Influence of Pilot-Fueling and Nozzle-Opening Pressure on Performance and Tailpipe Emissions of WCO Biodiesel in a CRDi Engine

Abstract

Pilot-fueling and nozzle-injection pressure are significant injection parameters, and they have significant impacts on modern vehicles for enhancing the engine output, in addition to meeting rigorous tailpipe-exhaust emission standards. In this current work, the influence of the pilot-fueling pressure and nozzle-opening pressure (NOP) on the engine performance and tailpipe outcomes from a compression-ignition (CI) engine at a higher injection pressure and varying load conditions was investigated using a waste cooking oil (WCO) biodiesel (B20). The experiments were executed in a high-pressure CRDi-fitted diesel engine at the start of pilot fueling (SOPF) (timing: 23° bTDC), and at the start of the main fueling (SOMF) (timing: 33° bTDC). The results showed that the combined influence of the pilot-fueling and nozzle-opening pressure induced a remarkable enhancement in the BTE, by 25.13%, and the BSFC decreased by 13.88%, compared with diesel at 10% pilot fueling. Carbon monoxide, hydrocarbon, and smoke emissions were drastically reduced for the higher pilot-fueling quantity by 21.05%, 16.66%, and 33.10%, respectively, compared with the diesel at 10% pilot fueling. With the implementation of the pilot-fueling strategy, there is no effect on the NOx reduction.

1. Introduction

Rising industrial energy requirements and depleting crude-oil resources have sparked a worldwide search for environmentally friendly alternative transportation fuels. Biodiesel is a favorable sustainable alternative fuel option for partially replacing mineral diesel, according to many studies conducted on biodiesel in compression-ignition (CI) engines derived from various feedstocks [1,2,3]. With little or no modification in the diesel engine, biodiesel is a sulfur-free, nontoxic, ecofriendly, highly oxygenated, and renewable source that can be employed in diesel engines. Biodiesel imparts better fuel efficiency and horsepower. Biodiesel, as a substitute for diesel, lowers the tailpipe pollutants, such as soot, CO, and UBHC, regardless of the potential advantages of lower emissions, while the NOx emissions shoot up.

Split-injection strategies aim to simultaneously reduce engine tailpipe emissions and noise by forming an ideal injection-rate profile, which can be easily achieved by common rail direct-injection systems. Edible vegetable oil is commonly used in deep frying and cooking. The repeated usage of oils at high temperatures during cooking can generate them into polycyclic aromatic HCs, dioxins, and other varieties of toxic compounds that can cause severe health problems in humans if consumed. As a result, the value-added applications of WCO include soaps, lubricants, and domestic fuel. WCO biodiesel has properties that are similar to mineral diesel. As a result, biodiesel is a replacement for the diesel engine [4].

The influence of the injection parameters of WCO-biodiesel blends in a CRDi engine were investigated by Kannan et al. [5]. They noticed a drastic enhancement in the BTE, as well as a depletion in NOx and smoke. Yesilyurt [6] inspected the influence of the fueling-injection parameters on the engine characteristics of a diesel engine using WCO biodiesel. In this study, biodiesel blends (from 5% to 30% vol.), varying injection pressures, and the engine speed were examined. They concluded that increased injection pressure tends to enhance the brake thermal efficiency and nitrogen oxide emissions, with marginal decreases in the HC and smoke-emission levels in comparison with mineral diesel. Joonsik Hwang et al. [7] analyzed both the effects of the fuel-injection pressure and injection timings of a WCO-biodiesel blend in a CRDi engine. The results showed increasing NOx emissions, while other emissions were reduced. Rajesh Kumar et al. [8] analyzed the impact of the nozzle-opening pressure using WCO biodiesel in a diesel engine. The results showed that, as the nozzle-opening pressure increased, emissions such as HC, CO, and the smoke opacity decreased slightly, while the oxide of nitrogen emissions increased marginally. How et al. [9] examined the impact of the fueling strategy on the engine characteristics of a coconut biodiesel blend in a CRDi engine. They observed the significant diminishing of NOx and smoke emissions. Indudhar et al. [10] investigated single and split fueling using six-hole and seven-hole injectors fueled with Honge biodiesel in a CRDi engine. Multiple (pilot-main-post) injection quantities in the ratios of 40:30:30 and 40:20:40 were injected at 20° bTDC and TDC and 5° ATDC, respectively. Their results revealed that multiple injections decrease emissions such as smoke and HC, with a minimum BTE penalty. The injection quantity ratio of 40:30:30 exhibited a higher BTE compared with the 40:20:40 ratio. Atul Dhar et al. [11] studied the outcome of pilot injection for the engine characteristics in a CRDi diesel engine produced from 10%, 20%, and 50% blends of Karanja biodiesel. Their investigations showed that the pilot-quantity concentration of biodiesel increased with an increasing BTE and lower CO emissions.

