The findings indicate that while hydrogen-fueled engines present a clear advantage in terms of emissions reduction,challenges such as cost and space requirements for hydrogen storage rem
Overview of hydrogen
Properties of hydrogen
Hydrogen fuel is based on hydrogen, a chemical element which has symbol H and atomic number 1 It is the lightest element and, at standard conditions, is a gas of diatomic molecules with the formula H , sometimes called2 dihydrogen, but more commonly called hydrogen gas, molecular hydrogen or simply hydrogen.[1] To understand how hydrogen fuel is applied in transportation, the properties of hydrogen is shown: a Physical properties:
Hydrogen has physical properties of a colorless, odorless, tasteless, harmless element The majority of Earth’s hydrogen is present in molecular forms, including water and organic compounds The commonly seen isotope of hydrogen embraces one proton, one electron, and no neutrons in each atom.
Hydrogen fuel has a very low density of 0.0899 g/L at standard temperature and pressure, which is about 14 times lighter than air It also has a very low viscosity of 0.0085 cP making it easy to flow It has a very low boiling point of -252.9°C and a very low melting point of -259.1°C It can be liquefied under low temperature and high pressure, but it requires a lot of energy to do so. Liquefied hydrogen has a density of 70.8 g/L at -253°C and 1 atm.
Another physical property for hydrogen to be potential for fuel usage is its very high specific heat capacity of 14.3 kJ/kg K This means it can absorb or release a lot of heat without changing its temperature much It also has a very high latent heat of vaporization of 445 kJ/kg, making its absorption or release of heat occurs in large amount when it changes from liquid to gas or vice versa. b Chemical properties:
Hydrogen is also a potential source of clean and renewable energy because of its important chemical properties that affect its use as a fuel First of all, it has a high energy content per weight, but a low energy density per volume. This means that hydrogen gas can store a lot of energy in a small mass, but it occupies a large volume at standard temperature and pressure.
Hydrogen also has the property of being flammable and explosive when mixed with air or oxygen Its flammability range is between 4% and 75% by volume in air which makes it ignite spontaneously when exposed to a spark, heat, or sunlight Even though the combustion of hydrogen gas with oxygen produces water and a large amount of heat which can be used to generate electricity or power vehicles, it also produces nitrogen oxides (NO ) asx pollutants.
Moreover, hydrogen is very reactive and can form compounds with almost all elements, except some of the noble gases It can react with metals, nonmetals, and organic compounds, forming hydrides, acids, bases, and hydrocarbons The property cause hydrogen embrittlement, which is the weakening or cracking of metals due to the absorption of hydrogen atoms The hydrogen atoms can diffuse into the metal lattice and create internal stresses or defects that reduce the strength and ductility of the metal.
Therefore, hydrogen is a great source of fuel both for SI and CI.Specifications of the fuel properties of hydrogen are given in figure 1:
Figure 1: Fuel properties of hydrogen[1]
Color code of hydrogen
To produce hydrogen molecules, energy sources and methods are crucial factors determining the color of hydrogen Although hydrogen itself is a colorless gas, there are approximately nine color codes assigned to it, reflecting the sources or processes used in its production These colors include green, blue, grey, brown, black, turquoise, violet, purple, red, and white Green hydrogen is generated through water electrolysis using electricity from renewable sources, earning its name due to the absence of CO emissions during production.2
Electrolysis, also known as water splitting, requires an energy input While it is an environmentally friendly process, electrolysis is energy-intensive but far more eco-friendly than grey hydrogen production.
Blue hydrogen originates from fossil fuels, yet CO emissions are2 captured and stored underground (carbon sequestration) Many companies are striving to employ carbon capture, utilization, and storage (CCUS) technology to utilize carbon capture for fuel Since it emits no CO , blue hydrogen production2 is classified as carbon-neutral The capture and storage of carbon dioxide instead of releasing it into the atmosphere allow blue hydrogen to become a low-carbon fuel Two primary production methods are steam methane reforming (SMR) and coal gasification, both involving carbon capture and storage Blue hydrogen serves as a cleaner substitute for grey hydrogen, although it is expensive due to carbon capture technology usage.
Green hydrogen is produced by utilizing electricity from clean energy sources It is considered a low-emission or emission-free fuel because it uses energy sources such as wind and solar power, thus emitting no greenhouse gases during electricity production Green hydrogen is created when water (H O) is2 split into hydrogen (H ) and oxygen (O ) Water splitting, also known as2 2 electrolysis, is an energy-intensive process but significantly more environmentally friendly than grey hydrogen production.
Grey hydrogen is derived from fossil fuels and commonly utilizes steam methane reforming (SMR) with water In this process, CO is generated and2 ultimately released into the air In short, grey hydrogen is produced from fossil fuels such as natural gas or coal, accounting for about 95% of global hydrogen production today The two primary production methods are steam methane reforming and coal gasification Both processes release carbon dioxide (CO ) If2
CO2 is emitted into the atmosphere, the hydrogen produced is called grey hydrogen Grey hydrogen is not considered a low-carbon fuel.
