A Review of the Use of Hydrogen in Compression Ignition Engines with Dual-Fuel Technology and Techniques for Reducing NO x Emissions

: The use of compression ignition engines (CIEs) is associated with increased greenhouse gas emissions. It is therefore necessary to research sustainable solutions and reduce the negative environmental impact of these engines. A widely studied alternative is the use of H 2 in dual-fuel mode. This review has been developed to include the most recent studies on the subject to collect and compare their main conclusions on performance and emissions. Moreover, this study includes most relevant emission control strategies that have not been extensively analyzed in other reviews on the subject. The main conclusion drawn from the literature is the negative e ﬀ ect of the addition of H 2 on NO x . This is due to the increase in temperature during combustion, which increases NO x formation, as the thermal mechanism predominates. Therefore, to reduce these emissions, three strategies have been studied, namely exhaust gas recirculation (EGR), water injection (WI)


Introduction
The global demand for energy is increasing annually, mainly due to the growth of the world's population.This is directly related to the use of fossil fuels, which have become the primary source of energy in certain sectors, such as power generation, transportation, heating, industry, and building.The use of these fuels is directly linked to the emission of CO2, one of the main greenhouse gases (GHGs).According to the European Commission's report on global GHG emissions [1], in 2022, CO2 emissions were more than twice as high as in 1970 (53.8 Gt vs. 25 Gt).In that year, CO2 emissions reached their highest annual average concentration in the atmosphere at 417 ppm, which was about 50% higher than at the beginning of the Industrial Revolution in 1760 (280 ppm) [2,3].
To reduce these emissions into the atmosphere and their contribution to global warming, governments have implemented stringent emission regulations.The Paris Agreement, signed in 2015 at COP21, sets long-term goals for countries to take action to limit the global temperature increase to 2 °C above pre-industrial levels.It also includes working with developing countries to mitigate climate change [4].However, both COP26 and COP27 concluded to set this limit at 1.5 °C.In addition, the European climate legislation (called "Fit for 55") is part of the European Green Deal, which aims to reduce its net GHG emissions by at least 55% by 2030 compared to 1990 levels and to become carbon neutral by 2050 [5].These measures have a major impact on the transport sector, which is responsible for GHG emissions as well as other pollutants, such as CO, NOx, unburned hydrocarbons (HC), and particulate matter (PM).The European Commission has imposed stricter CO2 emission targets for 2020 for this sector.These targets are 95, 93.6, and 49.5 g CO2/km for the periods of 2020-2024, 2025-2029, and 2030-2034, respectively.From 2035, the target is a 100% reduction, meaning carbon neutrality [6].
In this regard, extensive efforts are being made to search for different techniques to either improve the efficiency of thermal engines or alternative forms of propulsion, such as electrification.Regarding the latter, it seems that the transportation sector is moving towards electrification, according to reports by British Petroleum [7], the International Transport Forum [8], and McKinsey & Company [9].However, thermal engines, particularly compression ignition engines (CIEs), still have a key role to play in transportation and other sectors (such as agriculture, aviation, and power generation).To improve the efficiency of thermal engines, various studies focus on two different strategies that reduce pollutants and do not require significant engine modifications [10][11][12].
The first is the use of biofuels to replace carbon-based fuels.For more than 25 years, to improve their performance and production, research on biofuels has been developed worldwide.The use of biofuels has several advantages, including the use of fuel properties close to those of diesel fuel, the fact that they can be used directly in CIEs without significant engine modifications, and the fact that there is no production of sulfur oxides [13].Biodiesel is the most widely used biofuel for CIEs; the most common feedstocks are vegetable oils derived from edible plants, such as rapeseed, palm, soybean, sunflower, and other oleaginous crops [14].However, the use of the above biofuels may be affected by global regulations (such as the European Green Deal) affecting the production and sale of first-generation biofuels (from feedstocks that may cause land use change).However, research continues into blending these fuels with other components, i.e., alcohols, as well as the development of second-generation (solid urban and agricultural waste, such as used oil, plastic, and biomass) and third-generation (algae carbohydrate) biofuels.
The second strategy is the use of a carbon-free gaseous fuel, such as hydrogen (H2).H2 is an energy carrier with a wide range of fuel applications, such as direct use in internal combustion engines (ICEs), heaters, or fuel cells.In addition, the adaptation of existing ICEs to this gas makes possible the transition of the transportation sector to a hydrogenbased economy.The properties of this gas are discussed in Section 2, but some of the advantages of H2 over other fossil fuels are its zero-carbon content and high energy density in terms of mass (120 MJ/kg, about three times that of carbon-based fuels) (see Table 1).Despite its zero-carbon content, the vast majority of H2 production, about 90 Mt in 2020, is derived from fossil fuels through steam methane reforming (SMR), partial oxidation and autothermal reforming, or steam-oxygen gasification of coal, resulting in the generation of 900 Mt of CO2 [15,16].However, H2 production from renewable energy and natural sources, such as water, which can split O2 and H2 molecules through electrolysis, reduces CO2 emissions to zero [17].