KaturuBala Prasad et al. [12] investigated the engine characteristics of tamarind biodiesel from split-fuel injection strategies in a CRDi diesel engine for a pilot-injection quantity of 30%, with 100% main injection. The results disclosed that the split-injection strategy enhances the BTE and lowers the BSFC. In comparison with diesel fuel, tailpipe discharge, such as HC, CO, and smoke, were found to be decreased by 12.16%, 32.14%, and 19.71%, respectively. Senthil Kumar et al. [13] examined the effects of pilot-injection quantities (5%, 10%, 15%) and retarded injection (2° CA and 4° CA) in a common rail direct-injection engine with cashew-nutshell (CNSL) biodiesel blends. The CNSL biodiesel blend reduced the NOx, smoke, and carbon monoxide emissions, in addition to increasing the hydrocarbon emissions. Plamondon et al. [14] examined the impact of pilot–main-fueling strategies on the engine characteristics of WCO-biodiesel blends in a diesel engine. The results concluded that the implementation of pilot-main-fueling strategies drastically reduced the ignition delay, with a significant decrease in NOx emissions, in contrast to a single injection. The impact of fuel-injection strategies on palm oil biodiesel were inspected by Naresh Kumar et al. [15]. The experimentation results concluded that the pilot-injection strategy reduces emissions, with a significant rise in the BTE, owing to the reduction in the ignition delay. They showed that the EGR technique is a practical way to reduce NOx for palm oil methyl ester. Pathikrit Bhowmick et al. [16] implemented the fuel-injection strategy and EGR to study the engine characteristics of Calophyllum inophyllum biodiesel. They concluded that implementing varying EGR rates reduced the nitrogen oxide emissions by 14.4% and 27.6%, respectively, while increasing the carbon monoxide, hydrocarbon, and smoke emissions. D Babu et al. [17] studied the engine parameters of a palm-munja biodiesel blend on a diesel engine using the split-injection technique. The pilot-injection timing varied from −30° bTDC to −50° bTDC, and the pilot-fuel ratio varied from 5% to 15%, using mineral diesel and palm munja biodiesel. Their results revealed that pilot-injection timing advancement yields lower emissions, such as CO, NOx, and soot, but HC emissions increase. An increase in the split-injection ratio increases exhaust-gas emissions. Pavan et al. [18] studied the engine parameters for a palm oil methyl ester diesel blend on a CRDi engine using injection strategies such as split injection and injection pressure. It was observed that both injection strategies reduced emissions such as NOx, HC, and CO. Senthil et al. [19] conducted experiments on different blends of cashew-nutshell biodiesel and varying EGR rates in a CRDi engine. The results concluded that split injection and the EGR technique reduce NOx and smoke emissions. Nanthagopal et al. [20] investigated the engine parameters in a CRDi engine using Calophyllum inophyllum methyl ester biodiesel blends (10% vol. and 20% vol.) and an injection pressure of 600 bar, with variations in the pilot injections (5%, 10%, and 15%) and EGR rates (10% and 20%). The experimental study indicated that the increase in the biodiesel blend enhanced the engine characteristics. An increased pilot-injection quantity enhances complete combustion, while EGR implementation lowers NOx and carbon dioxide emissions. Prem Anand et al. [21] studied the impact of fuel-injection strategies on the engine characteristics in a mahua methyl ester blend in a CRDi engine. They observed that high fuel-injection pressure enhanced the BTE and combustion characteristics. The implementation of split-injection techniques exhibits lower tailpipe emissions, with only a minor reduction in the brake thermal efficiency. Nivin Chacko et al. [22] studied the influence of the multiple fueling of a waste-cooking-oil biodiesel blend on a light-duty diesel engine. They concluded that using pilot and post-fueling, as well as a higher fuel-injection pressure, resulted in emissions that were significantly reduced at part load. Teoh et al. [23] investigated the impact of the dwell angle on the engine characteristics in a CRDi engine powered by coconut oil methyl ester blends. They observed that the retarded start of the ignition timing, along with a long dwell angle, can reduce the NOx emissions without significantly increasing the smoke emissions. Gautam Edara et al. [24] analyzed fuel-injection strategies (split injections + cooled EGR) in a CRDi diesel engine. Split injection along higher EGR flow rates was found to lower the combustion duration, ignition-delay time, and tailpipe gas temperatures. Chiavola et al. [25] analyzed the engine characteristics of WCO blends (20% and 40% vol.) with ultralow sulfur diesel in a diesel engine. An increase in NOx emissions for the biodiesel blends was noticed.

