Effect of Producer Gas from Redgram Stalk and Combustion Chamber Types on the Emission and Performance Characteristics of Diesel Engine

: The engine performance has been improved by modifying the combustion chamber shape of the diesel engine for dual-fuel operation with liquid fuel and producer gas (PG). The combined effect of gaseous fuel from redgram stalk and combustion chamber type on the emission and performance of blended-fuel of diesel and HOME biodiesel–PG has been investigated. In this experimental study, four varieties of combustion chambers hemispherical (HCC), low swirl (LSCC), dual swirl (DSCC), and toroidal re-entrant (TRCC) were analyzed comprehensively. The results presented that the TRCC conﬁguration with a given nozzle geometry has 9% improved brake thermal efﬁciency (BTE) and 10.4% lower exhaust gas temperature (EGT). The smoke, unburnt hydrocarbon (UBHC), and carbon monoxide (CO) decreased by 10–40%, but a 9% increase in nitrogen oxides (NO X ) emission levels was observed with TRCC. The delay period and combustion period were decreased by 5% and 7%. The fuel replacement of about 71% for the diesel–PG combination with HCC and 68% for the HOME–PG combination with TRCC was achieved.


Introduction
Biomass fulfils different needs of humans as food, fodder, energy, and shelter since ancient days. In the present scenario, biomass-derived fuels for energy applications play a vital role due to their several benefits [1][2][3][4]. Studies on biomass showed that biomass was the primary source of energy in rural areas and more than 50% in the urban areas of India. In rural India, about 70% of the people depend on farming for their livelihood.
India has a considerable prospective for bio-derived fuels and agriculture waste which was around 500 million metric tons of biomass per year during the years 2016-17, and the predicted biomass production during the years 2024-2025 will be approximately 1127 million metric tons. As far as biomass energy is concerned, India's Central Electricity Authority (CEA-2017) reports that electricity generation during 2016-2017 was about 7000 MW from agricultural residues, 18,000 MW from forest residues in India. At present, 32% of primary energy is from biomass in the country. The biomass-based power plant's total installed capacity in India is about 2559 MW, and it was observed that energy from agricultural residues and various woody biomasses was found to be about 1800 MW approximately. By 2050, all countries will meet the greenhouse gas reduction targets by utilizing greater renewable energy potential that provides sustainable energy since the estimated world production of biomass is about 146 billion metric tons every year [5,6]. Increased use of biomass can mitigate the negativity of energy supply from conventional fuels. Hence, for engine and thermal applications, biomass and biodiesel energy play a role in reducing GHG emissions and providing a solution to energy security and socio-economic issues suitably [7][8][9][10][11][12][13][14].
In this context, the use of biomass for the production of electricity is more appropriate because the simple equipment gasifier can generate the producer gas. Low calorific value producer gas is generated from biomass, and that can be used to run IC (internal combustion) engines to generate electrical or mechanical power. The SI engines can run on producer gas only but the CI (compression ignition) engines cannot run on PG only because of high self-ignition temperature. Therefore, little quantity of liquid fuel (diesel or biodiesel) is used to initiate the combustion. Many researchers reported that diesel engines using producer gas emit lower smoke and NO X with little compromise in thermal efficiency [1,[15][16][17]. It was reported that the operation of the engine in dual fuel mode at lower loads causes underutilization of gaseous fuel resulting in lower thermal efficiency and higher CO, HC emissions than the operation with diesel [18][19][20][21][22]. Using the producer gas in dual-fuel CI engines will save about 60-80% of pilot fuel and 100% fossil fuel if biodiesel is used. Additionally, the diesel/biodiesel-PG dual-fuel engine operation does not require significant engine modifications [23][24][25][26][27].
The highest pilot fuel replacement was achieved for operation with triple fuel or the addition of hydrogen to producer gas, but higher combustion temperature leads to higher NO X emissions [20,28,29]. Some authors reported using liquefied petroleum gas in dual-fuel diesel engines for optimizing compression ratio (CR) and NO X emissions [30]. Higher CR is more significant in the performance and emissions of the diesel/biodiesel-PG dual-fuel operation [31]. Further, the addition of hydrogen resulted in improved combustion at higher load conditions [32]. Several researchers reported the use of CO and H 2 mixture similar to PG and studied the performance and emissions. Results of the study showed higher HC and CO emissions and lower NO X emissions than the operation with diesel [33,34]. Some investigators have studied producer gas generated from downdraft gasifiers using various agricultural residue feedstocks in the dual-fuel engine and their effects on the emission and performance characteristics [16,35]. Some literature studies show that gaseous fuels CH 4 , OH, CO, H 2 , CNG, and LPG could be used along with biodiesel/diesel in a dual-fuel operation to investigate the emission and performance characteristics [36][37][38][39][40]. However, literature shows that very few studies are available on the use of producer gas from the agricultural residue as a secondary fuel in dual-fuel diesel engine operation. In these few studies, investigators have used a variety of gasification feedstocks like coir-pitch [24], wood chips [41], corn cobs, pigeon pea stalk [42], coconut shells, and babul wood [43] for gasification. Barriers to using PG for engine application are biomass blockage within gasifiers leading to lower gas generation and converting to tar at higher temperatures which causes stagnant gas and poor performance [44]. Instead of wood chips, the use of biomass from agricultural residue in the form of briquettes will reduce the clogging at the throat [45]. The development of tar depends on the raw material type used and the type of the gasifier but it can be reduced by using a suitable cleaning and cooling system [46]. Since the producer gas was derived from agricultural residue, i.e., redgram waste, there is minimal effect on the environment. The residue of the redgram waste after generating the producer gas was used as cattle feed and fertilizer. Hence, there is total utilization of the biomass feedstock without affecting the environment. A literature review shows that diesel engine using PG has improved performance during dual-fuel operation compared to the single fuel with lower smoke and NO X emissions [47]. Hence the present work was undertaken to study the effect of PG from redgram stalk agricultural residue and diesel/HOME combination with different combustion chamber shapes. Finally, a comparison of results with the baseline operation was completed.