Property
Unit Hydrogen (Gas) Natural Gas Diesel Fuel (Liquid) Gasoline (Liquid) Density at normal conditions kg/m 3 0.0899 0.7-0.However, H2 cannot be used as the sole fuel in CIEs due to its high autoignition temperature (328 K above that of diesel fuel, see Table 1).Therefore, the addition of another fuel with a lower autoignition temperature to act as a combustion activator (usually in liquid form, such as diesel fuel or biofuels) is needed.This mode of engine operation, in which two different fuels are used simultaneously, is called dual-fuel operation.Typically, H2 is introduced into the intake manifold while diesel fuel is injected directly into the combustion chamber, although other methods can be used, as shown in Section 2. The use of H2 provides significantly different combustion and emission characteristics.
The study of H2 as a fuel in CIEs in dual-fuel mode is a growing topic.Rueda-Vázquez et al. [20] conducted a bibliometric study and found an increase in the number of publications and citations per year on this topic in the Web of Science from 2010 to 2021.However, studies on this technology started in the 1990s.The results, updated to 2023, are shown in Figure 1.According to the literature, a main drawback of the use of H2 as a fuel in CIEs is the generation of NOx [21][22][23][24][25]. Due to higher peaks in the heat release rate (HRR) and combustion chamber temperature, NOx emissions are increased.The Zeldovich mechanism is one of the main mechanisms that contributes to NOx formation [23,25].Various strategies are used to control NOx emissions, such as exhaust gas recirculation (EGR) and water injection and either direct water injection (DWI) or water injection into the intake manifold (WI).Another strategy developed to control NOx emissions in CIEs is the reduction of the compression ratio (CR) or the addition of NH3 [26,27].
The aim of this review is to provide a detailed review of recent work on the addition of H2 to CIEs.This work distinguishes between multi-cylinder engines, whose main application may be in the transport sector, and single-cylinder engines used for other purposes, i.e., agricultural machinery.The majority of the works consulted are those in which the ignition source is diesel fuel, with the exception of biofuels.
The objectives of this research are as follows: 1. To evaluate the main properties of H2 with respect to other carbon-based fuels.It also includes advantages and disadvantages of its use in CIEs, enumerating different strategies for introducing the gas into the engine.

2.
To analyze in detail the influence of the use of H2 in a CIE in dual-fuel mode in terms of performance and emissions.3. To evaluate different strategies to reduce the increase in NOx emissions associated with the use of H2.

