Zero Emission Hydrogen Fuelled Fuel Cell Vehicle and Advanced Strategy on Internal Combustion Engine: A Review

Abstract

Global energy consumption has gradually increased as a result of population growth, industrialization, economic development, and rising living standards. Furthermore, as global warming and pollution worsen, the development of renewable energy sources is becoming more essential. Hydrogen is one of the most promising clean and sustainable energy carriers because it emits only water as a byproduct without carbon emission and has the highest energy efficiency. Hydrogen can be produced from a variety of raw resources, including water and biomass. Water electrolysis is one of many hydrogen production technologies that is highly recommended due to its eco-friendliness, high hydrogen generation rate, and high purity. However, in terms of long-term viability and environmental effect, Polymer Electrolyte Membrane water electrolysis has been identified as a potential approach for producing high-purity, high-efficiency hydrogen from renewable energy sources. Furthermore, the hydrogen (H2) and oxygen (O2) produced are directly employed in fuel cells and other industrial uses. As a result, an attempt has been made in this work to investigate hydrogen synthesis and utilization in fuel cell vehicles. Low-temperature combustion technology has recently been applied in engine technology to reduce smoke and NOx emissions at the same time. The advantages and limitations of homogeneous charge compression ignition, partially premixed charge compression ignition, premixed charge compression ignition, and reactivity regulated compression ignition are described separately in low-temperature combustion strategy.

1. Introduction

Global energy consumption has increased gradually in recent years due to population growth, and economic development and industrialization. Also, global warming and environmental pollution worsened everyday too much of automobile vehicles and industrialization. Hence, the development of renewable energy sources became increasingly important. Hydrogen is one the most promising clean and sustainable energy sources because it emits only water as a byproduct and generates no carbon emissions [1]. Hydrogen has a quality of high energy carrier including high energy density that is more than ordinary petroleum and diesel fuel [2]. At the moment, global hydrogen production is estimated to over 500 billion cube meters per year [3]. It can be used in much industrial application including fertilizer, petroleum refining operation, fuel cell, chemical industries [4]. Hydrogen can be generated from variety of renewable and non-renewable sources like water and fossil fuels [5], oil reforming [6], coal gasification [7], biomass [8], water electrolysis [9].

Many approaches for manufacturing hydrogen are currently available however water electrolysis is one of the most capable methods for producing hydrogen as a product and oxygen as a by-product. At the moment, only 4% of hydrogen can be obtained by electrolysis of water [10]. Water electrolysis also provides a number of advantages, such as high cell efficiency and a greater hydrogen generation rate with excellent purity, making it a better method for converting water to electrical energy via low-temperature fuel cells. The water molecule is the reactant in the electrolysis process, and under the influence of electricity, it is split into hydrogen (H₂) and oxygen (O₂). Based on the electrolyte, operating conditions, and ionic agents (OH⁻, H⁺, O₂⁻), water electrolysis is separated into four categories: alkaline water electrolysis (ii), solid oxide electrolysis (SOE), microbial electrolysis cells (MEC), and PEM electrolysis of water [11]. The phenomenon was first described by Troostwijk and Diemann in 1789 [12], and it is a well-established technique for commercial hydrogen production up to the megawatt range in the world.

The hydroxyl ions (OH⁻) flow through the porous diaphragm to the anode under the effect of the electrical circuit between anode and cathode, where they are discharged to 12 molecules of oxygen (O₂) and one molecule of water (H₂O). Alkaline electrolysis is performed at lower temperatures, such as 30–80°C, with an aqueous solution (KOH/NaOH) as the electrolyte and a 20–30% concentration. Alkaline water electrolysis uses an asbestos diaphragm and nickel materials as electrodes [13]. In the 1980s, Donitz and Erdle proposed solid oxide electrolysis (SOE). Solid oxide electrolysis has attracted a lot of interest since it converts electrical energy into chemical energy while also producing ultra-pure hydrogen with a higher efficiency. Solid oxide electrolysis runs at high pressures and temperatures of 500–850°C and consumes water in the form of steam. Nickel/zirconia is commonly utilized as an O₂ conductor in solid oxide electrolysis [14].