Black or brown hydrogen is produced from coal Black and brown refer to bituminous (black) and sub-bituminous (brown) coal types Coal gasification is a primary method used to produce hydrogen However, this process is highly polluting, with CO and carbon monoxide-CO being generated as by-products2 and released into the atmosphere.
Turquoise hydrogen can be extracted by using thermal methane decomposition through the methane pyrolysis process Although in the experimental stage, this process removes carbon in solid form rather than CO2 gas Violet hydrogen is produced using nuclear energy and heat through high temperature electrolysis combined with water splitting.
Red hydrogen is derived from biomass Biomass can be converted to produce hydrogen through gasification Depending on the type of biomass and the use of carbon capture technology, red hydrogen may have lower CO2 emissions than grey hydrogen If CO is completely captured and no other2 emissions occur, it may be considered green hydrogen.
Pink hydrogen is produced through water electrolysis using electricity from nuclear power plants Red hydrogen is produced through water splitting by using a catalyst at high temperatures with nuclear power providing the energy source Pink hydrogen refers to hydrogen produced through electrolysis powered by nuclear energy Pink hydrogen is sometimes considered green because it does not emit or release CO during operation.2
Yellow hydrogen may indicate hydrogen produced through electrolysis using solar energy However, it is also used to indicate that electricity for electrolysis comes from mixed sources This combination of sources comes from the grid and relies on availability and may include renewable energy sources to fossil fuels.
Hydrogen production
Hydrogen can be produced from diverse, domestic resources, including fossil fuels, biomass, and water electrolysis using electricity The environmental consequences and energy efficiency of hydrogen production are contingent on the specific method employed Multiple initiatives are currently in progress to reduce the costs associated with producing hydrogen. a Natural gas reforming/Gasification
The process involves the reaction of natural gas with steam at high temperatures (700℃ - 1,000℃), resulting in a mixture of hydrogen, carbon monoxide, and a small amount of carbon dioxide Carbon monoxide further reacts with water to produce additional hydrogen This method is widely employed due to its cost-effectiveness, efficiency, and popularity.
The synthesis gas can also be obtained by subjecting coal or biomass to high-temperature reactions with steam and oxygen in a gasification furnace.
This process, known as gasification, transforms coal or biomass into gas components The resulting synthesis gas contains hydrogen and carbon monoxide, which can be separated by reacting with steam to obtain hydrogen. The steam-reforming process is characterized by being endothermic, which means that heat must be supplied for the reaction to take place.
This well-established production process in which high-temperature (700℃ - 1,000℃) is utilized to produce hydrogen from methane sources like natural gas In the steam-methane reforming process, methane reacts with steam at a pressure of 3 -25 bar (1 bar = 14.5 psi), facilitated by a catalyst, resulting in the production of hydrogen, carbon monoxide, and a minor amount of carbon dioxide The steam-reforming process is characterized by being endothermic, which means that heat must be supplied for the reaction to take place.
Afterward, in a process known as “water-gas shift reaction,” carbon monoxide and steam are chemically reacted with the aid of a catalyst to yield carbon dioxide and a higher amount of hydrogen In the final stage of the process, referred to as “pressure swing adsorption,” carbon dioxide and various contaminants are eliminated from the gas stream, leaving behind pure hydrogen Steam reforming can also be employed to generate hydrogen from alternative fuel sources (such as ethanol, propane, or even gasoline).
In the partial oxidation process, methane and other hydrocarbons present in natural gas undergo a reaction with a limited supply of oxygen,typically obtained from air This limited oxygen quantity is insufficient for the complete oxidation of hydrocarbons into carbon dioxide and water.
Consequently, the reaction primarily yields hydrogen, carbon monoxide (and nitrogen when air is used instead of pure oxygen), along with a small amount of carbon dioxide and other compounds.
Subsequently, in the water-gas shift reaction, carbon monoxide reacts with water to produce carbon dioxide and a larger quantity of hydrogen The partial oxidation process is exothermic and generally faster than steam reforming, requiring a smaller reaction vessel Comparatively, the partial oxidation process initially produces a lower amount of hydrogen per unit of input fuel when compared to steam reforming of the same type.
Partial oxidation of methane reaction:
2 O 2 → CO+2 H 2 (+ eat ℎ ) Water-gas shift reaction:
CO+ H 2 O →CO 2 + H 2 (+small amount of eat ℎ ) b Electrolysis
Electrolysis is an essential process that uses electricity to split water into hydrogen and oxygen with a device called an electrolyzer Electrolyzers can range in size from small-scale devices suitable for decentralized hydrogen production to large-scale facilities that can be directly integrated with renewable power sources.
Similar to fuel cells, an electrolyzer consists of a positive electrode(anode) and a negative electrode (cathode) separated by an electrolyte Different electrolyzers operate in various ways, primarily determined by the specific electrolyte material used and the type of ions it conducts.
