Numerical Study on the Combustion Process of the First Cycle of Diesel Engine Start-Up Based on Target Torque Control

Abstract

During the diesel engine start-up phase, low rotational speed and coolant temperature result in poor fuel atomization and prolonged ignition delay. This impedes the in-cylinder combustion process and directly impacts the engine’s emission performance. As the first combustion cycle during the starting process, the initial starting cycle significantly influences subsequent combustion cycles and overall starting performance. This paper proposes a target-torque-based control strategy for fuel injection quantity during the starting process. It optimally determines the target acceleration curve for the starting process, thereby calculating the optimal fuel injection quantity for the initial starting cycle. Based on this, a combustion system simulation model of the diesel engine was established using the 3D CFD software AVL FIRE v2010. The simulation investigated the impact of first injection speed on the combustion process and performance of the first firing cycle under different ambient temperatures: normal temperature (20 °C), low temperature (5 °C), and cold start (−10 °C). The results indicate that the optimal first cycle injection quantities under normal, low, and cold start conditions are 17.3 mg, 18.5 mg, and 20.4 mg, respectively. The impact of first injection speed on the first firing cycle combustion process primarily manifests in the mixture formation rate and time, and higher speeds do not necessarily yield better results. The optimal first injection speeds at normal temperature (20 °C), low temperature (5 °C), and cold start (−10 °C) were 220 r/min, 240 r/min, and 220 r/min, respectively. Corresponding indicated thermal efficiencies were 30.74%, 28.67%, and 28.7%, with relatively low emissions of pollutants such as CO, NOx, and HC.

1. Introduction

As global emission regulations such as China 6, Euro 6d, and EPA Tier 3 become increasingly stringent, the importance of diesel engine starting performance is growing. The “Emission Limits and Measurement Methods for Light-Duty Vehicles” (China Stage 6) mandates exhaust emission tests for start-up at normal temperatures, as well as CO, THC, and NOx emission tests during cold starts at low temperatures for light-duty vehicles equipped with compression-ignition engines. Incorporating the starting process into testing undoubtedly imposes higher demands on all aspects of diesel engine starting performance [1].

Researchers have extensively studied the spray and mixture formation characteristics [2,3,4,5], ignition and combustion characteristics [6,7,8], and emission characteristics [9,10,11] during cold start conditions. Yang et al. investigated the effects of cyclic injection quantities on instantaneous engine speed, spray characteristics, equivalence ratio, and combustion behavior during cold starts at −5 °C in compression-ignition engines through experimental and numerical simulations. Results indicate non-monotonic trends in engine starting and combustion performance with varying injection quantities, where both excessive and insufficient injection quantities lead to performance deterioration [12]. Liang et al. analyzed spray and combustion characteristics during the first cycle of diesel engine starting using an environmental simulation chamber and a three-dimensional simulation model. Tests were conducted across a temperature range of −20 °C to 10 °C and altitudes from 0 to 4000 m. The study revealed that first cycle spray and combustion characteristics are significantly influenced by altitude and ambient temperature. As altitude increases or ambient temperature decreases, insufficient atomization and combustion efficiency result in reduced starting performance [13].

Building upon this foundation, researchers have employed methods such as intake air preheating [14,15], fuel performance enhancement [16,17,18,19,20], auxiliary ignition [21], electric glow plugs [22], and optimized fuel injection parameters [23,24,25] to improve diesel engine starting performance. Li et al. employed intake air preheating to enhance cold starting performance, investigating the minimum preheating power required under varying ambient temperatures and densities. They developed an environment-adaptive method for estimating preheating power during cold starts [26]. Sun et al. incorporated CeO₂ nanoparticles into diesel fuel to facilitate the transition from low-temperature to high-temperature reactions, enhancing ignition performance during the first firing cycle. This effectively reduced the critical temperature required for cold starting and improved overall cold start performance [18]. Xu et al. discovered that dual injections under cold start conditions enhance main injection ignition and combustion processes—specifically reducing ignition delay, decreasing ignition location dispersion, and increasing flame area [23]. Lu et al. improved cold start performance at varying altitudes by adjusting pilot injection strategies, demonstrating that pilot injections effectively optimize in-cylinder combustion environments to enhance cold start capability [24]. Optimizing pre-injection timing and quantities constitutes an effective measure for enhancing cold start performance. Yang et al. investigated the effects of six electronic control parameters on spray characteristics, mixture formation, ignition-combustion behavior, and parameter sensitivity during start-up conditions. Their findings indicate that appropriately increasing pre-injection quantities, selecting suitable injection timing, and moderately elevating injection pressure contribute to improved diesel engine start-up performance [25].

