Experimental study on the effect of load and air+gas/fuel ratio on the performances, emissions and combustion characteristics of diesel–LPG fuelled single stationary ci engine

Due to the issue of combustion stability when using natural gas and the problem of knocking when using both natural gas and hydrogen, liquefied petroleum gas (LPG) is then a good candidate to use for the dual‐fuel concept since it has been proven to be a good solution to limit the pollutants and the excessive use of fossil resources in the aim to support and advance the African Union's and UN's sustainable development goals. In this paper, the effect of load as well as the air + gas/fuel ratio on the performance, emission, and combustion characteristics of a dual‐fuel diesel–LPG engine, single‐cylinder, four‐stroke, direct injection diesel engine with a rated power of 3.5 kW at a speed of 1500 rpm has been carried out. Experiments have been performed in dual‐fuel mode for a range of loads from 0 to 12 kg and a range of volume flow of LPG from 1 to 5.5 L/min, and the results were compared with those obtained from the single‐fuel mode. Results show that the dual‐fuel mode gives better performance and fewer pollutants than the single‐fuel mode. For example, at low load, Brake thermal efficiency, the indicated thermal efficiency, and mechanical efficiency increased by 83.79%, 24.36%, and 41.77%, respectively, and by 57.48%, 19.84%, and 24.37% at high load when we moved from the single‐fuel mode to the dual‐fuel mode. The smoke, carbon monoxide, and NOx decreased by 24.3%, 94.2%, and 96.2% respectively at low load and by 62.3%, 89.8%, and 91.4% at high load. And, no knocking came up during this research compared to natural gas or hydrogen dual‐fuel engines.


