg/h) of diesel, and ${\dot{m}}_M$ is that of methanol. LHV_D and LHV_M are the lower heating values of diesel (42.50 MJ/kg) and methanol (19.66 MJ/kg), respectively.

2.4.2. Heat Release Rate

The HRR is calculated based on the first law of thermodynamics through a single-zone heat release model, and three main assumptions are as follows: The first is that the temperature and mixture of the air–fuel is homogeneous in the cylinder. The second is that the working medium in the cylinder is ideally a gas state. The last is that there is no leakage and blow-by from the cylinder. In this model, the heat-transfer effect is taken into consideration to calculate the HRR and one of the most important parameters, the heat-transfer coefficient, is obtained by using Woschni’s expression. The HRR is expressed as

$$\frac{dQ}{d\phi }=\frac{dW}{d\phi }+\frac{d{Q}_w}{d\phi }+\frac{dU}{d\phi }$$

(2)

where Q is the heat released. W is the work of the working medium. Q_w is the heat transfer from the cylinder wall. U is the internal energy. φ is the crank angle.

2.4.3. Start of Combustion

In general, the ignition delay is the interval from the start of direct diesel injection to the start of combustion (SOC), whereas the DF model with pilot injection gives rise to the occurrence of two-stage combustion, so there are two SOCs. As seen in Figure 2, the two SOCs are determined according to the d²p/dφ² curve. The first peak of d²p/dφ² caused by pilot injection is SOC1, and the second peak of d²p/dφ² caused by the main injection is SOC2.

In this study, for each test condition, the pressure data of 120 consecutive cycles were recorded and averaged. COV_PRR before and after SOC2 was calculated to evaluate the combustion stability of the two combustion stages caused by the pilot injection strategy. The COV_PRR was calculated based on the study by Li [28].

3. Results and Discussion

3.1. Effects of Pilot Injection Timing

3.1.1. Effect on PRR under Different CCRs

Figure 3 shows the effects of pilot injection timing on PRR curves under different CCRs. As seen in Figure 3, the variation trend of peak PRR in the first combustion stage is consistent. At first, with the advancement of pilot injection timing, the first peak of PRR rises, and its corresponding crank angle is obviously advanced. The PRR reaches its maximum when pilot injection timing is 19 °CA before the top dead center (BTDC). As the pilot injection timing is advanced to 24 °CA BTDC and continues to increase, the peak PRR is decreased and the corresponding crank angle is slightly advanced. However, under the higher CRR and earlier pilot injection timing, the corresponding crank angle is slightly delayed. This is because, in the beginning, with the advancement of pilot injection timing, there is enough time and T for the formation of the diesel–air mixture. Additionally, the main diesel injection crosses through the oil mist wake of the pilot injection, and the jet effect accelerates the diffusion of the pilot diesel spray to both sides, producing a wider spray area, especially under the condition of high CCR. All of these aspects accelerate the combustion process. While pilot injection timing continues to advance, the combustion in the first stage becomes worse. This is because the T is significantly reduced, and the jet effect of the main diesel injection on the pilot injection is weakened, which is not conducive to the evaporation and atomization of diesel.

For the second combustion stage, as the pilot injection timing is advanced, the peak PRR slightly decreases under low CCR, while there is no significant change in the peak PRR with the further advancement of pilot injection timing. This is due to the almost unchanged SOC2 and the low concentration of the methanol–air mixture. Under a higher CCR of 42%, the declining trend in peak PRR becomes more noticeable, which is mainly attributed to the rich concentration of the methanol–air mixture and the advanced ignition of the main injection diesel. As is known, the flame-propagating speed of methanol–air mixtures is faster than that of diesel, which could accelerate the rapid combustion in the second combustion stage.

