Effects of hydrogen and methane addition on combustion characteristics, emissions, and performance of a CI engine

Highlights

• Effect of hydrogen and methane addition on performance of a CI engine was tested.

• Between 15% and 75% H₂ + CH₄ mixture in energy basis was injected into the engine.

• Single phase combustion similar to gasoline was observed with H₂ + CH₄ addition.

• An important decrease on NO_x was obtained with 15% and 40% H₂ + CH₄ addition.

• NO_x emissions were dramatically increased with 75% H₂ + CH₄ addition.

Abstract

In this study, while a hydrogen + methane mixture in the gas phase (30% H₂ + 70% CH₄ in moles) is injected from an intake port by a gas injector, ignition is supplied with diesel fuel sprayed from a diesel injector. The energy content of the sprayed gas fuel is modified to constitute 0% (only diesel fuel), 15%, 40%, and 75% of the total fuel energy content. All tests are conducted at 1500 rpm stable engine speed and in full-load condition. The effects of energy content modification of hydrogen + methane gas fuel on engine performance, emissions, and combustion are observed. Both NO_x and soot emissions are taken under control with 15% and 40% energy content rates in the gas fuel compared to the diesel-only condition. Although an increase is observed in CO and THC emissions with gas fuel addition compared to the diesel-only condition, an improvement is considered compared to the results obtained for only methane fuel usage with diesel fuel. Moreover, gas fuel mixtures reach a high value (75% energy content) as peak cylinder gas pressure decreases. A single-phase heat release was observed, and the diesel combustion characteristic of the engine was converted into gasoline combustion with the addition of 75% hydrogen + methane as the energy basis.

Introduction

Fast depletion of fossil fuels due to increasing energy demand is causing the price of petroleum fuels to increase. Also, stringent emission regulations for energy efficiency and environmental concerns are accelerating the number of studies on alternative fuel. According to a European Union Commission White Paper Report, a 60% decrease is targeted on greenhouse gas emissions generated from transportation in the year 2050 compared to 1990 [1]. Even though renewable energy consumption between 2010 and 2011 in transportation grew from 3.5% to 3.8%, this rate should be at least 4.1% to arrive at the determined target [1]. Emission limits in regulations continuously decrease with technological developments, but NO_x and particulate matter emissions have increased [2], [3], [4], [5]. With respect to the 2013 air quality report in Europe, NO₂ emission passed the limit value in 2011 in 42% of measurements made at traffic control points [1]. In 2011, particulate matter (≤10 μm) emission exceeded 43% in traffic jams, 38% in city traffic, 25% in industrial areas, and 15% in rural areas [1]. The main reason for the increase in NO_x and especially in particulate matter is the increasing number of diesel engine vehicles. Studies on alternative environmentally friendly fuels in the automotive industry are continuing in parallel with this progression. Researchers have recently leaned toward alternative fuels such as hydrogen, LPG, CNG, LNG, biogas, and ethanol. Hydrogen is a carbon-free alternative fuel that generates only water after combustion [6]. Some vehicle manufacturers have tried to develop proton exchange membrane fuel cell (PEMFC) systems to use in vehicles. But compared to hydrogen-fueled internal combustion engines, the cost of PEMFCs is high, and they require hydrogen of extremely high purity (>99.99%). They must improve in durability, and price reduction is also necessary [7]. Hydrogen's usage in internal combustion engines will act like a bridge for transition to fuel cell vehicles from hydrocarbon derivatively fueled vehicles. On the other hand, methane is the closest environmental alternative fuel to hydrogen thanks to its low carbon/hydrogen ratio.

There are several ongoing researches on electrical vehicles [8], [9]. Because of their low energy density and insufficient infrastructure problems, electric vehicles are not widespread, so dependency on internal combustion vehicles will continue in the near future [5]. Hydrogen can be produced from fossil fuel or biomass conversion, electrolysis, or direct thermochemical solar conversion [6]. Furthermore, hydrogen is an extremely clean and environmentally friendly fuel when produced from renewable energy sources, and it can also be used as a carrier of secondary energy such as electrical energy [10].

Some researchers have used hydrogen in homogenous charged compression ignition engines (HCCI), but difficulties in controlling the onset of ignition and narrow working ranges make its commercial usage impossible in the near future [11]. Most researchers have studied usage of hydrogen on Otto engines. During these researches, nearly 30% power inferiority, pre-ignition, flare-back, and backfiring problems were seen [12], [13], [14].

Despite their high thermal efficiency and low specific CO₂ emission, diesel engines are disadvantageous due to high NO_x and soot emissions [15]. In order to limit NO_x, particulate matter, and soot emissions from diesel vehicles, after-treatment equipment (catalytic converter, SCR, LNT, DPF, etc.) is utilized, but these systems increase the cost. Usage of hydrogen as a dual fuel in diesel engines is an alternative way to control the emissions in question [16]. Because of hydrogen's high self-ignition temperature (576 °C), it is not possible to use hydrogen directly in diesel engines as fuel [10]. An energy source is needed to provide ignition [11]. Ikegami et al. [17] used a glow plug gas injector, whereas Antunes et al. [11] heated intake air to provide ignition.

