Experimental analysis on the influence of compression ratio, flow rate, injection pressure, and injection timing on the acetylene — diesel aspirated dual fuel engine

The predicted scarcity, increasing cost of petroleum fuels, and environmental degradation are encouraging researchers to search for alternative fuels throughout the world. Hence, it is intended to utilize acetylene-based DF in the compression ignition (CI) engine with minor modifications. An engine of 5 Hp, four stroke, single-cylinder, water-cooled operated in dual-fuel (DF) mode (acetylene gas-diesel), aiming to reduce the emissions, was deployed to investigate its characteristics. In DF mode, gaseous fuel is injected through intake air manifold with 2, 4, and 6 lpm constantly. According to the research findings, the gas rate of 6 lpm provides the best results, having a superior BTE of 30.7%. Various compression ratios (16:1, 18:1, and 20:1) were used to determine the optimal compression ratio (CR) under a volume flow rate of 6 lpm with diesel. Fuel injector pressure (200, 220, and 240 bar) with injector intervals (19°, 23°, and 27°bTDC) were changed consecutive sequence while adjusting CR, and the best outcomes for improved CI fuel efficiency were determined. From the investigational analysis, the peak in-cylinder pressure and net HRR (heat release rate) are assessed for being better by the increment in CR in DF mode of operation with an acetylene gas of 6 lpm at all operating settings. At a 240 bar injection pressure, the BTE is recorded highest (35.1%), and smoke was decreased. An IT of 23obTDC, the CO and HC were found as to be minimum as 28 ppm and 0.04 ppm.