As seen in the literature survey, many researchers have conducted investigations on WCO biodiesel in different blend ratios with the effects of variations in the nozzle-opening pressure and injection timings. There are very few studies that focus on the influence of biodiesel blends with a split-injection strategy in order to achieve a reduction in engine emissions and enhance performance. In the present research work, the objective was to evaluate the influence of pilot-main fueling and the variation in the pilot-injection quantity on a CRDi diesel engine fueled with WCO B20 (80% diesel + 20% waste cooking oil) at a nozzle-opening pressure of 600 bar. The pilot-fueling and main-fueling timings were maintained at 33° bTDC and 23° bTDC, respectively. Initially, neat diesel fuel was evaluated at a pilot-fueling quantity of 10%. Later, the effects of the pilot-fueling quantity in the range of 10%, 20%, and 30% were investigated for WCO B20, and were compared with neat diesel at a 10% pilot-fueling quantity.

2. Experimental Setup

The experimental tests were carried out in a 3.5 kW single-cylinder diesel engine.

Table 1. Test engine specifications.

Parameter	Specification
Make	Kirloskar
Engine type	Single cylinder, 4-stroke
Bore and stroke	87.5 mm and 110 mm
Compression ratio	18:1
Rated power	3. 5 kW at 1500 rev/min
Speed	1500 rev/m
Nozzle-opening pressure	600 bar
Injection timing	23° bTDC
Type of injection	Common rail direct injection
ECU	Nira i7r with solenoid injector

indicates the engine specifications. The engine was loaded with a water-cooled eddy-current dynamometer. The engine setup included a high-pressure CRDi system, and the nozzle-opening pressure was adjusted using a programmable open electronic control unit (ECU) and computer software. Figure 1 shows the diagram of the experimental engine setup.

An AVL gas analyzer (Digas 444N) and AVL standard (437) smoke-measuring system were used to note the exhaust tailpipe pollutants and smoke-level data, respectively. The K-type thermocouples (make: Radix, model: PT100), air-mass-flow sensor, burette, and rotameter were used to note the exhaust tailpipe temperature, air discharge, fuel discharge, and cooling-water-flow rate, respectively. The crank-angle shaft encoder (make: Kubler, model: 3700) and piezo sensor (make: PCB-USA, model: 111A22) were used to note the crank angle and cylinder combustion pressure, respectively.

The readings noted down from the measuring instruments tended to have some errors due to the selection of the instruments, calibration, environment conditions, observations, and equipment defects. An uncertainty analysis was required to increase the accuracy of the experiment readings.

Table 2. Uncertainty values of various measured parameters.

Measurement	Range	Accuracy	Percentage Uncertainty
Speed	0–1500 rpm	±1 rpm	±1.0
Load	0–50 kg	±0.1 kg	±0.5
Pressure sensor	0–34,475 kPa	10 kPa	±0.5
Crank-angle encoder	0–6000 rpm	1 rpm	±0.1
NOx	0–20,000 ppm	±10 ppm	±1.0
CO	0–15% vol.	±0.01% vol.	±0.2
HC	0–20,000 ppm	±10 ppm	±0.15
Smoke opacity	0–100%	±0.1%	±1

shows the accuracies and uncertainty values of the measuring instruments.

