Effects of injection timing, injector opening pressure and nozzle geometry on the performance of cottonseed oil methyl ester-fuelled diesel engine

Abstract

Increasing petroleum prices, increasing threat to the environment from exhaust emissions and global warming have generated intense international interest in developing renewable and alternative non-petroleum fuels for engines. Evolving technology and a recurring energy crisis necessitates a continuous investigation into the search for sustainable and clean-burning renewable fuels. In this paper, cottonseed oil methyl ester (COME) was used in a four-stroke, single-cylinder variable compression ratio diesel engine. Tests were carried out to study the effects of fuel injection timing, fuel injector opening pressure (IOP) and injector nozzle geometry on the performance and combustion of COME biodiesel fuel used in a compression ignition engine with a single fuel mode. Fuel injection timing varied from 19° to 27° before top dead centre (bTDC) in incremental steps of 4° bTDC; fuel IOP varied from 210 to 240 bar in incremental steps of 10 bar. Fuel nozzle injectors with three, four and five holes, each of 0.3 mm size, were selected for the study. The results suggested that with retarded injection timing of 19° bTDC, increased IOP of 230 bar and a four-hole nozzle injector of 0.3 mm size resulted in overall better engine performance with an increased brake thermal efficiency and reduced HC, CO and smoke emission levels.

Keywords::

• cottonseed oil methyl ester
• injection timing
• injector opening pressure
• nozzle geometry
• emissions
• heat release rate

1. Introduction

All nations in the world are confronted with the problem of energy. There is an ever-increasing demand for energy in the industrial, transport and domestic sectors. Sustainable source of energy is essential for economic and social development. The existing sources of energy from fossil fuels may not be adequate to meet the ever-increasing energy demand. The diminishing and continuously increasing cost of petroleum resources along with their alarming pollution levels emitted from diesel engines has led to an intense interest in finding renewable, alternative and sustainable fuels to diesel fuel. India's share in global oil reserves is about 0.5%, whereas its share in global consumption is about 3%. India ranks among the top 10 largest oil-consuming countries importing about 70% of its oil needs. The total oil consumption in India is about 2.5–2.8 million barrels per day. The demand for fossil fuels will rise higher and higher with every passing decade and is expected to reach an amount of nearly 250 million tons by the year 2024. Consequently, to reduce the huge amount of import bills of crude oil and to become a self-reliant country in the energy sector, India has initiated the use of renewable energy over the last three decades (Agarwal 2006).

Renewable fuels have advantages in terms of renewability, biodegradability, energy security, environmental concerns, foreign exchange savings and socio-economic issues compared with fossil fuels. The use of renewable energy from non-edible oils reduces greenhouse gas emissions, gives better food and energy security to the nation and ensures sustainable development (Banapurmath et al. 2005; Banapurmath and Tewari 2010; Murugesan et al. 2009).

Therefore, renewable fuels such as biofuels can be used predominantly for both transportation and power generation sectors (Atadashi, Aroua, and Abdul Aziz 2010; Basha, Raja Gopal, and Jebaraj 2009; Murugesan et al. 2009).

A number of vegetable oils have been used for biodiesel production and are also being used in diesel engine applications. These include biodiesel fuels produced from jatropha, honge (karanja), honne, palm, rubber seed, rapeseed, mahua and neem seed oils (Banapurmath, Tewari, and Hosmath 2008, 2009; Bari, Lim, and Yu 2002; Gajendra Babu, Kumar, and Das 2006; Karnwal et al. 2010; Morshed et al. 2011; Onga et al. 2011; Ramadhas, Muraleedharan, and Jayaraj 2005; Ramadhas, Jayaraj, and Muraleedharan 2005; Raheman and Phadatare 2004; Sahoo et al. 2007, 2009; Venkanna and Venkataramana Reddy 2011; Yang et al. 2011). Biodiesel is a clean-burning alternative fuel produced from domestic renewable resources. It is simple to use, biodegradable, non-toxic and essentially free of sulphur and aromatics. Biodiesel improves lubricity and reduces premature wearing of fuel pumps.