Fuel Characteristics
The proximate analysis gives the moisture, volatiles, ash, and fixed carbon by difference according to ASTM standards (D-3172-73 through D-3174-82 and D-3175-82). The ultimate analysis gives the chemical composition of carbon, hydrogen, nitrogen, sulfur, and oxygen (CHNSO) using the CHN element analyzer. The compositions of the redgram stalk and liquid fuel properties are presented in Tables 1 and 2. The redgram stalk is available in good quantity in the north Karnataka region and is a potential gasifier feedstock. Redgram stalk has suitable characteristics for gasification and gives the quality producer gas to run CI engines. The redgram stalk briquettes are used in the downdraft gasifier.

Experimental Setup
The experimentation was carried out on a single-cylinder, 4-stroke, VCR CI engine test stand, as shown in Figure 1 experimental setup (a) and (b). An eddy current dynamometer was used to measure the load on the engine using the electric bulbs. The rotor rotates with the help of the diesel engine shaft and the voltage is applied to the coil or stator housing. Since such magnetic flux is generated and the rotor cuts off these magnetic fluxes, the eddy current opposite to the change in magnetic flux is generated in the rotor. Therefore, the rotor obtains a reverse force and attempts to reduce the rotational speed of the rotor. However, the torque provided by the diesel engine maintains the speed. This torque is measured by a sensor at the end of the arm on the dynamometer. The gasifier was appropriately connected to the diesel engine. The producer gas was generated in the gasifier of downdraft type and fed to the engine. The PG flow rate was measured with a digital venturi meter, and the flow rate of diesel or HOME was measured continuously. The constant speed of the engine at 1500 rpm is maintained by varying the flow rate of pilot fuel operating the regulator. The PG and air are inducted into the engine cylinder and compressed; the pilot fuel is injected to ignite the mixture at the end of the compression stroke. The piezoelectric transducer mounted on the cylinder head was used for measuring cylinder pressure and gas analyzer, and the Hartridge smoke meter for measuring emissions. The specifications of the engine and the gasifier are presented in Table 3.

