In order to explore the effects of exhaust gas recirculation (EGR) rates, hydrogen blending ratios, and ignition timings on the combustion and emission performance of a natural gas engine under partial load conditions, combustion and emission characteristic experiments were conducted on a six-cylinder spark ignition natural gas internal combustion engine under different EGR rates, hydrogen blending ratios, and ignition timings. The tests were performed under stoichiometric conditions with the engine speed kept constant at 900 rpm and the throttle opening fixed at 30%. Cylinder pressure signals were collected by a Kistler 6117N piezoelectric sensor, and crankshaft angle signals were measured by a Kistler 2613B crankshaft angle encoder with a minimum counting interval of 0.1 °CA, and 101 cycles were recorded. During the experiment, an oxygen sensor was installed on the exhaust side to monitor the oxygen content in the engine exhaust in real time, transmitting the air-fuel ratio signal to the electronic control unit (ECU), which maintained the excess air coefficient φ_a=1 under different EGR rates and hydrogen blending ratios. The results showed that with increasing EGR rates, the peak values of cylinder pressure, temperature, and total heat release rates decreased. When the ignition timing was set at 18 °CA bTDC, as the EGR rates increased from 0 to 16.3%, the peak values of cylinder pressure, temperature, and heat release rates decreased by 28.4%, 17.7%, and 53.6%, respectively. Conversely, the peak values of cylinder pressure, temperature, and heat release rates increased with increasing hydrogen blending ratios and advanced ignition timings. As the hydrogen blending ratios increased from 0 to 50%, the peak values of cylinder pressure, temperature, and heat release rates increased by 13.1%, 11.3%, and 27.9%, respectively. As the ignition timing advanced from 12 °CA bTDC to 30 °CA bTDC, the peak values of cylinder pressure and temperature increased by 27.5% and 10% respectively. The crankshaft angles corresponding to the peak values of these parameters shifted away from the top dead center (TDC) with increasing EGR rates, and shifted closer to TDC with increasing hydrogen blending ratios and advanced ignition timings. The coefficient of variation of the indicated mean effective pressure (COV_IMEP) showed an opposite trend to cylinder pressure with changes in EGR rates, hydrogen blending ratios, and ignition timings. The brake thermal efficiency of engine increased with increasing EGR rates and hydrogen blending ratios. When the hydrogen ratio was 50%, as the EGR rates increased from 0 to 16.3%, the peak brake thermal efficiency increased from 26.4% to 28.2%. When the EGR rate was 0, as the hydrogen blending ratio increased from 0 to 50%, the peak brake thermal efficiency increased from 24.5% to 26.4%. Within a certain range, the brake thermal efficiency first increased and then decreased with advancing ignition timings. The ignition timings corresponding to the peak brake thermal efficiency shifted away from TDC with increasing EGR rates. When the EGR rates increased from 0 to 16.3%, the corresponding ignition timing advanced from 8 °CA bTDC to 26 °CA bTDC. As the hydrogen blending ratios increased, the ignition timings shifted closer to TDC. When the hydrogen blending ratios increased from 0 to 50%, the ignition timing was delayed from 8 °CA bTDC to 16 °CA bTDC. As the EGR rates increased, the emissions of CO and NO_x decreased, while the emissions of THC increased. When the ignition timing was set at 18 °CA bTDC, as the EGR rates increased from 0 to 16.3%, the CO and NO_x emissions decreased by 32.1% and 88.7%, respectively, while THC emissions increased by 30.7%. With increasing hydrogen blending ratio, the NO_x emissions increased, while CO and THC emissions decreased. When the ignition timing was set at 18 °CA bTDC, as the hydrogen blending ratios increased from 0 to 50%, the NO_x emissions increased by 34.2%, while CO and THC emissions decreased by 14.4% and 28.6%, respectively. The emissions of CO, NO_x, and THC all increased with advancing ignition timings. The research findings can serve as a reference for optimization of hydrogen-blended natural gas engines based on stoichiometric air-fuel ratio.

This study aims to explore the performance and combustion characteristics of a direct-injection hydrogen engine. A 36.8 kW non-road China IV diesel engine was modified to alter its intake manifold and cylinder head. A four-stroke hydrogen engine was then developed to investigate its performance under full-load conditions. A systematic investigation was also implemented to explore the impact of ignition timing on its combustion characteristics and knocking tendency. The hydrogen engine was operated stably to fully meet the requirements of the power output during testing. The throttles were also kept fully open. The optimal ignition timing was selected for each full-load condition. While the engine speed was varied from the idle at 800 r/min to the rated speed at 2 500 r/min. The test results indicated that the torque initially increased and then decreased, as the speed increased within the entire speed range of the hydrogen engine, thus reaching the maximum of 147 N·m at 1 500 r/min. The maximum power of 36.8 kW was achieved at the rated speed of 2 500 r/min. The hydrogen consumption ranged between 90 and 95 g/(kW·h) under specific full-load conditions. The exhaust temperature also rose, as the speed and load increased. The coefficient of variation of the mean indicated pressure (COVIMEP) was highest at the rated power, but still only 5.36%, indicating stable engine operation. At the maximum torque operating point, the ignition timing was varied to analyze its impact on the combustion characteristics of the hydrogen engine. Once the ignition timing was advanced from -7.5 to -8.3 °CA, the combustion centroid was shifted earlier from 6.7 °CA after the top dead center (ATDC) to 6.1 °CA ATDC. The duration of the full combustion decreased from 13.4 to 12.6 °CA, whereas, the peak heat release rate increased from 67.3 to 72.0 J/(°CA). The process was more concentrated closer to the constant-volume combustion. The more intense in-cylinder combustion was then obtained to rapidly increase the in-cylinder pressure and temperature. The fuel combustion rate was accelerated to cause instability in the combustion process between cycles, which further increased the COVIMEP. Additionally, the ignition timing increased the crank angle duration of the in-cylinder temperatures above 1 800 K from 30.7 °CA to 33.1 °CA. The longer durations of the high in-cylinder temperatures were observed to increase the NOx emissions from 11.51 to 12.03 g/(kW·h). The operating condition was selected with the average maximum amplitude of pressure oscillations (MAPO) under external characteristic conditions (1 400 r/min, throttle fully open). The ignition timing was then varied to analyze the effect of the ignition timing on the knock characteristics. The ignition timing increased the average MAPO of the hydrogen engine from 0.015 to 0.020 MPa, and the peak in-cylinder pressure from 5.99 to 6.16 MPa. The individual knocking cycles revealed that the knocking depended mainly on the high in-cylinder temperatures, which induced the spontaneous ignition of unburned mixtures. The ignition timing caused more fuel to combust and release heat before the top dead center. The increasing degree of the constant-volume combustion raised the in-cylinder temperatures and pressures. The increasing tendency was found for the spontaneous ignition of the unburned mixtures, thereby increasing the knocking tendency of the hydrogen engine.