3.1.2. Effect on HRR and T under Different CCRs

Figure 4 depicts the effects of pilot injection timing on HRR and T. With the advancement of pilot injection timing, the peak HRR of the first combustion stage and T increase first and then decrease at low and high CRRs, which is the same as the experimental results of other researchers [31]. This is attributed to the combined effect of diesel evaporation and atomization quality. Due to the increased CRR, the pre-injection methanol quantity increases, and the latent heat of vaporization is increased, which leads to the decrease in T and ulteriorly deteriorates the combustion of the pilot diesel. Thus, under a high CCR, the decreases in peak HRR and T are particularly obvious.

In addition, it can be seen from the figure that, as the pilot injection timing is advanced, the HRR curve of the second combustion stage distinctly advances at 42% CCR, which is negligible at a low CCR. With the advanced pilot injection timing, the interval between the first and second combustion stage becomes longer, and then, the effect of the heat released by the pilot diesel on the second combustion stage is weakened, leading to the SOC1 being almost unchanged. Therefore, the variation trend is not obvious. However, under a high CCR condition, the combustion phase of the pilot diesel is delayed due to the high concentration of the methanol–air mixture. Therefore, the released heat of the first stage has a significant effect on the combustion of the main injection diesel, reducing its ignition delay. Thus, a noticeable variation trend of HRR curves can be observed in Figure 4b.

As seen in Figure 4b, it can also be found that the double peak increases in HRR when the pilot injection time is 14.5 °CA BTDC at 42% CCR. There are two reasons for the increase in double peak in HRR during the second combustion stage. The first peak comes from the unburned methanol of the pilot injection combustion. Previous research shows that methanol consumes OH with high activity to HO₂ with low activity, which inhibits ignition below 1000 K [36]. In addition, when the temperature exceeds 1000 K, HO₂ will convert to OH and more ignitions occur. It can be seen in Figure 4 that, at the 19, 24, and 29 °CA BTDC pilot injection timing conditions, the temperature at the late phase of the pilot combustion exceeds 1000 K, and more methanol burns before the main injection. At the 14.5 °CA BTDC pilot injection timing condition, the temperature approaches 1000 K at the end of the pilot combustion, which leaves more methanol to the main injection combustion and increases the first combustion (premixed combustion) peak. The increase in the second peak is because of the unburned diesel of the first premixed combustion peak. The more specific reason is that, at a high CCR condition (42%), methanol combustion produces a high concentration of CH₃, which inhibits the pyrolysis of diesel during the premixed combustion.

3.1.3. Effect on SOC under Different CCRs

Figure 5 shows the variations in SOC1 and SOC2 under different pilot injection timings. It is clear that as the pilot injection timing is advanced, the SOC1 advances, except for when the pilot injection timing changes from 24 °CA BTDC to 29 °CA BTDC at 42% CCR. The advancement of SOC1 is due to the high T at a 66% load, which is beneficial for the ignition of the pilot diesel. However, at 42% CCR, the retardation of SOC1 is the result of remarkably decreased T. As for SOC2, it initially advances and then retards as the pilot injection timing is advanced at 18% CCR. At first, with the advancement of pilot injection timing, the quantity of heat released by the pilot diesel contributes to the increase in T, which shortens the ignition delay of the main injection diesel. As the pilot injection timing is further advanced, the combustion phase of the first stage advances. Hence, more heat released by the first combustion stage is dissipated through heat transfer, which causes a decline in T and weakens the promoting effect on the ignition of the second stage, leading to the retardation of SOC2. When the CCR is 42%, the SOC2 gradually advances due to the retarded combustion phase of the first stage.

3.1.4. Effect on COV_PRR under Different CCRs

Figure 6 shows the COV_PRR of the first combustion stage under different pilot injection timings. Overall, the COV_PRR of the first combustion stage increases with advanced pilot injection timing. There is a considerable decline when the pilot injection timing is 29 °CA BTDC at high CCR. On the one hand, the decreased in-cylinder ambient temperature is not conducive to the stable combustion of pilot diesel spray. On the other hand, the quantity of pilot diesel is small and does not have enough energy to ignite the methanol–air mixture, resulting in a small quantity of heat released during the first combustion stage. These combined factors lead to an increase in COV_PRR in the first combustion stage. As for high CCR of 42%, the high concentration of methanol–air mixture leads to the decrease in T. When the pilot injection timing is excessively advanced, the in-cylinder ambient temperature is too low, even leading to the misfire of the pilot diesel, thereby having little effect on the peak PRR. This can be verified by the marked decrease in HRR and T shown in Figure 4b.