In this paper, cottonseed oil methyl ester (COME) is used as a fuel in a diesel engine. In India, cottonseed oil is categorised as vanaspati oil. Also, India has been ranked as the second largest producer of cotton seeds and cottonseed oil in the world followed by China as the top producer (Ministry of Agriculture, India and U.S. Department of Agriculture). The estimated values of 1190 and 10,600 MT for cottonseed oil and cotton seed production in 2012 reveal that India is self-reliable for consumption of cottonseed oil. Consequently, in the last 3 years, there has been a rise in the capital growth of cotton seed production in India. The values for cottonseed oil production have been estimated to be 1045, 1150 and 1210 MT, which accounted for capital growth rates of 1.46, 10.05 and 5.22% in 2009, 2010 and 2011, respectively. Therefore, it can be concluded that India has a vital option of utilising cottonseed oil as a marginal substitute for the conventional petroleum diesel fuel to meet its energy demand. However, the Indian government should come up with strong and well-furnished polices for oil seed production.

Many investigators have studied the feasibility of COME and their blends in diesel engine applications. They have reported lower CO, CO₂, NO_x and smoke emission levels for COME fuel. Lower brake thermal efficiency (BTE) with COME and their blends operation is mainly due to their lower heating value compared with neat diesel fuels (Karabektas, Ergen, and Hosoz 2008; Nurun Nabi, Mustafizur Rahman, and Shamim Akhter 2009; Naga Sarada et al. 2010; Xiaohu et al. 2008; Yucesu and Ilkilic 2006). Lower and medium percentages of COME fuel can be partially substituted for diesel fuel without any major modifications in the diesel engine (Huseyin and Bayindir 2010; Martin and Edwin 2011). The effects of using neat cottonseed oil or its neat methyl ester (ME) biodiesel on the combustion behavior of a standard, high-speed, direct injection (HSDI) hydra diesel engine has been reported in literature (Rakopoulos et al. 2010). A heat release analysis of an experimentally obtained cylinder pressure diagram has been developed and used. The maximum cylinder pressure remained the same for raw cottonseed oil and slightly increased for the biodiesel with cottonseed oil (Rakopoulos et al. 2010). Approximately 20–40% of cottonseed oil can be substituted for diesel as fuel without any major modifications in the engine required.

Several researchers have conducted engine tests on a compression ignition (CI) engine with different vegetable oils and their esters at different injection pressures and injection timings (Gajendra Babu 2007). Varied fuel injection timings change the position and magnitude of the cylinder pressures and temperatures. The retarded injection timings showed a significant reduction in diesel and biodiesel NO_x emission levels (Hountalas et al. 2001; Tao et al. 2005). Cylinder pressures and temperatures gradually decrease when injection timings are retarded (Roy 2009). The results on the effect of fuel injection timings for waste cooking oil as a fuel used in a direct injection diesel engine with an advanced injection timing of 4° bTDC has shown better efficiency, reduced CO and higher NO_x emission levels (Bari, Yu, and Lim 2004).

Furthermore, other investigators have performed experiments on a CI engine with different vegetable oils and their esters at different injection pressures. The engine showed better performance and higher peak cylinder pressures and temperatures were obtained (Bari, Yu, and Lim 2004; Puhan Sukumar Jegan, Balasubbramanian, and Nagarajan 2009; Rosli, Semin, and Ismail 2008; Roy 2009).

From the literature survey, it follows that only limited work has been carried out to investigate the effects of injection timings, injector opening pressure (IOP) and nozzle geometry on the performance, combustion and emission characteristics of a COME-fuelled diesel engine.

The present study reports the effects of fuel injection timings, fuel IOP and injector nozzle geometry on the performance and combustion characteristics of a CI engine fuelled with COME.