Introduction
Rapid industrial growth and motorization of the world are due to the internal combustion engines. Power developed by the CI engines is used for generating electricity, irrigation, operating locomotive and marine, defense, etc. Depletion of hydrocarbon resources, crude oil price increase and environmental degradation due to the emission of sulfur oxides, unburnt hydrocarbons, and CO 2 has created an attention of the researchers to search for alternative fuels. Several research works are undertaken to identify suitable alternate fuel which should be compatible in modern vehicles, adequate in energy content and low in pollutants emission. DF mode is one among the prominent techniques to preserve conventional fuels. Several fuels viz. methanol, ethanol, CNG, LPG, hydrogen (H 2 ), producer gas, acetylene, and vegetable oils are being tested to use in IC engines. Literature reveals that the research on the utilization of gaseous fuels for replacing diesel and petrol either as a partial or full replacement in IC engines has got greater attention owing to its reduction in emissions. Gaseous fuels could be regarded as the substitute fuel for diesel engines in the DF mode without engine modifications. Because gaseous fuels cannot be used in an auto-ignition mode due to their high self-ignition temperature, they must be mixed with air at the manifold intake or injected directly into the cylinder and compressed. Acetylene could be identified with the garlic-like smell, as it is colorless and whose properties are found to be in line with the hydrogen gas. Proper utilization of acetylene gas, as an alternate fuel, in IC engines might be possible because of its significant properties such as developed flame speed and extensive flammability range. Lakshmanan and Nagarajan (2009) have conducted research in the dual fuelled engine supplied with acetylene gas at the different flow rates. HC, CO, and NO X had been reduced due to leaner operation, smoke emission was increased, and BTE was nearer to diesel operation. Lower BTE was reported by Lakshmanan and Nagarajan (2010a) in DF mode of operating the CI engine, while adopting TMI (timed manifold injection) and TPI (timed port injection) methods. NO X emissions reduced with improved thermal efficiency. HC, CO, and NO X were reduced with rise in the smoke level while operating on TMI technique compared to the diesel with diesel-acetylene mode Lakshmanan and Nagarajan (2010b). Lakshmanan and Nagarajan (2011b) observed more CO, CO 2 besides HC, and inferior oxides of nitrogen emission while operating the engine with acetylene gas related with diesel, deploying EGR. Sudheesh et al. adopted reverse flow water cooling technique and found that there is an increase in BTE and drop in the emissions viz. NO X and smoke (Sudheesh and Mallikarjuna 2010).
Experiments have been performed in acetylene-aspirated CI engine at 7lpm flow rate by varying the CRs (18:1,18.5:1,19:1,and 19.5:1) in the DF mode by Choudhary et al. (2018) and reported the increase in BTE and peak in-cylinder pressure for higher CR, whereas EGT decreases. Srivastava et al. (2017a, b) explored the CR impact in the acetylene-diesel-fuelled CI engine for various flow rates (60, 120, 180, and 240 Lph) of acetylene gas and observed higher BTE, net HRR, and peak in-cylinder pressure at the CR of 21 and gas flow rate of 120 Lph, in comparison with baseline experiments that uses diesel as a fuel. Exhaust pollutants of HC, CO, and smoke remained reported as lower, whereas NO X was at its higher, when operated at the optimum CR and flow rate. Srivastava et al. (2018) examined the influence on IP (180,190,200, and 210 bar) on operating characteristics of diesel-acetylene gas for 120 Lph flow rate. The outcomes revealed that operating the engine on DF mode produces higher BTE compared with the diesel, at an IP of 200 bar. Haragopala Rao et al. (1983) investigated engine performance on DF mode, by injecting H 2 at the inlet manifold and reported that the thermal efficiency was approximately nearer with diesel mode by decrease in smoke and temperature of gaseous pollutants, whereas peak pressure and NO X were increased on maximum load. Tomita et al. (2002) investigated the performance of the CI engine by introducing H 2 into the intake port and found a reduction in HC, CO, CO 2 , NO X , and smoke, as well as a modest gain in BTE, as a result of lean and premixed combustion. It was found that when engines run on biodiesel blends, emissions, performance, and combustion all improved significantly, with enhanced engine characteristics such as IT, load, and CR (Raheman and Ghadge 2008).
With increased CR and advanced IT, BTE, peak combustion pressure, then HRR of the DF engine powered by CNG and hinge oil biodiesel improved, whereas the combustion period and ignition delay decreased (Hosmath et al. 2016). After analyzing experimental findings, Swami Nathan et al. (2008) reported that acetylene may be utilized in HCCI engine. When compared to the diesel operation, they reported equal thermal efficiency, lower NO x emissions, smoke, and higher HC. Jaichandar and Annamalai (2013) carried out studies on diesel engine and observed that at higher IP: (i) BTE and HRR attains maximum with higher peak in-cylinder pressure, (ii) specific fuel consumption decreases, (iii) ignition delay period is less, and (iv) increase in NO X was observed. Atomization of fuel and duration of combustion was better in varying IP; unburned HC was lower in higher IP (Jindal et al. 2010). Reduction in volumetric efficiency and concentration of HC and increase in concentration of NO x was reported while advancing IT (Huang et al. 2007).
Analysis on the CI engine characteristics fueled with ethanol blend (Gumus et al. 2012;Leo et al 2021) revealed that with the delay in IT (i) concentration of NO x and CO 2 decreases and (ii) unburnt HC and CO increases, whereas it follows an opposite trend with the advanced IT. From the analysis of experimental outcome it is inferred that (i) the increase in biodiesel content in the biodiesel blend results in increase in BSFC, BSEC, and NO x , further decrease in BTE, smoke opacity, CO and HC emission; (ii) higher CR, IP, and timely IT associated with higher BSFC, BSEC, BTE, and NOx emission, whereas smoke opacity, CO, besides HC were lower.
A DF engine combines the features of both CI and SI engines (Wagemakers and Leermakers 2012). Engine characteristics could be improved by operating in DF mode as it enhances combustion and thermal efficiency with lower emission of smoke.
Natural gas, LPG, and hydrogen along with diesel and biodiesel derived from vegetable oils enhance the CI engine performance. Dual fuelling of CI engines with gaseous fuels needs specific arrangements such as gas storage, gas flow meter, control valve, and flame arrester (Bora and Saha 2015). Raman and Kumar (2019) investigated the engines characteristics using acetylene and n-butanol/diesel blends and inferred, (i) increase in BTE and peak cylinder pressure while injecting B10 blend as a pilot fuel, (ii) lower emission of CO and exhaust gas temperature, and (iii) efficient utilization of acetylene when blending 10% of n-butanol with diesel.
Acetylene-alcohol used as a DF in engines has been reported as a good alternative to gasoline and diesel. Behera et al. (2014) reported that engine operation using acetylene and used transformer oil, as dual fuel, has lesser ignition delay.
Thus, from the above discussion, it could be inferred that the minimal research work on dual fuelled engine using acetylene-diesel was performed. Internal combustion engines use acetylene gas and can operate even with a lean mixture as the gas has wide flammability range. Acetylene, as a fuel, is highly desirable and is used in the CI engines for transportation, power generation and in agricultural machinery. Acetylene is fed into CI engine adopting any of the four techniques viz. carburetion, manifold injection, port injection, and direct injection. Among the techniques, port injection technique is regarded as the simplest and flexible technique for operating CI engine in DF mode and reduces pre-ignition and backfire. Dual fuelling of existing conventional CI engines is the better option, as it is flexible in switching over to operate in pure diesel mode on demand. Acetylene gas could be used in IC engines due to its wider flammability range. Dual fuel a CI engine using diesel and acetylene gas as fuels and identify the acetylene flow rate that yields maximum efficiency. Whereas researching on utilization of acetylene in CI engines was found to be minimal. Hence, it is intended that the effects of changing CR, flow rate, IP, and IT on engine performance of a dual fuel stationary CI engine supplied with acetylene kept in a gas cylinder and diesel have been examined. Figures 1 and 2 show a line representation of the test design used to study the engine characteristics of acetylene-diesel engines. The research was conducted using a 5-horsepower, four-stroke, single-cylinder, water-cooled VCR diesel engine. The dynamometer was connected to the CI engine, which serves as a loaded unit. Control mechanism at the fuel intake was provided for operating the CI engine both in diesel and DF mode. Solenoid gas injector was positioned just above the inlet valve in the engine cylinder head for supplying the acetylene gas. Table 1 illustrates technical specifications of the engine.

