The effects of different combustion chamber geometries on the performance and emissions of a internal combustion engine

Internal combustion engines are systems that convert the chemical energy contained in fuel to mechanical energy. This energy is revealed by burning the fuel in the engine. Internal combustion piston engines are basically divided into two classes. These are spark ignition engines (these engines can be called Otto engines or gasoline engines) and compression ignition engines (these engines can also be called diesel engines). Internal combustion piston engines are widely used in land, sea and air transport and other power generation systems. Today, studies on internal combustion engines are generally aimed at improving engine fuel consumption and reducing harmful exhaust emissions. Improving performance and reducing harmful exhaust emissions in spark-ignition engines depend on the combustion process. In the combustion process, chemical and physical variables are very important. Physical variables in internal combustion engines such as extreme turbulence affect combustion more than chemical variables. Rapid combustion in spark-ignition engines expands its stable operating conditions so that engine can run at different air fuel ratios. Also, increasing the burning speed allows the engine to use a higher compression ratio by reducing the knock tendency. The burning rate depends mainly on the air motion in the cylinder. Air motion in the cylinder is created by geometry of the intake manifold and combustion chamber. In spark-ignition engines, the effective efficiency reaches up to 35% in the full load regime whereas this value can drop to 10% in the partial load regime. The biggest cause of losses in the partial load regime is flow losses due to throttle valve. Alternative solutions are being studied to reduce these losses. Flow losses can be reduced at stable operation of the engine with poor mixtures. These losses can be reduced partially if the engine operates stably with poor mixtures. In this study, the effect of the combustion chamber geometry, which is one of the important parameters affecting the performance and emissions of the internal combustion engines, was experimentally investigated using different combustion chamber geometries. Three different types of combustion chamber geometries were designed with different compression ratios. Two of the selected combustion chamber types are flat and SO (cylindrical bowl) geometries used in spark ignition engines. In addition to these geometries, The MR type combustion chamber geometry proposed by Prof. Dr. Rafig Mehdiyev has been tested. Increasing the turbulence level is expected from the MR type combustion chamber geometry thus the engine is able to operate more stable with poor mixtures. By increasing the burning speed with the help of turbulence, the engine will run without knock at higher compression ratios. The MR combustion chamber geometry was produced at three different compression ratios (10.5, 12 and 14). The flat combustion chamber geometry was produced only for the lowest compression ratio of 10.5, since the combustion rate is expected to be low. In the SO type combustion chamber geometry, there is a bowl in the middle of the piston. For this reason, squish velocities will occur in this combustion chamber geometry. As burning rates are expected to be increased by air motion, they are produced at compression ratios of 10.5 and 12. Experiments were performed a single cylinder water cooled engine converted from compression ignition to spark ignition. In the experiment engine, fuel is injected to intake port. Injection and ignition system is controlled by an electronic control unit designed as a part of a master thesis. The experiments were tested at three different engine speed (1500, 2000 and 2500 rpm), different loads (1, 3, 5 and 7 bar for 1500 and 2000 rpm and full throttle for 2500 rpm) and different excess air coefficients (1.00, 1.20, 1.40 and 1.60 for 1500 and 2000 rpm and 1.00, 1.10, 1.20, 1.30, 1.45, 1.60 and 1.70 for 2500 rpm). Cylinder pressure, intake and exhaust manifold pressures were collected at each test point by the pressure collection system. Fuel consumption, engine power, exhaust emission values, average temperature and pressures were recorded during 90 seconds for each test point with the data acquisition system of the engine testing laboratory. Totally 200 experiments were carried out for all pistons. Experimental results were analyzed in three different sections. These are the basic parameters of the engine such as fuel consumption, torque and emission values; the indicated values such as in-cylinder pressure, amount of heat released and duration of combustion; finally, the results of the theoretical model to investigate the burn speed and the turbulence coefficient. When the test results are investigated, the NO emissions decrease significantly after a certain value of excess air coefficient (HFK) while the stoichiometric mixture goes to the poor mixture, as expected. If the excess air coefficient was changed from 1.00 to 1.60, the NO value decreased about 80%. When combustion chamber geometries compared to each other for the same compression ratio, NO emissions value