When the electric current is passed through the water, several chemical reactions occur at the electrodes At the cathode (negative electrode), water molecules ( H 2 O) gain electrons, resulting in the reduction of water and the formation of hydrogen gas ( H 2):
At the anode (positive electrode), water molecules lose electrons through the process of oxidation, leading to the production of oxygen gas (O 2 ) and positively charged hydrogen ions ( H +¿¿ ):
Liquid derived from biomass sources, such as ethanol and biofuels, can be converted to produce hydrogen in a process similar to natural gas reforming. Liquid biomass-derived fuels can be more easily transported than their bulk biomass counterparts, enabling centralized production or decentralized hydrogen production at fueling stations.
The liquid fuel reacts with steam at high temperatures in the presence of a catalyst to create synthesis gas, consisting mainly of hydrogen, carbon monoxide, and some carbon dioxide Additional hydrogen and carbon dioxide are generated by reacting carbon monoxide (produced in the first step) with steam at high temperatures in a "water-gas shift reaction." Finally, hydrogen is separated and refined.
CO+ H 2 O →CO 2 + H 2 (+small amount of eat ℎ ) d Microbial biomass conversion
Biomass can be converted into sugar-rich feedstock, which can be fermented to produce hydrogen Microbial biomass conversion processes exploit the ability of microorganisms to consume and digest biomass, releasing hydrogen in the process Depending on the pathway, this research could lead to commercially viable systems in the medium to long term.
In fermentation-based systems, microorganisms, such as bacteria, break down organic matter to produce hydrogen The organic matter can be refined sugars, raw biomass sources like corn stalks, or even wastewater Because they don't require light, these methods are sometimes referred to as "dark fermentation".
In direct hydrogen fermentation, bacteria naturally produce hydrogen. These bacteria can break down complex molecules through various pathways, and the byproducts of some pathways can be combined by enzymes to produce hydrogen Scientists are researching ways to make hydrogen fermentation systems faster (improve reaction rates) and more productive, generating more hydrogen from the same amount of organic matter (increase yield).
Method for storing and transporting hydrogen fuel
Hydrogen fuel is a potential alternative to fossil fuels, but it also carries risks of combustibility, electric shock, embrittlement… To apply this superior energy source in vehicle, storing and transporting technology is needed to be considered precisely.
Main methods for storing hydrogen fuel are:
There is a method of storing hydrogen gas in high-pressure tanks, typically 350–700 bar (5,000–10,000 psi) This method is widely used for hydrogen vehicles, but it requires expensive and heavy tanks and reduces the cargo space.
Hydrogen can be stored as a liquid in cryogenic tanks at very low temperatures, around -253°C This way allows for higher density and longer range than compressed hydrogen However, it requires a lot of energy to maintain the low temperature and causes boil-off losses.
Another way of storing hydrogen is absorbing it into certain metals or alloys at moderate temperatures and pressures This method can store a large amount of hydrogen in a compact and safe way, but it also requires high heat to release the hydrogen with a higher cost.
Hydrogen is stored by binding it to chemical compounds that can release it upon demand Although the method can achieve high densities and low pressures, it involves complex reactions and catalysts and may produce harmful by-products.
The adsorption of hydrogen occurs on the surface of porous materials such as activated carbon or metal-organic frameworks This method can store hydrogen at low temperatures and moderate pressures, but it also has a low capacity and a slow kinetics.
Moving on to hydrogen fuel transportation, this is the process of delivering hydrogen from the point of production to the fueling station or power generator Hydrogen can be transported in different ways, depending on the distance, demand, and cost and the main methods are:
Pipeline: Hydrogen can be transported as a pressurized gas through pipelines, similar to natural gas This method is suitable for regions with high and stable demand, such as industrial clusters or urban areas. Pipelines can offer low-cost and reliable delivery, but they require high initial investment and maintenance.
Cryogenic liquid tanker: Hydrogen can be transported as a liquid in super-insulated, cryogenic tanker trucks This method is suitable for longer distances and lower demand, such as remote or rural areas Liquid hydrogen has a higher energy density than gaseous hydrogen, but it requires energy-intensive liquefaction and vaporization processes.
Gaseous tube trailer: Hydrogen can be transported as a compressed gas in tube trailers, which are trucks with multiple cylinders This method is suitable for shorter distances and smaller demand, such as backup or emergency supply Gaseous hydrogen has a lower energy density than liquid hydrogen, but it does not require liquefaction or vaporization.
Chemical carrier: Hydrogen can be transported as a chemical compound that can release hydrogen on demand, such as metal hydrides, ammonia, or methanol This method is suitable for long distances and large-scale applications, such as export markets or maritime transport. Chemical carriers can offer high energy density and safety, but they require additional conversion steps and may have environmental impacts.
At the point of use, hydrogen fuel needs additional infrastructure components, such as storage tanks, compressors, dispensers, meters, and purifiers Therefore, the choice of storing and transporting method with its infrastructure depends on various factors, such as the source, quality, quantity,and end use of hydrogen.
Overview of hydrogen combustion engine
History of hydrogen combustion vehicles
The journey of hydrogen as a fuel for internal combustion engines spans over two centuries, marking a fascinating evolution in automotive technology. Back in the early 19th century, visionaries like Franỗois Isaac de Rivaz and Rev.