Microbial electrolysis cell (MEC) technology may produce hydrogen from organic matter such as renewable biomass and wastewaters. MEC technology is similar to microbial fuel cells (MFCs), however the operational concept is reversed [15]. In 2005, two independent research institutions, Penn State University and Wageningen University in the Netherlands, established the first Microbial electrolysis cell (MEC) method. Electrical energy is turned into chemical energy in microbial electrolysis cells (MECs). Under the influence of an electric current, MECs created hydrogen from organic molecules. Microbes oxidize the substrate at the anode side of the microbial electrolysis process, producing CO₂, protons, and electrons. The electrons move to the cathode side via the external circuit, while the protons travel to the cathode via a proton conducting membrane (electrolyte), where the protons and electrons combine to form hydrogen [15]. However, this MEC technology is still in the early stages of development, and various issues like as high internal resistance, electrode materials, and intricate design must be addressed before the technology can be commercialized [16].

In the early 1950s, Grubb achieved the first PEM water electrolysis, and General Electric Co. was created in 1966 to overcome the drawbacks of alkaline water electrolysis. PEM water electrolysis technique, which is similar to PEM fuel cell technology [17], used solid poly sulfated membranes (Nafion®, fumapem®) as an electrolyte (proton conductor). Lower gas permeability, strong proton conductivity (0.1 0.02 S cm⁻¹), thinness (20–300 m), and high-pressure functionality are all advantages of these proton exchange membranes. In terms of sustainability and environmental impact, PEM water electrolysis is one of the most environmentally benign methods for converting renewable energy to high purity hydrogen. Another prospective PEM water electrolysis device has a small footprint, high current density (over 2 A cm⁻²), high efficiency, fast responsiveness, and operates at lower temperatures (20–80°C) while producing ultrapure hydrogen as a byproduct [17].

2. Fuel cell vehicles

Fuel cell technology is gaining popularity in the automotive industry due to its ease of use, quiet operation, high efficiency, and modular structure. According to Mustafa et al., recent investigations have showed that the usage of fuel cells in vehicles has expanded rapidly, causing a revolution, and will be an alternative to conventional vehicles in the future (2021). Configuration, system components, control/management, technical obstacles, marketing, and future aspects are all categories for fuel cell cars. Based on chemical characteristics and operating temperature, fuel cells are classed as proton exchange membrane FCs, solid oxide FCs, direct methanol FCs, alkaline FCs, molten carbonate FCs, and phosphoric acid FCs. FCs are used in both commercial and research & development applications. Common stack size, theoretical cell voltage, operating temperature, electrical efficiency, benefits, and downsides are used to classify FC features [18]. In this environment, FCs are used in distributed generation, mobile power, backup power, military, space, and vehicle applications. Low temperature and pressure PEMFCs are the most used FCs in vehicle applications because of their high power density, lower working temperature (60–80°C), and reduced corrosion than other FCs [18].

In the construction of fuel cell hybrid electrical vehicles (FCEVs), fuel cell vehicles (FCs) are coupled to electric motors via controlled electronic interfacing components [19]. The basic components of traditional FCEVs are a voltage regulation converter, motor drive, electric motor, and auxiliary energy generation units [20]. For interfacing components and energy management algorithms, FCEVs vehicles have a variety of configuration topologies [21]. The powertrain structures, voltage regulation topologies, motor drive converters, and energy management technologies can all be used to classify FCEVs. In the operation of FCEVs, the FC stack feeds energy to the dc-bus and maintains the required DC bus voltage [22]. The FC is then connected directly to the Unidirectional DC-DC converter (UDC) as a system element to maintain the dc-bus voltage and send the energy generated for vehicle propulsion to the motor drive converter. A DC-AC converter checks the motor speed and torque for safe operation. Finally, the drive controller is in charge of monitoring the electric motors as they convert electrical energy into kinetic energy [23].

