A quasi-dimensional model for combustion performance prediction of an SI hydrogen-enriched methanol engine

https://doi.org/10.1016/j.ijhydene.2016.07.146Get rights and content

Highlights

  • A quasi-dimensional model is developed for H2 methanol fueled engine.

  • A laminar flame speed correlation of H2 methanol blends is used.

  • The H2 methanol engine performance is simulated at various operating conditions.

  • Hydrogen addition is favor to be used at lean and low load conditions.

Abstract

A two-zone quasi-dimensional model is developed and validated for predicting the performance of a hydrogen-enriched methanol engine. The fractal-based turbulent entrainment model was applied to hydrogen-enriched methanol combustion simulations with a laminar flame speed correlation of hydrogen-methanol-air mixtures. The model accuracy was evaluated under different hydrogen volume fractions, equivalence ratios, loads and speeds. Satisfying agreement between numerical and experimental results was achieved. The validated model was applied to investigate the flame propagation speed, exhaust loss and brake thermal efficiency of hydrogen-enriched methanol engines. The results showed that the hydrogen enrichment could improve the flame propagation speed, reduce the exhaust loss and enhance brake thermal efficiency. Moreover, the results suggested that, for the actual vehicular engine operation, the hydrogen addition should be coupled with the lean burn strategy under low speed and part load conditions.

Introduction

In recent years, the limited fossil fuel reserves and stringent environment constraints have inspired efforts on exploring alternative fuels for internal combustion engines (ICE). Methanol is a renewable fuel that can be produced from kinds of materials [1], [2]. Meanwhile, the high octane number of methanol enables the engine to adopt larger compression ratios which benefit enhancing the engine thermal efficiency and reducing the toxic emissions [3], [4]. Moreover, compared with gasoline, the high oxygen content and relative high flame speed of methanol could help the fast and complete combustion of fuel air mixtures in the cylinder [5], [6]. For the above reasons, methanol is widely used as a feasible fuel candidate for spark ignition (SI) engines. Balki et al. [6], [7] found that brake mean effective pressure, brake thermal efficiency and volumetric efficiency of the pure methanol fueled engine were higher than those of gasoline engines. From the research of Celik et al. [8], for the same engine operating conditions, when the gasoline engine was converted to be fueled with the methanol, CO, CO2 and NOX emissions could be reduced by 28%, 37% and 35%, respectively. Besides, the brake thermal efficiency was increased by about 16%. However, as the latent heat of methanol is relatively high, homogeneous methanol-air mixtures are difficult to be formed on the low load and start conditions where the in-cylinder temperature is low, which leads the methanol engine to produce more emissions [1]. According to the experimental results from Gong et al. [9], the methanol engine couldn't be started when the ambient temperature is below 16 °C.

Comparatively, hydrogen is another promising green alternative fuel for SI engines [10], [11], [12]. Huang and Tang et al. [13], [14], [15], [16], [17] investigated laminar burning velocities of the hydrogen-enriched fuels in the constant volume bomb. Their investigations showed that the flame speed of hydrogen could reach 15 m/s, which is about five times of that of methanol under the same equivalence ratio, ambient temperature and pressure conditions. Because of the unique physicochemical properties of hydrogen, the hydrogen enrichment avails improving the fuel-air mixtures homogeneity, promoting the turbulent combustion and heightening the thermal efficiency [18], [19]. Raviteja et al. [20] studied the performance of hydrogen-enriched butanol engines. Their researches confirmed that the hydrogen enrichment benefited reducing the fuel consumption, decreasing the emissions and increasing the cylinder pressure. According to the experimental results from Lorio et al. [21], the hydrogen/methane blends fueled engine could gain raised thermal efficiency and shortened combustion duration with the increase of hydrogen addition fraction. Ji and Zhang et al. [22], [23] studied the hydrogen-enriched methanol engines. Their investigations showed that the addition of hydrogen not only benefited reducing the engine cyclic variation and extending the engine lean burn limit, but also availed elevating the peak engine speed and cylinder pressure during the cold start. Thus, the hydrogen-enriched methanol engine tends to be a promising approach for future SI engines.

The vehicular engines are always operated under largely and frequently varied operating conditions, resulting in the various boundary conditions for in-cylinder combustion, including in-cylinder turbulent flow, mixture thermodynamic state, and the amount of residual gas. As a consequence, for the hydrogen-enriched methanol engines, the combustion behaviors differ with operating conditions, indicating that the effect of hydrogen enrichment on promoting engine combustion varies as functions of engine operating parameters such as engine speed, absolute manifold pressure, equivalence ratio, and spark timing. For this reason, the hydrogen enrichment strategy should be carefully designed and optimized before its commercialization. However, considering the large test works for optimizing the engine controlling parameters, the design and optimization for the hydrogen enrichment strategy are hard to be finished only with experimental approaches.