W Cecil pioneered the concept of hydrogen-powered engines Rivaz's innovative design in 1807 birthed the first hydrogen-oxygen combustion engine, propelling a prototype vehicle with a manual control mechanism for fuel injection and ignition, reminiscent of modern combustion engines.
The momentum continued with Etienne Lenoir's breakthrough in 1860, introducing a practical small horizontal gas engine fueled by hydrogen, derived from water electrolysis Lenoir's engine, though initially hydrogen-based, later accommodated other gases like coal gas, showcasing versatility Subsequently, a hydrogen-fueled vehicle equipped with Lenoir's engine completed a significant test drive in 1863, marking a milestone in automotive history.
Despite early promise, hydrogen engines faced formidable challenges, including lower efficiency compared to hydrocarbon engines and storage issues. The dominance of fossil fuels, economically supported by established extraction and refining methods, hindered widespread adoption of hydrogen as an automotive fuel.
However, the tides shifted during and after World War I and II, igniting renewed interest in hydrogen as a viable alternative Rudolf Erren's pioneering work in the 1920s laid crucial groundwork, despite setbacks caused by wartime destruction Erren's experimentation with direct hydrogen injection in gasoline and diesel engines set a precedent for future advancements.
Remarkably, even amidst global conflicts, instances like Boris Shelishch's conversion of trucks to hydrogen in 1941 during the siege of Leningrad demonstrated hydrogen's resilience and efficacy in dire circumstances.
The 1970s witnessed a resurgence of interest in hydrogen engines, fueled by environmental concerns stemming from fossil fuel emissions Initiatives by researchers like Roger Billings and institutions in Japan and Germany propelled hydrogen combustion engine development, culminating in the introduction of the first Japanese hydrogen vehicle in 1974 and subsequent technological refinements.
Collaborative efforts between BMW, Mazda, and research institutions further propelled hydrogen engine technology into the 20th century This era not only showcased the feasibility of hydrogen in existing engine designs but also set the stage for future innovations.
Entering the 21st century, the evolution of hydrogen-powered vehicles unfolds with a plethora of new models and advancements, driving the boundaries of automotive engineering forward The trajectory of hydrogen as a fuel for internal combustion engines embodies human ingenuity, perseverance, and an unwavering commitment to sustainable transportation solutions.
Figure 4: The history of hydrogen vehicles
1.2.2 Classification hydrogen internal combustion engine (HICE)
There are several types of hydrogen internal combustion engines (HICE) that have been developed and explored for various applications. a Hydrogen in spark ignition (SI) engines
The operating principles of hydrogen SI engines are similar to those of gasoline engines Hydrogen is introduced into the combustion chamber along with air in a stoichiometric or slightly lean mixture The mixture is then ignited by a spark plug, leading to combustion and the expansion of gases This expansion generates power, which is transmitted to the crankshaft and ultimately to the vehicle's wheels The exhaust gases, primarily water vapor, are expelled through the exhaust system.
In spark ignition ICEs, hydrogen can be introduced by three methods: manifold induction, port injection and direct Injection.
Manifold induction involves introducing hydrogen into the intake manifold along with the incoming air The hydrogen mixes with the air, forming a homogeneous fuel-air mixture that is drawn into the combustion chamber during the intake stroke.
Port injection involves injecting hydrogen directly into the intake manifold, just upstream of the intake valve, rather than fuel in at a central point. The hydrogen is injected in a controlled manner, allowing for a more precise fuel-air mixture Port injection systems typically include dedicated fuel injectors that deliver hydrogen in a measured and consistent manner.
Hydrogen direct injection involves injecting hydrogen directly into the combustion chamber This method allows for precise control over the fuel-air mixture and can optimize combustion efficiency Direct injection systems typically operate at higher pressures to ensure effective fuel atomization and distribution.
Figure 5: Hydrogen induction in a spark-ignition engine[3] b Hydrogen in compression ignition (CI) engines
CI hydrogen engines, also known as hydrogen-diesel engines, operate on the principle of compression ignition, similar to diesel engines In this type of engine, hydrogen is injected into the combustion chamber along with air that has been highly compressed The high temperature and pressure cause the hydrogen to auto-ignite, resulting in combustion and power generation.
In compression ignition ICEs, hydrogen can be introduced by two methods: dual fuel mode operation and direct injection.
A dual fuel hydrogen engine is an innovative type of engine that combines the use of hydrogen and another primary fuel (diesel), to power the combustion process In this engine configuration, hydrogen is used as a supplemental fuel alongside the primary fuel, offering unique benefits and characteristics.
In the Direct Injection method, hydrogen is injected directly into the cylinder at the end of the compression stroke The gas impinges on the glow plug in the combustion chamber and hence surface ignition burning of fuel occurs Moreover, it is possible to introduce lean hydrogen–air during the intake of an engine and then inject the bulk of hydrogen toward the end of compression The advantage of this method is that surface ignition is not dependent on the compression ratio for ignition Literature shows that a very small difference in brake thermal efficiency was observed in the compression ratio 12–18.