To improve cold starting performance in diesel engines, methods such as intake air preheating and glow plugs are employed. Their advantage lies in rapidly elevating intake air temperature, thereby enhancing fuel atomization conditions and improving cold start capability. However, their drawback is the need for additional electric heating or flame heating devices, resulting in higher energy consumption. Furthermore, their slow preheating response makes them ill-suited for scenarios involving frequent starts and stops. The advantage of fuel performance enhancement lies in improving fuel atomization and combustion efficiency. Its drawback is that using alternative fuels or fuel additives increases operating costs, hindering widespread adoption. Auxiliary ignition technology effectively boosts starting and ignition success rates. Its disadvantages include a complex structure, requiring additional engine modifications, and high maintenance costs. Compared to auxiliary measures like intake air preheating, optimized fuel injection control does not require additional auxiliary equipment to enhance starting performance. Proper calibration of key control parameters, such as the first injection speed and fuel injection quantity, can improve starting performance. However, research on the first cycle of compression-ignition engine starting remains relatively scarce.

During the diesel engine start-up process, low rotational speed and coolant temperature, combined with the inability to establish rail pressure rapidly, result in poor fuel atomization when the fuel is injected into the combustion chamber. This hinders the formation of a combustible mixture, resulting in inefficient combustion that directly affects start-up reliability and emission characteristics. Particularly under cold start conditions, the combustion conditions during the first injection cycle are even more unfavorable, making misfires more likely and further deteriorating emission performance during the start-up process. The first combustion cycle during startup—corresponding to the first ignition speed—directly influences subsequent startup processes through its combustion quality and control strategy. Optimizing this cycle is crucial for enhancing startup performance and emission characteristics. The characteristics of in-cylinder gas flow and fuel concentration distribution during the first cycle have a significant influence on ignition behavior, combustion efficiency, and pollutant formation. Three-dimensional simulation techniques can replicate the initial conditions, boundary conditions, and combustion process of the first cycle, yielding critical data—such as velocity, concentration, and temperature fields within the cylinder—that are difficult to obtain experimentally. This facilitates microscopic research into the combustion characteristics of the first cycle [13]. The first injection speed is engine speed at the first injection event. The first injection speed is a critical control parameter for diesel engine starting, as it directly affects first cycle performance. Different first injection speeds result in varying mixture formation rates and times, and changes in mixture formation conditions alter first cycle combustion efficiency and pollutant generation. Therefore, the first injection speed must be appropriately calibrated under different ambient temperatures. This helps improve the economy and emission characteristics of the diesel engine starting process.

Building upon the research group’s prior work on target torque control strategies for the start-up process [27], this paper employs a three-dimensional computational fluid dynamics (CFD) simulation model. Based on start-up control strategies with different target acceleration profiles, it simulates the fuel economy and emissions performance of the first start-up cycle under various target acceleration curves. This approach identifies the optimal fuel injection quantity for the first start-up cycle. Furthermore, we investigated the impact of first injection speed on first cycle fuel economy and emissions performance. Analysis of the first cycle velocity field, concentration field, and temperature field revealed key factors influencing fuel economy and emissions during the first cycle. Under three ambient temperature conditions—20 °C (normal temperature), 5 °C (low temperature), and −10 °C (cold temperature)—the optimal first injection speed for each condition was determined by comparing first cycle thermal efficiency and emission characteristics at different speeds. Results demonstrate that the optimized control strategy effectively improves automotive diesel engine starting performance, holding significant implications for the low-carbon development of automotive diesel engines.

2. Research Protocol

The engine startup process consists of three stages: starter drag, initial fuel injection and acceleration, and entering idle speed, as shown in Figure 1. When the starter motor drives the engine to reach a predetermined speed, the engine begins to inject fuel. This speed is defined as the first injection speed, and the combustion cycle corresponding to this speed is the first cycle of the engine’s start-up process. After ignition in the first cycle, the engine enters the starting acceleration stage, causing the speed to rise rapidly. When the speed reaches the set successful starting speed, the starting process is deemed complete, and the engine begins the idle control stage.

Regarding the control of fuel injection quantity during the engine start-up acceleration phase, our research group has previously proposed a target-torque-based fuel injection control strategy for the start-up process. During the engine ignition acceleration phase, the target torque required by the engine is divided into starting resistance torque and starting acceleration torque, as shown in Figure 2. The starting resistance torque is used to overcome the internal friction resistance of the engine and drive accessories during the starting process, while the starting acceleration torque is used to overcome the inertial resistance during the transition from the first injection speed to the successful starting speed. After determining the target torque, the fuel injection quantity for each cycle during the starting process can be calculated based on the torque–fuel quantity conversion coefficient, thereby achieving control of the starting acceleration phase.

