The impact of compression ratio on performance and exhaust emissions in a 100 cc spark ignition engine for green technology

The compression ratio has a significant role in vehicle performance. The arrangement of air entering and leaving the combustion chamber is determined by the design of the air inlet and outlet locations. The design of the combustion chamber is crucial to prevent backflow in the remaining combustion air. Backpressure is the emission of gases flowing back into the combustion chamber, causing non-stoichiometric combustion. The purpose of this research is to find the effect of compression ratio on performance and exhaust emissions of motorcycles, especially spark ignition engines. The research procedure begins with a standard piston modification process to get compression ratios of 9:1, 10:1, 11:1, and 12:1. In order to get uniform weight, pistons with low compression ratios will be equipped with holes at the bottom. Experimental data taken were; dynamometer, gas analyzer, and SFC. An increase in compression ratio causes an increase in vehicle performance in the form of torque and power. The air-fuel mixture is compressed to a smaller volume, leading to increased density. This increased density promotes better flame propagation and faster combustion. When the combustion process is faster and more efficient, there is less time for unburned hydrocarbons to be released into the exhaust gases.


Introduction
Motorcycles are classified in the spark ignition engine (SIE) category, where the combustion process occurs due to the ignition produced by the spark plug.SIE machines are commonly used in society due to the compact Size and Weight Reduction.SIE vehicle performance is determined by several parameters: Air-Fuel Ratio, Ignition Timing, Combustion Chamber Design, and Compression Ratio.The air-fuel ratio is the ratio between air and fuel (AFR) that enters the combustion chamber.The fuel mixture is classified into rich, lean, and Stoichiometric-ratio.For SIE engines, the AFR Stoichiometric ratio is 14.7:1.The lean combustion may lead to increased combustion temperatures, potentially affecting engine durability and producing higher nitrogen oxide (NOx) emissions [1].The rich AFR can provide enhanced power output and cooling effects.However, this condition results in incomplete combustion and higher emissions of unburned hydrocarbons (HC) and carbon monoxide (CO).Meanwhile, Ignition timing has a critical role in the combustion process of internal combustion engines, including determining: Peak Cylinder Pressure, Efficiency, Emissions Load, and RPM of the engine.Opening the inlet and exhaust valves at the wrong time can cause misfires or incomplete combustion, which is characterized by the appearance of knocking [2].The determination of ignition timing is closely related to the combustion chamber design and compression ratio.The compression ratio is determined by the total cylinder volume when the piston is at the bottom dead centre (BDC) compared to the total volume when the piston is at the top dead centre (TDC).Ignition timing must be adjusted so that the spark plug ignites when the piston is at TDC, and the compressed air is at high pressure [3].The arrangement of air entering and leaving the combustion chamber is determined by the design of the air inlet and outlet locations.The design of the combustion chamber is crucial to prevent backflow in the remaining combustion air [4].Backpressure is the emission of gases flowing back into the combustion chamber, causing non-stoichiometric combustion [5].Injection timing and AFR settings are being made adaptively using the Engine Control Unit (ECU).The ECU functions to monitor and diagnose ignition in the combustion chamber.Thus, the combustion process can produce optimum speed with low emission [6] & [7].
The compression ratio has a significant role in vehicle performance.The size of the vehicle's compression ratio is determined during the combustion chamber design process as the basis for the manufacturing process.As previously explained, the compression ratio is the ratio between the compressed fuel volume and the combustion chamber's total volume.The compression ratio of the vehicle is adjusted to the engine's ignition timing [8].Determination of the compression ratio must also consider several things, such as Piston Design and Material, Cylinder Head and Gasket, Fuel Octane Rating, and Cooling System.High compression ratios engine increases pressure and heat within the combustion chamber.Therefore, the piston material should have sufficient strength and durability to withstand these higher stresses [9].The piston base material must resist high heat to prevent drastic changes in physical size due to thermal expansion.The outside of the piston is coated with thermal barrier coatings (TBC) materials such as Yttria stabilized zirconia (YSZ), Mullite, Alumina (Al2O3), AlSi, and NiCrAl.The use of TBC material outside the piston serves to reduce engine heat loss, thereby reducing the percentage of unburned fuel [10].It is important to recognize that excessive porosity levels can diminish the durability of the coating and may require a means to seal off the open surface pores.The proprietary solution precursor plasma spray (SPPS) process enables the discrete organization of the porosity structure into layers, forming inter-pass boundaries and producing reduced thermal conductivity [11].The piston manufacturing process used in Spark Ignition (SI) considers weight, geometry: porosity, bowl head size, length, and diameter.Porous medium combustion over free-fame combustion increases burning speed, reduces lean mixture flammability restrictions, and reduces air pollutant emissions.Due to the inherent properties of the porous medium structure (for example, surface area per unit volume), the radiation heat transfer mechanism between the solid design and the reacting gas significantly increases the overall heat transfer rate.The flammability, stability limits, and ranges of porous medium combustion contradict those in conventional systems.Through external heat recirculation, the porous medium can use the energy lost from downstream products of fame to preheat the fresh mixture upstream.As a result, fame stability can be enhanced, and the ability limit increases.The porous medium can be considered a heat recovery element, heating either the reactants or the combustion air directly with the heat rooted in the fame zone [12].Porosity material is added near the TDC or piston head on SI engines to speed up the mixing of fuel and air in the combustion chamber.In addition, the porous medium can be applied near the injection valve on premixed SI machines.
The piston used in the SI combustion process should be lightweight with a compact design.Reducing piston weight by 13% can increase power by up to 35.9% [13].Piston shape optimization can be simulated using Finite element analysis (FEA) dynamically or statically.In general, geometry reduction can be done on the skirt piston [14].However, the piston's weight can be reduced at the crown to get a bowl shape.In addition to reducing the weight of the piston, the bowl shape on the crown can also speed up the process of mixing fuel and water, and burning occurs evenly in the combustion chamber [15].But the depth of the bowl needs to be adequately calculated and adjusted to the angle of the fuel injectors.The bowl on the piston crown that is too deep causes the injected fuel to miss the bowl and will flow directly into the exhaust valve [16].The shape of the crown piston is classified into flat, dome, and dish shapes, as shown in Figure 1.The diameter and length of the piston affect the combustion duration, Velocity of the Air-Fuel Mixture, and Torque Characteristics.These two components are determined at the start of the piston manufacturing design, related to other engine components, such as the crankshaft design, connecting rod length, etc. Changes in piston length cause changes in piston weight when applied to engines with the same bore diameter.Several studies on the effect of the compression ratio on motorbikes have been carried out, among others, by researchers [15] & [16] by varying the shape of the piston head and increasing the cylinder volume.However, there are still a few researchers who vary the piston length with the same bore diameter to obtain the compression ratio.Several methods that are often used include numerical simulation methods with CFD and experimental methods.These two methods, both numerical simulation with CFD [17] and experimental methods [18], have their respective advantages.This research was conducted experimentally to determine the impact of changing piston length with the same bore diameter on vehicle performance.