Technology of hydrogen fuel engine 2.1 Hydrogen fuel in spark ignition engine
Manifold induction
The hydrogen manifold induction is a system that allows hydrogen gas to be injected into the intake manifold of a spark ignition engine.
Spark ignition engine using hydrogen manifold induction does not require major modifications to the existing engine design, it is adapted to optimize the combustion for hydrogen with additional hydrogen injection system Crucially, it has air-fuel mixing and intake manifold components to direct the flow of air and fuel into the combustion chamber.
In the fuel mixture stage, a dual-fuel approach is employed by the intake system The primary fuel remains petrol, while hydrogen serves as the secondary fuel Hydrogen is injected into the intake air stream using hydrogen injectors which release controlled amounts of gaseous hydrogen into the incoming air, creating a homogeneous mixture Then in the induction process, both fuels are introduced into the engine’s inlet within the manifold When hydrogen is induced into spark ignition engine, a flame arrester is used for safety.
In manifold induction system, hydrogen is injected to the manifold at low temperature which leads to several of its characteristics as a fuel in spark ignition engine Inducting hydrogen fuel allows quality governing, ultra-lean operation, high thermal efficiency, and low exhaust emissions in the equivalence range of 0.3–0.5 for the engine.[1] Besides, this system also has other properties like every hydrogen fuel engine such as: higher flame speed, knock suppression and dual fuel capability.
Port injection
Hydrogen port injection in spark ignition engines involves supplying hydrogen gas directly into the intake port of the engine, usually alongside the conventional air-fuel mixture.
This method is designed to enhance combustion characteristics and improve overall engine performance with the usage of hydrogen fuel The distribution of injectors is organized so that the fuel is introduced directly to each intake port rather than rather than drawing fuel at a central point. [1]
At the onset of the intake stroke, air is introduced to dilute the heat of the remaining gases and to lower the temperature of any overheated areas within the combustion chamber Subsequently, hydrogen is administered into the manifold following the starting of the intake stroke.Properties:
As the hydrogen is delivered less in the manifold at any one time,the probability for premature ignition is reduced Coming with this property is the reduction of the backfire risk,[1] and other traits of most hydrogen fuel engine.
Spark ignition direct injection
In additional to the port injection delivery method, hydrogen direct injection is also a way of introducing hydrogen promptly but is direct to the combustion chamber skipping the intake component.
Direct injection systems has an advanced injector system that require more sophisticated injectors capable of handling high-pressure hydrogen with exact precision in control Apart from the enhancement, its system still keep the common design of every hydrogen fuel engine. Operating principle:
Early injection typically denotes the administration of hydrogen direct injection soon after the intake valve closes, during the initial phase of the compression stroke Conversely, late direct injection introduce hydrogen towards the end of the compression stroke, usually concluding right before the ignition spark is initiated In order to avoid displacement of a fresh charge by hydrogen of low density, the start of injection is normally set after intake valve closing.[1] During the process of fuel injection, the inlet valve is fully sealed to prevent early ignition throughout the suction phase Last but not least, it is important to consider the condition of pressure inside the injection system compared with the cylinder to ensure hydrogen be capable of flowing directly to the combustion chamber.
Hydrogen direct injection system was developed to achieve near zero emission with an increase in power.[1] Coming with the critical condition to supply hydrogen directly to the combustion chamber, the system offers the range of pressure about 5 to 20 bar and up to 100 bar for recent and early direct injection, respectively.
Hydrogen fuel in compression ignition engine
In dual fuel mode, hydrogen is mixed with fresh air and introduced into the combustion chamber along with diesel fuel This method typically involves a conventional swirl chamber type diesel engine.
Hydrogen is ignited by a spray of injected liquid fuel (diesel), utilizing the autoignition characteristics of hydrogen The combustion process relies on a combination of hydrogen's high combustion rates and diesel's ignition properties.
Hydrogen addition enhances combustion efficiency due to its high flame speed, resulting in faster combustion rates and reduced combustion duration However, challenges such as incomplete combustion and uncontrollable combustion arise when insufficient fuel is injected or when the hydrogen-gas mixture becomes too rich.
Approximately 30% hydrogen can be replaced using this method,with higher percentages leading to uncontrolled pressure rise Endurance studies have shown that material compatibility is not a significant issue, making this method viable for adoption.
In surface ignition, hydrogen is directly introduced into the cylinder at the end of compression The gas impinges on the glow plug in the combustion chamber, initiating combustion of the fuel.
Surface ignition occurs when hydrogen impinges on the glow plug, leading to the burning of fuel This method does not depend on the compression ratio for ignition, offering flexibility in engine design. Properties:
Surface ignition results in higher power output compared to premixed mode, with a peak power approximately 14% higher than conventional diesel-fueled operation Hydrogen direct injection allows for better control of engine operation compared to port injected modes,offering enhanced fuel-air mixing and reduced local peak temperatures in the combustion chamber Direct injection enables better control of injection timing and duration, influencing factors such as the rate of pressure rise and maximum combustion pressure.