The resistance torque two-dimensional map was obtained from engine reverse-drain tests. The map records the friction resistance torque corresponding to each operating point as coolant temperature ranges from −30 °C to 100 °C and engine speed from 0 to 1200 r/min. To accommodate the phenomenon where engine oil fails to form an adequate oil film at low temperatures and speeds, leading to increased friction resistance torque, the resistance torque is intentionally elevated at low temperatures. This action increases the target starting torque, ultimately boosting the cyclic fuel injection quantity. By enhancing fuel injection quantity, power output during the starting process is improved, thereby counteracting the negative impact of increased friction resistance and ensuring adequate starting power delivery.

The starting resistance torque can be obtained through an engine reverse drag test, while the starting acceleration torque is calculated using a target acceleration curve based on the target acceleration and the engine’s rotational inertia. By adjusting different target acceleration curves, various starting acceleration characteristics can be obtained. In this paper, the target acceleration curve is set as a two-segment linear function, as shown in Figure 3 (The single rotational speed trace retained in the figure serves only to illustrate the logic of acceleration driving rotational speed, not to compare speed differences under varying accelerations). The first segment is the rapid acceleration phase, during which the engine speed increases rapidly. The second segment is the slow acceleration phase, where the target acceleration gradually decreases to zero as the engine speed approaches the successful starting speed, ensuring a smooth transition from the starting acceleration phase to the idle phase.

The focus of this study is on optimizing the first segment, which encompasses the entire startup acceleration phase, including the first cycle. To save simulation computation time, the first cycle, mid cycle (500 r/min), and late cycle (650 r/min) during the injection acceleration process were selected to form the three node cycles of the injection acceleration phase. To optimize and achieve the optimal target acceleration, four sets of target acceleration curves with varying slopes were designed, as shown in

Table 1. Slope of the target start acceleration curve.

Target Start Acceleration Curve	Slope (rad/s2) *	Initial Acceleration Value (rad/s)
a1	−20.93	47.10
a2	−10.47	39.25
a3	10.47	23.55
a4	20.93	15.70

* Unit Conversion Notes: 1 rpm/s² = π/30 rad/s² ≈ 0.10467 rad/s².

. Simulation calculations were then performed over the three cycles.

The first combustion cycle is the initial combustion cycle during the starting process. At this stage, the engine speed and coolant temperature are at their lowest, resulting in the poorest combustion conditions, which are highly prone to misfiring or incomplete combustion. Therefore, optimizing the combustion process during the first cycle is crucial for improving starting performance. The first injection speed, as a key parameter for controlling starting conditions, influences the formation of the first cycle mixture and the combustion process. This directly impacts starting reliability, fuel economy, and emissions performance. To investigate the mechanisms by which different first injection speeds affect the first cycle combustion process, based on the previously established simulation calculation model of the first cycle during the starting process under normal temperature conditions, four groups of different first injection speeds were set: 200 r/min, 220 r/min, 240 r/min, and 260 r/min. The study investigated the impact of varying first injection speeds on the combustion process and emission performance.

Ambient temperature has a significant impact on the starting process of diesel engines, particularly during cold starts. Increased fuel viscosity and lower air temperatures make the fuel–air mixing process more challenging. Therefore, determining the optimal first injection speed under different ambient temperatures is crucial for improving the combustion performance and emission characteristics of the first cycle during starting across various environmental conditions. This study simulates the first cycle combustion process at three ambient temperatures (20 °C, 5 °C, and −10 °C) to analyze how the first injection speed affects first cycle combustion, fuel economy, and emissions performance under varying environmental conditions.

3. Simulation Model Establishment and Verification

3.1. Establishment of Simulation Model

Table 2. The tested engine specifications.

Description	Specification
Engine type	4D20
Number of cylinders	4
Bore	83 mm
Stroke	92 mm
Displacement volume	1.99 L
Compression ratio	17.2
Rated power at speed	110 kW @ 3800 r/min
Maximum torque at speed	310 N·m @ 2000–2800 r/min
Injection system	Bosch common rail
Intake boost system	VNT turbo charge

presents the primary technical parameters of the test prototype, which utilizes a 2.0 L high-pressure common-rail light-duty diesel engine (Great Wall Motors, Baoding, China).

Table 3. Main equipment and apparatus.