Hydrogen as Fuel in Compression Ignition Engines
H2 is the lightest gas in the periodic table of elements (14 times lighter than air).It is highly flammable and odorless, and it burns with a colorless flame [28].It is usually found in its molecular form under normal conditions.Some of the most important properties of H2 with respect to other carbon-based fuels are shown in Table 1.
H2 is a carbon-free fuel, and one of its main advantages is its lower heating value (LHV).Its LHV is about double that of natural gas (NG) and triple that of diesel fuel and gasoline.However, due to its low density, a large amount of H2 is required to produce the same amount of energy as these fuels, which can lead to a storage problem.The most common method of storing H2 is to compress it in high-pressure vessels.H2 vessels are mainly divided into four different types (Type I to Type IV).Type I (all metal) and Type II (metal liner with hoop wrapping) cannot be used in vehicles due to their lower H2 storage density and serious H2 embrittlement problems [29].Currently, Type III (metal liner with full wrapping) and Type IV (plastic liner with full wrapping) are commonly used in fuel cell vehicles (FCV).Other vehicle-compatible and widely studied H2 storage systems include cryogenic or cryo-compressed storage [30,31].
Also worth mentioning are its high flammability limits, making it suitable for a wide range of air-fuel mixtures and engine powers, with the advantage that H2 can run on a lean mixture [32].Its high diffusivity allows for reducing the heterogeneity of a diesel spray, resulting in a better and more uniform premixing with air [33].This better mixture implies better conditions for the complete combustion process.As mentioned above, H2 cannot work exclusively in a conventional CIE, because its high auto-ignition temperature (858 K) is significantly higher than that of other fuels, which makes ignition difficult when the temperature rises during compression stroke.Therefore, this gas must always work together with another fuel, with a lower autoignition temperature, to act as an ignition source.The amount of H2 supplied to the engine can be indicated in two ways; directly, by specifying the amount of H2 supplied, or indirectly, by using a parameter that relates the energy supplied by H2 to the total energy supplied by both fuels.This parameter is commonly referred to as the hydrogen energy share (HES), and it is calculated using Equation (1).
where m and m represent the mass flow rates of H2 and diesel fuel, in kg/h, respectively.LHV and LHV are the lower heating values of H2 and diesel fuel, in J/kg.However, the utilization of H2 encounters two challenges requiring attention: knocking and backfiring.H2 influences the combustion process, as explained subsequently.On occasion, the combustion can arise prematurely, igniting before the piston reaches the top dead center (TDC) and causing sudden vibrations.This occurrence, known as knocking, can be detected through engine vibration and the audible pinging sound caused by combustion [34,35].It presents a significant obstacle to determining the maximum HES, as higher H2 levels increase the likelihood of knocking.For example, Castro et al. [36] detected knocking in a marine four-cylinder diesel engine at 80% HES under all load conditions.For other operating modes, there was no evidence of knocking.Bakar et al. [37] conducted tests on a single-cylinder diesel engine using H2 flow rates up to 49.6 L/min and engine loads ranging from 5 to 25 Nm.In general, the higher the load, the lower the maximum amount of H2 that can be supplemented.
Backfiring is the combustion of fresh air and H2 during the intake process in the intake manifold or combustion chamber.When the fresh mixture enters the combustion chamber, the remaining hot residual gas can generate an ignition.This ignition is similar to pre-ignition, with the difference that pre-ignition occurs during the compression stroke, with the intake valves closed.Backfiring is an undesirable situation that occurs when the intake valves are open, and it can seriously damage the intake manifold [19].This issue can be resolved by correctly injecting H2 into the engine.Therefore, it is essential to select the most suitable method for combining H2 with air.
The approach to injecting H2 into the engine significantly impacts engine performance [38].One main advantage of H2 use is the ability to employ the same fuel injection methods utilized for compressed natural gas (CNG) and liquified natural gas (LNG), which include (i) carburation, (ii) inlet manifold injection, (iii) inlet port injection, and (iv) direct injection.Figure 2 provides a schematic representation of these techniques.Carburation is the earliest and most straightforward technique employed in spark ignition engines (SIEs), wherein the fuel is mixed with the air via a carburetor before it enters the intake manifold.This approach has an advantage over the methods below, as high fuel supply pressure is not required [32,39,40].However, carburation use implies the presence of an air-H2 mixture in the intake manifold.This remnant can pose a significant risk in CIEs if the intake valve closes and sudden ignition occurs with a premix.If the intake valve opens, the flame can spread through the intake manifold, resulting in a backfire and severely compromising the engine [32,38,40].