Advantages and disadvantages of hydrogen fuel
Clean and non-polluting operation:
Hydrogen is considered a clean fuel because it only produces water vapor as a byproduct during combustion, therefore, hydrogen fuel engines do not produce carbon monoxide (CO), hydrocarbon (HC) and carbon dioxide ( CO 2 ¿ emissions, a significant greenhouse gas that contributes to climate change This helps reduce negative impacts on the environment and human health.
Low ignition energy and high efficiency:
Hydrogen internal combustion engines (ICE) have a lower ignition energy compared to traditional gasoline engines This is due to the combustion of hydrogen in these engines, which operate at lower flame temperatures and heat transfers As a result, hydrogen ICE can effectively run on lean mixtures while still enabling quick combustion Moreover, the high diffusivity of hydrogen, meaning it mixes with air more rapidly than gasoline, helps reduce risks associated with potential leaks.
To achieve a driving range comparable to conventional vehicles, hydrogen, with its low volumetric energy density, needs to be stored as a compressed gas This necessitates the use of high-pressure tanks capable of storing hydrogen at either 5,000 or 10,000 pounds per square inch (psi) At retail dispensers located at gas stations, these tanks can be filled in approximately 5 minutes Research and development efforts are exploring alternative storage technologies, such as chemically bonding hydrogen with materials like metal hydrides or low-temperature sorbent materials These advancements aim to address the challenges associated with hydrogen storage and enhance its viability as a fuel source for vehicles.
Several countries are actively pursuing initiatives to promote the production of hydrogen as a fuel for various applications, including airplanes, ships, and power generation By adapting internal combustion engines to burn hydrogen alongside or instead of gasoline, these countries aim to reduce the reliance on fossil fuels If the hydrogen is produced using alternative energy sources, such as renewable energy, it has the potential to be a cost-effective solution for rapidly decreasing the use of fossil fuels in multiple sectors. 2.3.2 Disadvantages
Due to its lower energy density compared to other fuels, hydrogen requires a larger quantity to generate the same amount of work When combined with the inherent inefficiencies of piston engines, hydrogen engines do not provide a substantial energy advantage in general.
Hydrogen poses challenges in terms of handling due to its low volumetric energy density To store enough hydrogen to power a car, it needs to be highly compressed at high pressures ranging from 350 to 700 bar and packed into a tank Approximately 5/6 kg of compressed hydrogen is needed to cover a distance of about 600 km Without compression, the amount of hydrogen that could fit in a car's tank would only be sufficient to cover a mere 5 km.
Although hydrogen engines do not produce carbon emissions, the heat generated in the combustion chamber can lead to the formation of nitrogen oxide(NOx) as a byproduct NOx is detrimental to the environment, which means that despite hydrogen engines having zero carbon emissions, they are not completely free from emissions.
The combustion of hydrogen with oxygen produces water as its only product:
Applications and challenges of hydrogen combustion engine 3.1 Fuel efficiency and emission of hydrogen fuel engine in reality
Fuel efficiency of hydrogen fuel engine
Hydrogen-powered internal combustion engines hold significant promise for enhanced efficiency The broad combustibility range and swift combustion of hydrogen with air enable diverse strategies for managing engine load By having the high auto-ignition threshold, hydrogen fuel engine permits greater compression ratios Present research on sophisticated hydrogen ICEs is concentrated on surpassing 45% brake thermal efficiency and simultaneously maintaining low NO emission levels.x
To consider more clearly about hydrogen efficiency, its volumetric efficiency and brake thermal efficiency are two most significant factor that bring various differences compared to other internal combustion engines From recent science research, the volumetric efficiency and brake thermal efficiency of hydrogen fuel engines is shown in Figure 1 and Figure 2:
Figure 7: Volumetric and brake thermal efficiency of CI engine[5]
Figure 8: Volumetric and brake thermal efficiency of SI engine[5]
As it was indicated, the volumetric efficiency of hydrogen fuel both CI and DI engine decreased as the percentage of hydrogen mixture increased This efficiency creates an impact on the engine power and fuel consumption which becomes challenges that scientist need to improve for hydrogen fuel system. Beside that is also the decline of brake thermal efficiency which leads to combustion efficiency.
In reality, a test of a 67 ml 4-stroke hydrogen engine examined by a dynamometer at the best operating point with a similar hydrogen engine but with 26% gasoline efficiency was made After being installed in a lightweight vehicle and driven up known gradients while measuring speed and hydrogen flow, the result yielded an average decrease in efficiency of 3.5% to 5.9% of the fully used hydrogen com
Hydrogen combustion engines, when powered by green hydrogen, produce zero CO emissions from the fuel itself during operation1 However, it’s2 important to note that the CO emissions associated with the production of2 hydrogen can vary significantly, depending on the method used For instance,emissions are higher when hydrogen is produced from fossil fuels without carbon capture and storage (CCS) compared to when it’s produced via electrolysis using renewable energy sourcespared to the other one.[3]
Emission of hydrogen fuel engine
Hydrogen fuel engines are being increasingly developed because of its significant contribution to nearly zero emissions When powered by hydrogen, these engines emit only water vapor and warm air as exhaust during operation. This makes them a true zero-emission technology when the hydrogen is sourced sustainably.