2. Experimental set-up

Table 1 lists the properties of cotton seed oil and COME used in the study. A schematic diagram of the engine test rig used in the present study is shown in Figure 1. The engine tests were conducted on a four stroke, single-cylinder, water-cooled direct injection CI engine provided with a displacement volume of 662 cc, compression ratio of 17.5:1 and a rated power of 3.7 kW running at 1500 rpm. The engine was always operated at a rated speed of 1500 rpm and had a conventional fuel injection system. The IOP and the static injection timing were 205 bar and 23° bTDC, respectively, as specified by the manufacturer. The engine was provided with a governor that maintains a constant engine speed at all load levels. It was also used to control the speed of the engine. The engine comprised a hemispherical combustion chamber with overhead valves that were operated through push rods. The engine was cooled by circulating the water through the jackets in the engine block and cylinder head. A piezoelectric pressure transducer was mounted onto the cylinder head surface to measure the cylinder pressure. Table 2 outlines the specification of the engine used for the present study. Figure 2 shows fuel injectors with three different holes having an orifice diameter of 0.3 mm. The exhaust gas analyser and smoke meter employed for the measurement of exhaust gas emission levels are shown in Figures 3 and 4, respectively. Their specifications are given in Tables 3 and 4, respectively.

Table 1 Properties of the fuels used.

Sl no.	Properties	Diesel	Cottonseed oil	COME
1	Chemical formula	C13H24	–	–
2	Density (kg/m^3)	840	900	885
3	Calorific value (kJ/kg)	43,000	38,049	39,648
4	Viscosity at 40°C (cSt)	2–5	50	5.00
5	Flashpoint (°C)	75	210	167
6	Cetane number	45–55	48	45
7	Carbon residue (%)	0.1	0.66	–
8	Cloud point	–2	–	7
9	Pour point	–5	–	4
10	Carbon residue	0.13	0.55	0.01
11	Molecular weight	181		227
12	Auto-ignition temperature (°C)	260		470
13	Ash content, by mass (%)	0.57		0.01

Figure 1 Experimental set-up. 1 – Control panel; 2 – computer system; 3 – diesel flow line; 4 – airflow line; 5 – calorimeter; 6 – exhaust gas analyser; 7 – smoke meter; 8 – rotameter; 9, 11 – inlet water temperature; 10 – calorimeter inlet water temperature; 12 – calorimeter outlet water temperature; 13 – dynamometer; 14 – CI engine; 15 – speed measurement; 16 – burette for fuel measurement; 17 – exhaust gas outlet; 18 – outlet water temperature; T1 – inlet water temperature, T2 – outlet water temperature, T3 – exhaust gas temperature.

Table 2 Specifications of a CI engine.

Sl no.	Parameters	Specifications
1	Type	TV1 (Kirlosker make)
2	Software used	Engine soft
3	Nozzle opening pressure	200–225 bar
4	Static injection timing	23° bTDC
5	Governor type	Mechanical centrifugal type
6	No. of cylinders	Single cylinder
7	No. of strokes	Four strokes
8	Fuel	HS Diesel
9	Rated power	3.7 kW (5 HP) at 1500 rpm
10	Maximum engine speed	1500 rpm
11	Cylinder diameter (bore)	0.0875 m
12	Stroke length	0.11 m
13	Compression ratio	17.5:1

Figure 2 Fuel injectors with different holes having a 0.3 mm orifice diameter.

Figure 3 Exhaust gas analyser.

Figure 4 Hartridge smoke meter.

Table 3 Specifications of the exhaust gas analyser.

Parameters	Specifications
Type	DELTA 1600S
Object of measurement	Carbon monoxide (CO), carbon dioxide (CO2) and hydrocarbons (HC)
Range of measurement	HC = 0–20,000 ppm as C3H8 (propane)CO = 0–10%CO2 = 0–16%O2 = 0–21%NOx = 0–5000 ppm (as nitric oxide)
Accuracy	HC = ± 30 ppm HCCO = ± 0.2% COCO2 = ± 1% CO2O2 = ± 0.2% O2NOx = ± 10 ppm NO
Resolution	HC = 1 ppmCO = 0.01 vol.%CO2 = 0.1 vol.%O2 = 0.01 vol.%NOx = 1 ppm
Warm-up time	10 min (self-controlled) at 20°C
Speed of the response time	Within 15 s for 90% response
Weight	800 g

Table 4 Specifications of the smoke meter.

