Performance Assessment of a Diesel Engine Fueled with Biodiesel in a Plateau Environment

Abstract

Biodiesel has a higher oxygen content and a higher cetane number, which can compensate for the intake oxygen deficiency in diesel engines in a plateau environment to a certain extent. However, the decreased air density makes biodiesel fuel spray atomization and evaporation more difficult due to its higher density and kinematic viscosity, reducing the quality of the air-fuel mixture. The investigations in this study focus on the effects of biodiesel blending ratios and their coupling with injection timing on diesel engine performances under varying altitude conditions. The results show that as the altitude increases, using a high proportion of biodiesel-blended fuel results in a lower degree of torque reduction. The torque reduction of B100 is 14% lower than that of baseline at an altitude of 4500 m. In addition, when the altitude increases by 2000 m, the optimal fuel injection timing is delayed by 4° CA, regardless of the biodiesel blending ratio. The low-temperature combustion heat release ratio of biodiesel engines slightly increases with the delay of injection time, which is increased with the biodiesel blending ratio. For B100 fuel, increasing the pilot injection quantity under high-altitude conditions helps to improve the heat release rate during the early and late stages of combustion and reduce expansion losses.

1. Introduction

Biodiesel is a liquid fuel produced from animal and vegetable oils, which has the advantages of being renewable and environmentally friendly [1,2]. As an alternative fuel to diesel, biodiesel offers advantages such as biodegradability, renewability, safe storage and transportation, and the same sales channels as existing fuels [3]. In recent years, the production and consumption of biodiesel have grown exponentially, gradually highlighting its importance in the fuel sector [4].

Compared to diesel, most biodiesel fuels and their blends with diesel have lower compressibility, higher viscosity, and higher cetane numbers, resulting in earlier combustion initiation and shorter ignition delay [5,6]. These characteristics of biodiesel lead to combustion properties that differ from those of pure diesel. Daho et al. studied the combustion characteristics of cottonseed oil and its blends with diesel fuel in diesel engines [7]. The results showed that with the increase in the blending ratio of cottonseed oil, the proportion of premixed combustion decreased, the maximum in-cylinder pressure increased, and the thermal efficiency improved under high load conditions exceeding 50%. An et al.’s experimental results indicated that low speed and low load significantly affect the process of burning biodiesel in diesel engines [8]. To address the issues caused by the high viscosity and low volatility of vegetable oils when used long-term in diesel engines, such as poor atomization, carbon deposits, stuck rings, and fuel pump failures, Ramadhas et al. converted high-viscosity vegetable oil into various blends of rubber seed oil and diesel [9]. They found that mixtures containing 50–80% rubber seed oil exhibited optimal performance. Murugesan et al. found that a blend containing 20% biodiesel achieved the highest effective thermal efficiency [10]. It is evident that the higher cetane number and rich oxygen content of biodiesel effectively improve the combustion process, reducing emissions of CO, HC, particulate matter, and smoke. Low-blend biodiesel fuels can be used directly without modifying the original engine, making them an ideal alternative fuel. However, the lower calorific value and density of biodiesel and its blends result in slightly higher specific fuel consumption compared to diesel. Additionally, biodiesel has issues such as poor low-temperature fluidity, higher viscosity, and higher NOx emissions, which limit its application in diesel engines [11].

When diesel engines operate in a plateau environment, the reduced atmospheric pressure leads to a decrease in the intake air volume, resulting in more rich mixture regions in the combustion chamber and deteriorating combustion [12]. This causes a series of problems, such as reduced power, increased fuel consumption, worsened emissions, increased thermal load, and difficulty in cold starting. Research results show that for every 1000 m increase in altitude, the maximum combustion pressure of a turbocharged diesel engine decreases by 3% to 4%, the minimum specific fuel consumption increases by 3% to 7%, and the rated power decreases by 4% to 7% [13,14,15]. Compared to diesel, biodiesel has a higher oxygen content and a higher cetane number, which can compensate for the intake oxygen deficiency in diesel engines in a plateau environment to a certain extent, thereby improving the combustion process [16]. It is beneficial to achieve the goal of improving combustion efficiency, reducing pollutant emissions, and enhancing diesel engine performance without modifying the engine structure. However, under high-altitude conditions, the decreased air density in the cylinder makes fuel spray atomization and evaporation more difficult. The higher density and kinematic viscosity of biodiesel hinder the diffusion of fuel jets into the surrounding air, reducing the quality of the air-fuel mixture [17]. This not only increases the fuel consumption rate of diesel engines but also hampers performance improvement.