The percentage of the overall uncertainty was determined by the following equation:

$$x_R={\left({\left[\frac{\partial W}{\partial y_1}{x}_1\right]}^2+{\left[\frac{\partial W}{\partial y_2}{x}_2\right]}^2+\dots +{\left[\frac{\partial W}{\partial y_n}{x}_n\right]}^2\right)}^{1/2}$$

(1)

Considering the above equation, the overall percentage of the uncertainty was found to be ±2.19%.

We procured WCO biodiesel from the local biofuel development park.

Table 3. Fuel properties.

Properties	ASTM Standards	Diesel	Neat WCO Biodiesel	B20 Biodiesel
Density (kg/m3)	ASTM D287	816	875	833
Viscosity, 40 °C (cSt)	ASTM D445	2.09	4.07	2.78
Flash point (°C)	ASTM D93-58T	53	160	78
Calorific value (MJ/kg)	ASTM D4809	45.30	41.82	44.19

shows the fuel properties. Before conducting a new test trial, the engine was permitted to run for about 10 min to burn the residual fuel from the previous testing. Measurements began when the new fuel attained a stable condition. Each test had 25 cycles, and the mean value of the recorded data was noted down.

Table 4. Experimental test matrix.

Abbreviations	Fuel	Nozzle-Opening Pressure (bar)	Pilot Fueling (%)	Main Fueling (%)	Pilot-Fueling Timing	Main-Fueling Timing
DieselP10	Diesel	600	10	90	33° bTDC	23° bTDC
B20P10M90	80% Diesel + 20% WCO		10	90
B20P20M80	80% Diesel + 20% WCO		20	80
B20P30M70	80% Diesel + 20% WCO		30	70

presents the test matrix.

3. Results and Discussion

The following section describes the influence of the pilot-fueling and nozzle-opening pressure on the engine characteristics, such as the performance, emission, and combustion parameters.

3.1. Performance

In the performance parameters, the BSFC and BTE were obtained for different pilot-fueling quantities with varying engine loads, which are discussed in detail.

3.1.1. Brake Thermal Efficiency

For all the B20 blends, as the load varied, substantial changes in the BTE were noted. Figure 2 illustrates the deviation of the BTE for the WCO B20 blend for varying pilot-fueling quantities. The brake thermal efficiency increased as the pilot-fueling quantity increased. For maximum load conditions, the BTEs for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were observed as 20.73%, 23.28%, 24.23%, and 25.94%, respectively. This shows that the biodiesel blends are enriched with oxygen content, which results in higher temperatures and the proper combustion of the fuel. Dhar et al. [11] reported the same conclusions for Karanja biodiesel of 10% and 20% blends. Naresh et al. [15] proposed the same results for B20 palm-biodiesel blends. As the combustion advances with pilot fueling, there is a shorter ignition delay. As a result, the main injected fuel burns more efficiently, which leads to an increase in the BTE with increased pilot-fueling quantities. The B20P30M70 exhibited an optimum of 25.94%, which was noted to be 20.08%, 10.25%, and 6.59% higher compared with the DieselP10, B20P10M90, and B20P20M80, respectively.

3.1.2. Brake-Specific Fuel Consumption

Figure 3 illustrates the deviation of the brake-specific fuel consumption (BSFC) for the WCO B20 blend for varying pilot-fueling quantities. It was seen that the BSFC deteriorated with the increases in the pilot-fueling quantity and load. For peak load conditions, the BSFC values for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were observed as 0.31 kg/kWh, 0.36 kg/kWh, 0.34 kg/kWh, and 0.33 kg/kWh, respectively. This shows that biodiesel blends are enriched with oxygen content, which results in higher temperatures and the proper combustion of the fuel. Biodiesel blends have a lower heating value, which leads to the injection of a higher amount of fuel for the same nozzle-opening pressure to produce the necessary power. Nanthagopal et al. [20] observed that the BSFC for biodiesel blends deteriorates with the increment of the pilot-fueling quantity. The B20P30M70 exhibited a lower BSFC, which was noted to be 16.13%, 9.67%, and 6.45% lower compared with the DieselP10, B20P10M90, and B20P20M80, respectively.