Methodology
The agricultural residue redgram stalk (RGS) is used for the generation of the producer gas for experimentation. The producer gas derived from redgram stalk is used to run the engine under dual-fuel mode with diesel/HOME as injected fuel. The nozzle used for experimentation has three holes and a 0.25 mm diameter. In the present work, advanced injection timing of 27ºbTDC and 240 bar IOP was used to investigate the effect of diesel/HOME and producer gas on the diesel engine. The different shapes of the combustion chamber such as HCC, LSCC, DSCC and TRCC were used for the dual-fuel engine operation and performance, emission characteristics were obtained.
All experiments were carried out at a constant loading from 0 to 80% with varying amounts of inducted fuel supply. The producer gas was measured by a venturi-meter with a digital flowmeter and flow is controlled by a control valve. Load, speed, airflow, EGT, cylinder pressure, HC, CO, NO X , and smoke were recorded for each test. At least five sets of observations were collected to ensure the accuracy of the measured data. In addition, the experimental arrangements were prepared to be compatible with repeatable measurements.
In the transesterification process, the oil was combined with methanol by a 6:1 ratio, the 98 percent of sulfuric acid concentration by an additional 1 percent v/v honge oil was added to the catalyst and heated on a hotplate by 60 • C in a water-chilled mixing chamber. The esterification reaction is therefore improved. The period to react to the fusion was 60 min. The resulting blend results in the synthesis of methyl ester [50][51][52][53][54]. Figure 2 illustrates the preparation of biodiesel and Table 4 shows the fatty acid content of honge biodiesel [50]. Figure 1 illustrates the (a) Gasifier (b) gasifier-engine setup.

Results and Discussion
The present work describes the effect of the shapes of the combustion chamber, illustrated in Figure 3a-c, on the emissions and performance of the internal combustion engine in dual-fuel mode using diesel/HOME-PG combinations at various loads. The injection opening pressures of 205 and 240 bar were used for diesel-PG and HOME-PG, respectively. The injection nozzle of six holes and 0.25 mm size for HOME-PG and four holes and 0.25 mm size for diesel-PG was adopted.
In this section, the results of the dual-fuel engine operation using diesel/HOME as injected fuel and PG as inducted fuel with different combustion chamber shapes such as HCC, LSCC, DSCC, and TRCC were presented.