Category	Measuring Instruments	Manufacturer	Measurement Range	Accuracy
Dynamometer	CW260	CAMA (Luoyang, China)	Speed: 0–7500 r/min Torque: 0–1395 Nm	Speed: <±1 r/min Torque: <±0.2% FS
Fuel consumption meter	FCMM-2	Dongfeng (Shiyan, China)	0–500 kg/h	<±0.3% FS
Cylinder pressure sensor	6056 A	Kistler (Winterthur, Switzerland)	0–250 bar	<±0.3% FSO
Charge amplifier	5018 A	Kistler (Winterthur, Switzerland)	±2 … 2,200,000 pC	<±0.3% FS
Combustion analyzer	DS9100	Onosokki (Yokohama, Japan)

shows the main testing equipment and their measurement accuracies.

The computational domain within the prototype cylinder was meshed using AVL-FIRE v2010 software. The specific number of mesh layers is shown in

Table 4. Number of grid layers.

Crank Angle (°CA)	Number of Grid Layers
180~320, 400~540	20
320~340, 380~400	10
340~360, 360~380	5

, and the mesh model is illustrated in Figure 4. The solver was configured with computational boundary conditions, initial conditions, and the selected computational model, as shown in

Table 5. Solver parameter settings.

Project	Setting Value
Cylinder top temperature	355 K
Cylinder wall temperature	355 K
Piston top temperature	385 K
Initial pressure	1.14 bar
Initial temperature	303 K
Average eddy ratio	2.0
Combustion model	ECFM 3Z
NO emission model	Extended Zeldovich
Soot emission model	Kinetic Soot
Turbulence model	k-ζ-f
Particle interaction model	O_Rouke
Wall interaction model	Walljet2
Breakup model	KH-RT
Evaporation model	Dukowicz

. The injector parameters were set to match those of the test prototype. The injection method employed a three-stage injection process, with injection timings at 354.5 °CA, 356 °CA, and 358 °CA, respectively. At −10 °C, the ignition delay period increases due to low temperatures, so the injection timing is advanced to 350 °CA, 352 °CA, and 354 °CA. The compression top dead center is 360 °CA. The injection ratio for pre-injection 1/pre-injection 2/main injection = 1:2:7.

3.2. Verification of the Simulation Model

By adjusting the parameters of the aforementioned models, the cylinder pressure curves and heat release rate curves obtained from the simulation calculations were compared with experimental data under identical conditions to evaluate the validity of the simulation model. Each operating condition was repeated 20 times in the experiment. After excluding abnormal cycles such as misfires, cylinder pressure data from 10 valid cycles were selected for cyclic synchronous averaging. The averaged cylinder pressure data were used to validate the effectiveness of the simulation model. As shown in Figure 5 and Figure 6, the first cycle cylinder pressure curves and heat release rate curves for both the experimental and simulation calculations are presented, respectively. The simulation and experimental comparison results for the first cycle cylinder pressure and heat release rate show that the model-predicted cylinder pressure peak error is less than 3%, the peak error in heat release rate is less than 3%, the combustion start point error is less than 0.5° CA, and the combustion duration error is less than 0.3° CA. All quantitative metrics meet the engineering simulation accuracy requirements, fully validating the reliability of this simulation model.

The model selection and mesh division for the mid cycle and late cycle are the same as those for the first cycle, but the model parameters are modified based on the actual test results. As shown in Figure 7 and Figure 8, the simulation results and test data for the cylinder pressure curves of the mid cycle and late cycle are compared, respectively. The model predicts peak cylinder pressure with an error of less than 3%, combustion initiation with an error of less than 0.5 °CA, and combustion duration with an error of less than 0.3 °CA. Simulation values closely match experimental data for core combustion characteristics during the first, mid, and late cycles, showing no systematic deviation. This fully validates the reliability of the simulation model, providing robust support for deriving subsequent research conclusions.

The emission generation model employed in this study has been calibrated by commercial software developers through extensive steady-state engine testing. Given that this model represents a mature, commercially calibrated version, no additional secondary calibration was performed in this research. Predictions were generated only by invoking the model for start-up conditions. It should be noted that the first cycle emission results in this study primarily reflect the trend patterns of emissions under different start-up control strategies, rather than precise predictions of absolute concentration values. Future research will utilize transient emission test systems to obtain actual first cycle measurement data, further validating and refining the model’s predictive accuracy to enhance the reliability of the results.

4. Result

4.1. Determining the Start Injection Quantity Based on the Target Acceleration Curve

The control method for fuel injection quantity during the start acceleration phase of the starting process directly impacts starting performance. A control strategy is employed where target acceleration is used to determine the target required torque, which in turn defines the fuel injection quantity.

Table 6. First cycle, mid cycle, and late cycle fuel injection quantities at different target accelerations.

