Effect of Turbocharger Compression Ratio on Performance of the Spark-Ignition Internal Combustion Engine

Internal Combustion Engines (ICE) are one of the most important engineering applications that operate based on the conversion of chemical energy from fuel into thermal energy as a result of direct combustion. The obtained thermal energy is then turned into kinetic energy to derive various means of transportation, such as marine, air, and land vehicles. The efficiency of ICE today is considered in the range of the intermediate level, and various improvements are being made to enhance its efficiency. The turbocharger can support the ICE, which works by increasing the pressure in the engine to enhance its efficiency. In this investigation, the effect of the turbocharger pressure on ICE performance was studied in the range of 2 to 10 bar. It was found that the increase in turbocharger pressure enhanced the pressure inside the engine, positively affecting engine efficiency indicators. Therefore, the increase in turbocharger pressure is directly proportional to the ICE efficiency. validation, S.S.A., A.M.A. and T.K.M.; formal analysis, S.S.A.; investigation, S.S.A.; resources, A.M.A.; data curation, A.M.A.; writing—original draft preparation, S.S.A.; writing—review and editing, A.M.A.; visualization S.S.A.; supervision, S.S.A.; project administration, S.S.A.; funding acquisition, S.S.A.

on the place of fuel-burning, combustion engines are classified into two types: external and internal combustion engines. The former is the oldest type and depends on completing the process of burning the fuel in a specific area and then transferring energy to be used in another place [31]. While the latter relies on completing the burning of the fuel process and using it in the same place. The internal combustion engine is the latest and most widely used type. Several advantages such as simplicity of mechanics, higher power per working cycle, low cost, and efficiency make the internal combustion engine the best candidate [32][33][34].
Numerous studies have addressed the development of combustion engines, especially the internal type. Jafari et al. (2020) [35] developed an active thermo-atmospheric combustion concept. Negoro et al. (2013) [36] tried to improve Toyota cars by controlling the combustion toward an auto-ignition to ensure the stabilization inside the internal combustion engines. Doppalapudi et al. (2021) [37] analyzed the effect of thermal stress on some parts of the internal combustion engines such as the piston, connecting rods, and pins; they found that the pressure on the pistons of the engine plays a major role in transforming the engine load from the combustion chamber which lies inside the cylinder to the crankshaft of the engine through the connecting rod.
Concerning the development of the engines, 1862 Alphonse described an efficient four-stroke cycle engine and the connection between the operating conditions through investigating the ignition system with high pressure during the compression cycle and the relation between the compression cycles by increasing the volume of the burned mixture [38]. Numerous works addressed the design and improvement of internal combustion engines to improve their efficiency and performance. Examples of improvements are camshaft improvements, fuel injection system improvements, and improvements to the design of the combustion chambers. The cylinder balance is the main factor in the reduction of the torsional vibrations that comes from the unbalanced cylinders in the engines. In this frame, the injectors of the fuel cylinder are automated to control the injection times, through controlling the dynamic loads which aid in a uniform distribution of the mechanical strain all over the internal combustion engine parts [39][40][41].
Fuel injection systems are considered one of the most important support systems in internal combustion engines [42][43][44]. These systems have faced many developments and improvements. The first engines had direct-flow and traditional injection systems which were then developed into systems with direct and indirect injection systems to ensure supplying the sufficient amount of fuel to run the engines at the highest efficiency and fuel economy. In this work, the performance of the spark-ignition internal combustion engines is investigated based on a turbocharger with different pressure compression ratios starting from 2 to 10 bars. The investigation is aimed to assess the correlation between the pressure ratio of the turbocharger and the ICE performance. Figure 1 indicates the basics of the engine cylinder in which combustion takes place. The energy conversion process starts due to the linear motion that results from combustion inside it. The engine cylinder includes a piston, connecting rods, crank, and crankshaft. The process of the energy conversion starts from the fuel injection point to the cylinder where the piston begins to move from the top dead center (TDC) to the bottom dead center (BDC) what is known as the intake stroke followed by the compression process which is a reversible motion of the piston from BDC to TDC. With increasing the pressure, the spark comes to make the firing to let the piston expand from the TDC to BDC in what is known as the expansion stroke. Finally, the piston comes to its origin to start over again with what is known as exhaust stroke. All these steps complete the conversion process of the energy [26].