INTRODUCTION
Heat engines are essential equipment in the vehicle propulsion sector (automotive, naval, aeronautics, aerospace, etc.) and the electrical energy production sector. In recent decades, these engines have faced a lot of criticism, particularly about their high level of pollutant emissions. The major challenge for designers of combustion engines is then to produce highly energy-efficient engines with a low level of pollutant emissions. Diesel engines enjoy an indisputable lead in long-haul heavy-haul, and power generation due to their heritage of superior fuel economy, high reliability, durability, and simplicity. 1,2 A relevant alternative for reducing pollutant emissions in diesel engines is dual-fuel operation. Several researchers have therefore taken an interest in dual-fuel diesel combustion. Some have turned to studying a dual-fuel alcohol/diesel combination. Thus, Zunquing et al. 3 conducted an experimental study of the combustion and emissions of n-butanol/biodiesel pollutants under both blended and RCCI modes. This study showed that dual-fuel combustion reduces pollutant emissions compared to the combustion of conventional biodiesel. In addition, combustion in both blended modes generally gives better results in terms of pollutants, than combustion in the RCCI mode. Zhancheng et al., 4 based on work, 5,6 conducted a study on the effect of engine parameters (tailpipe length, MSP, injection timing, and EGR rate) on the production of ultrafine particles in a dual-fuel methanol engine /diesel. This study showed that when the injection delay is reduced, the increase in the MSP leads to a decrease in the number of particles emitted. Similarly, Baodong et al. 7 did research on dual-fuel methanol/diesel combustion investigating the energy balance of different parameters. Experimental results suggested that methanol temperature and coolant temperature have a significant impact on the engine performance. Yaoyuan et al. 8 also studied dual-fuel bioethanol/diesel operation in an intelligent charge compression ignition engine. From their work, they found out that for a bioethanol/diesel ratio of 64%-68%, the thermal efficiency indicates a value of 50.1%, and pollutant emissions drop considerably for NO x (1 g/kWh) and solid particles (particle number close to 1 × 106/cm 3 ).
Other researchers have exploited the benefits of hydrogen 2,9,10 when combined with diesel. In this regard, Zhaoju et al. 11 showed that by combining hydrogen and diesel with a hydrogen/diesel ratio of 20%, a 7.7% increase in peak pressure and 3.7% in heat release are obtained. For their part, Sarthak et al. 6 have shown that an optimum between a hydrogen/diesel ratio of 20% in an engine with exhaust gas recirculation (EGR rate of 10%) is an optimum between the emission of pollutants and the achievement of good performance.
Another alternative heavily explored by researchers is dual-fuel gas/diesel operation. In addition to ecological and energy considerations, this approach also considers the economic criterion. Indeed, gas is a resource available in abundance, easy to exploit, and cheaper. 12 Among the pioneering works of this approach is the work of Patterson et al., 13 in which a comparison is made between the combustion of methane, propane, or butane associated with diesel. This work showed that the highest gas substitution levels were achieved when using methane under all test conditions, the emissions of NO x and smoke were lower when using propane, and butane gave the least satisfactory results. The teams of Abagnale et al., 14 Cameretti et al., 15 and Arkadiusz et al. 16 carried out an experimental and numerical study of combustion in an engine running on dual-fuel natural gas/diesel. They found that increasing the NG rate to between 70% and 90% levels induces an engine behavior comparable to one of the spark ignition engines close to knocking conditions with a sudden consumption of natural gas in the reacting mixture and, consequently, a steep pressure rise. More recently, Emad et al. 17 investigated the use of LPG consisting of a mixture of propane and butane in different proportions. The experimental data indicates that the engine parameters play a significant role in the engine's performance. Different LPG fuel compositions did not significantly affect the engine efficiency but directly impacted the levels of generated combustion noise.
Due to the instability of the combustion of natural gas and the problem of the knocking using natural gas 18 and hydrogen, 2 the LPG is then a good candidate to use as the alternative fuel for engines, and the analysis of its combustion is then essential. In other works done in the dual-fuel LPG-diesel, [19][20][21] authors have found sometimes contradictory results because they have not studied the same combustion parameters at the same time and in the same boundary conditions.
In this work, the novelty of this study is to observe the effect of diesel/LPG dual fuel on engine performance parameters, combustion characteristics, smoke and exhaust emissions of a direct injection diesel engine at constant engine speed (1500 rpm) and different loads. Although many studies of LPG/diesel dual fuel have been found in the literature, none of them have ever tried to investigate at the same time the effect of A/F ratio and load on combustion characteristics, engine performance parameters (11 performance parameters in total), smoke and pollutants. In fact, by studying at the same time all those performance parameters (engine torque, engine powers, engine efficiencies), combustion characteristics (pressure, net heat release rate, cumulated heat release rate, pressure rise rate, combustion duration), pollutants (NO x , NO, CO 2 , CO), and smoke, the combustion process will be better understood and this will help to analyze in depth relationships between these parameters. A better understanding of the combustion process and the relationships between these parameters will be a good step towards the development of better engine control maps which is deeply linked to the experimental study. In addition, to get accurate results, 10 engine cycles have been done and compared to some works where just one cycle was done for each experiment. [22][23][24] 2 EXPERIMENTAL METHODOLOGY

Experimental setup
The research engine setup used in this study consists of a single-cylinder, four-stroke, multi-fuel connected to an eddy current type dynamometer for loading. The operation mode of the engine can be changed from diesel to ECU petrol or from ECU petrol to diesel mode by following some procedural steps. In both modes, the compression ratio can be varied without stopping the engine and without modifying the combustion chamber geometry by a specially designed "tilting cylinder block arrangement". In diesel mode, the one we have used, the fuel injection point as well as pressure can be manipulated for research tests. 25 These signals are read by a computer via a data acquisition card National Instrument USB-6210. Instruments were provided to interface airflow, fuel flow, temperature, and load measurements (Figures 1 and  2). The set-up has a stand-alone panel box (see Figure 3) consisting of an air box, two fuel tanks for dual-fuel tests, a F I G U R E 1 Schematic arrangement of the engine setup (two parts: engine block and dynamometer).

F I G U R E 2
Single-cylinder four-stroke multifuel engine at JKUAT.