Experimental setup and methodology
Piezo electric sensors were placed in both fuel injector and cylinder head for measuring pressure in the fuel line and combustion pressure respectively. The crank angle was determined by an electronic encoder with a photo emitter and a sensor that was fixed in the flywheel. A non-contact tachometer was used for measuring the engine speed. A water filled "U" tube type manometer in succession with an orifice meter was connected to surge tank (to support precise measurement) for determining the rate of airflow. A burette connected to fuel delivery line was used to measure the fuel delivered to the CI engine. A Chromel-Alumel (K type) thermocouple was used to measure EGT. AVL make gas analyzer and smoke meter were deployed for measuring flue gas composition (CO, HC, NO x , CO 2 , O 2 ) and smoke density respectively. Experimental data, while operating the CI engine, was logged into the computer using a national instruments make data acquisition device. The engine was tested using diesel and then in DF mode for comparing its performance. In DF mode, the engine was initially started with diesel, and then, acetylene was supplied into the engine through the intake port using gas injector. The injector was opened and closed using a proximity sensor and a voltage signal monitored through an electronic control unit. The diesel was injected at 23° bTDC at the pressure of 220 bar. A metered quantity of acetylene gas was supplied from a high pressure cylinder that is regulated through double stage gas regulator at 2 bar. A fire trapper and a flame limit switch were used for the acetylene pipeline system to avoid backfire from the engine. Table 2 compares the physiological, chemical, and thermal parameters of acetylene to those of other fuels. Brief description of the methodology adopted is given in Table 3.

3
Solenoid gas injector with a non-return valve was positioned just above the inlet valve in the engine cylinder head for supplying the acetylene gas while operating the CI engine in DF mode. Opening and closing of the injector was performed using a proximity sensor by sending the voltage signal controlled by an electronic control unit. Details:

Make quantum technologies
Supply voltage 8-16 volts Peak current 4 ampere Holding current 1 ampere Flow capacity 0.8 g/s @ 483-552 kPa Working pressure 103-552 kPa Baseline experiments had been conducted in the CI engine using diesel as a fuel. Then, the engine was operated in DF mode using diesel and acetylene gas at various acetylene flow rates of 2 lpm, 4 lpm, and 6 lpm (D + A2L, D + A4L, and D + A6L). CR, IP, and IT were maintained as 18:1, 220 bar, and 23°CA bTDC as the engine standard conditions during the entire experimentation.
Having analyzed the influence of acetylene flow rates with diesel on dual fuelling of CI engine, it was decided to investigate the effect of variable compression ratio (VCR), varying injection pressure (VIP), and varying injection timing (VIT), for using D + A6L as fuel that resulted in better performance of CI engine in DF mode. It was decided to experiment the CI engine with.

Error analysis of the experimental data
Error in the data observed during experimentation could be attributed to the environmental and working conditions, instruments used, calibration and experimental methods adopted, etc. Analysis of errors is indispensable for determining the accuracy of the measured parameters. The error pertaining to the engine speed was estimated approximately by considering the error in tacho generator. Fuel consumption rate was considered for estimating the error in mass flow rate of fuel. Similarly, an error of EGT was calculated from the error in the thermocouple. Statistical analysis and standard analytical techniques (root mean square method) were used to calculate the errors in the parameters to be established. The uncertainty analysis reveals that the uncertainties, mostly in measured parameters, have a negligible impact on the final results. Uncertainties in the parameters measured are presented in Table 4.  1 3 The uncertainty occurs due to fixed or random errors. The uncertainties in the measured parameters were estimated based on analytical methods. The total uncertainties computed for the measured quantities are listed below.

Results and discussion
Experiments on engine performance analysis of a dualfueled engine were performed to adjust its flow of acetylene, CR, IP, and IT. The results of the experiment are being investigated intensively.