Injection allows for timed and precise control of H2 input into the engine, unlike carburation, which is dependent on throttle position.At the start of the intake phase, injectors deliver H2 into the intake manifold, leaving no remnants and decreasing the likelihood of backfire [38].Injection can be controlled either mechanically or electronically.Electronic injectors possess a sturdy construction and offer precise control over injection timing and duration at high speeds [32,41].Mechanical injectors are better-suited than carburation due to their sluggish response time.
As indicated earlier, three types of injections can be distinguished for hydrogen.H2 may be injected directly into the manifold without an intermediate gas mixer.In this scenario, the fuel supply pressure will be higher compared to carburation, as there is no increase in the intake air velocity and, consequently, the air pressure is not reduced.Two methods can be identified based on the location of the H2 injection.In port injection, the injection point is located near the intake valve, typically within a range of 10-15 mm, while in manifold injection the distance is much higher, usually between 100 and 200 mm [41].
When dealing with a poly-cylinder engine, such as the one illustrated in Figure 2, in the case of port injection, as many injectors as cylinders must be used.Conversely, in the case of manifold injection, only one injector is required, thus simplifying the process.Port injection offers the advantage of ensuring that each cylinder contains the same amount of H2.However, with manifold injection, this guarantee cannot be made because the size of the common rail can impact the amount delivered to each cylinder.
Saravanan and Nagarajan [35,41] conducted several studies comparing the performance and emissions of a single cylinder direct injection diesel engine with different H2 injection techniques (carburetor, manifold, and port injection) and found that port injection provided superior performance and emissions compared to carburetor and manifold injection techniques.
Finally, in direct injection, hydrogen directly enters the combustion chamber via high-pressure direct injection (HPDI) or low-pressure direct injection (LPDI) [32].LPDI occurs near the bottom dead center (BDC) at the end of the intake phase, while HPDI takes place after the compression phase.As the injection occurs inside the combustion chamber, the risk of backfire decreases [42].The primary limitation of this injection technique is that the injectors are ill-equipped for the severe thermal stress it produces, particularly in highpressure injection scenarios.As a result, this method may prove impractical, especially if significant engine modifications are unwanted [32,39,43,44].Liu et al. [45] conducted an experiment wherein they bypassed three cylinders of a traditional four-cylinder engine and injected H2 directly into the combustion chamber using a commercial spray-guided gasoline direct injector.Although there were no errors observed during the testing, oil needed to be added to the injector every day of testing to lubricate the injector needle.
The upcoming sections will investigate the effects of hydrogen addition on engine performance and emissions.To track trends in the analyzed parameters, a table of recent work (from 2016 onwards) will be assembled for each parameter.The studies will be categorized by engine type, starting with poly-cylinder engines (which relate more closely to transport) and then moving on to single-cylinder engines used in other sectors.For each study, additional information regarding test conditions (amount of supplied H2, used technique, engine specifications, test speed, and load) will be provided.The main results will be analyzed to draw a conclusion regarding the effect of H2 addition on the parameter being investigated.Findings may vary based on their design and execution.When conducting a test, certain variables, such as HES, m , m , and engine load, will be either set or varied to achieve specific outcomes.To categorize the tests performed in the studies, various types of tests will be classified for a particular engine speed.Table 2 shows a schematic representation of the differences between each type of test: • Type 1: In this scenario, the values of m and m are adjusted to acquire the desired HES for a specific engine load.In case the HES is increased, m will be increased while m will be decreased while keeping the engine speed and load constant, as before.Rocha et al. [23] kept the engine load at 60% and carried out experiments for various HESs (ranging from 0 to 24%).When no H2 was injected (0% HES), the value of m was recorded as 1.1072 kg/h.When the HES increased to 5%, the diesel fuel mass flow rate decreased to 1.0394 kg/h, while the hydrogen mass flow rate increased by 0.0194 kg/h.At 24% HES, the diesel fuel mass flow rate decreased to 0.9318 kg/h, while the hydrogen mass flow rate increased to 0.1021 kg/h, with no change to the initial 60% engine load.