3.2. Emissions

Tailpipe emissions from the CRDI diesel engine with different pilot-fueling quantities were tested with the engine load and are discussed in detail.

3.2.1. Carbon Monoxide

Partial combustion, an inadequate air supply, and a low in-cylinder temperature result in the emissions of carbon monoxide. Figure 4 illustrates the deviation of carbon monoxide for the WCO B20 blend for varying pilot-fueling quantities. For all the fuel trials, it was observed that the CO emissions increased as the load increased. At partial loads, the conversion of CO into CO₂ was less, and hence, the CO emissions increased. As the load increases, the CO₂ formation will be greater because of the availability of oxygen and temperature, which reduces the CO formation. With a further increase in the load, CO emissions increase because of the dissociation of CO₂, and the resident time is less, and hence, the availability of oxygen will be less [26]. For maximum load conditions, the CO emissions for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were observed as 0.38%, 0.35%, 0.33%, and 0.3%, respectively. DieselP10 releases a higher amount of CO emissions compared with WCO blends. The enriched oxygen content of the WCO blends and the implementation of a higher nozzle-opening pressure and the split-injection strategy resulted in a higher temperature and complete combustion, which thus resulted in the lowering of the CO emissions. The B20P30M70 emits the lowest amount of CO emissions, which was noted to be 26.66%, 16.60%, and 10% lower compared with the DieselP10, B20P10M90, and B20P20M80, respectively. Nanthagopal et al. [20] observed that the implementation of high injection pressure and the split-injection strategy drastically reduced the CO emissions by increasing the pilot-injection quantity.

3.2.2. Hydrocarbon

Partial combustion in an engine cylinder causes the formation of hydrocarbon emissions. Hydrocarbon emissions affect the engine efficiency, as well as the chemical composition of the air-fuel mixture. Figure 5 illustrates the deviation of the hydrocarbon for the WCO B20 blend for varying pilot-fueling quantities. For all the fuel trials, it was observed that the HC emissions increased as the load increased. For maximum load conditions, the HC emissions for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were observed as 30 ppm, 28 ppm, 27 ppm, and 25 ppm, respectively. Excess oxygen content in the biodiesel blends produced higher temperatures, which contributed to complete combustion. The B20P30M70 had the lowest emission level, which was noted to be 20%, 12%, and 8% lower compared with the DieselP10, B20P10M90, and B20P20M80, respectively. Naresh et al. [15] reported similar outcomes for B20 palm-biodiesel blends.

3.2.3. Oxide of Nitrogen

Excess oxygen amounts in the biodiesel blend and a high combustion temperature resulted in the formation of nitrogen oxide. Figure 6 shows the deviation of nitrogen oxide for the WCO B20 blend for varying pilot-fueling quantities. It was observed that the nitrogen oxide emissions increased as the load increased. For maximum load conditions, the NOx emissions for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were observed as 1322 ppm, 1369 ppm, 1503 ppm, and 1643 ppm, respectively. Excess oxygen content in the biodiesel blends produced a better air-fuel mixture, which resulted in the maximum cylinder temperature. Thus, the peak cylinder temperature facilitated the NOx formation. The B20P30M70 had the highest emission levels, which were noted to be 19.53%, 16.67%, and 8.52% higher compared with the DieselP10, B20P10M90, and B20P20M80, respectively. Naresh et al. [15] reported similar outcomes for B20 palm-biodiesel blends.

3.2.4. Smoke Opacity

The oxygen content and cylinder temperature are attributed to the smoke formation. Figure 7 illustrates the deviation of the smoke opacity for the WCO B20 blend for varying pilot-fueling quantities. It was observed that the smoke-opacity emissions increased as the load increased. For maximum load conditions, the smoke-opacity emissions for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were observed as 43.8%, 39.6%, 30.4%, and 29.3%, respectively. The split-injection strategy, excess oxygen content in the biodiesel blend, and a high cylinder temperature are the reasons for the lower smoke emissions. The B20P30M70 had the lowest emission levels, which were noted to be 49.48%, 35.15%, and 3.75% higher compared with the DieselP10, B20P10M90, and B20P20M80, respectively. Naresh et al. [15] and Nanthagopal et al. [20] reported similar outcomes.