For CO emission, the engine using hydrogen fuel do not emit CO but the2 2 production process of hydrogen can vary in terms of emissions Hydrogen produced from fossil fuels without carbon capture can have higher associated
CO2 emissions compared to hydrogen produced via electrolysis using renewable energy sources.
Consider NO emission, hydrogen combustion still produce NOx x emissions due to the high temperatures involved in the process However, advancements in engine technology and design are aimed at reducing these emissions For instance, a study on a hydrogen-fueled dual-cylinder free-piston engine showed potential for significant NO reduction, with the largest reductionx being 57.37% at 6 Hz compared to a conventional gasoline engine.
For unburned hydrocarbon (UHC) exhaust, it is important to note that in practical engine conditions, some minor UHC emissions can occur due to incomplete combustion or during the cold-start phase when the engine and catalyst are not yet at optimal operating temperatures But still, these emissions are significantly lower compared to conventional fossil fuel-powered engines.Therefore, hydrogen fuel engines have the potential for zero tailpipe emissions Combustion engine will not only contribute to cleaner environment by reducing the emission in its operation but also by using a renewable energy source that can replace fossil fuel.
Economic and safety of hydrogen fuel
The economic landscape of hydrogen energy systems is multifaceted, with critical considerations spanning storage, production, utilization, and transportation costs At present, the cost of hydrogen production emerges as a pivotal determinant, heavily contingent upon the prevailing costs of primary energy sources In this intricate calculus, new hydrogen production technologies leveraging renewable energy sources exhibit promise but presently entail higher costs compared to conventional fossil fuel-based methods.
The cost dynamics of hydrogen production from non-fossil fuel sources are intricately intertwined with the cost of electricity Electrolysis, notably, stands as a prominent method for generating hydrogen from renewable sources. However, its economic feasibility is tethered to factors such as the upfront investment in electrolyzer technology and its subsequent utilization Despite the initial financial outlay, electrolysis offers a compelling proposition, given its capacity to yield green electricity devoid of carbon emissions.
Conversely, the economics of fossil fuel-based hydrogen production are inextricably linked to the market prices of these resources Thermochemical methods, while holding promise, encounter hurdles such as pressure swing absorption and catalyst expenses, which contribute to elevated production costs. Nonetheless, these methods can mitigate biomass expenses and bolster overall efficiency.
Water electrolysis, owing to its favorable economics, presents a viable solution for small-scale hydrogen production However, as production scales up, costs escalate proportionally, with the expenditure on electrolyzer utilization emerging as a significant component.
When assessing the cost landscape, it becomes evident that dark and photo fermentative methods offer relatively lower production costs, ranging from $2.5 to $18.7 USD per kg In contrast, steam methane reforming and coal gasification prove to be more cost-effective options compared to electrolysis methodologies.
Solar electrolysis, while holding immense promise for sustainable hydrogen production, currently commands the highest cost among the methods discussed, with production expenses reaching $5.0 USD per kg Nonetheless, the trajectory of technological advancement instills optimism, suggesting that these costs are poised to decrease over time Consequently, solar electrolysis stands as a compelling option for future consideration, particularly from an environmental standpoint, as it offers the potential to yield hydrogen with minimal ecological footprint.
In summation, the intricate interplay of various hydrogen production technologies underscores the importance of continued research and technological innovation As advancements progress and economies of scale are realized, the economic viability of hydrogen as a sustainable fuel source for automotive applications is expected to improve significantly, paving the way for a greener and more efficient transportation landscape Therefore, the commercial development of these methods with time, and, according to their negligible environmental impact, the cost of H production is anticipated to be less or2 comparable to that of conventional methods using fossil fuels
Figure 9: Costs of hydrogen production technologies
Figure 10: Fossil fuels dominate world’s energy system
Figure 11: The hydrogen cycle 3.2.2 Safety of hydrogen fuel
The adoption of hydrogen as a fuel source for automotive engines presents a promising avenue towards sustainable transportation However, ensuring the safety of hydrogen-powered engines is paramount to their widespread implementation This essay delves into a comprehensive analysis of the safety considerations associated with such engines.
Explosion Potential: Hydrogen's propensity to form explosive mixtures when combined with air mandates stringent precautions Due to its high volatility and flammability, hydrogen poses a notable risk of explosion if not handled with care Safety measures such as the incorporation of safety valves and venting systems are imperative to mitigate explosion risks within engine systems.
Combustibility: The unique nature of hydrogen combustion, characterized by the absence of a visible flame, introduces challenges in fire detection This inherent property underscores the need for sophisticated fire detection systems and sensors to promptly identify and address potential fire incidents.
Flammability: Hydrogen's lower ignition temperature relative to conventional fuels escalates the likelihood of fire and explosion events, particularly during the various stages of production, storage, and transportation. Addressing these risks demands meticulous attention to safety protocols and operational procedures.