F I G U R E 3 Engine control panel.
F I G U R E 4 Schematic diagram of the modified experimental setup. manometer, a fuel measuring unit, transmitters for air and fuel flow measurements, and a process indicator and hardware interface. Rotameters were provided for cooling water and calorimeter water flow measurement. For this experimental set-up, the LabVIEW-based engine performance analysis software package "Enginesoft" has been used for online engine performance evaluation. Figure 4 shows the schematic diagram of the experimental setup with the modifications made so that the engine can operate on dual-fuel LPG/DIESEL mode. Table 1 shows the engine specification.

Measurements
In this work, the main objective was to investigate the effect of the air+gas/fuel ratio as well as the load on the combustion and performance of a dual-fuel LPG/diesel engine. The first stage of the experiment concerned the test of the engine running with diesel alone as a reference fuel. Next, the primary part of the experimental work included tests of the engine working in a dual-fuel mode with the engine fed with diesel fuel and compressed liquefied petroleum gas (LPG). Many parameters have been recorded during the tests. The combustion parameters (in-cylinder pressure, net heat release rate, cumulated heat release rate, mass fraction burnt, and in-cylinder temperature, etc.) have been recorded every 1 degree of crank angle whereas the performance parameters (torque, brake power, brake mean effective pressure, Indicated Power, indicated mean effective pressure, specific fuel consumption (SFC), brake thermal efficiency, indicated thermal efficiency, mechanical efficiency, etc.) have been recorded for each load. For this work, we have done 50 measurements containing 10 subsequent engine cycles each. To change the value of the air+gas/fuel ratio, the gas flow has been changed from 1 L/min to 5.5 L/min with a step of 0.5 L/min. At each gas flow, the combustion and performance of the dual-fuel engine have been evaluated for five loads in total (0.5; 3; 6; 9; and 12 kg). The tests were carried out at a constant compression ratio (18:1); constant injection timing (23 • BTDC) and an EGR rate of 0%. The amount of diesel injected at each experiment was calibrated by the software itself according to the load, speed, and many other engine parameters, and the record time was calibrated to 1 min. The pollutants (NO x , CO 2 , CO, unburnt hydrocarbons C x H y ) and smoke were recorded using a Testo 350 gas analyzer shown in Figure 5 and filter paper smoke meter (BANZAI-XEL) for each gas flow at different loads. The record time for each test was different because depending on the case (load and amount of gas flow), the pollutants were recorded since the amount of CO 2 stopped increasing. The concentration of CO 2 is difficult to measure if it does not reach a certain value. In this regard, CO 2 is taken as the reference for when the recording starts/stops for all pollutants. Table 2 shows the parameters of the gas analyzer Testo 350.

Engine Im240PE Apex Innovations
Type of engine Four stroke compression ignition and multi fuel  In Appendix C, Table C1, specifications of the measuring instruments used are presented.

Engine calculations
During this experimental study, engine power output, performance and emissions were measured. The engine performance parameters were analyzed using parameters such as engine powers, engine efficiencies, fuel consumption, engine torque. The formulas used by the engine software to compute all the engine performance as well as combustion characteristics analyzed in this work can be seen in Appendix A.