3.3. Combustion

Combustion parameters are an important factor to understand the in-cylinder behavior. The performance and emission characteristics depend on the combustion parameters. The in-cylinder pressure, temperature of the exhaust gas, and rate of maximum heat release are discussed in detail.

3.3.1. Cylinder Pressure

Figure 8 shows the deviation of the engine-cylinder pressure for the WCO B20 blend for varying pilot-fueling quantities. For peak load conditions, the maximum in-cylinder pressures for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were 68.86 bar, 69.15 bar, 69.78 bar, and 69.82 bar, respectively. It was noted that the maximum cylinder pressure of B20P30M70 for the peak load was 1.39% higher compared with the DieselP10. This is due to the advanced combustion of pilot fueling, which shortens the delay period and increases the cylinder pressure. An increase in the pilot-fueling quantity resulted in advanced combustion, causing the maximum in-cylinder pressure to rise. Naresh et al. [15] performed the experimentation with palm biodiesel and obtained similar results.

3.3.2. Exhaust-Gas Temperature

Figure 9 illustrates the deviation of the exhaust-gas temperature for the WCO B20 blend for varying pilot-fueling quantities. For peak load conditions, the exhaust-gas temperatures for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were 315 °C, 309 °C, 308 °C, and 304 °C, respectively. For the peak load, the exhaust-gas temperature for the B20P30M70 was 3.49% less compared with the DieselP10. It was seen that the exhaust-gas temperature of the WCO-biodiesel blend decreased as the pilot-fueling increased. The low heating value of biodiesel results in a reduced exhaust-gas temperature.

3.3.3. Net Heat Release

Figure 10 illustrates the deviation of the HRR for the WCO B20 blend for varying pilot-fueling quantities. The rise in the in-cylinder temperature is owing to the heat-release rate, which results in the formation of nitrogen oxide. Contrarily, a lower heat-release rate tends to lower the nitrogen oxide formation. For peak load conditions, the maximum heat-release rates for the DieselP10, B20P10M90, B20P20M80, and B20P30M70 were 72.45 J/deg, 73.46 J/deg, 74.72 J/deg, and 75.23 J/deg, respectively. It was observed that the higher pilot-fueling quantity, additional oxygen amount in the biodiesel blend, and increased temperature due to advanced combustion from pilot fueling reduced the delay period for the main fueling [15].

4. Conclusions

In the present work, the influence of the pilot-fueling and nozzle-opening pressure on a CRDi diesel engine were investigated. Based on the experimental results, the main conclusions can be summarized as follows:

• The implementation of the pilot-fueling and nozzle-opening pressure at 600 bar was found to be more effective on the performance and emission characteristics;
• At a 50% engine load, the CO emissions were lower for the WCO-biodiesel blends, and later increased for the full-load conditions. Moreover, the exhaust-gas temperature decreased under partial-load conditions. The CO emissions, smoke opacity, NOx emissions, and exhaust-gas temperature curves were not similar among the different engine loads due to differences in the ambient conditions from change over time of the fuel, and the rich content of oxygen in the biodiesel blends;
• For the higher pilot-fueling quantity of the WCO B20 blend, the BSFC decreased and the BTE increased by 13.88% and 25.13%, respectively, compared with the diesel at 10% pilot fueling. The brake-specific fuel consumption for the B20P30M70 was lower compared with the B20P10M90 and B20P20M80. The BTE for the B20P30M70 was higher compared with the B20P10M90 and B20P20M80;
• The carbon monoxide, hydrocarbon, and smoke emissions were drastically reduced for the higher pilot-fueling quantity by 21.05%, 16.66%, and 33.10%, respectively, compared with the diesel at 10% pilot fueling;
• The nitrogen oxide emissions were found to be higher as the pilot-fueling quantity increased. The NOx level for the B20P30M70 was higher compared with diesel at 10% pilot fueling.