Figure 1. The basics of the engine cylinder
The geometry of the piston, connecting rods, and the crank is shown in Figure 2 with some dimensions indicating the geometry of the energy conversion of the engine. (1) (2) where, Ø is angle of the connecting rod relative to the center line of the cylinder, is crank angle from the TDC position, a is crank radius, and is connecting rod length.
The distance s between the crank axis and the wrist pin axis can be determined by: The piston displacement from the TDC position is given by: Instantaneous piston speed: Mean piston speed: The power developed inside the cylinder of the engine is called indicated power. While the actual power available from the crankshaft is called the break power. The frictional power is the difference between the indicated and break powers. Figure 3 shows the work algorithm, which started from determining the type of engine (an internal combustion engine) that works by a spark. Then, the effect of different pressure ratios (2-10 bar at 2-bar intervals) was assessed by using a turbocharger. The efficiency parameters and the effect of increasing the pressure inside the engine were also explored. The work methodology of this investigation relied on the selection of the internal combustion engine operating with spark. The turbocharger served as an auxiliary factor. The work began by changing the supplied pressure ratio of the engine using a turbocharger (2-10 bar); then, a wide range of performance indicators was studied, including the engine power, friction power, fuel consumption rate, temperature, and pressure. At the end of each stage, an extensive comparison was made based on every new compression ratio. The relationship between the compression ratio supplied by the turbocharger to the engine and the engine efficiency was also extracted as the final result.

3-Results and Discussion
The selected engine of this study was a four-stroke SI engine with injection inlet port valves. The engine is an in-line engine with 4 cylinders cooled by liquid water. The bore diameter is 150 mm, the piston stroke is 180 mm, and the nominal speed rate is 3000 rpm with a compression ratio of 15. The ambient condition of this study involved the atmospheric pressure of 1 bar and temperature of 288 K. This investigation is based on changing the compressor pressure ratio of the turbocharger from 2 to 10 at 2-bar intervals. The performance parameter of the engine was studied and recorded at each time the turbocharger compressor pressure ratio was changed. Table 1 shows the parameters of the efficiency and power of the investigated engine with the different turbocharger compressor pressure ratios. The engine speed was fixed at 3000 rpm while the turbocharger compressor pressure ratios were varied from 2 to 10 bar. The piston engine power increased with enhancing the turbocharger compressor pressure ratio (TCPR) as well as the brake mean effective pressure and the brake torque. The previous values of the listed parameters were 603 kW, 18.95 bar, and 1919.6 N.m at the 2 TCPR and 2655.6 kW, 83.5 bar, and 8453.7 N.m at the 10 TCPR. Figure 4 shows the three types of engine efficiency: piston engine efficiency, indicated efficiency, and mechanical efficiency of the piston engine. The mechanical efficiency of the piston engine increased with the raising TCPR; while the indicated efficiency remained approximately constant. There was a slight increment in the efficiency of the piston engine with the increase of the TCPR. Overall the engine performance was affected by the increase of the TCPR.   Table 2 presents the turbocharger and exchange parameters of the investigated engine. As seen, the volumetric efficiency of the turbocharger remained constant with the increase of the turbocharger compressor pressure ratio. The turbocharger efficiency, however, increased from 0.55 for 2 TCPR to 0.6 for 10 TCPR. The total air-fuel equivalence ratio and the total fuel-air equivalence ratio remained unchanged with the increase of the TCPR as well as the coefficient of the scavenging and the burnt gas fraction backflow into the intake percent. Figure 5 depicts the pressure of the turbocharger before the inlet manifold and the average total turbine inlet pressure. Both the mentioned pressures increased with the enhancing the TCPR. But, the pressure before the manifold showed a sharp increment with the increase of the TCPR. The engine performance was affected by increasing the TCPR as the engine temperature was elevated from 309.3 ℃ to 375.7 ℃ with the increase of the TCPR from 2 to 10 bar. The temperature elevation raised the total mass airflow to the piston from 0.66 to 2.8 m-air when the TCPR rose. The scavenging coefficient remained constant with increasing the TCPR as well as the total air-fuel equivalence of piston engine and the total fuel-air equivalence ratio.   Table 3 shows the intake and exhaust parameters of the investigated engine. The average exhaust manifold temperature was higher than the average intake manifold temperature. Both of them rose with increasing the TCPR. The average gas velocity at the exhaust was higher than the intake stroke; both of which incremented with the increase of the TCPR. The rest of the exhaust parameters were higher than the intake stroke and all increased with the elevation of the TCPR. The only opposing parameter was the total effective valve port throat area which was higher in the intake stroke as compared with the exhaust stroke. It, however, rose with the increase of the TCPR. Figure 6 shows the average manifold pressure of the intake and exhaust strokes. Accordingly, the average manifold pressure of the intake stroke was higher than the average pressure of the average exhaust manifold pressure and both of them increased with the increment of the TCPR. The results of the intake system show that the total effective valve port throat area remained constant with the increase of the TCPR; the same trend held for the max velocity in the middle section of the inlet port and the average gas velocity. The rest of the parameters exhibited an incremental pattern with increasing the TCPR.    Table 4 shows the parameters of the combustion and the heat exchange in the engine cylinder. As TCPR was increased, the air-fuel and fuel-air ratios remained constant during the combustion process. The combustion pressure and the temperature increased by enhancing the TCPR. The ignition timing, as well as the combustion duration, also remained constant during the combustion process as the TCPR was incremented. The rest of the combustion process parameters rose with elevating TCPR. The heat exchange in the engine cylinder was enhanced by the increase of the TCPR. Based on the enhancement of all the heat exchange parameters in Table. 4, the Boiling Temp. in Liquid Cooling System and Average Factor of Heat Transfer from head cooled surface to coolant remained constant when TCPR was enhanced in the heat exchange process. Table 5 lists the compressor parameters of the studied engine. The parameters showed an enhancement in the engine's performance upon increasing the TCPR. Noteworthy, the adiabatic efficiency of the high-pressure compressor, the thermal efficiency of the high-pressure air inter-cooler, and the inlet total temperature of the high-pressure compressor remained constant by increasing the TCPR.   Based on the results, a rise in the turbocharger-supplied pressure ratio of the engine significantly enhanced the engine efficiency. There is congruence with a large group of researchers around the world. For example, Wang et al. (2022) [45] explored the effect and load demand of the Miller cycle based on using a low-speed marine engine and turbocharger and found that the use of a turbocharger with different designs and pressures can influence the engine performance. Lu et al. (2022) [46] conducted a parametric investigation into a marine diesel engine with a recirculation exhaust gas and turbocharger system. They expressed the major role of the recirculation and turbocharger systems in the performance of the engine. Other researchers shared the same idea about the use of the turbocharger to enhance engine performance [47][48][49].

