Effect of hydrogen addition on combustion and emissions performance of a spark ignition gasoline engine at lean conditions

Abstract

Hydrogen has many excellent combustion properties that can be used for improving combustion and emissions performance of gasoline-fueled spark ignition (SI) engines. In this paper, an experimental study was carried out on a four-cylinder 1.6 L engine to explore the effect of hydrogen addition on enhancing the engine lean operating performance. The engine was modified to realize hydrogen port injection by installing four hydrogen injectors in the intake manifolds. The injection timings and durations of hydrogen and gasoline were governed by a self-developed electronic control unit (DECU) according to the commands from a calibration computer. The engine was run at 1400 rpm, a manifold absolute pressure (MAP) of 61.5 kPa and various excess air ratios. Two hydrogen volume fractions in the total intake of 3% and 6% were applied to check the effect of hydrogen addition fraction on engine combustion. The test results showed that brake thermal efficiency was improved and kept roughly constant in a wide range of excess air ratio after hydrogen addition, the maximum brake thermal efficiency was increased from 26.37% of the original engine to 31.56% of the engine with a 6% hydrogen blending level. However, brake mean effective pressure (Bmep) was decreased by hydrogen addition at stoichiometric conditions, but when the engine was further leaned out Bmep increased with the increase of hydrogen addition fraction. The flame development and propagation durations, cyclic variation, HC and CO₂ emissions were reduced with hydrogen addition. When excess air ratio was approaching stoichiometric conditions, CO emission tended to increase with the addition of hydrogen. However, when the engine was gradually leaned out, CO emission from the hydrogen-enriched engine was lower than the original one. NO_x emissions increased with the increase of hydrogen addition due to the raised cylinder temperature.

Introduction

The limited fossil fuel resources and toxic emissions exhausted from internal combustion (IC) engines have pushed the researches to focus on alternative fuels. In the previous studies, hydrogen has been proved to be a green alternative fuel that can be applied on vehicles [1], [2]. Due to the high autoignition temperature of hydrogen (approximately 858 K [3]), it is more suitable to adopt hydrogen on SI engines rather than compression ignition (CI) engines [4], [5], [6], [7]. Hydrogen possesses many unique combustion properties that benefit the engine efficiency and emissions performance. The diffusion coefficient of hydrogen (0.61 cm²/s) is about four times as large as that of gasoline (0.16 cm²/s), which improves the mixing process of fuel and air, and also helps in improving the homogeneity of the combustible mixture. The adiabatic flame speed of hydrogen (237 cm/s) is five times as large as that of gasoline (42 cm/s) which may contribute to improving the engine operating stability. Meanwhile, the high adiabatic flame speed of hydrogen indicates that the combustion of hydrogen engines is much closer to ideal constant volume combustion, which is beneficial for higher thermal efficiency [8]. However, due to the high adiabatic flame temperature of hydrogen, the pure hydrogen-fueled engine always suffers a poor NO_x emissions performance, which has become the biggest barrier for its wide commercialization [9]. Since the energy density of hydrogen on volume basis is much lower than that of gasoline, the hydrogen-powered engines sometimes also suffer a weak torque output [10]. Compared with the pure hydrogen-fueled engines, using a small amount of hydrogen as an additive to hydrocarbons-fueled engines takes the positive properties of hydrogen and hydrocarbon fuels. Such a strategy has been applied to various hydrocarbons-fueled engines and has got relatively good test results [11], [12], [13], [14], [15], [16], [17], [18].

Flame limit range with hydrogen in the air is 4.1–75% by volume, which is much wider than that of gasoline (1.5–7.6% on volume basis). So hydrogen-fueled engines are able to work under much leaner conditions [19]. Since the ignition energy of hydrogen is ten times lower than that of gasoline, the hydrogen–gasoline mixture can be more easily ignited than pure gasoline, so a hydrogen-enriched gasoline engine can gain a smooth start and a good operating stability under lean conditions. It has been commonly agreed upon that the proper lean combustion is an effective way to improve engine thermal efficiency and emissions performance [20]. At the same time, the lower combustion temperature at lean conditions also contributes to decreasing cooling and exhaust losses [20]. Furthermore, NO_x emissions can also be eased by the reduced in-cylinder temperature [21].

In view of the excellent combustion properties of hydrogen and positive features of lean combustion, there have been many publications focusing on the performance of hydrogen-enriched engines. Andrea et al. [4] investigated the effect of various engine speeds and equivalence ratios on combustion of a hydrogen-blended gasoline engine. He carried out the experiment on a modified carburetor gasoline engine. The hydrogen and air was premixed in a specific tank before entering the cylinder. The experiment results showed that the combustion duration decreases with the increase of hydrogen blending fraction. The cyclic variation was also eased by hydrogen addition. Li et al. [22] studied the mechanism of the toxic emissions formation process for the engine fueled with hydrogen–gasoline mixture, and validated it on a modified carburetor gasoline engine. From the experiment, he found that NO_x, HC and CO emissions from the hydrogen-enriched gasoline engine were lower than the original one. Varde [23] carried out an experiment on a single-cylinder carburetor engine to investigate the effect of hydrogen addition on improving the engine lean operating stability. He found that flame development and propagation durations were decreased with the increase of hydrogen addition level. And the engine lean burn limit was also extended by hydrogen addition. Dimopoulos [24] performed a well-to-wheel assessment for a hydrogen-enriched natural gas engine. He proved that green house emissions can be effectively reduced by hydrogen addition, whatever hydrogen is produced by gas reforming or electrolysis. Ji and Wang [25] investigated the effect of hydrogen addition on a gasoline-fueled SI engine performance at idle and stoichiometric conditions. The test results demonstrated that engine cyclic variation in indicated mean effective pressure was steadily decreased with the increase of hydrogen addition fraction. The engine indicated thermal efficiency and emissions performance were also improved after hydrogen enrichment, except for that HC and CO emissions were slightly increased when hydrogen volumetric fraction in the intake exceeded 4.88%.

There are plenty of publications studying the effect of hydrogen addition on the compressed natural gas (CNG) engine performance [26], [27], [28], [29]. However, only limited studies were related to hydrogen-enriched gasoline engines and many researchers still used carburetor engines and mechanically inducted hydrogen or the premixed air–hydrogen mixtures into the intake manifolds. But such a method intensified the possibility of backfire in the intake manifolds due to the wide flammability and low ignition energy of hydrogen [20]. Thereby, the quantitative effect of hydrogen addition on the modern electronically controlled gasoline engine combustion and emissions at lean conditions still needs to be investigated in detail.

For the application of hydrogen addition to modern port fuel injection (PFI) gasoline engines, a multi-point hydrogen port injection system is needed. In this study, we modified the intake manifolds to dispose four electronically controlled hydrogen injectors near the intake ports to avoid the backfire as hydrogen can be quickly inhaled into the cylinders. A self-developed electronic control unit (DECU) has been used to govern the injection timings and durations of hydrogen and gasoline to obtain the expected hydrogen volume fractions in the intake and various excess air ratios. The effect of hydrogen addition on engine lean burn performance and emissions was experimentally investigated at an engine speed of 1400 rpm and a manifold absolute pressure (MAP) of 61.5 kPa with three hydrogen volume fractions in the intake and many excess air ratios.