Pressure vs crank angle
In the diesel-acetylene DF mode, Fig. 3 depicts the change in-cylinder pressure as a function of crank angle for diesel with acetylene flow of 2, 4, and 6 lpm. CR, IP, and IT are maintained as 18, 23°CA bTDC, and 220 bar respectively. The peak pressures measured at full load are 46.91 bar, 47.65 bar, 48.39 bar, and 49.2 bar in diesel mode of operation (baseline), 2lpm, 4lpm, and 6lpm acetylene induction correspondingly. The cylinder peak pressure rises as the acetylene gas flow rate rises, as shown in the graph. In DF mode of operation, peak pressure increases by 4.9% at 6lpm, 3% at 4lpm, and 1.5% at 2lpm as compared to diesel mode. This higher peak pressure observed during acetylene combustion could be attributed to the premixed combustion of acetylene in comparison with the diesel fuel (Lakshmanan and Nagarajan 2011a). Higher flame speed leading to the higher HRR might be the reason for the higher peak pressure observed (Lakshmanan and Nagarajan, 2010b). According to the results of the investigation, a 6lpm acetylene flow rate could result in a greater peak pressure. Figure 4 outlines the impact of CR on cylinder pressure variation at a 6lpm acetylene flow, IT of 23°CA bTDC, and IP of 220 bar. When its CR is raised, the gas temperature within the chamber goes up, resulting in a decrease in ignition delay, which improves rapid and wide-ranging fuel burning (Ramachandran et al. 2020). The figure revealed that peak pressure increases while CR increases. Peak pressure was witnessed as 43.32 bar, 49.2 bar, and 54.31 bar for the CRs of 16, 18, and 20 and is achieved at 15°, 10°, and 9° CA respectively, at 6 lpm acetylene induction in DF mode of operation. As a result of the CR of 20, its maximum combustion pressure is increased. At increased CRs, the increase in pressure might be influenced by the availability of more secondary energy for burning (Choudhary et al. 2018). In comparison to lower CRs, the higher peak pressure within the cylinder at higher CRs leads to premixed combustion and the induction of more oxygen and acetylene gas (Srivastava et al. 2017a, b). Figure 5 shows the influence of IPs on cylinder pressure at a flow rate of 6lpm, a CR of 18, and an IT of 23°CA bTDC for acetylene. At 200 bar, 220 bar, and 240 bar injected pressures, peak in-cylinder pressures of 48.18 bar, 49.2 bar, and 49.69 bar were measured at 6lpm of acetylene gas in DF mode. This higher peak in-cylinder pressure found at higher IPs could be due to effective atomization, improved air interaction, shorter delay period, and enhanced burning (Srivastava et al. 2018). The development of a rich mixture within the cylinder that ignites very quickly in the initial phase of ignition, effective fuel atomization, and premixed combustion, all of which result in a higher cylinder temperature and a shorter delay period when the IP is increased by 200 to 240 bar (Syed et al. 2017). The maximum cylindrical pressure was inferred to reach 240 bar at IP.
At CR18 and an IP of 220 bar, Fig. 6 depicts the effect of various ITs on in-cylinder pressure. When the IT is advanced at the same CR, the engine develops a higher chamber pressure (Hosmath et al. 2016). Maximum incylinder pressure was observed to be increasing (at an earlier crank angle) from 49.2 to 51.74 bar, while evolving the IT on 23 to 27°bTDC. Earlier starting of combustion and occurrence of shorter ignition delay period for gaseous fuel were observed while advancing the injection timing (Kannan and Anand 2012). When injection timing was retarded and moves towards TDC, ignition delay becomes shorter that leads to higher fuel fraction burning in the region of diffusion combustion, thereby lowering the maximum in-cylinder pressure (Thodda et al. 2020). Hence, minimum peak pressure was observed owing to this shorter ignition delay compared to that of earlier SOI conditions (Murugapoopathi and Vasudevan 2019a).