Leakage Potential: Hydrogen's ability to permeate through materials and cause leaks amplifies the risk of fire and explosion Preventive measures in system design and maintenance are essential to mitigate the potential hazards associated with hydrogen leakage.
Environmental Impact: while hydrogen combustion produces water vapor as a by-product, its emission of carbon dioxide and other pollutants is negligible. This environmentally-friendly characteristic underscores hydrogen's potential as a clean energy alternative, provided safety concerns are adequately addressed.
In summary, the integration of hydrogen-powered engines in automotive systems holds immense promise for reducing greenhouse gas emissions and fostering sustainable transportation However, ensuring their safety necessitates
Potential and challenges in hydrogen fuel used for vehicles
3.3.1 Potential of using hydrogen fuel as a replacement for fossil fuels
The world's energy demand is increasing significantly due to population growth, higher living standards, and the desire for better air quality Meeting these demands poses challenges Currently, most of the world's energy needs are met by oil and natural gas
The dominance of fossil fuels in the global energy market over the past century is due to their high energy density For example, one ton of oil is equivalent to approximately 42.7 GJ, one thousand cubic meters of natural gas is approximately 37 GJ, and one ton of coal is around 25 GJ One million tons of oil can generate about 4.5 TW.h of electricity in modern power plants The high energy density of fossil fuels, along with their good combustion properties, has made oil and natural gas ideal fuels for specialized combustion systems like spark-ignition engines (Otto) and compression-ignition engines (Diesel). Additionally, many fossil fuels exist in a liquid state at ambient temperature and atmospheric pressure, making processing, storage, and transportation convenient Their relatively low and readily available cost further adds to their advantages
Unfortunately, burning hydrocarbon fuels for transportation and heating contributes to over half of greenhouse gas emissions and a significant portion of air pollution Furthermore, the risk of depleting fossil fuel resources is a pressing concern
Therefore, the world is facing an urgent need to develop alternative energy sources Among various alternatives, hydrogen fuel offers the highest potential benefits in terms of diversified supply and minimizing greenhouse gas emissions and hazardous waste Hydrogen possesses most of these desirable qualities and is being evaluated and developed worldwide as an environmentally friendly substitute for gasoline, diesel, natural gas, and other fuels in both transportation and non-transportation applications.
3.3.2 Barriers and challenges of hydrogen fuel
Hydrogen holds promise as a future sustainable fuel, offering the potential to reduce global reliance on fossil fuels and decrease emissions from the transportation sector However, there are several barriers and challenges that this technology needs to overcome to optimize operation and efficiency a Hydrogen storage tank
High Hydrogen Density: Hydrogen has an extremely low density, which means that a large storage tank is required to store a sufficient amount of fuel. The challenge is to find a material that can achieve a high hydrogen density and allow for compact storage.
Strong Chemical Bonds and Atomic Packing: The material used in the storage tank must have strong chemical bonds and close atomic packing to ensure the containment of hydrogen without leakage At the same time, the material should have loose atomic packing to facilitate fast diffusion of gaseous hydrogen.
Thermal Conductivity: The storage material should have adequate thermal conductivity to prevent decomposition due to the heat released during the hydrogen storage process.
Gravimetric and Volumetric Density: Both the gravimetric (mass of hydrogen per unit mass of storage material) and volumetric (volume occupied by hydrogen per unit volume of storage tank) densities are important factors in determining the efficiency of the storage system for both mobile and stationary applications.
Safety concerns: Hydrogen-powered vehicles with internal combustion engines utilize pressurized fuel tanks to store hydrogen While these tanks are designed to prioritize safety, it is important to acknowledge that in the event of a leakage, the highly flammable nature of hydrogen can pose a significant risk. One potential solution to address this concern is the installation of specialized sensors in the vehicles to detect any leaks However, this implementation comes with associated costs. b Hydrogen transportation
Infrastructure Development: Establishing a robust infrastructure to deliver hydrogen from production plants to refueling stations is essential for a hydrogen-based transportation system This involves the construction of pipelines, cryogenic liquid trucks, and compressed tube trailers.
Inefficiency of Transportation: The low density of hydrogen poses challenges in the transportation of the fuel Both liquid and pressurized hydrogen require specialized equipment and may result in inefficiencies during transport.
Pipeline Material Requirements: High-purity stainless steel piping with specific hardness criteria is preferred for hydrogen pipelines to ensure safety and prevent hydrogen embrittlement The selection of appropriate materials is crucial to withstand the unique physical and chemical characteristics of hydrogen. c Hydrogen-fueled internal combustion engines
Lean Mixture Operation: HICE can operate effectively on lean mixtures due to the high energy content and unique physical and chemical properties of hydrogen However, achieving stable combustion and optimizing the performance of lean-burn HICE is a challenge that requires careful control of the air-fuel mixture.
Power Output: The energy density of the hydrogen/air mixture is lower compared to conventional fuels like gasoline This can result in lower power