Target Acceleration	First Cycle Injection Quantity (mg)	Mid Cycle Injection Quantity (mg)	Late Cycle Injection Quantity (mg)
a1, k = −20.93	17.3	12.4	11.9
a2, k = −10.47	16.3	12.2	12.6
a3, k = 10.47	14.4	12.2	14
a4, k = 20.93	13.4	12.3	14.7

shows the fuel injection quantities for the first cycle, mid cycle, and late cycle phases at different target accelerations under normal temperature conditions of 20 °C. Figure 9 illustrates the indicated thermal efficiency for the first cycle, mid cycle, and late cycle phases. The figure reveals that different acceleration slopes significantly affect the indicated thermal efficiency in the first and late cycles, while the mid cycle efficiency remains essentially unchanged. Furthermore, as engine speed increases, the indicated thermal efficiency improves to varying degrees across different target acceleration curves, indicating enhanced in-cylinder combustion conditions during the acceleration phase. On the other hand, for the first cycle, the indicated thermal efficiency reaches its maximum of 31% when the acceleration slope a1 is used; however, the indicated thermal efficiency for the late cycle is the lowest, exceeding 40%. Conversely, when the target acceleration slope is a4, although the indicated thermal efficiency for the late cycle is the highest at 45%, the indicated thermal efficiency for the initial cycle is the lowest at only 25.8%.

Therefore, according to the calculation method of the cycle fuel quantity based on the target torque, the fuel injection quantity during the start-up process is positively correlated with the acceleration. As the first cycle is the initial stage of the start-up acceleration phase, it requires a large acceleration, so the first cycle requires a large fuel injection quantity. When the cycle speed reaches 650 r/min in the later stage, due to the different target accelerations, the calculated fuel injection quantities are q_a4 > q_a3 > q_a2 > q_a1, and the corresponding indicated thermal efficiencies also show the same trend, i.e., η_a4 > η_a3 > η_a2 > η_a1. It is worth noting that, compared with the effect of different target accelerations on the indicated thermal efficiency of the first cycle, the effect on the indicated thermal efficiency of the later cycles is relatively small. This shows that as the speed increases during the start-up process, the combustion conditions in the cylinder improve, and the weight of the fuel injection quantity on the indicated thermal efficiency of the start-up process gradually decreases.

Considering that the generation of NO and soot during the diesel engine startup process is at a very low level compared to high-speed, high-load operating conditions, and that the start-up condition is the primary operating condition for CO and HC generation in diesel engines, the generation of CO and HC directly reflects the combustion quality during the startup process. Therefore, the emission levels of CO and HC are included as evaluation criteria for selecting the optimal target acceleration. Figure 10 shows the comparison results of CO emission levels across three cycles under different target acceleration conditions. In the first cycle, the CO emission level is lowest when the acceleration slope is a1 and highest when the slope is a4. However, in the later cycles, the CO emission level shows an opposite trend compared to the first cycle, with the lowest CO emission level at a4 and the highest at a1, indicating that the trend in CO emission levels decreases as the indicated thermal efficiency increases. Figure 11 shows the comparison results of HC emission levels for three cycles under different target accelerations. It can be seen that the impact of different target accelerations on HC emissions is relatively small, and the overall emission levels are generally low. This indicates that HC emission levels during the start-up process are less affected by engine speed.

Based on the above analysis, it can be seen that during the entire start-up process, the economy and emission levels of the prototype improved with the increase in fuel injection quantity. However, the impact of changes in fuel injection quantity on the combustion process varied at different stages of the start-up process. In the early stages of the starting process, such as the first cycle, a larger fuel injection quantity has a significant effect on improving combustion, which is particularly effective in enhancing economy and reducing emissions. In the later stages of the starting process, such as the later cycles, a larger fuel injection quantity has no significant effect on combustion economy and emissions. Therefore, the fuel injection quantity can be appropriately increased in the early stage of the starting process without exceeding the limit, which is crucial for improving the combustion process of the first cycle of starting. In the late stage of the starting process, a smaller fuel injection quantity can be used to reduce fuel consumption while satisfying power and emission requirements. Therefore, the final target acceleration curve is selected as the a1 acceleration curve when the slope is k = −20.93.