FCs have a higher energy density and efficiency than other power sources such as photovoltaics, batteries, ultra capacitors, and super conducting magnetic energy storage. Because of its modular design, FCs are also suitable for electric vehicle applications. Furthermore, FC has a 20–30 year lifespan [24]. As a portable/rechargeable energy storage system, the battery is also a preferred power source for FCEV hybridization. However, it has a short lifespan and is only useful for a short length of time [25]. Ultra capacitors (UC) are a type of storage element that can be used in FCEV applications to increase the dynamic response of the system. Photovoltaic (PV) is a gadget that generates energy, however it is too large to carry. The output of super conductive magnetic energy storage (SMES) generates a lot of power, however it has a low energy density. Short-duration energy storage is also included in SMES, albeit at a high expense [26]. Based on this, several hybridization topologies are recognized in the literature. Full FC, partial FC, and hybrid FC cars are classified as FC + battery hybridization, FC + UC hybridization, FC + battery + UC hybridization, FC + battery + PV, FC + flywheel hybridization, and FC + SMES [18]. FC + battery + PV, FC + battery + PV, FC + flywheel hybridization, and FC + SMES are all examples of FC-powered cars.

The FCEV scheme clearly shows that this topology’s energy generation is exclusively dependent on the FC stack. It simple construction includes a fuel tank, FC stack, DC-DC power converter, inverter, and electric motor [27]. These cars feature a long driving range, a fast charging time, high efficiency, cold start capabilities, silent operation due to the lack of mechanical components, energy supply continuity, and low emissions [27]. Full FCEVs are a suitable fit for low-speed vehicles including forklifts, busses, airline vehicles, trams, and marine vehicles. The combination of FC + battery units is the most common topology in FCEV hybridization [18]. A unidirectional DC-DC converter (UDC) connects FC to the DC bus, while a bidirectional DC-DC converter connects the battery to the DC bus. In the operating procedure of FC + battery hybridization, an initial start-up with the battery is provided to avoid the FC running in the low-efficiency zone. As a result, a huge amount of current is generated to start the electric motor [25]. When the car is turned on for the first time, the FC is activated to keep the electric motor going. After then, the battery is charged according to the charge status criteria. The UC only allows FC to be utilized in emergency situations to meet transient power demands. UC, on the other hand, has a low energy density and is not used to give energy on a long-term basis [28].

In contrast to earlier hybridization topologies, FC + battery + UC hybridization has a primary energy source (FC) and two secondary energy sources (battery and UC) (battery and ultra capacitor). In this design, the FC is connected to the DC bus through a one-way DC-DC converter. The energy storage units, battery and UC, are connected to the DC bus by bidirectional DC-DC converters (BDCs). This architecture combines the advantages of FC + battery and FC + UC systems to provide continuous energy while also boosting FC dynamic response during transient events [29]. In recent years, PV panels have been incorporated with FC-based electric vehicles for hybridization. In FC + battery + PV hybridization, PV panels generate DC voltage that is coupled to the DC bus via a unidirectional converter. The FC is the primary energy source in an FC + battery + PV system, with the PV panel acting as a backup. Both the FC and PV busses are connected to the DC bus by unidirectional converters. PV panels generate varying amounts of power based on the intensity of solar radiation, the temperature, and the sun’s direction. As a result, the PV electricity generated is fed directly into the electric motor or is used to charge the battery [30].