4-Conclusion
This study was aimed at modifying the performance of the internal combustion engine by changing the turbocharger compressor pressure ratio and examining the parameters and criteria that can reverse the performance of the engine depending on the intake and exhaust strokes as well as the combustion with the heat transfer parameters. The findings revealed that all the performance parameters were positively affected by increasing the turbocharger compressor pressure ratio as the performance of the engine was enhanced by raising the TCPR. The turbocharger efficiency also exhibited an enhancement with the increase of the TCPR as it increased from 0.55 to 0.6 with the increase of the TCPR from 2 to 10. The pressure before the inlet manifold was enhanced from 2 to 10 bars with an equivalent increment in the TCPR. One of the important factors affected by the increase in TCPR is the emission of NOx as Wet NOx or NOx which is converted to NO. Both NOx types were decreased by the increasing TCPR. The thermal efficiency of the high-pressure air inlet cooler of the compressor remained constant (0.75) upon the increase of the TCPR. This study can contribute to enhancing the performance of the internal combustion engines by improving the engine design to face the new needs.

5-2-Data Availability Statement
Data presented in this study are available in article.

5-3-Funding and Acknowledgements
I gratefully acknowledge the funding of the project (428/2021) by the deanship of academic research -Mutah University. This work is done by the researcher under Mutah University regulation and policy.

5-4-Conflicts of Interest
The authors declare that there is no conflict of interests regarding the publication of this manuscript. In addition, the ethical issues, including plagiarism, informed consent, misconduct, data fabrication and/or falsification, double publication and/or submission, and redundancies have been completely observed by the authors.