A series of studies have been conducted on the combustion characteristics of biodiesel, either used alone or as a blended fuel in diesel engines under high-altitude conditions [18]. Shen et al. studied the combustion and emission characteristics of biodiesel blends in high-altitude areas [19]. The results showed that compared to diesel, biodiesel has significant advantages in high-speed and high torque conditions. Under high-altitude conditions, the oxygen elements contained in biodiesel can offset some of the combustion deterioration caused by high altitude. Wang et al. found that the output torque of diesel engines running on biodiesel decreased under full load conditions, but this difference diminished with increasing altitude [20]. Yu et al. studied the performance of diesel engines using diesel and soybean oil biodiesel at different altitudes of 1608~4560 m [21]. The results indicated that as the altitude increased, the BSFC (Brake Specific Fuel Consumption) of both fuels increased by a similar magnitude. Chen et al.’s research indicated that at higher altitudes, diesel engine power and economy decrease, but the impact of altitude on biodiesel fuel is less significant compared to pure diesel fuel [22]. Additionally, Liu et al. found that when using biodiesel/diesel blends, BSFC decreases with the increase in oxygen content in the blend [23]. Wang et al.’s study results showed that the reduction in intake pressure at higher altitudes delays the initial moment of combustion for both fuels [20]. However, due to the oxygenated properties of biodiesel, this effect is somewhat mitigated. Furthermore, the differences in ignition delay characteristics result in a lower proportion of premixed combustion for biodiesel in a plateau environment. Benjumea et al. found that the combustion duration for both diesel and biodiesel increased with altitude, but the effect was more significant for diesel [24].

In summary, biodiesel can be blended with diesel in any proportion and has strong compatibility with diesel engines, making it an ideal alternative fuel. The use of biodiesel in diesel engines under high-altitude conditions helps reduce dependence on petroleum resources and alleviates the pressure of insufficient oil supply. However, due to the influence of altitude on fuel and spray characteristics, there are still issues of increased fuel consumption rate and reduced power performance in practical applications. It is necessary to research the combustion control strategy involving the synergistic regulation of biodiesel blending ratios and high-pressure injection technology for diesel engines at altitudes above 4000 m. This paper investigated the effects of biodiesel blending ratios and their coupling with injection timing on diesel engine performance under varying altitude conditions. The study aims to reveal the mechanism of the coupled effects of biodiesel blending ratios and injection strategies on the combustion process under variable altitude conditions, assess their improvement on combustion performance, and provide theoretical support for better utilizing biodiesel as a diesel engine fuel in a plateau environment.

2. Construction and Calibration of Simulation Models

GT-Suite V2016 software was used to carry out the modeling and simulation of the one-dimensional thermal cycle. Figure 1 shows the simulation model constructed based on the actual structure and pipeline dimensions of a six-cylinder turbocharged high-pressure common rail diesel engine (as shown in

Table 1. Main technical parameters of engine.