Performance Parameters
The variation of BTE with BP using different forms of combustion chambers is shown in Figure 4. The BTE for the dual-fuel diesel-PG operation was superior to the HOME-PG operation for the whole load range. This is because of the lower HV of the HOME and PG. Experimental results show that the HOME-PG combination with TRCC offers improved performance than all other forms of combustion chambers tested. This is because the TRCC prevents the propagating flame in the squish area, thus ensuring an improved HOME and PG-air mixture and increased airflow by increasing the vortex. From the results, it was observed that the TRCC was able to direct the flow properly into the combustion chamber, despite the load. The BTE for the HOME-PG combination with HCC, LSCC, DSCC, and TRCC was 17%, 17.2%, 17.8%, and 18%, compared with the 19.1% for the PG-diesel combination with HCC.
The variation of the EGT with BP using different forms of combustion chambers is shown in Figure 5. For the dual-fuel diesel-PG operation, the EGT was lower than the operation with HOME-PG for a given load range. This is due to the diesel and PG-air improved mixing, which will lead to almost complete combustion of the fuel. Most of the heat released during combustion is used for work, hence lowers EGT. Due to the higher viscosity of HOME, it will not mix properly with PG, and air causes combustion to take place in later stages resulting in higher EGT. The EGT for the HOME-PG combination with HCC, LSCC, DSCC, and TRCC was 402, 391, 382, and 360 • C, compared to 330 • C for the PG-diesel combination with HCC. Figure 6 shows the variation of smoke opacity with BP for different forms of combustion chambers. The incomplete combustion occurs due to poor air supply and improper air-fuel mixing. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide leading to production of carbon as soot. The operation with diesel-PG has lesser smoke opacity than the operation with HOME-PG for the given load range. This is as a result of the viscosity and free fatty acid in HOME, which causes a lower fuel-air mixture [48]. However, the experimental results show lower levels of smoke with TRCC than all other combustion chamber types tested. This is on account of the improved mixing of air-fuel and turbulence within the combustion chamber, leading to absolute combustion. Soot plays an unwanted part in the energy generation from combustion processes, whereas soot decreases total combustion process efficiency. Soot formation is linked to the HC chains creating carbon solid clusters. These reactions happen in the gaseous phase and compete with the energy discharge during the oxidation process of the HC [55]. The use of the modified CC, biodiesel-producer gas combination reduces the soot formation considerably [56][57][58][59][60]. For the operation with HOME-PG, the smoke levels with HCC, LSCC, DSCC, and TRCC were 56.6, 46.5, 45, and 31.3 HSU compared to 27.5 HSU for operation with diesel-PG and HCC.

Figures 7 and 8 illustrate variations of UBHC and CO emissions for operation with
diesel/HOME-PG for different combustion chambers. For HOME-PG operation, the UBHC and CO emissions were higher than the operation with diesel-PG for load range. This is because of the partial combustion of the HOME-PG combination due to insufficient oxygen for combustion, a lower HV of HOME and PG, and higher viscosity of HOME. However, the lower UBHC and CO emission levels resulted in TRCC compared to all other combustion chambers for the HOME-PG operation. This may be due to higher turbulence and higher temperature with TRCC, less heat loss, and improved oxidation, reducing CO and UBHC emissions. For operation using HOME-PG, the levels of UBHC with HCC, LSCC, DSCC, and TRCC were 46, 45, 36.3, and 35 ppm, compared to 33 ppm for the diesel-PG combination with HCC. Likewise, CO levels for the PG-HOME combination with HCC, LSCC, DSCC, and TRCC were 0.345%, 0.34%, 0.32%, and 0.31% compared to 0.26% for the PG-diesel combination with HCC.   Figure 9 shows the NO X variation with BP using different forms of combustion chambers. For operation with the HOME-PG, NO X levels were lower than the diesel-PG combination with HCC for the entire load range. There is a higher heat release rate in the diesel-PG mode in the premixed combustion phase than in the HOME-PG mode. HOME-PG operation with TRCC results in a slightly higher NO X emission due to improved combustion of the homogeneous mixture formed and greater heat release before TDC than all other combustion chamber forms tested. The presence of oxygen in the HOME leads to improved combustion and higher peak temperature, and NO X . The NO X emissions for a combination of PG-HOME with HCC, LSCC, DSCC, and TRCC were 100, 105, 106, and 110 ppm, compared to 115 ppm for operation with diesel-PG and HCC.