Combustion characteristics at different air+gas/fuel ratio
The parameters of interest in this research related to the combustion characteristics are the in-cylinder pressure, combustion duration, cumulative heat release rate and the net heat release rate; all of them as a function of the crankshaft angle (CA).
In cylinder pressure, net heat release rate, and cumulative heat release rate In this paper, the evaluation of those parameters has been made for pure diesel first and then for the diesel+LPG at different amounts of mass of LPG injected and different loads to evaluate the impact of the use of the dual-fuel mode instead of the single-fuel mode. The volume flow of LPG was varied from 1 to 5.5 L/min, whereas the load was varied from 0.5 to 12 kg. Figure 7a presents the evolution of the in-cylinder pressure as well as the net heat release rate for both the singleand dual-fuel modes. It can be seen from that figure that the peaks of both the cylinder pressure and net heat release rate increase with the increase in the volume flow of LPG. For the cylinder pressure, the peak in the single-fuel mode (Diesel mode) is higher than the ones for the dual-fuel mode up to a volume flow of LPG of 5 L/min. On the other hand, the peak of the net heat release rate is higher in single-fuel mode than in dual-fuel mode for volume flow of LPG less than 1 L/min. For example, we moved from a peak of cylinder pressure of 29.22 bar in single-fuel mode to 32.62 bar in dual-fuel mode (corresponding to an increase of 9.07%). Regarding the net heat release rate, the peak increased from 15.24 J/CA to 63.23 J/CA (corresponding to an increase of 75.9%). The percentages of increase of the cylinder pressure as well as the net heat release rate can be seen in Table 3. From that table, it can be seen an increase of 29.76% in cylinder pressure and 78.68% in the net heat release rate in dual-fuel mode when we moved from a volume flow of LPG of 1-5.5 L/min. Figure 7b shows the evolution of the cumulative heat release rate in both single-and dual-fuel modes as a function of LPG/diesel fuel ratio. As can be seen, the peak of the cumulative heat release rate increases with the increase in the LPG/diesel fuel ratio. In addition, more energy is released in dual-fuel mode compared to single-fuel mode and this can be  justified by the fact that in dual-fuel mode two fuels are used and then it seems normal to see more energy being released. This result collaborates the one observed for the net heat release rate. Many reasons can justify those observations. For example, for the increase of the peak of the cylinder pressure when the amount of LPG used increases, this can be justified through the ignition delay. 16,26 The more the ignition delay increases, the more the peak of the cylinder pressure is decreasing due to the prolonged combustion. That means, from the volume flow of LPG of 1-5.5 L/min, the combustion process is becoming increasingly rapid and particularly for the last case (the case of 5.5 L/min), the combustion process is faster than the one for the diesel mode justified by the highest peak of pressure obtained in that case. In the case of the net heat release rate, in dual-fuel mode, the energy is not just coming from only the combustion of the diesel but from both the combustion of the diesel and the LPG. This can justify the fact that with the dual-fuel mode more energy has been released compared to the single-fuel mode in that research.

Combustion duration
Combustion duration is defined as the period from 10% to 90% of mixture combust up, which is mainly a turbulent combustion and greatly be accelerated by turbulence in cylinder. Many works [27][28][29][30] showed that combustion duration has a big impact on combustion characteristics, pollutants as well as engine performance. In this study, effect of LPG/diesel fuel ratio on combustion duration were investigated. Figure 7c shows that combustion duration decreases with the increase in the ratio. This is because of the less thermal energy liberated from the leaner mixture which increases the ignition delay and slows the flame propagation. In addition, the flame temperature is low at lean and rich mixtures. 29 The result obtained in this work collaborates with those obtained in previous works. 28,29 In addition, the observation made on the variation of combustion duration as a function of LPG/diesel fuel ratio strengthen the results obtained during this work on the peak of in cylinder pressure, emitted smoke, engine performance and NOx. In fact, the peak of in cylinder pressure increases with a decrease in the combustion duration. This is due to an increase in the ratio.

Effect of using LPG on the emitted smoke
Smoke occurs when there is incomplete combustion (not enough oxygen to burn the fuel completely). Smoke is then a collection of these tiny unburned particles. Many parameters can affect the apparition of smoke, but in this research, just the effects of both load and air+gas/fuel ratio have been investigated for the dual-fuel LPG/diesel engine. In Figure 8, it can be seen the evolution of the smoke emitted for both single-and dual-fuel modes at different loads and volume flow of LPG. From that figure, many observations can be made. First, the percentage of smoke released in dual-fuel mode is less than the one released in single-fuel mode whatever the load and the amount of LPG used. Second, the percentage of smoke released in either dual-fuel mode or single-fuel mode increases with the load and decreases with the volume flow of LPG globally. This can be justified by the fact that when the ratio increases, the combustion duration decreases. 29 The minimum of smoke released in dual-fuel mode is observed in the case of the volume flow of LPG of 5 L/min. In that case, the percentage of smoke decreased by 19.4% at 0.5 kg load, 24.3% at 3 kg load, 63.3% at 6 kg load, 76.9% at 9 kg load, 62.3% at 12 kg load when we moved from the single-fuel mode to the dual-fuel mode.