Methodology
The research was carried out experimentally by varying the compression ratio of the piston spark ignition engine (SIE).The research procedure begins with a standard piston modification process.Compression ratio variation is done by cutting the crown piston with a dome shape.The pistons used have compression ratios of 9:1, 10:1, 11:1, and 12 :1.All pistons weigh the same 70 gr.In order to get uniform weight, pistons with low compression ratios will be equipped with holes at the bottom.After the piston is punched, it will be weighed to ensure that the weight of each piston is the same.The piston will be installed in the combustion chamber of a standard motorized vehicle with a cylinder volume of 97 cc.Experimental data taken were: dynamometer, gas analyser, and SFC.SFC retrieval is done by recording the initial position of the fuel.Then the vehicle is run until the engine consumes 10 ml.Calculation of fuel consumption time is done by using a stopwatch.Furthermore, the vehicle performance test is carried out using a dynamometer test.Data is collected when the machine runs for 5 minutes with a steady rotation.The test is carried out at idle when the gear is in neutral.Further testing was carried out at idle rpm at 3000 and 5000 rpm.Retrieval of dynamometer data is described in previous study [19].Emission testing is carried out with a gas analyser to determine the amount of CO and HC from the exhaust manifold.

Result and Discussions
Preliminary research was conducted by comparing the performance of vehicles.The relationship between engine speed and engine power at 9:1 compression is shown in Figure 3.The graph shows the change in vehicle power at engine speed from 3000 rpm to 7000 rpm.The dynamometer test results show that the vehicle power has increased along with the increase in vehicle speed.An increase in the throttle valve inlet opening causes fuel to enter the combustion chamber.Thus, there is an increase in the air-fuel ratio in the combustion chamber, creating combustion conditions that approach stoichiometric conditions.Increasing the fuel entering the combustion chamber only sometimes increases vehicle performance.In this test, an increase in fuel mass flow causes an increase in vehicle power until it reaches the optimum point at 7000 rpm.Moreover, the vehicle performance will decrease due to too high AFR in the combustion chamber.Rich combustion conditions in the engine can prevent the incoming fuel from burning optimally.This condition can cause an increase in combustion emissions.