Parameters	Specifications
Type	Hartridge Smokemeter-4
Object of measurement	Smoke
Measurement range for opacity	0–100%
Accuracy	± % Relative
Resolution	0.1%
Smoke length	0.43 m
Ambient temperature range	− 5 to +45°C
Warm-up time	10 min (self-controlled) at 20°C
Speed of the response time	Within 15 s for 90% response
Sampling	Directly sampled from the tail pipe
Power supply	100–240 V AC/50 Hz10–16 V DC at 15 A
Size	100 mm × 210 mm × 50 mm

3. Results and discussion

The effects of injection timings, IOP and nozzle geometry on the performance of a diesel engine fuelled with COME are discussed in the following sections.

3.1 Effect of injection timings

Tests on engine performance were conducted at three injection timings of 19°, 23° and 27° bTDC. Compression ratio of 17.5:1 and an IOP of 205 bar (20.5 MPa) were maintained during the test period. A fuel injector with three holes each having an orifice diameter of 0.3 mm was selected for the experimental test.

3.1.1 Brake thermal efficiency

The effect of injection timings on the BTE of the CI engine fuelled with COME at the three injection timings is shown in Figure 5. For 80% load, highest BTE of 25.57% was obtained at a static injection timing of 23° bTDC in the diesel engine. The optimum injection timing for a standard diesel was found to be 23° bTDC, which also matched with the manufacturer's specification. However, BTE of the engine when fuelled with COME for the injection timing of 23° bTDC was found to be 21.50%. BTEs were lower for COME biodiesel fuel than for the standard diesel fuel at all the three injection timings tested. The decreased BTE for COME biodiesel might be attributed to its lower energy content and higher fuel consumption for the same power generation. Due to the high viscosity of COME, the formation of its mixture with air and subsequent combustion were poorer compared to diesel fuel operation. However, the retarding injection timing by a crank angle (CA) of 4° exhibited an improved BTE. The BTE at the injection timing of 19° bTDC was 23.01%. Based on the BTE values, the optimum injection timings for the standard diesel fuel and COME biodiesel fuel were 23° and 19° bTDC, respectively.

Figure 5 Effect of the injection timings on BTE for COME biodiesel engine.

3.1.2 HC and CO emissions

Figures 6 and 7 show the effect of the injection timings on HC and CO emission levels, respectively, for the standard diesel fuel and COME biodiesel fuel. HC emission from diesel engines is mainly caused due to lean mixture formation during the delay period and under-mixing of fuel leaving the fuel injector nozzle with a lower velocity. A general trend of increased HC and CO emission levels for COME biodiesel fuel was observed when compared with the standard diesel fuel at all the three injection timings tested. This trend may be attributed to the poor spray characteristics of COME and its poor mixing with air and results in poor combustion efficiency. At the 80% load, HC emission levels were found to be 64, 79 and 94 ppm at the injection timings of 19°, 23° and 27° bTDC, respectively. The lowest HC emission levels were found at the optimum injection timing of 23° bTDC for the standard diesel fuel. However, for COME biodiesel fuel, the lowest levels were found to be at the injection timing of 19° bTDC.

Figure 6 Effect of the injection timings on HC emission levels.

Figure 7 Effect of the injection timings on CO emission levels.

Carbon monoxide is a toxic by-product and is a clear indication of incomplete combustion of the pre-mixed mixture. The levels of CO emission decreased at part loads and increased at higher loads at all the injection timings. The levels of CO emission at the 80% load were 0.25, 0.33 and 0.45% at the injection timings of 19°, 23° and 27° bTDC, respectively. The lowest CO emission levels were found at the optimum injection timing of 23° bTDC for the standard diesel fuel. It may be concluded that at the injection timing of 19° bTDC, improved combustion and increased cylinder pressures and temperatures resulted in lower HC and CO emission levels when compared with the other injection timings for COME biodiesel fuel.

3.1.3 NO_x emissions

The effect of the injection timings on NO_x emission levels for the standard diesel fuel and COME biodiesel fuel is shown in Figure 8. In general, the retarded injection timings result in a substantial reduction in NO_x emission levels. When the injection timing is retarded, the combustion process also becomes retarded. NO_x emission levels were lower when the peak temperature was lower. On the other hand, NO_x emission levels were higher at the injection timings of 23° and 27° bTDC, thereby leading to a sharp pre-mixed heat release due to a higher ignition delay (Figure 10).