FC+ flywheel hybridization is similar to the preceding approach in that the FC serves as the major energy source and the flywheel, rather than batteries, serves as an energy storage method. Flywheels and generators are connected to store energy mechanically with a high rotating speed and transform that mechanical energy into electricity when EM requires a lot of it. Flywheels have a faster charging capability, higher efficiency, and higher power rating than batteries [30]. Flywheels are also environmentally friendly, as they operate over a wide temperature range, have a big energy storage capacity, and have a long lifespan [66]. There are three types of static FC models accessible in the literature. Chamberlin-Kim and Amphlett, Larminie, and Dicks models [31] are examples. The most common static model published in the literature is the Amphlett model, which is based on Nernst and Tafel equations. This model takes into account physical parameters like as pressure, temperature, and concentration. The other static model is the Larmine and Dicks model. This model calculates the FC voltage–current characteristic using empirical equations. This model yields the FC voltage versus current amplitude curve. Three zones can be found in this curve. The three zones are electrochemical activation, linear part, and gas diffusion kinetics [32]. The third static FC model is the Chamberlin-Kim model. In this approach, the FC voltage is described in terms of current density. In addition, the fuel-oxidant rate, local temperature, and humidity all affect five factors in this model [32].

Dynamic modeling of FC is described in the literature such as the impedance model, Becherif-Hissel model, and Dicks-Larminie model have been reported [33]. Layer capacitance, diffusion impedance, and ions transport, membrane, and contact resistances are all included in the impedance model [34]. The Nernst voltage, ohmic polarization, concentration, and activation are all modeled in the Dicks-Larminie model. A voltage supply, two resistances, and a capacitor make up this model. The Nernst voltage is demonstrated via the voltage source. The resistances represent electron-hydrogen flow and activation-concentration losses. The charge layers are represented by the capacitance. The pneumatic feature is taken into account in the Becherif-Hissel model to obtain the comparable model for electrical components. The conservation of mass, energy, and charge is taken into account in pneumatic properties [35].

3. Introduction to low temperature combustion techniques

Conventional diesel engine running on petroleum and diesel fuel emits more oxides of nitrogen (NO_x), oxides of carbon (CO_x) and particulate matter (PM) around the world. Low-temperature combustion (LTC) technology in engine development has dropped the environmental effects by providing better combustion efficiency, and increased the engine efficiency and fuel economy. Several low-temperature combustion strategies are available such as homogeneous charged compression ignition (HCCI), premixed charged compression ignition (PCCI), and reactive controlled compression ignition (RCCI). Before combustion, the entire air and fuel is premixed in the LTC combustion mode. The combustion is controlled by a predetermined equivalency ratio and cylinder temperature which leads to reduce the soot formation, PM, and NOx emissions. In LTC mode, the combustion temperature could be maintained between 1800 and 2200 K, which means no NOx emissions are produced in the rich mixing region and no soot is formed below 1800 K in the lean mixing by Hoekman and Robbins.

3.1 Homogeneous charge compression ignition (HCCI)

The homogeneous charge compression ignition (HCCI) engine combines the combustion characteristics of both SI and CI engines in an IC engine. The fuel is premixed in the HCCI engine in the same in SI engines, and the fuel is auto-ignited to start the combustion in the same way in CI engines. Before combustion begins, the fuel is vaporized and homogeneously premixed with air. Due to lean-burn combustion, the HCCI has the ability to reduce NO_x emissions and increased the brake thermal efficiency. The in-cylinder temperature is reduced via lean-burn combustion, resulting in decreased NOx emissions as observed by Komninos and Rakopoulos [36]. In addition, due to the increased displacement capacity, HCCI combustion improves brake thermal efficiency by 50%, while emitting less smoke than conventional diesel combustion. The HCCI engine’s compression ratio and premixed fuel combustion has improved the brake thermal efficiency of engine and lower the smoke emissions as noticed by Desantes et al. [37]. The multi-zone auto ignition and spontaneous combustion of the entire mixture is promoted by the homogenous mixture and uniform equivalence ratio in the cylinder. Furthermore, flame propagation has little effect on combustion in the HCCI mode [38].