Engine Parameters (Unit)	Parameter Value
Type	V-type, turbocharged and intercooled
Displacement (L)	8.6
Geometric compression ratio	17:1
Rated power (kW/r·min−1)	258/2100
Bore (mm)	112
Number of cylinders	6
Diameter of injection holes (mm)	0.18
Number of injection holes	8

). The model mainly includes the turbocharging system, intake and exhaust systems, injection system, cylinder system, and crankcase system. To evaluate the impacts of biodiesel combustion and fuel injection strategies on the combustion process, a predictive combustion model is adopted. This combustion model takes into account factors such as the shape of the combustion chamber, airflow motion, and the state of the mixture, enabling precise prediction of the influence of injection timing and fuel quantity on combustion rate. For heat transfer calculations, a modified Woschini heat transfer model is adopted to simulate transient in-cylinder thermal behavior. The InjMulti Profile Conn injector model is selected, which requires input parameters such as the diameter of the injection holes, the number of holes, the injection quantity per cycle for each segment, and the injection timing. This model facilitates the setting of single or multiple injections conveniently.

It refers to relevant literature to set the various physical and chemical parameters of liquid and gaseous biodiesel [25]. The physical and chemical parameters of the blended fuels of biodiesel and diesel used in the study were obtained using the Fluid Mixture module. These parameters were derived by linear interpolation based on the parameters of diesel and biodiesel, according to the blending ratio. B[m] refers to a biodiesel volume blending ratio of m%. For example, B20 indicates that the fuel contains 20% biodiesel by volume and 80% diesel.

To ensure that the simulation results accurately reflect the actual working conditions of the diesel engine, the simulation model needs to be validated through experiments. The layout of the test bench is shown in Figure 2, and the engine specifications are shown in Table 1.

Table 2. Main instruments and technical parameters of test.

Equipment Name	Model	Manufacturer
Electric dynamometer	S33-4/1400-1BS-1	AVL (Graz, Austria)
Transient fuel consumption meter	AVL733S	AVL (Graz, Austria)
Cylinder pressure sensor	GH14P	AVL (Graz, Austria)
Data acquisition and analysis system	PUMA	AVL (Graz, Austria)

shows the main instruments, and

Table 3. Main test parameters and accuracies.

Test Parameters	Test Error
Rotational speed	±0.05%
Torque	±0.01%
Mass airflow	±0.15%
In-cylinder pressure	±0.16%
Fuel consumption	±0.12%

shows the main test parameters and their accuracy during the experiments. To ensure the accuracy of the test data, all instruments and equipment have been calibrated before use. Based on the engine test bench shown in Figure 2, external characteristic tests of the diesel engine from 1400 r/min to 2000 r/min, as well as tests over the full operating range at 1000 r/min,1500 r/min, and 2000 r/min, were conducted at an altitude of 4500 m.

The calibration results are shown in Figure 3 and Figure 4. It can be seen that the simulation results are in good agreement with the experimental results, indicating that the model and the calculation boundary conditions used in the simulation process are relatively accurate. Therefore, this model can be used for simulating the combustion process of diesel engines in high-altitude environments.

3. Results

3.1. Effects of Biodiesel Blending Ratio on Diesel Engine Performance Under Varying Altitude Conditions

To study the effects of biodiesel blending ratio on diesel engine performance under varying altitude conditions, simulations were conducted under full load conditions at altitudes of sea level, 2500 m, 3500 m, and 4500 m, as shown in

Table 4. Boundary Conditions for Simulation Calculations.

Altitude/m	Fuel	Speed/(r/min)	Load
0	B0/B10/B20/B50/B100	1500/1800 rpm/2000 rpm	100%
2500
3500
4500

.

The external characteristic torque results of B100 fuel at different altitudes are shown in Figure 5. When the altitude increases from sea level to 2500 m, the external characteristic torque remains largely unchanged. At this point, the impact of altitude on diesel engine in-cylinder combustion has not yet manifested. However, as the altitude further increases, the dynamic performance significantly deteriorates. Taking 1500 r/min as an example, the full load torque at an altitude of 4500 m is 23.6% lower than that on the plain. As shown in Figure 6, the variation of the external characteristic torque of different blending ratios of biodiesel/diesel mixed fuels with altitude is generally consistent. In addition, at the same altitude, the dynamic performance decreases with the increase in the biodiesel blending ratio. The lower calorific value of biodiesel-diesel blended fuels is the main reason for the decrease in diesel engine torque and power, as well as the increase in fuel consumption. However, as shown in Figure 7, with the increase in altitude, the reduction of torque using a high biodiesel blending ratio is lower than that under plain conditions. Taking 1800 r/min full load condition as an example, the torque reduction of B100 is 14% lower than that of baseline (diesel) at an altitude of 4500 m.