Evaluation of pollutants
As well as smoke released, the effects of load and air+gas/diesel fuel ratio have been evaluated for pollutants such as NO x , NO, CO 2 , and CO. One of the interesting remarks made was that NO x and NO behaved the same way in the function of the load and fuel ratio; the same for CO 2 and CO. Figures 9 and 10 show the amount of NO x and CO released, respectively, in function of the load and the volume flow of LPG used. As can be seen, pollutants are higher in single-fuel mode than in dual-fuel mode and this is whatever the load and/or the volume flow of LPG used. In Figure 9, it can be seen that the amount of NO x increases with the load and decreases with the volume flow of LPG globally. This can be explained by the fact that the more the load increases the more the amount of O 2 increases too. Compared to the smoke where the minimum has been found for a volume flow of LPG of 5 L/min, for NO x , the minimum has been found at 4 L/min. By moving from Diesel mode to dual-fuel mode, the NO x has decreased by 100% at 0.5 kg load, 96.2% at 3 kg load, 93.6% at 6 kg load, 92.3% at 9 kg load, and 91.4% at 12 kg load.
In Figure 10, it can be seen that the amount of CO released decreases both with the load and the volume flow of LPG globally. As for the NO x , globally, the minimum of CO released is located at the case of the volume flow of LPG of 4 L/min. By moving from Diesel mode to dual-fuel mode, the CO has decreased by 94.6% at 0.5 kg load, 94.2% at 3 kg load, 85.91% at 6 kg load, 87.5% at 9 kg load, and 89.8% at 12 kg load.
As we know, two main parameters contribute to the formation of NO x : the in cylinder temperature and the amount of oxygen in the combustion chamber. In addition, when the combustion duration increases, the peak of the in cylinder temperature decreases which leads to a decrease in NO x . It is then normal to see NO x increase with an increase in the load because of an increase in combustion duration. But contrarily to other works where NO x increase with an increase in the ratio, [27][28][29][30] in this work NO x decrease with an increase in the ratio (increase in the peak of in cylinder temperature due to the increase in the combustion duration). This can be justified by the type of fuel used as well its composition. More investigations should be then done to better understand the effect of the fuel composition on the combustion duration and then on NO x .

Engine performance at different fuel ratios
In this paper, 11 performance indicators and engine efficiencies were investigated depending on the volume flow of LPG and the load. As can be seen, engine performance is improved when the load increases. What can justify that is the decrease in the combustion ratio with the increase in the load. More results on the variation of engine performance as a function of the load can be seen in Figures B1-B3, Appendix B.

3.4.1
Investigation of the effect of load and fuel ratio on the indicated, brake and friction power The indicated power (IP) is the energy remaining after the loss in the exhaust, coolant, and radiation, and the brake power (BP) is the energy remaining after the loss in friction and pumping. Figures 11-14 present the variation of the IP, BP, as well as the energy lost FP in function of the load and the ratio. It can be seen that IP, BP, and FP increase when the load increases too and for high loads (6, 9, and 12 kg), it is clearly shown that IP and BP in dual-fuel mode are higher than those in single-fuel mode. In addition, in most cases, the friction F I G U R E 11 Indicated, brake, and friction powers for different volume flow of LPG at 3 kg load.

F I G U R E 12
Indicated, brake, and friction powers for different volume flow of LPG at 6 kg load. F I G U R E 13 Indicated, brake, and friction powers for different volume flow of LPG at 9 kg load.

F I G U R E 14
Indicated, brake, and friction powers for different volume flow of LPG at 12 kg load. power in dual fuel is found to be the smallest one for a volume flow of LPG of 5.5 L/min whatever the load, which means for a volume flow of LPG of 5.5 L/min, less energy is lost to overcome the friction between the piston and cylinder walls, between the crankshaft and camshaft and their bearings. Table 4 shows the improvement of engine power that we can achieve when we move from the single fuel to dual fuel.