Heat release rate
The net HRR is described as the variation among gross heat released during biodiesel combustion and heat transfer through cylinder walls (Murugapoopathi and Vasudevan 2021), where dQ n dt , net heat release rate, dQ in dt , heat released by combustion and dQ loss dt , heat lost to cylinder walls due to convection and radiation etc.
k , specific heat ratio,p , combustion pressure, V , stroke volume, and t, residence time.
In DF mode, Fig. 7 illustrates how HRR changes when the crank angle changes at various acetylene flow rates. CR, IP, and IT are maintained as 18, 23°CA bTDC, and 220 bar respectively. The figure reveals that (i) the highest HRR occurs at 4° CA, 3° CA, and 3° CA for 2lpm, 4lpm, and 6lpm of acetylene gas rate at full loading, and (ii) at the same CR, the DF mixture with 6lpm of acetylene fluid velocity produces the maximum HRR of 42.93 J/°CA when compared to the diesel mode (38.82 J/°CA). During the regulated phase of combustion, the HRR first drops, then rises to roughly 40°CA. During the first two phases of the process of combustion, the heat energy generated by the fuel is roughly 80%. Combustion occurs in two phases (phase 1-combustion of baseline fuel, phase 2-ignition of acetylene gas by injecting diesel) in dual-fuel engine. In DF mode, the high power intensity of the acetylene-diesel-air composition results in a higher HRR than in diesel phase (Lakshmanan and Nagarajan 2011b). The figure reveals that HRR decreases in the region of diffusion combustion for diesel-acetylene DF mode compared with diesel mode of operation. Improved combustion could also be due to a faster gas flow rate and a broad range of acetylene combustibility limitations. Figure 8 shows the effect of crank angle on HRR variation for different crank angles at a 6lpm acetylene flow, IT of 23°CA bTDC, and IP of 220 bar. As the CR increases, the HRR goes up as well, which could be due to a rise on chamber pressure, leading to greater reaction temperature. From Fig. 8, it is inferred that the maximum HRR takes place at 0°CA (32.36 J), 4°CA (42.93 J), and at 9°CA (49.27 J) when acetylene was supplied at 6 lpm for 16, 18, and 20 CR at 100% loading respectively. Due to the decline in gas temperature during compression stroke, HRR drops as CR falls, which appears to be similar to the variation in chamber pressure as CR changes (Banapurmath et al. 2014). Higher in-cylinder pressure, rapid combustion, and additional intake of oxygen as a result of increase in CR could be the reason for higher net HRR (Choudhary et al. 2018).
Effect of IP on the variations in HRR by varying the acetylene gas velocity at 6lpm, CR of 18 and IT of 23°CA bTDC is demonstrated in Fig. 9. The figure revealed that the higher HRR is occurring at 3°CA, 3°CA, and 4°CA for IPs of 200 bar, 220 bar, and 240 bar in full load condition. This increased HRR occurs during the premixed combustion stage itself, as a result of improved gas spray with adequate oxygen mixing due to higher IP, which promotes improved burning and higher combustion temperature (Jaichandar and Annamalai 2013). The variation in HRR for varying IP seems to follow similar pattern of variation in in-cylinder pressure for the increase in IPs. Figure 10 exhibits the influence of IT on HRR for acetylene-diesel DF mode of operation. The figure revealed that the maximum cylinder pressure and premixed combustion could be obtained by providing longer delay period while advancing SOI. Whereas lower peak cylinder pressure prevails due to higher fraction of fuel burning diffusion combustion, while shortening the ignition delay at the time of retarding the IT. Because of the early availability of additional pilot fuel for combustion, the temperature inside this chamber reaches its maximum due to advanced IT. The main reason for 27°bTDC to exhibit highest HRR could be possibly due to improved fuel spray characteristics (spray angle and spray penetration distance) followed by better fuel-air