The unanticipated pressure rise and cycle to cycle variation are exacerbated by multi-zone combustion and unexpected ignition location. Also, knocking is caused by high oscillation frequency and unanticipated pressure surge as noticed by Ganesh and Nagarajan [39]. Contino et al. [40] reported that some of the techniques such as early direct injection, early multiple injection, water injection, port fuel injection, external cold EGR, variable valve timing, variable compression ratio, air preheating, and alcohol injection are commonly employed in HCCI to control combustion and emission. The biofuel auto ignition temperature and viscosity are higher than diesel hence a higher compression ratio was used in HCCI engine. The compression ratio for the various loads can be adjusted to enhance the combustion efficiency as noticed by Zhang, et al. [41]. By modifying the spark timing and spark plug placement, the spark aided HCCI engine was able to achieve combustion phasing and emission reduction [42]. The key factors that have been employed to detect the combustion phenomena in the HCCI engine are the pressure increase rate, combustion noise, and ringing intensity. In a real-time combustion application, the ringing intensity is primarily employed to detect the combustion noise for the needed cylinder pressure [43].

Because of the increased stroke volume, the higher compression ratio HCCI engine improves brake thermal efficiency by achieving the auto-ignition temperature of the fuel. High to low octane fuels can be utilized as a port fuel to solve knocking and NOx formation. In HCCI engine, keeping the inlet charge temperature is critical. Similarly, the HCCI engine’s compression ratio could be maintained effectively between 10:1 and 28:1. Compression ratios of 10:1 were favored for higher cetane fuels like n-heptane, and 28:1 were preferred for high octane fuels like iso-octane. For biodiesel, the intermediate compression ratio was favored [44]. Alternative method for achieving lower emission in HCCI engine includes use of alcoholic fuels such as ethanol, n-butonal, and methanol. Due to oxygen enrichment, alcohol fuel accelerated premixed burning and complete oxidization of fuel. Also, because of the latent heat of vaporization is higher, it lowers the combustion temperature, enhancing the quenching effect [41]. The HCCI combustion’s power output is mostly determined by the equivalency ratio and fuel intake. For the higher power production, the equivalence ratio should remain at 1 as noticed by Vinod Babu et al. [45].

3.2 Premixed charge compression ignition (PCCI)

Too early injection of fuel with a higher injection pressure can result in premixed charge compression ignition. Due to early fuel injection, the time between commencement of injection and start of combustion has been extended, considerably improving the homogeneity of the air-fuel mixture prior to combustion [41]. With a slightly higher intake charge temperature maintained at 170°C, the PCCI engine may operate from a minimal air-fuel ratio of 34:1 to an excessively lean air-fuel ratio of 80:1 [46]. In comparison to a standard SI engine, the PCCI combustion strategy uses lean-burn technology and operates on a higher compression ratio engine. After all of the fuel had been injected, the PCCI began to burn. Also, unlike traditional combustion, the combustion events are primarily identified by chemical kinetics and do not follow the diffusion mixed combustion and speed of burning. As a result, the injection pattern and fuel combustion do not overlap, reducing the odds of direct combustion control [47]. To achieve the premixed charge in the PCCI combustion, a single stage fuel injection pattern with an earlier start of injection was adopted. However, starting the injection too early causes wall impingement and wall wetness, resulting in incomplete combustion and higher HC and CO emissions. The fuel injection pattern has been adjusted with a split and multiple injection method to alleviate these issues. Despite the fact that the period of the many injections is completed before combustion begins. Controlling auto ignition by early injection is also a critical job in PCCI combustion. To manage the auto ignition and lengthen the ignition delay interval, a higher amount of EGR is used. EGR also aids in lowering in-cylinder temperature and NOx generation due to the dilution of a fresh charge mixture [48].

PCCI combustion has performed better than HCCI combustion due to the stability of the combustion by partially premixed charge and controlled auto ignition rage and temperature. The phasing of combustion in the PCCI is mostly determined by chemical kinetics, but it can also be influenced by altering the inlet charge temperature, EGR rate, and fuel injection time and pressure. PCCI combustion has used a variety of fuel patterns, including early single pulse injection, port fuel injection, advanced multiple injections, and advanced injection with a tiny amount of late injection. In the previous section, the effects of early and late injection timings were explored. The modest amount of late injection is mostly used to reduce smoke emissions [49]. The spray angle of 70° was employed to atomize the fuel within the combustion chamber in order to eliminate wall wetness during advanced injection [49]. To avoid the generation of HC and CO emissions, the compression ratio of the PCCI engine was kept at the same level as that of a regular diesel engine. Due to the low volatility and strong flammability of the fuel, PCCI combustion has several limitations, according to a few studies [50].