3.4.2
Investigation of the effect of load and fuel ratio on the indicated, brake and friction mean effective pressures The indicated, brake, and friction mean pressures (IMEP, BMEP, FMEP) are pressures calculated by using indicated, brake, and friction powers. It is then normal to see here the same variations as for the powers (IP, BP, FP) in function of the load and the ratio. It is important to mention that among those pressures, only the brake mean effective pressure takes into account the engine efficiency since it is the actual output of the internal combustion engine at the crankshaft. In the following table, the improvement of the mean effective pressures has been presented when we moved from the diesel to the dual-fuel mode (Table 5 and Figures 15-18).

3.4.3
Investigation of the effect of load and fuel ratio on the indicated and brake thermal efficiencies as well as the mechanical efficiency (ITHEFF, BTHEFF, Mecha Eff)  show the evolution of the engine efficiencies in function of the load and fuel ratio. It can be seen first that the engine efficiencies are better in dual fuel than in single fuel. In addition, when the load increases, the engine TA B L E 4 Comparison of engine powers for single-and dual-fuel modes at different loads.

Load
Single efficiencies increase too and globally the engine efficiencies increase too when the volume flow of LPG increases. Table 6 shows the improvement of engine efficiencies when we moved from the single fuel to the dual fuel.

Investigation of the effect of load and fuel ratio on the engine torque and SFC
This section presents the evaluation of the impact of the load and fuel ratio on the engine torque and SFC (see . From those figures, it can be seen that the torque is increasing with the load whatever the ratio. This result is normal since when the load is increasing, the engine becomes heavier than before and more energy is then necessary to move on the vehicle. About the SFC, when the load increases, the SFC decreases whatever the ratio. The decrease in the F I G U R E 19 Brake thermal, indicated thermal, and mechanical efficiencies for different flow volumes of LPG at 3 kg load.

F I G U R E 20
Brake thermal, indicated thermal, and mechanical efficiencies for different flow volumes of LPG at 6 kg load.
SFC is an indicator of good performance, which means also fewer pollutants. This can then strengthen the results gotten before about the pollutants which are decreasing with the increase of load and about the other performance parameters and engine efficiencies. Globally, it can be seen that when the volume flow of LPG is increasing, the evolution of the torque is not constant or standard (it is changing every time) whereas for the SFC up to 3 or 3.5 L/min, the SFC is decreasing before increasing between 4 and 5 L/min. In addition, those results are showing that torque and SFC are better in dual-fuel mode than in single-fuel mode. By moving from Diesel mode to dual-fuel mode, the SFC has decreased by 84% at 3 kg load, 77.5% at 6 kg load, 70% at 9 kg load, and 57.6% at 12 kg load. Concerning the torque, it has increased by 15% at 3 kg load, 0.52% at 6 kg load, 7.2% at 9 kg load, and 10.5% at 12 kg load.

Discussion
Numerous experimental studies of dual-fuel combustion have been conducted either by using as primary fuel natural gas, LPG, hydrogen, alcohols, or a mix of the previously cited fuels. As mentioned previously, in those research, the authors F I G U R E 21 Brake thermal, indicated thermal, and mechanical efficiencies for different flow volumes of LPG at 9 kg load.

F I G U R E 22
Brake thermal, indicated thermal, and mechanical efficiencies for different flow volumes of LPG at 12 kg load.

TA B L E 6
Comparison of engine efficiencies for single-and dual-fuel modes at different loads.

F I G U R E 23
Torque and specific fuel consumption for different volume flow of LPG at 3 kg load.