Figure 7 displays the COV_PRR of the second combustion stage under different pilot injection timings. It can be seen that the COV_PRR of the second combustion stage firstly increases and then decreases at low and high CCRs. When the CCR is 18%, the initial increase in COV_PRR is derived from the elevated quantity of heat released from rapid combustion. However, when the pilot injection timing is adjusted from 24 °CA to 29 °CA BTDC, the combustion of the pilot diesel deteriorates, leading to the delay of SOC2. Meanwhile, the HRR curves and the crank angle of the peak PRR are almost unchanged, as observed in Figure 3a and Figure 4a. Thus, the quantity of heat released from rapid combustion is reduced during the second combustion stage, which results in a decrease in COV_PRR. Regarding 42% CCR, when the pilot injection timing is adjusted from 14.5 °CA to 19 °CA BTDC, the heat release quantity of rapid combustion is reduced, but the COV_PRR ascends. This may be because when the pilot injection timing is 14.5 °CA BTDC, the main injection diesel is injected into the cylinder during the combustion phase of the pilot diesel. Then, the heat released by the pilot diesel excellently facilitates the atomization and evaporation of the main injection diesel, which results in a higher concentration of the premixed diesel–air mixture, improving the combustion stability. Nevertheless, with the further advancement of the pilot injection timing, the quantity of heat released from rapid combustion decreases so that the COV_PRR is reduced.

3.2. Effects of Pilot Injection Period

3.2.1. Effect on PRR under Different CCRs

Figure 8 displays the effects of the pilot injection period on PRR under different CCRs. With the increase in the pilot injection period, the peak PRR of the first combustion stage ascends continuously, while the corresponding angle is almost the same. The increased pilot injection period means that the pilot diesel quantity ascends, and the diesel–air mixture becomes richer, leading to the increase in peak PRR. Moreover, the pilot injection timing is fixed at 19 °CA BTDC, and there is a small quantity of pilot diesel. Thus, the heat absorption of the pilot diesel has little effect on the T, which results in almost the same combustion phase of the first stage.

In addition, the peak PRR firstly decreases and then increases with the increase in the pilot injection period during the second combustion stage. As discussed previously, this is mainly due to the variation in the quantity of heat released from rapid combustion before the PRR reaches the maximum. From Figure 8, it can also be seen that there is a distinct rise in peak PRR, compared with a low CCR condition when the pilot injection period is 3 °CA. Firstly, the improved combustion of the first stage increases T, which contributes to the ignition of the main injection diesel and the increased ignition energy. Secondly, a high CCR causes a high concentration of the premixed methanol–air mixture in the cylinder. Then, the high concentration of the methanol–air mixture accelerates the HRR, leading to a distinct rise in peak PRR.

3.2.2. Effect on HRR and T under Different CCRs

Figure 9 describes the effects of the pilot injection period on HRR and T. With the increase in the pilot injection period, both peak HRR and T gradually increase during the first combustion stage, which is mainly caused by the increased quantity of pilot diesel. Under high and low CCRs, there is not enough ignition energy to ignite the methanol–air mixture due to the low pilot diesel quantity. The combustion of diesel plays a dominating role in the first combustion process. Therefore, with the increase in pilot injection, the in-cylinder concentration of diesel–air ascends, which results in the increase in the overall activity level of the in-cylinder mixture fuel, accelerating the combustion process. Regarding the second combustion stage, it could be found that the HRR and T show the trend of increasing BTDC as the pilot injection period is increased at a high CCR. Although the main diesel injection quantity decreases with the increased pilot injection period, the concentration of the in-cylinder premixed methanol–air mixture is essentially the same and high enough at a high CCR. In addition, the rapid combustion process of the second combustion stage is dominated by the premixed combustion of the methanol–air mixture. When the pilot injection quantity increases, the promoting effect of the pilot injection diesel on the ignition of the main injection diesel increases, the ignition of the main injection diesel is advanced, and the combustion of the methanol–air mixture is advanced. Therefore, the HRR and the T ascend. However, the trend is imperceptible at a low CCR, which is due to the almost unchanged SOC2 and the low concentration of the methanol–air mixture.