With the rapid development of computational technology, the numerical simulation has been widely used in optimizing the combustion of alternative fuel engines. Gharehghani et al. [24] numerically simulated the performance of a hydrogen-enriched CNG engine by coupling the computational fluid dynamics (CFD) to chemical kinetic models. Their investigations showed that the engine lean burn limit was extended after the hydrogen addition. The combustion duration was decreased from 70 to 45 °CA when the H2 mole fraction was increased from 0% to 50%. Gong et al. [25] studied the combustion and emissions performance of a direct injection methanol engine with the hydrogen addition, by using the CFD software and incorporating the detailed methanol oxidation and nitrogen oxides reaction. The test results indicated that CO, formaldehyde and the unburned methanol were significantly reduced when the hydrogen addition fraction was increased from 0% to 15%. However, the calculation of multi-dimensional model consumed plenty of computational time in satisfying the parameter studies.

The quasi-dimensional models are well suited to perform parameter studies and predict optimum controlling strategy on existing engines with less computational time. Many investigations have been carried out by using the quasi-dimensional two-zone combustion model to predict the performance of internal combustion engines. Verhelst et al. [26] developed a quasi-dimensional model to predict the gas dynamics, combustion and knock occurrence in methanol and ethanol engines. The breathing cycle model and new laminar burning velocity correlations of methanol-air and ethanol-air mixtures were applied in this model, which could effectively improve the model accuracy. Meanwhile, a new knock prediction sub-model was proposed for predicting the knock limited spark advance. Besides, they also proposed a knock model for SI engines fueled with the gasoline-methanol blends [27]. Altin et al. [28] built a quasi-dimensional model of a SI engine with dual-spark plugs. In this model, the flame propagation process was described by the flame maps which were acquired from the computer aided design (CAD) software under the different spark plug positions, spark timings and piston positions. It was proved that the centrally located single spark plug resulted in the fastest combustion. Thereby, the engine performance with different hydrogen enrichment strategies could be obtained easily and quickly by the quasi-dimensional model of hydrogen-enriched methanol engines.

However, up to now, there is no publicly reported quasi-dimensional model for hydrogen-enriched methanol engines. Thus, this paper developed and validated a quasi-dimensional model for predicting the combustion performance of the hydrogen-enriched methanol engines. The fractal-based turbulent entrainment model was applied to hydrogen-enriched methanol combustion simulations with a laminar flame speed correlation of hydrogen-methanol-air mixtures. The engine performance and combustion characteristics under various hydrogen volume fractions, equivalence ratios, manifolds absolute pressures (MAPs) and engine speeds were evaluated for validating accuracy of the proposed model. Correspondence between the calculated and experimental results was observed, which suggested that the proposed model could well predict the combustion performance for hydrogen-enriched methanol engines. Moreover, main engine combustion characteristics including turbulent flame speed, exhaust loss, and brake thermal efficiency were numerically investigated by the proposed quasi-dimensional combustion model.

Section snippets

Assumptions and thermodynamic model

The proposed quasi-dimensional model assumes that the combustion chamber is divided into two zones by a spherically flame front. One zone is the burned zone that contains burned gasses. The other zone is unburned zone that contains unburned reactants. At the same time, for the sake of simplicity, the model assumes the two zone gases are ideal gases and there is no heat transfer between two zones. Moreover, the pressure throughout the combustion chamber is uniform while the temperatures in two

Test engine

The quasi-dimensional combustion model is built based on constructions of a 1.6 L four-cylinder SI engine manufactured by Beijing Hyundai Motors. Specifications of the test engine are listed in Table 1. The original engine is modified by adding a hydrogen port injection system which ensures the hydrogen and methanol could be simultaneously injected in the intake manifolds. A hybrid electronic control unit is used to realize the electronically controlled hydrogen port-injection, as well as

Combustion performance prediction

Since the engines always operate under a wide range of conditions, it is of necessity to investigate the performance of hydrogen-enriched methanol engine under different hydrogen addition levels, equivalence ratios, speeds and loads. In the first group, the engine continuously run under a speed of 1400 rpm, a MAP of 61.5 kPa and a spark advance of 26 °CA BTDC. At the same engine operating condition, the effects of hydrogen addition level and equivalence ratio on the combustion of

Conclusions

A quasi-dimensional model was developed and validated for predicting the combustion performance of a hydrogen-enriched methanol engine. In this model, the fractal-based turbulent entrainment model was adopted to hydrogen-enriched methanol combustion simulations with the implementation of a laminar flame speed correlation of hydrogen-methanol-air mixtures.

The predictive accuracy of this model was evaluated by performing numerical calculation and experimental test under different hydrogen volume

Acknowledgements

This work was supported by National Natural Science Foundation (Grant No.51476002), National Key Basic Research Development Project (973) (Grant No.2013CB228403), Key Program of Sci & Tech Project of Beijing Municipal Commission of Education (Grant No. KZ201610005005) and Beijing Municipal Commission of Science and Technology (Grant No. Z141100003814017).

References (40)

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