Taking 1800 r/min full load condition at 4500 m altitude as an example, the variations of in-cylinder pressure and the burned mass fraction (BMF) with different biodiesel blending ratios were analyzed, as shown in Figure 8. It can be seen that the peak pressure of biodiesel-diesel blended fuels is lower than that of pure diesel. The crank angle of biodiesel-diesel blended fuel reaching the peak cylinder pressure is slightly ahead of that of pure diesel fuel. As the blending ratio of biodiesel increases, the decrease of in-cylinder pressure becomes more pronounced. Compared to pure diesel, biodiesel has a lower calorific value and higher kinematic viscosity. When the same mass of fuel is injected, the reduced calorific value of the biodiesel-diesel blended fuel limits the total heat release during combustion, leading to a decrease in the maximum combustion pressure in the cylinder.

As shown in Figure 9, from the combustion process perspective, at the initial stage of combustion, with the increase in the biodiesel blending ratio, the initial combustion heat release rate decreases. This is mainly due to the increasing density and kinematic viscosity of biodiesel, which makes the atomization and evaporation rate of fuel spray gradually slow down and, thus, reduces the mixing speed with air. In the diffusion combustion stage, as the oxygen content and cetane number of the mixed fuel gradually increase with the blending ratio of biodiesel, the ignition performance is improved, so that the heat release time in the middle and late stage of combustion is advanced, and the subsequent combustion process is improved.

Overall, to better leverage the inherent oxygen content advantage of biodiesel, it is necessary to appropriately increase the injection quantity to compensate for the power deficit. On the other hand, it is also essential to further couple the adjustment of injection timing to achieve better matching between the fuel and altitude operating conditions.

3.2. Effects of Biodiesel Blending Ratio Coupled with Injection Timing on Diesel Engine Performance Under Varying Altitude Conditions

With the development of electronic control technology in diesel engines, modern diesel engines adopt electronically controlled high-pressure common rail technology, allowing flexible control of injection parameters to achieve an optimal injection pattern and optimize the combustion process. By keeping the total injection quantity per cycle constant, the effects of injection timing on the power and economy of the diesel engine using biodiesel-diesel blended fuels were investigated.

The operating condition of the diesel engine selected for the simulation study is 1800 r/min at 100% load. The altitudes are chosen at 2500 m and 4500 m. At an altitude of 2500 m, the injection quantity per cycle is 151 mg, with an injection advance angle of 9° CA BTDC. At an altitude of 4500 m, the injection quantity per cycle is 132 mg, with an injection advance angle of 9° CA BTDC. Using the single injection strategy, the injection timing is varied while keeping the injection quantity per cycle constant to investigate the impact of main injection timing on diesel engine performance under varying altitude conditions.

The variation of in-cylinder pressure with different injection timing and biodiesel blending ratio at an altitude of 2500 m is depicted in Figure 10. It can be seen that compared with pure diesel, the proportion of low-temperature combustion heat release of a biodiesel engine increases slightly with the delay of injection time. With the increase in the biodiesel blending ratio, the low-temperature heat release is more obvious.

The results of torque and BSFC with different injection timing and biodiesel blending ratios at 2500 m and 4500 m are shown in Figure 11. It is indicated that the torque of the diesel engine first increases and then decreases as the injection timing advances. The BSFC of the diesel engine first decreases and then increases as the injection timing advances. This is because increasing the injection advance angle within a certain range makes the heat release closer to the top dead center (TDC), increasing the degree of constant volume combustion, enhancing the work capacity, and reducing fuel consumption. However, an excessively advanced injection timing causes too much fuel to burn before TDC, increasing the negative work during compression, reducing torque, and increasing fuel consumption. Comparing the results at different altitudes, it can be seen that as altitude increases, the optimal injection timing is delayed by 4° CA, bringing it closer to TDC.