Hydrogen or methane pulverization into an intake port, hydrogen pulverization directly into a cylinder, or transmitting hydrogen continuously from intake manifold and pilot diesel pulverization are methods of ignition that are listed in the literature [18], [19]. When hydrogen or methane is transmitted continuously from the intake manifold, problems like preignition, backfiring, and flare-back in the intake manifold are seen. During hydrogen or methane pulverization in the intake port method, gas fuel is replaced by air taken into the cylinder intake stroke and volumetric efficiency decreases, which causes a drop in engine power [20]. During hydrogen or methane pulverization directly into the cylinder, special high-pressure hydrogen injectors resistant to high temperature are required. Also, some modifications in the engine and placement of a high-pressure gas injector on the cylinder head are needed. This is an important obstacle to commercialization of hydrogen pulverization into the cylinder. As a result, hydrogen pulverization into the intake port and providing ignition by the diesel fuel method is the most convenient method in dual fuel (hydrogen + diesel fuel) diesel engines [10].

Bose and Maji [10] operated a 4-stroke, water-cooled diesel engine, and a 0.15 kg/h hydrogen flow rate was introduced into the intake manifold. The different load conditions were experimentally investigated on in 0% (without EGR), 10%, and 20% EGR conditions. The test results were compared with each other, and an important rise in the thermal efficiency and a drop in CO, CO₂, THC, and smoke emissions were obtained with the addition of hydrogen. The NO_x emissions dropped thanks to the EGR system. Liew et al. [21] tested the effect of H₂ enrichment on the combustion characteristics depending on the brake engine power and H₂ enrichment level for a 4-stroke diesel engine. At high engine loads and in high H₂ level conditions, the combustion duration value was decreased and the maximum rate of the heat release value was dropped. Bari and Esmail [22] experimentally investigated the effects of different levels of a hydrogen + oxygen gas mixture produced by an alkaline water electrolyzer as an additional fuel in a 4-stroke, direct-injection diesel engine at different engine loads. However, the brake thermal efficiency and emissions (HC, CO, and CO₂) were improved with the addition of hydrogen + oxygen, the rise of NO_x emissions couldn't be prevented. Pan et al. [19] worked on a 2-stroke compression ignition engine and sent 0 SLPM, 22 SLPM, and 220 SLPM of hydrogen into the intake manifold as supplementary fuel. Different load conditions and idle operation conditions were experimentally investigated. A great improvement in NO_x emissions and PM emissions (up to 40% on NO_x emissions and 86% on PM emissions) was obtained with the addition of hydrogen in the idle operating condition. Miyomato et al. [15] did an experimental work on a 4-stroke, common-rail compression ignition engine. Hydrogen was injected into the intake port, and diesel fuel was injected into the cylinder. Both the use of low temperature combustion (changing diesel injection time) and the use of EGR NO_x emissions were controlled.

Studies made on methane fuel usage in ICEs (​Internal Combustion Engines) are summarized as follows:

Yang et al. [23] studied on a common-rail fuel system diesel engine and modified this engine into a dual-fueled engine (natural gas + diesel engine). They observed pilot injection pressure and its duration. They also observed its effect on emissions at partial loads. They realized that THC and CO emissions are taken under control. Yang et al. [24] observed the effect of natural gas pulverization into the intake manifold at different injection periods on engine performance and emissions with a 2.8 L, 4-stroke ve 4-cylinder CI engine. At low and partial loads, higher cylindrical gas pressure is obtained with natural gas pulverization delay but an NO_x emissions increase, whereas at full load a slight decrease is seen. Xiao et al. studied engine performance and emissions with the addition of different quantities of methane [25] in a 4-stroke, compression ignition engine at stable engine speed. The addition of methane with the same energy content decreased NO_x emissions compared to diesel fuel. Abdelaal and Hegab [26] studied the effects of EGR on a natural gas-diesel fuel engine. As EGR and natural gas addition decrease, the NO_x rate decreases even more. Although HC and CO emissions reduce in dual fuel mode with increasing EGR, they are still extremely high compared to those of diesel fuel. Thurnheer et al. [27] tested a methane + hydrogen gas mixture at stoichiometric conditions via an electronic controller in a 2 L cylindric volume SI engine. Whenever the quantity of H₂ in the gas fuel increased, the ignition advance was modified because combustion speed increased as well. Zhou et al. [28] tested a diesel-H2-CH4 fuel mixture in a 4-stroke CI engine. With the use of the diesel-H₂-CH₄ mixture fuel, an improvement was seen especially at low loads compared to a dual fuel condition. This improvement increased with an H₂ rate increase. The researchers recommended the usage of a 3-fuel system with a high H₂ rate in order to increase engine performance at low and medium loads and to reduce CO and HC emissions.

Up to now, only methane and hydrogen fuel have been used with diesel fuel in compression ignition engines. Although an improvement in combustion and emission characteristics has been observed after the use of hydrogen fuel together with diesel fuel, it is not possible to implement in vehicles due to the problems of storing hydrogen. The drop in efficiency and increase in THC and CO emissions are being inspected, and a significant improvement is obtained in emissions after the use of methane fuel together with diesel fuel. That's why a small amount of hydrogen (30% of volume but a small amount in energy content) and methane (70% of volume) gas mixture is utilized. On the other hand, methane has similar properties to hydrogen, such as its low carbon/hydrogen ratio, and it can be easily used in CI engines due to its high octane number. Not only is a gas mixture that is practically possible to use in reality being tested, but the prevention of THC and CO emissions increases after the use of methane together with diesel fuel in compression ignition engines is being targeted. Moreover, the energy content of gas fuel is changed from a very low amount (15%) to a very high amount (75%). In this way, the effect of hydrogen + methane gas fuel's energy content on emissions and determination of optimum value (especially for NO_x and soot emissions) is aimed for.