•
Type 2: When intending to maintain a constant engine load for an established m , it is necessary to decrease m relative to the scenario without H2, causing a higher HES.Subsequently, with higher m in a subsequent test, m will be further reduced, thereby increasing the HES and keeping the previously established engine load.Yilmaz et al. [46] conducted tests at three engine loads (50, 75, and 100 Nm) and two hydrogen volumetric flow rates (20 and 40 L/min).At 75 Nm, no H2 injection resulted in a 0% HES, increasing up to 8.93% and 17.35%, with 20 and 40 L/min, respectively, while keeping constant the initial engine load.

•
Type 3: In this case, the test is conducted at a specified speed without any hydrogen injection.The diesel fuel mass flow rate measured at this test point is kept for the remaining points, where a specified hydrogen mass flow rate is assigned, resulting in a higher fuel quantity.To maintain a constant speed, the engine load must be increased.In this sense, the authors conducted a study where the diesel fuel mass flow rate remained constant at 1500 rpm.An increase in the hydrogen mass flow rate resulted in higher hydrogen enrichment in the fuel mixture, which in turn led to greater power output and increased engine speed.To maintain a constant speed of 1500 rpm, the brake load needed to be adjusted accordingly [47].

Effect of Hydrogen as a Dual Fuel on Combustion Performance Characteristics
In this review, the main performance parameters analyzed are the brake thermal efficiency (BTE), volumetric efficiency (VE), ignition delay (ID), combustion duration (CD), and maximum in-cylinder pressure.

Brake Thermal Efficiency
BTE is a critical performance parameter in engine testing.Efficient use of fuel is essential for optimal engine performance.It measures the engine's ability to convert the fuel energy into power, and it is expressed as the ratio of power output to fuel energy input.For the calculation of the BTE for a dual-fuel engine, Equation (2) is used [48].
where T represents the engine torque, in Nm; n is the engine speed, in rpm; and m and m represent the mass flow rates of H2 and diesel fuel, in kg/h, respectively.LHV and LHV are the lower heating values of H2 and diesel fuel, in J/kg.Table 3 highlights recent and relevant studies exploring BTE with H2 additions.For conventional vehicle engines used in commercial applications, numerous studies have demonstrated that the incorporation of H2 enables a positive impact on the BTE across all load ranges.Hoang and Pham [50], Yilmaz and Gumus [51], and Wu et al. [52] reported an improvement in the combustion process, which increased the BTE, due to H2's high diffusivity and flame propagation.Barrios et al. [49], however, did not find a clear trend, arguing that the lack of EGR control would influence the combustion process.
There appears to be a disparity in results concerning lower-powered engines.Das et al. [53] and Kanth and Debbarma [21] found similar tendencies to those described above for more powerful engines based on the same facts.However, Nag et al. [54] concluded that the use of H2 has a positive effect on BTE, except for low loads, as the lower combustion efficiency of H2 causes a drop.Studies conducted by Sharma and Dhar [55] and Subramanian and Thangavel [56] suggested that at medium and high loads, the BTE also decreased.The reason for this is that the wall heat flux of the H2 flame was found to be higher than that of diesel fuel, which results in higher thermal losses and, consequently, lower BTE [57].Furthermore, Pinto et al. [22] reported that during a medium load, the reduction in BTE occurs because the prolonged ignition delay causes a significant portion of the fuel to burn during the expansion stroke.They concluded that modifying the pilot fuel injection is necessary to reduce this inefficient behavior.
Several studies, including those by Yilmaz and Gumus [51] and Das et al. [53], have suggested that the higher the HES, the lower the BTE.This could be due to the displacement of O2 by H2 in the intake manifold.These findings have been observed in both large and small engines, although other research indicates the opposite trend, stating that the higher the HES levels, the higher the BTE [21,53].The displacement may also be impacted in naturally aspirated engines.Most of the engines for commercial use are turbocharged, whereby the air enters at a higher pressure and reduces the displacement.

Volumetric Efficiency
VE is a metric that gauges the breathing efficiency of an engine.It gives the proportion of the volume of fresh air taken into the engine cylinder prior to the intake valves, with shutting relative to the cylinder-swept volume at atmospheric conditions (1 atm and 298 K) [58].VE can be calculated using Equation (3) [23].
where the mass flow of air is represented by m and ρ is the density of air calculated using the temperature and the pressure of the air within the intake manifold, in kg/h and kg/m 3 , respectively.V represents the displaced volume in m 3 .Table 4 presents recent relevant studies analyzing VE with the addition of H2.Only single-cylinder-engine studies have been found for VE calculation.In all cases, the use of hydrogen led to a reduction in VE at all load rates due to hydrogen's lower density than air.This resulted in a significant displacement of air when introducing it into the combustion chamber.Nag et al. [54], Rocha et al. [23], Sharma et al. [55], and Subramanian et al. [56] noted that the higher the HES, the lower the VE.This reduction arises from a greater amount of air being replaced by H2 in the combustion chamber.

