Simulation Study On Combustion Characteristics Of HCNG Engine Based On Internal EGR

With the rapid development of automobile industry, the energy crisis and vehicle pollution problem increase more and more serious. Hydrogen enriched compressed natural gas(HCNG) is one of the ideal alternative fuels, which can effectively relieve these problems. And exhaust gas recirculation(EGR) is one of the effective ways to reduce NOx emissions, which can effectively reduce NOx emissions of HCNG engine. The internal EGR can be achieved by negative valve overlap(NVO), which is more effective strategy for mixture dilution than external EGR.In this paper, a 1-D model of the engine performance and a 3-D model of in-cylinder working process for HCNG engine were established. The effects of symmetrical NVO and asymmetrical NVO on EGR in cylinder and gas exchange process were analyzed. Under the symmetrical NVO strategy, the engine working processes were studied, including the flow field characteristics in cylinder of intake and compression process and the combustion characteristics, ignition characteristics, temperature distribution rule, NO emission characteristics of combustion process.The simulation results show that, when the symmetrical NVO increases, the EGR rate increases and the volumetric efficiency decreases, and when the symmetrical NVO increases from 20°CA to 100°CA, the EGR rate increases from 8.6% to 20.6%, the volumetric efficiency decreases from 0.662 to 0.512. Under the asymmetrical NVO strategy, the EGR rate and volumetric efficiency are primarily affected by the exhaust valve closure timing(EVC) and the intake valve open event(IVO) has slight effect on the EGR rate and volumetric efficiency. For the internal EGR achieved by NVO, symmetrical NVO can obtain higher volumetric efficiency and engine power without turning exhaust back into the intake manifold.Under the symmetrical NVO strategy, the gas in cylinder flows quickly and forms vortexes at the beginning of air intake, and this flow slows after the maximum intake valve lift. The EGR concentration appeares gradient distribution at the intake valve closing, which has higher concentration in the center and the bottom of the cylinder. At the late of the compression stroke the temperature and EGR concentration are lower and HCNG concentration is higher near the spark plug location, which is beneficial to fire and flam spread. The maximum rate of pressure rise, maximum pressure and maximum temperature reduces by using NVO, but the engine power is reduced. The flame leans to the side close to intake valve by the gas flow at the flame spreading process. Increasing EGR rate not only make it difficult to fire, but also decrease combustion rate. Enhancing the ignition energy can shorten the ignition delay and combustion duration and accelerate combustion rate.The distribution of NO concentration is consistent with the distribution of temperature, and the temperature and NO concentration decreases with greater EGR rate.Based on engine 1-D simulation platform obtaines that the NVO=40°CA strategy is determined as the best compromised scheme from the symmetrical NVO were 20, 40, 60, 80 and 100°CA by simulation calculations and analysis. The operating range of HCNG engine are predicted under the NVO strategy, and the loss of power and economy are restored by optimizing the intake pressure.