For low volatile fuels like kerosene, diesel, and biofuels, spark assisted PCCI combustion has been applied. When compared to conventional CI combustion, the use of low-quality cetane fuel in the spark aided PCCI strategy engine enhanced engine performance [51]. The partially premixed combustion mixture is generated in PCCI-DI dual-mode combustion by injecting a large volume of fuel in the intake port or early pilot injection, followed by conventional direct injection of the same or another fuel. Due to the ignition delay interval, the combustion phasing of the PCCI-DI dual-mode combustion is primarily determined by the pilot fuel quantity, and the combustion rate is determined by the pilot fuel ratio [31]. For premixed compression ignition low-temperature combustion, port fuel injection is preferred (PCI-LTC). To create a premixed mixture with a proper air-fuel ratio, single fuel or dual fuel port injection is employed. Dual fuel premixed LTC has a better brake thermal efficiency than single fuel LTC and has achieved a significant reduction in NOx and soot emissions. The single fuel premixed LTC has a higher cycle to cycle variance due to the low temperature and lean air-fuel ratio [52, 53]. Reactivity controlled compression ignition (RCCI) is the name given to the dual-fuel premixed LTC, and a thorough description of the RCCI will be given in the following sections.

3.3 Partially premixed charge compression ignition (PPCI)

Partially premixed charge compression ignition is related to the PCCI method, which is a hybrid of traditional diesel and HCCI combustion. However, for low cetane fuels, PPCI combustion is favored. Similar to the PCCI combustion method, a longer ignition delay period and improved air-fuel mixing can be accomplished. Few studies have shown that improved and delayed injection strategies can result in extended ignition delay in PPCI combustion. To achieve a longer ignition delay, low and moderate compression ratio were used, as well as moderate to high EGR dilution. The key benefit of PPCI mode over HCCI mode is that it releases less particulate matter and NOx while providing better combustion phasing. PPCI is divided into two categories: early injection PPCI and late injection PPCI. The fuel is injected at the middle of the compression stroke in early injection PPCI, and at the end of the compression stroke in late injection PPCI. The fuel-injection gases of the early injected PPCI variant are denser and cooler due to partial compression. Similarly, in the late injected PPCI model, the fuel-injected gases are colder and denser due to injection occurring on the expansion cycle, which lowers the temperature in the later stage [54]. Due to incomplete oxidation and non-optimal combustion phasing, the PPCI combustion used slightly more fuel than standard diesel combustion [55].

At low load, a greater EGR rate and a delayed injection time reduce the power output of both low and higher power engines. In EGR assisted PPCI combustion, the advanced injection method was used to avoid a reduction in power output. Another disadvantage of PPCI combustion is that it produces more HC and CO because the amount of non-oxidized fuel in the piston bowl and high-pressure squish region increases [56]. The addition of gasoline to the PPCI is another way to achieve lower NOx and soot emissions without using EGR. The main benefits of adding gasoline to the PPCI are that it reduces HC and CO emissions by reducing residual products in the cylinder [57]. For longer ignition delay times, most of the premixed heat release phase was seen, resulting in higher peak cylinder pressure and noise levels. When the ignition delay periods shorten, the diffusion heat release phase occurs, resulting in a state similar to that of ordinary diesel combustion [58].