Figure 8 Effect of the injection timings on NO_x emission levels.

NO_x emission levels were slightly lower for COME biodiesel fuel than for the standard diesel fuel. From these results, the best injection timing was taken as 19° bTDC for COME biodiesel fuel.

3.1.4 Combustion analysis

Figures 9 and 10 show the effect of the injection timings on pressure variations and heat release rate for engine operation at a full load, respectively.

Figure 9 Effect of the injection timings on pressure variations during full-load engine operation.

Figure 10 Effect of the injection timings on heat release rate during full-load engine operation.

The delay period increased with an increase in the injection advance angle, as shown in Figure 9. This is because the pressures and temperatures are lower at the beginning of the injection. When the injection advance angles are small, the delay period reduces and the operation of the engine becomes smoother. The optimum angle of injection advance would cause the peak pressure to occur at a temperature range of 10–15°C after top dead centre.

For advanced injection timing, the ignition delay period also increases and heat release rate decreases during the pre-mixed combustion phase, with a minor effect being observed on the mixing-controlled combustion phase. From Figure 10, it is clear that this observed effect is more pronounced at the advanced injection timing of 27° bTDC.

3.2 Effect of IOP

An IOP of 205 bar for the diesel engine was recommended by the supplier. However, IOP was varied from 210 to 240 (21.0–24.0 MPa) in incremental steps of 10 bar to study its effect on the performance of COME. Also, nozzle with three, four and five-holes were considered for these ranges of IOP to study its effect on the engine performance. The part-load (80%) and full-load tests were conducted fort these ranges of IOP and the engine operated at a rated speed of 1500 rpm. At these loads, airflow rates, COME flow rates, exhaust gas temperatures and HC, CO and NO_x emission levels were recorded. Based on the results, the optimum injection pressures and nozzle geometries were identified and fixed for COME oil. Subsequently, the performance, emission and combustion parameters of COME biodiesel fuel were compared with the standard diesel fuel.

3.2.1 Standard diesel operation

The engine was operated at an IOP of 205 bar, injection timing of 23° bTDC and compression ratio of 17.5:1, as per the manufacturer’s specifications. It was observed that for 80% load, maximum BTE was found to be 25.57%, and was complying with the standard engine specifications provided.

3.2.2 Operation on COME

Performance parameters

The effects of IOP and varying nozzle geometry at the static injection timing of 19° bTDC and the maximum compression ratio of 17.5:1 are presented.

The effects of brake power on BTE at different IOPs and different nozzle geometries with three holes, four holes and five holes are shown in Figures 111213, respectively. Among all the IOPs tested, the highest BTE occurred at an IOP of 230 bar for the nozzle geometry with four holes, each having an orifice diameter of 0.3 mm. This is because at higher injection pressures, atomisation, spray characteristics and mixing with air are better, resulting in improved combustion. A higher IOP (240 bar) will lead to a delayed injection period negating the gain.

Figure 11 Effect of a three-hole nozzle and varying injection pressures on BTE.

Figure 12 Effect of a four-hole nozzle and varying injection pressures on BTE.

Figure 13 Effect of a five-hole nozzle and varying injection pressures on BTE.

The BTE was found to be 24.95% at the 80% load, and its maximum value was obtained for a four-hole nozzle geometry at an IOP of 230 bar. The BTE reported for the three-hole and five-hole nozzles were 24.50 and 24.25% at an IOP of 230 bar, respectively.

In view of this, the results revealed that among all the IOP and nozzle geometries studied, a higher BTE was found for a four-hole nozzle geometry at an IOP of 230 bar. The optimum IOPs for the other two nozzles (three holes and five holes) were also found at 230 bar, respectively.

The increased number of nozzle holes did not affect the ignition delay period much, but provided increased fuel–air mixing rate.

