Numerical Investigation On Combustion Characteristics And Vehicle Fuel Economy Performance Of Hydrogen-enriched Engines

With the vast consumption of fossil fuels and increasingly severe air pollution, energy saving and emissions reduction have become an inevitable development trend for future internal combustion engines. Hydrogen possesses many desirable physicochemical properties, ensuring that the enrichment of hydrogen to fuels such as gasoline, methanol and ethanol could effectively enhance the combustion process in internal combustion engines. However, there is still limited in-depth and systematic researches on the numerical modeling of combustion characteristics and vehicle fuel economy performance for hydrogen-enriched engines. The lack of numerical tools not only restraints developing an in-depth knowledge on the engine performance improvement mechanism after hydrogen enrichment, but also makes the configuration design and hydrogen fueling strategy optimization difficult for hydrogen-enriched fuel engines. Thus, this paper performed numerical modeling and simulation study on the in-cylinder turbulent combustion characteristics and vehicle fuel economy performance of hydrogen-enriched engines through theoretical analysis, numerical calculation, and experimental measurement.The laminar flame speed is a basic fuel property for developing turbulent combustion models. It was found that the contribution ratio for adiabatic flame temperature dominates the laminar flame speed of hydrogen enriched flames. Based on this observation, a theoretical model for determining the laminar flame speed of hydrogen-enriched fuels covering the thermal conditions encountered in engine combustion was developed, and laminar flame speed correlations for hydrogen-enriched gasoline, methanol, and ethanol were proposed with accuracies satisfying the turbulent combustion simulation. The calculation results demonstrated that, the variations of laminar flame speed with hydrogen enrichment are quite different for different basic fuels. For hydrogen-enriched gasoline flames, the laminar flame speed linearly rises with a small slope under low hydrogen mole fractions and increases sharply after the hydrogen mole fraction exceeds 80%. For hydrogen-enriched methanol flames, the enhancement of hydrogen mole fraction in the total fuel promotes laminar flame speed through a quadratic function. For hydrogen-enriched ethanol flames, the hydrogen enrichment raises the laminar flame speed linearly before hydrogen mole fraction exceeds 50% and promotes it exponentially afterwards.For the sake of exploring the microcosmic combustion behavior in hydrogen-enriched engines, CFD combustion models were built and experimentally validated based on ECFM and a self-developed laminar flame speed subroutine for hydrogen-enriched fuels. The CFD calculation results showed that, the effect of turbulent eddy on flame surface stretching and twisting is more pronounced after hydrogen enrichment, benefitting the accelerated flame propagation for hydrogen-enriched fuel engines. With the enrichment of 3% hydrogen to the engine intake, the peak mean turbulent flame surface density and flame speed are enhanced by 28.9% and 37.2%, respectively. Moreover, since the hydrogen enrichment effectively reduces the post combustion, the engine combustion efficiency is promoted, and the mole fraction of unburned gas and wall heat flux are both reduced with hydrogen enrichment.In order to clarify the combustion performance of hydrogen-enriched engines under various operating conditions and identify the high-efficiency region where the hydrogen enrichment effect on promoting the engine combustion is more pronounced, a quasi-dimensional model for predicting the combustion performance of the hydrogen-enriched engines under various operating conditions were established and experimentally validated using numerical analysis code Matlab on the basis of the two-zone thermodynamic model, the fractal-based entrainment combustion model and the laminar flame speed correlation for hydrogen-enriched fuels. The calculation results from quasi-dimensional model illustrated that, the turbulent flame speed at the time of 50% mass fraction burnt is sharply reduced when the lean mixture is adopted for gasoline engines. The hydrogen enrichment could generally promote turbulent flame speed for all equivalence ratios, whereas its promotion effect is more pronounced for lean mixtures with an equivalence ratio lower than 0.9 in comparison with rich mixtures. Besides, the effect of 10.0 L/min hydrogen enrichment on improving the turbulent flame speed and brake thermal efficiency is more pronounced for low loads and low speeds conditions. At an engine speed of 1000 rpm and a MAP of 20 kPa, the turbulent flame speed at the time of 50% mass fraction burnt and the engine brake thermal efficiency are enhanced by 106.6% and 23.7%, respectively.Aimed at optimizing the hydrogen enrichment and production strategies for hydrogen-enriched gasoline engine-powered vehicles, the models for vehicle dynamic system and on-board hydrogen generation and storage systems were built using the vehicle performance analysis code AVL Cruise. The engine operating range during the New European Driving Cycle was analyzed and the effects of the adoption of on-board hydrogen production, idle elimination, and hydrogen start strategies on vehicle fuel economy and emissions levels were investigated. The variations of the energy saving ratio of the hydrogen-enriched gasoline engine-powered vehicle with the energy conversion efficiency of the on-board hydrogen generation system and its controlling strategy were simulated, and the high-efficiency operating region for the on-board hydrogen generation system was observed. The simulation results showed that, when the on-board hydrogen generator works if the engine speed exceeds 1000 rpm, the hydrogen generation rate of a traditional lye-added hydrogen generator could well satisfy the requirement of 13 times hydrogen-fueled engine start during the NEDC. With the adoption of hydrogen start-idle elimination strategy, the vehicle fuel consumption during NEDC is reduced by 0.79 L/100 km, and the HC and CO emissions are decreased by 70.8% and 13.6% compared with the original vehicle without the idle elimination strategy and hydrogen production and storage systems. However, due to the high flame temperature of hydrogen, the NOx emissions are slightly increased by 8.8%. Moreover, for the hydrogen-enriched gasoline engine-powered vehicle equipped with a traditional lye-added hydrogen generator with an energy conversion efficiency of 45% and a hydrogen storage system with a maximum volume under standard temperature and pressure of 9 L, the vehicle energy saving ratio could achieve 6.48% when the hydrogen generator works when the engine speed is higher than 1300 rpm and the engine load is lower than 80 kPa.