Once the target acceleration curve is determined, the quantity of fuel injected during the first cycle is also determined. According to the control strategy for fuel injection quantity based on the target torque start-up process, the first cycle injection quantities for normal temperature starting, low temperature starting, and cold starting are determined to be 17.3 mg, 18.5 mg, and 20.4 mg, respectively

4.2. Effect of First Injection Speed on the Combustion Process and Emission Performance

Under normal operating conditions, Figure 12 displays the cylinder pressure curves at various ignition speeds, while Figure 13 provides an enlarged view of a specific section of the graph. It can be observed that when the first injection speed is increased within the range of 200 r/min to 260 r/min, there is no significant effect on the cylinder pressure during the compression phase. However, a certain degree of influence is observed on the peak combustion pressure. The peak pressure is highest when the first injection speed is 240 r/min, and lowest when the first injection speed is 200 r/min, with a difference of about 200 kPa between the two. Figure 14 shows the heat release rate curves for four sets of first injection speeds. The combustion start points are all located near 358.5 CA, i.e., 0.5 CA after the main injection, and the heat release rate peaks near 360° CA (compression top dead center). Combined with the cylinder pressure curve, it is evident that when the first injection speed is 200 r/min, the heat release is the lowest, and the heat-to-work conversion effect is poor.

To explore the mechanism, the concentration field and velocity field in the cylinder were analyzed under four different first injection speeds, and the results are presented in Figure 15 and Figure 16. From the velocity field, it can be seen that, from the main injection (358 °CA) to the top dead center, due to the low engine speed, the airflow intensity in the cylinder is weak, and thus has little effect on the spray. That is, changes in the first injection speed within a small range have little effect on the velocity field distribution in the cylinder, so there is no obvious change in the concentration field distribution. From TDC to 362 °CA, the spray hits the combustion chamber wall and spreads along it, with some of the fuel returning to the combustion chamber space. The fuel film on the wall evaporates and mixes with the air in the combustion chamber. At this point, changes in the first injection speed have some effect on the formation of the mixture. Affected by the combustion chamber structure and speed, a squish flow of a certain intensity is generated in the combustion chamber. From the velocity field distribution, it can be seen that when the first injection speed increases from 200 r/min to 260 r/min, the squish flow intensity is proportional to the speed. As the speed increases, the intensity of the squish flow increases, which helps the fuel droplets evaporate on the wall to mix with the air in the combustion chamber to form a combustible mixture. From the concentration field, it can be seen that the area of the overly concentrated mixture in the combustion chamber decreases as the intensity of the squish flow increases. However, the amount of combustible mixture in the initial stage of combustion is determined by both the mixture formation rate and the mixture formation time. Increasing the speed reduces the time required for mixture formation. When the negative impact of increased first injection speed on the combustion process—due to reduced fuel–gas mixing time—exceeds the positive impact of increased squish flow intensity on the combustion process, it is not appropriate to continue to increase the first injection speed. From the concentration field distribution and the heat release rate curve, we can see that when the first injection speed increases from 240 r/min to 260 r/min, the concentration field distribution in the cylinder no longer shows a significant optimization trend.

Due to the characteristics of the starting conditions, incomplete combustion and misfiring frequently occur during the starting phase, resulting in higher emissions of CO and HC during the starting process. Therefore, the cyclic emissions of NO, soot, CO, and HC were selected as evaluation criteria to analyze the impact of different first injection speeds on the emission characteristics of the first cycle during the diesel engine starting process.

The formation of NO is primarily concentrated in high-temperature, oxygen-rich regions, and its final emission volume depends on the size and duration of these regions. When a local region within the cylinder simultaneously meets the conditions of a temperature exceeding 1800 K and a relatively lean mixture concentration (typically around a stoichiometric ratio λ < 1), the NO formation rate will be higher. Figure 17 shows the temperature field distribution from the start of the main injection to 365 °CA at different first injection speeds, and Figure 18 shows the NO generation curve at different first injection speeds. It can be seen that NO begins to generate after the top dead center. At this point, the temperature inside the cylinder rises significantly after premixed combustion, resulting in the formation of a high-temperature, oxygen-rich area. NO generation peaked at 510 ppm when the ignition spray speed was 220 r/min. At 200 r/min, 240 r/min, and 260 r/min, NO generation decreased slightly to 432 ppm, 418 ppm, and 377 ppm, respectively. As the first injection speed increases, the improved combustion conditions within the cylinder cause the internal temperature to rise. The expansion of localized high-temperature, oxygen-rich zones promotes NO formation, which is the primary reason for the higher NO emissions at 220 r/min compared to those at 200 r/min. When the speed increases to 240 r/min and 260 r/min, the temperature distribution reveals a slight reduction in the localized high-temperature zones, resulting in a corresponding decrease in NO emissions compared to the 220 r/min condition.