Emission parameters

HC emissions: Figures 141516 show the effect of brake power on HC emission levels at different IOPs and different nozzle geometries for COME biodiesel fuel. A significant decline in HC emission levels was observed for a four-hole nozzle geometry at an IOP of 230 bar due to better combustion. Enhanced atomisation will also lead to a lower ignition delay period. This will enhance the performance of the engine fuelled with biodiesels, which normally have a higher ignition delay due to their higher viscosity. An improvement in spray characteristics will lead to a lower ignition delay period. HC emission levels decreased from 90 to 80 ppm after the IOP increased from 210 to 230 bar at a full load. The highest IOP of 240 bar led to an increased HC emission levels to 85 ppm, and was probably attributed to the reduced BTE. In addition, a very high IOP will lead to a considerable portion of the combustion occurring in the diffusion phase and is due to the small ignition delay period. A higher IOP (240 bar) will lead to a delayed injection period negating the gain.

Figure 14 Effect of brake power on HC emission levels at a three-hole nozzle and varying pressures.

Figure 15 Effect of brake power on HC emission levels at a four-hole nozzle and varying pressures.

Figure 16 Effect of brake power on HC emission levels at a five-hole nozzle and varying pressures.

HC emission levels were found to be lower for the four-hole compared with the three- and five-hole nozzle geometries at an IOP of 230 bar, respectively. It can be concluded that unburnt hydrocarbons are less during the operation of the engine with a four-hole nozzle at an IOP of 230 bar, which is due to improved atomisation and proper combustion of COME biodiesel fuel.

CO emissions: Figures 171819 show the effect of brake power on CO emission levels. The observed trend for CO emission levels was similar to that for HC emission levels, with lower CO emission levels occurring for a four-hole injector at an IOP of 230 bar IOP.

Figure 17 Effect of brake power on CO emission levels at a three-hole nozzle and varying injection pressures.

Figure 18 Effect of brake power on CO emission levels at a four-hole nozzle and varying injection pressures.

Figure 19 Effect of brake power on CO emission levels at a five-hole nozzle and varying pressures.

NO_x emissions: NO_x emission levels increased with the increase in IOP due to faster combustion and higher temperatures reached in the cycle, as shown in Figures 202122.

Figure 20 Effect of brake power on NO_x emission levels at a three-hole nozzle and varying injection pressures.

Figure 21 Effect of brake power on NO_x emission levels at a four-hole nozzle and varying injection pressures.

Figure 22 Effect of brake power on NO_x emission levels at a five-hole nozzle and varying pressures.

Combustion analysis

Figures 23 and 24 show the effect of injection pressures on pressure variations and heat release rates with CA during full-load engine operation.

Figure 23 Effect of injection pressures on pressure variations with CA during full-load engine operation.

Figure 24 Effect of injection pressures on heat release rate with CA during full-load engine operation.

Figure 23 shows that the peak pressure for COME biodiesel fuel throughout the combustion process increased with increased fuel injection rates as the fuel injection pressures increased from 210 to 230 bar. Figure 24 shows that heat release rates with COME throughout the combustion process increased with increased fuel injection rates when fuel injection pressures were increased from 210 to 230 bar. Heat release rates were lower at injection opening pressures of 210 and 220 bar than at 230 and 240 bar, respectively. Heat release rates were the highest at an IOP of 230 bar followed by IOPs of 240 and 220 bar, respectively. The rates were the lowest at an IOP of 210 bar where the BTE was also lower when compared with the other IOPs.

4. Conclusions

From this exhaustive study on the feasibility of a COME-fuelled diesel engine, it can be found that its performance is lower when compared with the standard diesel fuel. However, by suitably adjusting the engine parameters such as injection timings, IOPs and nozzle geometries, it can be found that the performance of the diesel engine fuelled with COME can be improved. Therefore, COME oil can be used as an alternative fuel in diesel engines.

The retardation of injection timings from 23° to 19° bTDC showed encouraging results for COME fuel. Increasing the injection pressures from 210 to 230 bar for COME fuel favoured the performance of the engine with reduced emission levels. Increasing the number of nozzle holes in the fuel injector from three to four improved the performance of the COME-fuelled engine with reduced emission levels. However, further increasing the number of nozzle holes reduced the performance of the engine.

Finally, it can be concluded that operating the existing diesel engine with slightly higher injection pressures, increased number of holes in the fuel injector and retarded injection timings improved the performance of the COME fuelled diesel engine considerably.