3.4 Reactive controlled compression ignition (RCCI)

HCCI, PCCI, and RCCI are examples of sophisticated low-temperature combustion technology that have recently been created. RCCI, for example, increases research focus due to its versatility. By achieving low-temperature combustion, HCCI and PCCI improve engine efficiency and reduce pollutants, according to previous studies. These two technologies, however, have considerable limits, and they are not ideal for low and high load settings due to knocking, misfire, and a faster rate of pressure rise. Fuel alteration is required in the HCCI and PCCI combustion to overcome the difficulties [59, 60]. They also stated that combustion quality had improved across a broad range of engine operations Bessonette et al. [61] investigated the effect of a partially mixed gasoline/diesel charge in a CI engine from low to high load. Raw diesel is favored for the lowest load situation, while a higher percentage of gasoline blend is suited for the highest load condition, according to them. In a subsequent stage, this dual fuel PCCI operation is referred to as RCCI combustion [62]. Adjusting the low to high reactive fuel ratio and the injection pattern of the high reactive fuels to achieve the NOx to smoke trade-off and higher efficiency. Reactivity stratification in RCCI combustion can also be influenced by fuel qualities such as viscosity, volatility, and ignite ability.

Biodiesel has been tested in a variety of engines and under a variety of operating circumstances all around the world. Due to the presence of oxygen in the biodiesel fuel, NOx emissions were higher for the engine [63, 64]. The RCCI engine driven by gasoline/biodiesel was mathematically analyzed by Li et al. [65]. When comparing raw biodiesel to gasoline/biodiesel, the study found decreased NOx emissions in the gasoline/biodiesel operation. As a result, using biodiesel under the RCCI method may be a better alternative for reducing NOx pollution than using biodiesel-powered diesel engines. Hanson et al. [58] study the RCCI combustion utilizing direct-injected diesel and biodiesel mixture (B20) as a direct-injected fuel and gasoline, E85 (85% ethanol and 15% diesel blend), and E20 as a port fuel. In the RCCI combustion, the findings of the E20/diesel mixture show that maximum pressure and HRR dropped, allowing the peak load to increase by 2 bar (from 8 bar to 10 bar BMEP). The usage of E20 improves combustion efficiency while lowering the heat release rate and exhaust leakage. The combustion efficiency of gasoline/B20 RCCI operation was also increased by lowering the UHC, albeit with a greater CO. Fuel efficiency also improved, resulting in a 1.68 percent increase in BTE. In comparison to the RCCI gasoline/diesel operation, E85/B20 allowed the RCCI operation to increase the BTE from 40 to 43%. The use of biodiesel as a pilot fuel has improved the stability of the cyclic operation of RCCI engine powered by natural gas/biodiesel, according to Gharehghani et al. [66]. This is due to the fact that biodiesel contains oxygen, which raises the cetane number. In comparison to natural gas/diesel, the mixture of natural gas/biodiesel produced 1.6% higher BTE as noticed by Gharehghani et al. [66].

3.5 Low-temperature combustion advantages and challenges

The combustion temperature in the LTC mode was always lower than the combustion temperature in a regular diesel engine. There are primarily two strategies to achieve low-temperature combustion: one is to operate the engine with higher EGR, and the other is to operate with an excess air ratio 0 greater than 1 [67]. Fuel combusted and oxidized at higher temperatures under stoichiometric operating conditions, resulting in more NOx production. Also, due to a reduction in oxygen availability in the fuel spray periphery, maximum soot emission was observed under the stoichiometric condition compared to normal diesel combustion [68]. Higher fuel injection pressure is usually a viable approach for overcoming the aforementioned concerns. Higher fuel injection pressure promotes atomization, mixing, and vaporization. However, the key duty to be remedied in modern injection technology in low-temperature combustion is the wall impingement of fuel caused by spray tip penetration at increased fuel injection pressure [69]. Furthermore, improved injection strategies such as high-pressure injection and CRDI approaches reduce the ignition delay period and boost premixed phase combustion, resulting in increased NO_x emissions. The ignition delay and combustion phasing will be lengthened by using a higher level of cold EGR, lowering the compression ratio, and using variable valve timing control to advance the exhaust valve opening. Increased ignition delay enhances air-fuel mixing, resulting in increased homogeneity in the air-fuel combination. Higher EGR rate and lower compression ratio reduce the cylinder peak pressure and temperature, which has a major impact on engine performance and higher fuel consumption.