3.2.3. Effect on SOC under Different CCRs

Figure 10 illustrates the SOC1 and SOC2 under different pilot injection periods. As the pilot injection period is increased from 1.4 °CA to 3 °CA, the SOC1 is almost unchanged under two CCRs. This may be attributed to the same pilot injection timing and a small quantity of pilot diesel. From this figure, it can also be found that the SOC2 exhibits a slightly advanced trend, which is due to the ascending quantity of heat released in the first combustion stage caused by the increased quantity of pilot diesel. Furthermore, it is noted that when the pilot injection period is increased from 2 °CA to 2.5 °CA, the SOC2 distinctly advances at 42% CCR. This is because, with the increase in CCR, the concentration of the methanol–air mixture ascends. Then, the high concentration of the methanol–air mixture competes with diesel for active radicals used for hydrogen abstraction reactions of diesel. Meanwhile, the T decreases. All these factors lead to the delay of the combustion phase of the pilot diesel; thus, the promoting effect of an increased quantity of heat released by the pilot diesel on the ignition of the main injection diesel is more obvious, accelerating the advancement of SOC2, compared with that of 18% CCR.

3.2.4. Effect on COV_PRR under Different CCRs

Figure 11 and Figure 12 present the COV_PRR of the first and second combustion stage, respectively. It can be seen that, as the pilot injection period is increased, the COV_PRR increases constantly in all of the test conditions. As discussed above, the COV_PRR of the first stage is mainly affected by the ambient temperature and quantity of heat released by the pilot diesel. However, when the pilot injection period increases, the combustion phase of the first stage is basically unchanged, which indicates that the COV_PRR is mainly related to the quantity of heat released. Therefore, with the increase in the pilot injection period, the COV_PRR increases due to the increased quantity of heat released. The variation trend of COV_PRR during the second stage is also based on the increased quantity of heat released from rapid combustion.

4. Conclusions

In this study, the effects of pilot injection timing and period on the combustion characteristics of a methanol–diesel DF engine were experimentally studied. The main findings are summarized as follows:

• At a low CCR, with the increase in pilot injection timing, the peak HRR and PRR of the first combustion stage all initially increase and then decrease. However, the declining trend shown in the peak PRR of the second combustion stage becomes more noticeable at 42% CCR, which is beneficial to the reduction in combustion noise. The COV_PRR in the two-stage combustion process generally first ascends and then decreases in the two CCR conditions tested.
• The advanced pilot injection timing leads to the advancement of SOC1, while it has no significant effect on SOC2. Additionally, a high CCR results in the delay of the combustion phase of the first stage, making the heat released by pilot diesel have a more distinctly promoting effect on the advancement of SOC2.
• With the increase in the pilot injection period, the peak HRR and peak PRR of the first combustion stage increase, leading to increased combustion noise. However, their corresponding crank angles almost always remain constant. The COV_PRR in the two-stage combustion process generally presents a tendency to ascend in the two CCR conditions tested.
• The increased pilot injection period has a negligible effect on the SOC1 and SOC2 at 18% CCR. Regarding 42% CCR, the variation in SOC1 is consistent with that in 18% CCR, while SOC2 obviously advances.
• A Small quantity of pilot injection diesel and appropriately advanced pilot injection timing could effectively ensure the combustion stability of the second combustion stage and improve the operation stability of the engine.