Brake thermal efficiency
The fluctuation on BTE with the dual fueled engine for varied acetylene flow rates and diesel is shown in Fig. 11. Because the LCV of acetylene gas is higher than diesel, the BTE of an engine run with dual fuel method was found to be more significant at about 1.67%, 3.08%, and 5.18% for the fluid velocity of 2lpm, 4lpm, and 6lpm, respectively. Furthermore, acetylene's augmented flame rate and broader combustibility features improve flame diffusion throughout the cylinder, resulting in improved engine torque (Hosmath et al. 2016). From the figure, it is evident that the acetylene gas actively participates in the process of combustion.
where, N is the engine speed (rpm), W is the load applied (N), R is the dynamometer arm length (m).
where, BP is the brake power (kW), m d is the mass flow rate of diesel (kg/h), m a is the mass flow rate of acetylene gas (kg/h), CV d is the calorific value of diesel (kJ/kg), CV a is the calorific value of acetylene (kJ/kg).
where, dQ n is the net HRR (J/°CA), γ is the ratio of specific heats = 1.35, P is the cylinder pressure (N/m 2 ), V is the cylinder volume (m 3 ). Figure 12 portrays the variation in BTE with respect to load by varying CR (16,18,and 20 CR) in diesel and acetylene-diesel DF modes. The improvement in BTE was witnessed at 6.51% by CR20 and reduced to 5.86% by CR16, in comprehend with CR18. The enhanced BTE with increased CRs could be due to (i) greater heating value within the chamber (Srivastava et al. 2017a, b), (ii) greater turning effect in the crank (Choudhary et al. 2018), and (iii) increase in combustion phenomena due to the existence of high pressure.
Variation in BTE with load by varying IPs in diesel-acetylene DF mode is illustrated in Fig. 13. Maximum BTE is inferred to be 29.1%, 30.7%, and 35.1% at 200 bar, 220 bar, and 240 bar correspondingly, by 100% (1) BrakepowerBP = 2 NT 60 * 1000 kW load. BTE was shown to be greater for increased IPs, which could be owing to improved vaporization at greater IPs, facilitating good ignition (Murugapoopathi and Vasudevan 2019b). Influence of ITs on BTE is described in the Fig. 14. BTE was inferred to be in an increasing trend with advanced IT and reverses its trend while the IT is delayed. BTE improved by 8.78% during the IT of 27°bTDC and decreased to 9.12% relative to the baseline IT for the acetylene circulation of 6lpm. When increasing its IT, BTE was presumed to be increased. This might be due to the ample duration of access and the higher burning speed that occurs during the ignition delay (Lakshmanan and Nagarajan 2010b). Thus, advanced IT resulted in higher combustion temperature that favors the higher BTE in the DF mode of operation. Figure 15 shows that HC emanations change with the percentage of maximum load at different acetylene gas flow rates (2, 4, and 6lpm). Decrease in the HC emission compared to diesel is 2.5%, 25%, and 30% by 2, 4, and 6lpm acetylene gas velocities in diesel-acetylene DF mode, on the maximum loading condition. Gaseous HC present in the boundary layer that is stagnant surrounding the wall of engine cylinder at low temperatures and in crevices due to lack of flame propagation could be the reason for unburnt HC emissions (Ramachandran et al. 2020). Because of broader combustion restrictions, higher combustion speed, lean engine running, and specific power for acetylene, HC emissions are reduced with the gradual rise in load (Lakshmanan and Nagarajan 2011a). Figure 16 shows that adjusting the CRs affects HC emissions with various loads. HC emissions decrease by 32.1% on CR20 and increases by 92.8% on CR16 correlated with CR18. At greater CR, the decrease in HC emissions could be attributed to an improvement in heat produced, the release of additional compressed heat to burning, as well as shorter combustion duration (Murugapoopathi and Vasudevan 2021). Higher HC emissions were observed at lower CRs owing to ignition delay resulting from insufficient heat of compression (Sayin and Gumus 2011).

Hydrocarbon emissions
Variation in HC emissions for different loading in diesel-acetylene DF mode by varying the IP is presented in Fig. 17. At full load operation, HC emissions drop when the IP rises and was recorded as 39 ppm, 30 ppm, and 24 ppm for 200 bar, 220 bar, and 240 bar of injection pressure, correspondingly. Inadequate atomization on the mixture, more HC emissions were observed with reduced IPs. Complete  Fig. 15 Effect of acetylene flow rates on HC emissions combustion of DF owing to leaner engine operation, higher burning velocity and larger energy released resulting from higher injection pressure could be the reason for lower HC emissions. Better fuel atomization and vaporization and proper mixing of air-fuel could result in minimal HC emissions (Gumus et al. 2012). Unvaporized emissions of HC were produced in lower IP at the exhaust because of poor atomization and droplets of large size (Jindal et al., 2010). Figure 18 indicates the significance of its IT variation in HC productions. The figure revealed that HC emissions is reduced by 17.8% and increased by 10.7% at the IT of 27°bTDC and 19°bTDC correspondingly, compared with standard IT of 23°. When increasing IT, increased input power with greater swirling level, higher combustion intensity, faster combustion rate, and a broader flame range of acetylene are attained, diminishing HC pollutants (Hosmath et al. 2016). Peak pressure occurring at an earlier CA and higher combustion temperature prevailing at the advanced IT could also be the reasons for lower HC emissions. Figure 19 depicts the influence of acetylene flow rates on the variation in CO emissions at different loading conditions. Incomplete combustion of DF results in CO emission and highly depends on air-fuel ratio. Emission of CO observed to be higher at lower loads for diesel and diesel-acetylene DF mode. CO emission decreases as the load increases due to the greater heat and pressure within the engine cylinder. CO emissions are reduced as the acetylene fluid speed is raised. When comparing diesel at 100% load to acetylene at 2, 4, and 6lpm gas velocity, CO levels drop by 5.09%, 11.66%, and 29.22%, respectively. The above reduction in CO emissions could be related to an engine running on DF operation during its low burning zone as a contrast to diesel (Srivastava et al. 2017a, b).