Combustion Parameters
The ignition delay (ID) variation with the BP for the use of different forms of combustion chambers is shown in Figure 10. The operation of HOME-PG results in a decrease of ID with an increase in BP for all forms of combustion chambers tested. The results show that the increase in BP with the rise in the quantity of fuel burnt inside the combustion chamber and the subsequent increase in temperature leads to a reduction in the ID for all the combustion chamber shapes. However, the ID was lower for the combination of diesel-PG than the HOME-PG with HCC. This may be due to the lower HV of PG and HOME, and the lower PG flame temperature, and the higher viscosity of HOME. However, for the HOME-PG operation, a lower ID was observed with TRCC than the HCC, LSCC, and DSCC. This might be certified to a superior air-fuel mixture and a higher temperature of combustion. The ID for the HOME-PG combination with HCC, LSCC, DSCC, and TRCC was 12.25, 11.75, 11.25, and 10.75 degrees CA, as compared to 10.25 degrees CA for the diesel-PG combination with HCC.
The variation in combustion duration as a function of BP is shown in Figure 10. Combustion duration increases with BP with all forms of combustion chambers due to the increased amount of fuel supply. The results showed a longer burning time with the HOME-PG combination than with the diesel-PG combination for the same form of the combustion chamber. This is because of the higher HOME viscosity, which causes inappropriate air-fuel mixing and requires additional time for preparation. Figure 11 illustrates the effect of the combustion chamber type on combustion duration. Figure 11. Effect of the combustion chamber type on combustion duration. However, the combustion duration observed for the HOME-PG combination with TRCC is lower than all other combustion chamber shapes tested. This might be certified to perfection in mixing injected fuel with an air-PG mixture due to an improved squish. The combustion duration of the HOME-PG combination with HCC, LSCC, DSCC, and TRCC was found to be 47.5, 46.5, 45, and 44 degrees CA compared to 42 degrees CA diesel-PG combination with HCC.
The variation of fuel substitution for dual-fuel diesel/HOME-PG operation at different BP is shown in Figure 12. The higher fuel substitution resulted in diesel-PG operation compared with the HOME-PG as conditions were the same. Cetane number, viscosity, and HV of fuel are accountable for this tendency. Experimental results show that, for the HOME-PG operation, fuel substitution for the TRCC is higher than all other forms of combustion chambers. The TRCC mixes air-fuel properly due to the higher vortex and squish in the chamber, allowing improved combustion of the diesel/HOME with the PG. Fuel replacement was higher at lower loads due to proper control of injected fuel supply, but less with higher loads. As a result, the injected fuel supply slightly increased at higher loads, which leads to improved combustion of the PG. The fuel substitution for the HOME-PG combination with TRCC, DSCC, LSCC, and HCC proved to be 67%, 65.5%, 64%, and 59% compared to 71% for the operation with diesel-PG and HCC. Similar results for BTE, EGT, HC, CO and NO X were found by 0D thermodynamic analysis for different fuel combinations [61][62][63][64][65]. The literature for 0D thermodynamic analysis of CI engines operating in dual-fuel mode using producer gas and liquid fuel combinations were not analyzed in the preceding literature. There is further scope for the study of the OD analysis for the dual-fuel engine operation.

Conclusions
The function of a dual-fuel engine with diesel/HOME-PG from redgram stalk showed that the performance is considerably affected by the type of the combustion chamber and liquid fuel type. However, comparing the shape of the combustion chamber to enhance performance and reduce emissions was successful. The engine was operated with the HOME-PG combination, and the TRCC combustion chamber results in a 9% increase in BTE, 10.4% reduction in EGT compared to HOME-PG with HCC. The operation with HOME-PG and TRCC resulted in a 44% reduction in smoke, 23.9% of UBHC, 10.14% CO but a 9% increase in NO X levels compared to HOME-PG with HCC. A 17% lower ignition delay and 11.5% shorter combustion duration were observed for operation with diesel-PG than the operation with HOME-PG and HCC. Operating conditions being the same diesel-PG operation has 71% fuel substitution with HCC, and the HOME-PG operation has 68% with TRCC which is 100% biofuel operation. Overall, it was concluded that the diesel/HOME-PG combination is an alternative and sustainable fuel combination for CI engine operation. Additionally, the diesel-PG combination does not need any significant engine modifications and has satisfactory performance with lower emissions but the HOME-PG combination requires some modifications.
In future research, to describe BTE, HC, CO and NO X parameters, 0D thermodynamic analysis based on measured in-cylinder pressure can be performed. It is crucial to get insight into the combustion process through heat release rate and in-cylinder temperature to be able to present underlying processes.