of MR combustion chamber geometry are less than other geometries, because it has lower optimum ignition advance than others. A theoretical model has been developed to explain the smaller ignition advance of MR geometry. The change of HC emissions with HFK is suitable for general literature results. As expected, HC emissions increased in very poor mixtures due to the incomplete combustion. HC emissions increased by about 60% at λ = 1.6 compared to the stoichiometric condition. A notable point about HC emissions is that the flat piston produces the lowest HC values especially in the full load regime. The design that obtains desired compression ratio can be shown as the reason for this. The crevice volume of the flat piston is much less than the other pistons. In extreme poor mixtures it was seen that this property was lost. This is due to the fact that this combustion chamber is unable to operate in poor mixtures because this geometry has low burn rate value. The CO emission is highly dependent on the air excess coefficient so negligible difference has occurred between the combustion chamber geometries. The fuel consumption values should be examined as well as the emission values to select suitable geometry. Increased air excess coefficient at constant mean effective pressure levels reduced specific fuel consumption values. The greatest effect here is the rise of volumetric efficiency, since the throttle valve was opened to achieve more poor mixture. According to the basic idea of the dissertation, the geometry of the flat combustion chamber with few turbulence movements was not stable at the values greater than 1.40 and so the specific fuel consumption values were badly influenced unlike the other pistons. If the specific fuel consumption values are investigated according to the compression ratio, the combustion chamber with compression ratio of 12 is preferred. In second step, the indication values were compared. Between the combustion chamber geometries which have the same compression ratios, the highest pressure value is obtained with the MR type. Also, the MR type combustion chamber geometry has the highest pressure rise rate. The pressure rise rate of SO type combustion chamber geometry is also found to be higher than flat type combustion chamber. The pressure rise rate is related to the burning rate. The highest pressure values rise with the increase of the compression ratio. No significant change occurred in pressure rise rates. The maximum pressure value and pressure rise rate decrease with the increase of excess air coefficient. Also, burning speeds decrease with the increase of excess air coefficient. Heat release rate results are similar to pressure rise rates. The maximum heat release rate is obtained for the MR type combustion chamber geometry. The value of the SO type combustion chamber geometry is greater than that of the flat type combustion chamber. Since heat release rate depends on the burning rate, it has been seen that the combustion speed of the flat combustion chamber geometry is lower than the other geometries. The heat release rate does not change significantly with the increase of compression ratio. The heat release rate decreases with the increase of excess air coefficients. Cycle to cycle variations are an important parameter for comparison of chamber geometries. It makes it easier to evaluate the stability of the engine operation. Cycle to cycle variations are investigated by comparing the COV (Coefficient of Variation) values of maximum pressure and indicated mean effective pressure. COV values of maximum pressure for 10.5 compression ratio are obtained 24% for flat, 7% for SO and 4% for MR type geometries at 3 bar of load and 1,00 value of excess air coefficient. Cycle to cycle variations are decreased with the increase of load. COV values of all geometries are increased with the growing of excess air coefficient. The maximum pressure COV values change from 6.9 to 15.8%, from 4.3 to 13.8% and from 3.8 to 8.1% for flat, SO and MR type geometries, respectively with the changing of excess air coefficient from 1.00 to 1.40 at 7 bar of mean effective pressure. The COV values of flat type combustion chamber geometry are obtained as maximum for all loads. COV values of MR type combustion chamber geometry do not increase greatly for higher values of excess air coefficient; since turbulence occurred in MR type chamber increased burn ratio. The obtained results show that the MR type combustion chamber geometry has lower COV values than the other geometries. COV values of MR combustion chamber geometry with 14 compression ratio is increased at high loads and stoichiometric mixture. A thermodynamic model was developed for the investigation of the results. MR type combustion chamber geometry has the highest burning volume growth rates. When the flame velocities, which were calculated from burning volume sizes, were compared, MR type has the highest flame speed. The MR type combustion chamber geometry has the highest turbulence coefficients. The MR type combustion chamber geometry reduces cyclic differences because it produces higher turbulence. Therefore, the engine is able to run stably at higher excess air coefficient and reduces NO emissions.