Carbon monoxide emissions
Variation in CO emission by varying ER at different loads is illustrated in Fig. 20. The amount of CO emitted reduces as the CR rises. Insufficient oxygen availability at higher speeds leads to formation of CO. For increased CRs, the in-cylinder temperature increases, resulting in a decrease in CO. Decreased CRs result in inadequate compressed temperature and combustion duration, leading to greater CO releases (Choudhary et al. 2018). Increase in CR reduces the ignition lag and also causes complete burning of the fuel, which decreases the CO emission (Sayin and Gumus 2011). Furthermore, at higher CRs, more air is sucked in, which improves lean burning and results in lower CO emissions (Hirkude and Padalkar 2014). Figure 21 describes the effect of IP on CO emissions at different loading conditions in diesel-acetylene DF mode. In case of DF mode, as a rise in IP, CO levels drop. At 100% load, concentration of CO in the exhaust gas is inferred to be 0.08 ppm, 0.07 ppm, and 0.04 ppm by 200 bar, 220 bar, and 240 bar IPs correspondingly. Increased IP provides tiny sprays of acetylene which burn easily and completely due to greater blending interaction, lowering CO pollutants (Lakshmanan and Nagarajan 2010b).
CO content in the exhaust gas produced at different ITs is depicted in Fig. 22. The figure revealed that the trend followed by CO emitted during combustion is similar to that of emission of HC. Concentration of CO is found to be lesser in DF mode of operation which could be attributed to leaner engine operation resulting in complete combustion of fuel. CO emissions are decreased when IT rises due to increased heat in the combustion chamber and an improvement in the oxidizing process among carbon molecules. Occurrence of premixed combustion phase that enhances better combustion of inducted acetylene could also be the reason for the reduced levels of CO at advanced IT.

NOX emissions
Variations of NO x releases caused by different acetylene flow rates in DF mode are depicted in Fig. 23. According to Zeldovich mechanism amount of oxygen, reaction period and reaction temperature are the factors that influence the NO X emission. When nitrogen and oxygen in the combustion air interact at high temperatures in a flame, thermal NOx is produced. The majority of NOx produced during the combustion of gases and light oils is thermal NOx. Above a flame temperature of 2800°F, the rate of NOx formation normally accelerates substantially. The figure revealed that NO X emitted at full load is 392 ppm when operated with  diesel fuel mode, whereas it is lowered for diesel-acetylene DF mode. NO X emission is decreased by 29.6% at 2 lpm, 25.5% at 4 lpm, and 10.2% at 6lpm flow rates of acetylene in comparison by baseline mode of operation. Though NO x emissions in the DF operating phase were found to be lesser than diesel mode, NO x pollutants in the DF phase surged to higher acetyl gas velocity due to lean operating and a shorter ignition period (Behera et al. 2014). Increased NO x pollutants on greater gas fluid velocities might also potentially be owing to a greater heat and efficient oxidation as a result of rapid energy output (Syed et al. 2017). Concentration of NOX measured is in line with that of the results reported by Lakshamanan and Nagarajan (2009).
The variance in NO x release at various CRs is depicted in Fig. 24. On DF operation, NO x output appears to accelerate as CR intensifies. Because of the predominance of greater combustible temperatures, when CR rises, NO x releases rise (El Kassaby and Nemit Allah 2013). Availability of excess oxygen at higher CRs and higher calorific value of acetylene could also increase the amount of NO X emitted (Srivastava et al. 2017a, b). The amount of NO X emitted are 265 ppm, 352 ppm, and 390 ppm at CRs of 16, 18, and 20 respectively, while operating at full load in DF mode of operation. At low CRs, NO X emission is lesser due to the lower cylinder temperature that reduces flame temperature which suppresses NO X emission (Sayin and Gumus 2011).
Variation in emission of NO X at 200 bar, 220 bar, and 240 bar IPs are described in Fig. 25. At DF operation, NO x proportion grows with raising IP and falls with lower IPs. Emission of NO x was observed to increase by 3.1% at 240 bar and decreases by 24.7% at 200 bar when compared to the standard IP of 220 bar. Increased injecting pressure may result in higher NO x releases due to increased compression, better fuel evaporation, high reaction speed, faster oxidation, and enhanced combustion rate (Sayin and Gumus 2011). Figure 26 depicts the impact of IT on NO X emissions for varying load conditions. NO x pollutants grow as the IT advances, probably related to an elevation in chamber pressure and improved burning temperature. Further increase in the IT, beyond 27°bTDC, results in more noisy operation of engine and detonation which could be attributed to the diffused combustion of air-fuel mixture during burning.