Figure 3. The Graphic of engine rotation and engine power at 9:1 compression compared with
previous study [19].Testing continued to compare vehicle performance by varying the number of vehicle compression ratios at 9:1, 10:1, 11:1, and 12 :1.Vehicle performance is expressed in power and torque graphs as a function of the increase in RPM shown in Figure 4.An increase in speed causes an increase in the power of the vehicle.This condition is caused by increased throttle opening when the vehicle is accelerated.The combustion process in the combustion chamber causes the AFR in the combustion chamber to approach stoichiometric conditions, increasing engine power.The increase in power occurs in all compression ratio variations until the vehicle reaches 7000 rpm.In the graph of power as a function of angular speed shown in Figure 4.a, an increase in compression ratio causes an increase in vehicle power.In this study, the pistons have the same mass despite having different compression ratios.The highest power is produced by a vehicle equipped with a piston with a compression ratio of 12: 1 of 8 hp.A higher compression ratio allows the engine to extract more functional work from combustion.It results in better combustion efficiency, which means a more significant proportion of the energy released by the burning fuel is converted into useful work instead of being wasted as heat.Improved thermal efficiency leads to better fuel economy.Figure 4.b shows a graph of the change in torque of a spark ignition engine vehicle against a function of time.The pressure inside the cylinder increases when the air-fuel mixture is compressed to a smaller volume.This higher pressure leads to a more aggressive expansion of the burning gases during combustion.The improved pressure results in a more significant push on the piston, generating higher torque output.This condition is to the effects of research, which show that vehicles with a compression ratio variation of 12: 1 produces the most increased torque.Differences in compression ratio cause drastic changes in HC emission levels.An increase in the vehicle compression ratio causes a decrease in vehicle HC levels.The air-fuel mixture is compressed to a smaller volume, leading to increased density.This increased density promotes better flame propagation and faster combustion.When the combustion process is faster and more efficient, there is less time for unburned hydrocarbons to be released into the exhaust gases.A higher compression ratio allows for better air-fuel mixing and more uniform combustion.The higher compression leads to stochiometric fuel burning, minimizing the formation of unburned hydrocarbon molecules [20] [21].When the fuel is thoroughly combusted, there are fewer HC emissions produced.

Conclusions
Modifications in the design and size of the combustion chamber cause changes in the chemical reaction of the combustion process.Research conducted with variations in compression ratios 9:1, 10:1, 11:1, and 12:1 shows that an increase in compression ratio causes an increase in vehicle performance in the form of torque and power.In addition, an increase in compression ratio causes a decrease in vehicle HC emission levels.A higher compression ratio allows for better air-fuel mixing and more uniform combustion.This condition causes an increase in the quality of combustion in the spark ignition engine.The limitation of this research is that a larger compression ratio cannot be done due to equipment limitations.

Figure 1 .
Figure 1.The design of crown piston.

Figure 4 .
Figure 4.The Graphic of (a) engine power and (torque) as the function of engine rotation at various compression ratio.Modifications in the design of the combustion chamber cause changes in the combustion reaction in the engine.This condition causes changes in the levels of emissions discharged into the environment.Emissions produced by combustion engines generally consist of HC, CO, and CO2.The results of the vehicle emission from the gas analysis test at various compression ratio variations are shown in Figure 5. Changes in vehicle compression ratios cause slight changes in vehicle CO and CO2 emission levels.Differences in compression ratio cause drastic changes in HC emission levels.An increase in the vehicle compression ratio causes a decrease in vehicle HC levels.The air-fuel mixture is compressed to a smaller volume, leading to increased density.This increased density promotes better flame propagation and faster combustion.When the combustion process is faster and more efficient, there is less time for unburned hydrocarbons to be released into the exhaust gases.A higher compression ratio allows for better air-fuel mixing and more uniform combustion.The higher compression leads to stochiometric fuel burning, minimizing the formation of unburned hydrocarbon molecules[20][21].When the fuel is thoroughly combusted, there are fewer HC emissions produced.

Figure 5 .
Figure 5.The Graphic residual gas as the function of angular velocity at various compression ratio and engine rotation.