Getting LTC mode to work in real-time settings with heavy engine load is difficult. It is impossible to manufacture engines with a larger amount of EGR. In addition, the engine’s higher BTE should compensate for the increased EGR. In the LTC condition, an external charge booster is necessary to produce higher BTE [70]. When the engine is running at a higher RPM, moderate EGR with an intake charge booster raises the cylinder peak pressure. The combustion process changes depending on the engine load, and it is influenced by the different equivalency ratio and fuel mixing zone, making the engine demanding and difficult to modify the operating state for each load [71]. The real-time modern diesel engine employs dual fuel technology, multiple injection method, and negative valve overlapping. However, these technologies are costly and difficult to implement across the board. By increasing the premixed charge quantity while lowering peak pressure and temperature, these innovations reduce the fuel-rich zone [72].

4. Conclusions

This study provides a comprehensive overview of hydrogen production techniques and fuel cell vehicle also described about the low-temperature combustion (LTC) techniques and how it is improve the reliability and fuel efficiency of the CI engine combustion cycle with low emissions and noise. The important findings are presented in this review can be summarized below:

• Even though fuel cells have demonstrated and shown to be a very promising fuel, there are still a number of limitations that prohibit them from being used on a bigger scale than other fuels. The following are some of the most pressing issues that must be addressed right now: Compared to other kinds of energy, the FC has lower overall efficiency. The material and fabrication of the FC have high production costs.

• One of the most pressing concerns is the cost of hydrogen, as well as its storage. Because hydrogen is a relatively light and dangerous gas, it must be stored in special containers. Thermal management in the case of high-temperature fuel cell like solid oxide fuel cell is a type of fuel cell in which the temperature is higher than the ambient temperature.

• The size and weight of current fuel cell systems must be further reduced to meet the packaging requirements for automobiles. This applies not only to the fuel cell stack, but also to the ancillary components and major subsystems.

• PCCI combustion efficiently decrease the CO and HC emission as compared to the HCCI engine, but NO_x and soot emissions were significantly increased with increase in premixed charge percentage. However, the smoke and NO_x emissions were identified as minimum level when compared with conventional diesel engine combustion.

• Higher cycle-to-cycle variation, unpredictable pressure rise, combustion noise and knocking were occurred in the HCCI mode of combustion due to higher homogeneity and unpredictable auto-ignition zone.

• RCCI combustion is preferable for higher load condition due to combustion phase control and higher brake thermal efficiency than PCCI and HCCI modes. The use of natural gas as a reactive fuel was extending the load limit and attained the efficient, clean combustion which significantly decreases the NO_x and soot emission as compared to other techniques.

• The double injection of high reactive fuel in the RCCI combustion decreases the peak pressure and ringing intensity which efficiently decrease the smoke and NO_x emission. The advanced second injection in the RCCI increases the reactive controlled combustion and late second injection increase the mixed controlled combustion.

• The combustion efficiency was increased while using the B20 as the high reactive fuel. Due to the oxygen availability in the biodiesel promotes the oxidization process, which decreases the HC and CO emission as compared to the diesel/gasoline RCCI combustion.

Many experiments have extensively demonstrated that there is a wide and unexploited scope for improving low-temperature combustion using different fuel injection parameter and different reactive fuel injection. The overall study infers that depending on the operating condition, engine configuration parameters, fuel injection mechanism and fuel mixing method influenced more on the engine performance and emission characteristics. Hence, further research work will be needed to the trade-off between the NOx and soot emission with improvement in the engine performance.

Acknowledgments

The authors would like to thank King Mongkut’s University of Technology North Bangkok (Grant Contract No. KMUTNB-KNOW63-28, KMUTNB-Post-65-09, KMUTNB-Post-65-05) for financial support during this work.