3.3. Effects of Biodiesel Blending Ratio Coupled with Pilot-Main Injection Timing on Diesel Engine Performance Under Varying Altitude Conditions

This part studies the influence of pre-injection and main injection twice injection strategy on the power performance and economy of diesel engine fueled with biodiesel diesel blended fuel. The main research is to explore the influence of injection parameters such as pre-injection, main injection interval, and pre-injection quantity while keeping the total injection quantity per cycle unchanged.

The pilot-main injection interval refers to the crank angle interval between the start of pilot injection and the start of main injection. Under the condition of 2500 m altitude, for baseline, B10, B20, B50 and B100, the pilot-main injection strategy is adopted. The pilot injection quantity is fixed at 10 mg per cycle, the total injection quantity per cycle is maintained at 151 mg, and the main injection timing is fixed at 5° CA BTDC. The pilot injection timing is varied to investigate the effect of the pilot-main injection interval on the performance of the diesel engine using biodiesel.

As shown in Figure 12 and Figure 13, compared to a single injection, the use of the pilot-main injection strategy results in part of the fuel burning and releasing heat before TDC, increasing the in-cylinder temperature and pressure and shortening the ignition delay period during the main injection phase. As the pilot-main injection interval increases, more heat is released during the pilot injection combustion phase, making the shortening of the ignition delay period more pronounced and lowering the heat release rate during the main injection phase. With the increase in the pilot-main injection interval, the torque and power of the diesel engine gradually decrease and are both lower than the torque and power corresponding to a single injection. The BSFC of the diesel engine gradually increases and is higher than that of a single injection strategy. This is because as the pilot injection timing advances, more heat is released from the combustion of the pilot fuel before TDC, increasing the negative work during compression, which leads to a decrease in engine power and an increase in fuel consumption. Furthermore, as the pilot injection timing advances, the in-cylinder temperature and pressure during the combustion of the pilot fuel are lower, resulting in less complete combustion and lower work capacity, leading to decreased engine power and increased fuel consumption.

Under the condition of 4500 m altitude, for baseline, B10, B20, B50 and B100 fuels, the pilot injection quantity is fixed at 10 mg per cycle, the total injection quantity per cycle is maintained at 132 mg, and the main injection timing is fixed at 4° CA BTDC. The pilot injection timing is varied to investigate the effect of the pilot-main injection interval on the performance of the diesel engine using biodiesel under higher-altitude conditions. As shown in Figure 14, the results are generally consistent with the trends observed under the 2500 m altitude condition, but the effects of the pilot-main injection interval on torque and specific fuel consumption are significantly reduced compared to those at lower altitudes.

Under the condition of 4500 m altitude, for B100 fuel, the pilot-main injection interval is fixed at 5° CA and 10° CA, the total injection quantity per cycle is maintained at 132 mg, and the main injection timing is fixed at 4° CA BTDC. The pilot injection quantity is varied to investigate its effect on the performance of the diesel engine using biodiesel under high-altitude conditions. As shown in Figure 15, a pilot injection helps to increase the in-cylinder temperature and pressure before the start of the main injection, shortening the ignition delay period. As the pilot injection quantity increases, the heat release rates during both the pilot and main injection phases gradually increase. Compared to the single injection results, under high-altitude conditions, increasing the pilot injection quantity slows down the heat release rate during the main injection phase but helps to increase the heat release rate during the initial and late stages of combustion, reducing expansion losses.