Soot is the primary component of diesel engine particulates. Its formation is closely related to cylinder temperature and fuel–air mixture concentration, primarily resulting from the thermal cracking of hydrocarbon fuels under high-temperature, oxygen-deficient conditions [28]. When combustion temperatures exceed 2100 K, soot formation significantly increases. During the formation of the fuel–air mixture in start-up conditions, the concentration distribution is extremely uneven, with severe local oxygen deficiency. Figure 19 shows the soot generation curve at different first injection speeds. It can be seen that soot generation begins at the top dead center. Combined with the temperature field distribution, we can see that at top dead center, the temperature in some areas of the combustion chamber has reached 2100 K, and the mixture concentration is high at this time. At 1 °CA after top dead center, soot production reaches its peak. However, it is subsequently oxidized, causing a significant reduction in soot content. By 365 °CA, emissions have dropped to very low levels. Overall, due to the generally lower combustion temperatures during the first cycle, the mass fraction of soot emissions in the first cycle remained below 30 ppm across all four first injection speeds. This represents a significantly lower level compared to heavy-load operating conditions in diesel engines.

CO is often generated in local areas where the mixture is too rich. Figure 20 shows the CO generation curve for four groups of first injection speeds. CO generation begins at 358.5 °CA, which is synchronized with the start of heat release. From the temperature field distribution characteristics, we can see that the temperature inside the cylinder is generally below 1000 K at this time, indicating incomplete combustion. Similarly to the trend of soot generation, a large amount of CO is generated in the early stage and subsequently oxidized in the late stage of the combustion process. The entire oxidation process can last until approximately 400 °CA. Finally, the mass fraction of CO emissions in the first cycle for the four groups of first injection speeds is approximately 4000 ppm, with a minimum value of 3600 ppm at 220 r/min.

HC emissions in diesel engines mainly consist of unburned fuel. In the event of incomplete combustion and misfiring during start-up, HC emissions will increase significantly. Figure 21 shows the HC generation curve for the first cycle at different first injection speeds. It can be observed that at this starting temperature, no misfires occurred during the first combustion cycle for all four groups of first injection speeds. A peak appeared during the initial combustion phase after injection, and as the combustion process progressed, most HC was oxidized. Consequently, HC emissions remained low, consistently below 100 ppm, with no significant variation across different first injection speeds.

4.3. Determining the Optimal First Injection Speed Under Different Ambient Temperatures

During the start-up process, economy and emissions are mutually restrictive, so the selection of the optimal first injection speed cannot be evaluated based on a single performance metric. To determine the optimal first injection speed under different ambient temperatures, the comprehensive effects of the first injection speed on combustion conditions, economy, and emissions should be considered.

Figure 22 illustrates the impact of varying first injection speeds on the indicated thermal efficiency of the first cycle at normal temperature (20 °C). Figure 23 and Figure 24 show the effects on CO and NO emission characteristics, respectively. It can be seen from the figures that the first cycle has the highest indicated thermal efficiency at a first injection speed of 220 r/min, which is 30.74%. That is, from an economic perspective, the first cycle combustion state is optimal at a first injection speed of 220 r/min. At this point, CO emissions are lowest, and NO emissions are slightly higher. Considering that the NO emissions during start-up are minimal compared to those at normal operating conditions, their impact on the engine’s emission performance under all operating conditions is minimal. Furthermore, CO emissions are relatively high at the start condition, which is the main operating condition for CO generation in diesel engines. The first injection speed of 220 r/min is also the best choice from an emission perspective.

Figure 25 and Figure 26 illustrate the impact of varying first injection speeds on the first cycle pressure and heat release curves at a low temperature (5 °C). We can see that, due to the influence of ambient temperature, the start of the first cycle combustion and the peak heat release rate are delayed at low temperatures compared to normal temperature conditions. Figure 27 illustrates the impact of varying first injection speeds on the first cycle indicated thermal efficiency at low temperatures. From an economic perspective, when the first injection speed is 200 r/min, the indicated thermal efficiency is the lowest, at approximately 27.8%. The indicated thermal efficiencies of the other three groups are similar, all exceeding 28.5%. Figure 28 and Figure 29 illustrate the impact of varying first injection speeds on CO and NO cycle emissions, respectively. It can be observed that CO emissions remain relatively high across different first injection speeds, increasing with higher speeds. The highest CO emission occurs at 260 r/min, reaching 5286 ppm, while the lowest emission is recorded at 200 r/min, at 4295 ppm. The other two groups fall between these values. The lowest NO emissions were observed during the first cycle at 200 r/min, at a concentration of 382 ppm. The remaining three groups showed slightly higher emissions, but all remained below 600 ppm.