F I G U R E 24
Torque and specific fuel consumption for different volume flow of LPG at 6 kg load.
did not check the effects of engine parameters on all the engine performance, pollutants, combustion characteristics, and smoke at the same time as it is the case in this work. Among those works, for instance in References 9,11,13,16,17 have worked on the experimental study of dual-fuel combustion using the primary fuels cited above or a mix of them. Reference 17 has studied the effect of the load and mass fraction of LPG on the brake thermal efficiency and the brake indicated mean pressure. Similar to the results presented in this paper, the authors have found the same results which are the increase in the engine performance with the increase of the load and mass fraction of LPG. These results are due to the same assumptions taken (compression ratio constant, injection timing constant, etc.) and the fact that it has been shown in Reference 17 that the LPG composition does not affect the results. Another work where different compositions of LPG have been used, 13 has shown that the smoke decreases when the proportion of gas increases (like in this paper), but at 3 kg and 6 kg of load, emissions, and smoke increase with the increase of the proportion of gas. The result obtained in this work is different. In fact, for a range of loads from 0.5 to 12 kg, the smoke decreases globally with the increase of the proportion of gas. These contradictory results can be explained by the fact that the composition of LPG used in this present paper is different from the one used by Reference 13. They have used either 100%butane +0%propane or F I G U R E 25 Torque and specific fuel consumption for different volume flow of LPG at 9 kg load.

F I G U R E 26
Torque and specific fuel consumption for different volume flow of LPG at 12 kg load.
100%propane+0%butane as their composition of LPG, whereas for this work we have used 50%butane+50%propane. The mixture or not of butane and propane can affect the ignition delay, which can explain the differences observed. References 9,31 have both studied dual-fuel combustion using hydrogen as the primary fuel. Reference 9 obtained the same result as us. The increase of the brake power with the increase of load. This can prove that whatever the primary fuel used, the increase of the load will be responsible for the increase of the brake power, but for the other performance parameters, more investigations should be done when using hydrogen as the primary fuel. Reference 31 compared to Reference 9 did the same work but the %EGR rate was taken into consideration. They have shown that the %EGR rate has no substantial decrement in brake power, but the impact on the pollutants is important (important reduction of pollutants). In this research where the EGR rate has been taken to zero, this previous research can prove that if the %EGR rate is used in our case, the same result can be obtained. The reason for that is the use of EGR decreases the amount of fresh air+gas in the cylinder and then the temperature during the combustion responsible for the formation of many pollutants will be reduced too.

CONCLUSION
In this paper, the effects of the load and the LPG/diesel fuel ratio on the performance, exhaust emissions, and combustion characteristics of a dual-fuel LPG/diesel engine were experimentally investigated and the results were compared with those in single-fuel mode. Based on the experimental results, the following conclusions were taken: • The LPG/diesel dual-fuel concept increases the peaks of in cylinder pressure, net heat release rate and cumulated heat release rate. The average increase in peak of in cylinder pressure is 29.76% and net heat release rate is 78.68% • NO x decreases by 96.2% at 25% of load, 93.6% at 50% of load, 92.3% at 75% of load, and 91.4% at 100% of load when the LPG/diesel dual-fuel concept is used • CO decreases by 94.2% at 25% of load, 85.91% at 50% of load, 87.5% at 75% of load, and 89.8% at 100% of load when the LPG/diesel dual-fuel concept is used • Combustion duration in dual-fuel mode, decreases with an increase in LPG/diesel fuel ratio and load as well • A decrease in combustion duration by means of an increase in load, improves engine performance and efficiencies but increases pollutants (NOx) • A decrease in combustion duration by means of an increase in LPG/diesel fuel ratio, reduces pollutants (NO x and smoke) As perspectives for this work, some improvements can be done. The increase in the number of subsequent cycles can improve the accuracy of the results given by the engine software. In addition, because one of the main reasons for this work at the end is the development of better engine maps for the control part, the consideration of the engine parameters taken constantly during this work (parameters related to the injection, the EGR rate, compression ratio) in addition to the A/F ratio and the load/speed will be essential since those parameters are the control parameters used by the ECU to ensure that the combustion process is good and the driving conditions are better.

FUNDING INFORMATION
This work was funded by the African Union Commission.

CONFLICT OF INTEREST STATEMENT
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

DATA AVAILABILITY STATEMENT
Data is available only on request to the corresponding author. The in-cylinder pressure variation was measured by a pressure transducer (piezo sensor) and the signal sent to the computer data acquisition system. The rate of heat release for each cycle was calculated using the enginesoft software using the following equation where Q, , P, V, and is the heat release, crank angle, cylinder pressure, cylinder volume and specific heat ratio respectively