As shown in Figure 16, under full load conditions in a plateau environment, the effect of pilot injection quantity on diesel engine performance varies with different pilot-main injection intervals. When the pilot-main interval is small, the torque of the diesel engine gradually increases with the increase in pilot injection quantity, but it remains lower than the torque corresponding to a single injection. The BSFC gradually decreases, but it is still higher than the effective fuel consumption rate corresponding to a single injection. This is because, with the increase in pilot injection quantity, the combustion rates in the initial and late stages of combustion significantly increase, leading to an improvement in power and a reduction in fuel consumption. When the pilot-main interval is large, the torque of the diesel engine gradually decreases with the increase in pilot injection quantity, but the decrease is relatively small, and it remains lower than the torque corresponding to a single injection. This is mainly because more heat is released from the combustion of pilot fuel before TDC, increasing the negative work during compression.

3.4. Cost and Medium-to-Long-Term Feasibility Analysis

From the perspective of direct costs, the raw material cost of biodiesel fluctuates significantly. If waste oils (such as gutter oil) are used, the raw material cost can be significantly reduced [26]. However, in actual production, there are issues such as an imperfect collection system and unstable raw material supply. For example, gutter oil has a higher profit when it flows back to the dining table, which leads biodiesel enterprises to switch to high-priced raw materials such as palm oil [27]. This drives up the cost to USD 549–1166 per ton. After adding production costs such as catalysts and energy consumption, the total cost may be higher than that of traditional diesel (USD 960–1166 per ton). However, government policy subsidies (such as value-added tax rebates and mandatory blending ratios) can partially offset the cost disadvantage. Moreover, the lubricating properties of biodiesel can reduce engine wear and lower long-term maintenance costs. In terms of medium-to-long-term feasibility, technological breakthroughs such as the increasing maturity of efficient conversion technologies and engineering microalgae methods, as well as large-scale production, are expected to further reduce production and usage costs. Policy promotion and the carbon trading market mechanism provide market security for biodiesel.

In summary, although there are short-term challenges related to raw material supply and technological bottlenecks, the short-term cost of using biodiesel in diesel engines in plateau areas is still higher than that of diesel (approximately 10–30% higher). However, through policy support, technological iteration, and large-scale production in the medium to long term, it is expected that the cost will approach or even fall below that of diesel [28]. At the same time, ecological benefits can be achieved, and sustainability can be ensured.

In the future, research on the improvement of biodiesel production processes, optimization of fuel properties and regulation of combustion control strategy involving the synergistic regulation of two-stage adjustable boosting and high-pressure injection technology should be conducted for diesel engines at altitudes above 4000 m, in order to further improve the application benefits of biodiesel in diesel engines in plateau areas.

4. Conclusions

This paper systematically studies the effect of biodiesel blending ratio and different injection strategies on the combustion performances under different altitude conditions, and reveals the combustion heating release mechanism of biodiesel fuel properties coupled with injection timing and pilot-injection mass in a plateau environment. The main conclusions are as follows:

(1)

At the same altitude, as the blending ratio of biodiesel increases, the low calorific value decreases, resulting in a decrease in diesel engine torque and power. However, as the altitude increases, using a high proportion of biodiesel-blended fuel results in a lower degree of torque reduction. At an altitude of 4500 m, the torque reduction of B100 is 14% lower than that of the baseline (diesel).

(2)

Adopting a single injection strategy, as the altitude increases, the optimal injection timing of the engine becomes closer to TDC. Compared with pure diesel, the low-temperature combustion heat release ratio of biodiesel engines slightly increases with the delay of injection time. With the increase in the biodiesel blending ratio, the low-temperature heat release becomes more obvious.

(3)

As the pilot injection timing is advanced, the in-cylinder temperature and pressure during the combustion of pilot injection fuel decrease, resulting in an increase in the incomplete combustion ratio and a decrease in power. Using the method of increasing the pilot injection quantity under high-altitude conditions may slow down the heat release rate during the main injection stage, but it helps to improve the heat release rate during the early and late stages of combustion and reduce expansion losses.