In summary, although the first cycle emissions performance is favorable at a low-temperature first injection speed of 200 r/min, its economic efficiency is poor. Conversely, the first cycle’s economic efficiency peaks at a first injection speed of 260 r/min; however, CO emissions are significantly higher than those of the other three groups. At a first injection speed of 240 r/min, the indicated thermal efficiency was relatively high at 28.67%, with CO emissions at 4612 ppm and NO emissions at 542 ppm. Emissions performance was also favorable compared to the other three groups. Therefore, 240 r/min was selected as the optimal first injection speed under low-temperature conditions.

Figure 30 and Figure 31 display the first cycle cylinder pressure curves and first cycle heat release rate curves corresponding to different first injection speeds under cold start conditions (−10 °C). Due to increased fuel injection quantity at low temperatures, the peak cylinder pressure also rises accordingly. As shown in Figure 30, the peak cylinder pressure reaches its highest value at 220 r/min, approximately 11.4 MPa. Peak cylinder pressures at 200 rpm and 240 rpm are slightly lower, while the peak at 260 rpm decreases to approximately 9.7 MPa. At low temperatures, the ignition delay period lengthens, allowing the majority of fuel to be injected into the cylinder during this phase. This results in a larger volume of mixture igniting simultaneously, leading to a higher pressure rise rate. As shown in Figure 31, the peak heat release rate gradually decreases with increasing speed but remains above 500 J°/CA. Furthermore, the combustion phase progressively lags as speed increases. This occurs because higher speeds shorten the duration during which the charge remains at elevated temperature and pressure near TDC, causing the ignition delay period (measured in crankshaft angle) to increase linearly with speed. This results in delayed combustion at 260 r/min, accompanied by incomplete combustion. Figure 32 displays the first cycle indicated thermal efficiency corresponding to different first injection speeds under cold start conditions. Thermal efficiency peaks at approximately 28.7% at 220 r/min, while it drops to only 25.8% at 260 r/min. The remaining two groups both achieve thermal efficiencies above 27%. Figure 33, Figure 34, and Figure 35, respectively, illustrate the first cycle CO, NOx, and HC emissions corresponding to different first injection speeds during cold starts. Due to incomplete combustion, the first cycle CO and HC emissions at 260 r/min were higher than the other three groups, while NOx emissions were lower. However, the higher pressure rise rate caused by the larger volume of mixture ignited simultaneously resulted in increased NOx emissions, exceeding 1500 ppm at 200 r/min, 220 r/min, and 240 r/min. Overall, 220 r/min achieved the highest thermal efficiency, lowest CO emissions, and relatively low HC emissions. Although NOx emissions were slightly higher, they remained within acceptable limits. Therefore, 220 r/min is selected as the optimal first injection speed under cold start conditions.

5. Conclusions

Building upon the research group’s prior work on target torque control strategies during the starting process, this paper employs three-dimensional computational fluid dynamics simulations to optimize target acceleration curves for different starting phases. It determines the optimal injection quantity for the first cycle. Subsequently, the study systematically investigated the impact of the first injection speed on the fuel economy and emission characteristics of diesel engines during the first cycle. By analyzing velocity, concentration, and temperature fields, the underlying mechanisms were revealed. Optimal first injection speeds were determined under different ambient temperatures (20 °C, 5 °C, −10 °C). The main conclusions are as follows:

(1)

Simulation calculations and analysis of the three node cycles of the starting process under different target acceleration curves show that appropriately increasing the fuel injection quantity can improve the thermal efficiency of the first cycle and the later cycles, with the former being more obvious and the mid cycle having basically no effect. The optimal target acceleration curve slope is k = −200. At this point, the calculated optimal fuel injection quantities for the first cycle under normal temperature start-up, low-temperature start-up, and cold start-up are 17.3 mg, 18.5 mg, and 20.4 mg, respectively.

(2)

The effect of the first injection speed on the first cycle combustion process is mainly reflected in the mixture formation rate and mixture formation time. The intensity of the gas flow in the cylinder increases with the increase in the first injection speed, which can promote fuel atomization and wall fuel film evaporation, and improve the mixture formation process in the cylinder. On the other hand, the increase in speed shortens the mixture formation time. Therefore, the first injection speed is not necessarily better when it is higher.

(3)

When starting at normal temperature and low temperature, by comparing the effects on indicated thermal efficiency and emission levels, the optimal first injection speeds for the first cycle are selected as 220 r/min and 240 r/min, respectively. During cold starts, prolonged ignition delays occur due to temperature effects. Excessively high rotational speeds result in insufficient time for fuel–air mixture formation, leading to incomplete combustion. By comparing indicated thermal efficiency and emission performance, 220 r/min was determined as the optimal first injection speed for the first cycle.