Smoke emissions
A variation in smoke density under various acetylene flow rates is depicted in Fig. 27. In general, smoke is produced by decomposition of HC within fuel-rich regions. At full load condition, smoke is increased by 10.2%, 8.9%, and 3.15% at 2lpm, 4lpm, and 6lpm of gas fluid velocity in DF mode, compared with diesel owing to homogeneous Though smoke seems more in DF operation than diesel, this was shown to be diminishing as the acetylene stream speed increases. When acetylene is added to the DF phase, the mixture is ignited immediately, resulting in a decreased dispersion burning process and a reduction in smoke (Srivastava et al. 2017a, b). Figure 28 depicts the change of smoke levels to loads at various CRs. The figure reveals that emission of smoke is increased with the decrease in CR and vice versa, while operating in DF mode. Increase in CR to 20 lowered the smoke by 2.5% and decrease in CR to 16 increased smokes by 3.57%, when compared to standard CR of 18. An engine in-cylinder heat rises as the CR rises, resulting in a reduction in smoke production (Jaichandar and Annamalai 2013). The use of acetylene decreases the quantity of jet fuel spray, resulting in faster combustion owing to the development of uniform composition, and so lowers smoke levels (Swami Nathan et al. 2008).
Outcome of varying IPs on smoke levels is presented in Fig. 29. From the figure, it is noticed that higher smoke emissions were observed at the lower injection pressures because of poor atomization of DF. The smoke was observed to be decreasing by 7.05% for the IP of 240 bar and increasing by 6% for 200 bar, when related to standard injection pressure of 220 bar. Smoke production is minimized with greater IP, which may be attributable to superior thermal decomposition of DF delivered and improved air blending, resulting in improper burning (Sayin 2011). Figure 30 depicts the impact on IT of smoke production under various loading conditions. It is clear by this diagram that as IT advances; the amount of smoke produced reduces, influencing the combustion mechanism. The increased ignition lag interval caused by advanced IT causes a rapid surge in the initial stage of burning (Murugapoopathi and Vasudevan 2021). Once the jet fuel is burnt, it starts the combustion of the fuel gas faster, allowing for thorough burning fuel in the DF method of operation (Choudhary et al. 2018). The smoke opacity lowered by 4.3% when IT was advanced and raised by 3.98% in retarding the IT.

Conclusions
In this current study, outcomes of the detailed analysis on the results obtained while using acetylene-diesel DF at the optimum acetylene fluid flow rate for 6lpm in CI engine are presented below.
• The performance and combustion characteristics of CI engine while operated on DF mode at 220 bar, 23°bTDC and 6 lpm of acetylene flow rate resulted in increased BTE, in-cylinder pressure and HRR by 5.19%, 4.9%, and 9.57%, respectively. • BTE increases as IP increases due to effective fuel atomization, better mixing of air-fuel mixture and better combustion. The highest BTE obtained is 35.1% at the IP of 240 bar in comparison to BTE of 29.1% for IP of 200 bar. • Peak in-cylinder pressure is increased as CR increases in acetylene-diesel DF mode of operation. At higher CRs, the duration of combustion is lesser. The maximum peak in-cylinder pressure was 54.31 bar for the CR 20:1, IP of 220 bar pressure, and IT of 23° bTDC, which is inferred to be higher compared to the CR of 16:1. • HC, CO, and smoke emission from CI engine were found to be lower in DF mode at 6 lpm of acetylene flow rate compared with other acetylene flow rates (2 lpm and 4 lpm) and diesel mode of operation with the drop in HC, CO, and smoke emission by 30%, 29.22% and 4.61%, respectively compared with baseline readings. • Results revealed that advancing the IT results in decrease in emissions of HC and CO, whereas nitric oxide increases. When the IT was advanced, emission of CO decreases owing to improvement in the combustion reaction and an advancement in combustion increase charge temperatures that leads to lower HC emissions. At these conditions, emissions of CO and unburnt HC were found as 0.04% and 23 ppm, respectively, at the IT of 27° bTDC. • Smoke emissions decrease with increase in IPs. Higher IP results in thermal cracking owing to high temperature prevailing in the mixture. The engine produces least smoke emission (88.3 ppm) at the IP of 240 bar. • Nitric oxide emission is found to be lower at the CR of 16 owing to the existence of lower temperature during premixed phase of combustion inside the combustion chamber.
Based on the analysis on the results, it could be construed that acetylene could be used in DF mode with improved performance at the IP of 240 bar without any major modifications. HC and CO emissions were also less while advancing IT and NO X was higher. Hence, acetylene gas could be used as an alternative fuel in DF mode of operating CI engines.
Author contribution Concept and design -Arun Kathapillai, data analysis and interpretation -Gavaskar Thodda, drafting the manuscript and critical revision -Venkata Ramanan Madhavan, material preparation and data validation -Murugapoopathi Saravanamuthu.
Data availability Not applicable.

Declarations
Ethical approval The submitted work should be original and should not have been published elsewhere in any form.

Consent for publication
The authors have consented to publish the article.

Competing interests
The authors declare no competing interests.