Ignition Delay
ID is defined as the time between the start of fuel injection into the cylinder (SOI) and the start of combustion (SOC) [59].This time can be measured as the difference between the crank angle where 0% and 10% of heat are released.The delay period consists of a physical and a chemical delay that occur simultaneously [59].Fuel properties and composition are responsible for the physical delay [60], while the chemical delay is dependent on the cylinder temperature, cylinder pressure, and fuel properties [61].Table 5 shows some of the most relevant recent studies in which ID is studied with the addition of H2.For commercial engines that can be used in conventional vehicles, research has shown that using H2 can cause ID.Dhole et al. [62] revealed that higher mixture-specific heat (Cp) and fewer ignition sources are responsible for reducing the temperature of the gaseous fuel-air mixture at low and medium loads (13% and 40%).This results in an increased ID compared to the diesel mode (2° CA).At 80% load, they observed that higher temperatures promote increased heat transfer to diesel fuel droplets.This resulted in faster evaporation and a smaller ID compared to low and medium load cases, but they were still higher than the diesel fuel mode (1.5° CA).Similar results were obtained by Lata et al. [63] at 10%, 40%, and 80% loads with IDs of 4, 3, and 2° CA, respectively.Yilmaz et al. [46] reported a significant ID when introducing H2 at 50 Nm (9° CA with respect to the diesel fuel case).However, this delay was reduced as the engine load increased (by 3° CA at 75 Nm and by a non-negligible amount at 100 Nm).The authors argued that ignition of the pilot fuel may be affected by the reaction with the air-H2 mixture.However, all studies have shown a consistent trend in the impact of HES on ID.At higher HES levels, the delay is reduced until values are equal to those of the diesel fuel case.Dhole et al. [62] noted that with a higher HES at a high load, flame propagation from multiple ignition centers of the pilot fuel was rapid and consumed the majority of the air-H2 mixture, leading to higher residual gas temperature and consequently reducing ID.
There seems to be a discrepancy in results for lower-powered engines.Das et al. [53] and Pinto et al. [22] reported a similar trend as above for more powerful engines, but with different explanations.Das et al. [53] stated that the increase in delay (1-2° CA) is due to H2's high auto-ignition temperature, which causes the air-H2 mixture to ignite more slowly.Pinto et al. [22] argued that there was a delay of 0.5° CA.This resulted from the lesser quantity of pilot fuel injected under the dual-fuel mode, which was replaced by H2.Nevertheless, contrasting with this claim are the findings of Juknelevicius et al. [64] and Rocha et al. [23], who reported that H2 speeds up the combustion of the blended fuel, thereby advancing the combustion initiation by 5° CA and 0.5° CA, which is the opposite of the findings mentioned above.Finally, Tutak et al. [24] found no significant differences between net diesel fuel operation mode and dual-fuel operation mode.

Combustion Duration
CD is the time span between the start and end of combustion in relation to the crank angle.It is measured by calculating the difference between the crank angle at which 10% and 90% of the heat are released [53].Decreasing CD has a positive impact on engine operations, as it restricts the amount of heat released to the cylinder and combustion chamber walls, thus boosting efficiency.In addition, it promotes reduced knocking and improved combustion, leading to lower HC emissions.In addition to the mentioned benefits, faster fuel burn also results in rough engine operation (higher pressure rise rate for one degree of rotation of the crankshaft).Table 6 illustrates recent studies exploring CD with the addition of H2.Studies have shown that for commercial engines used in conventional vehicles, hydrogen can have both positive and negative effects on CD, depending on the engine load.According to Dhole et al. [62], at a low load (13%), lean mixtures lead to fewer ignition centers being formed.However, the addition of H2 reduces diesel fuel and increases CD (by 2.5° CA).At higher levels of HES, CD decreases by 1.5° CA when transitioning from 30% to 50% HES.It is suggested that this decrease could be attributed to the higher flame velocity of H2.At higher loads, increased fuel quantity for the pilot enhances the ignition of H2, which has a higher laminar velocity and results in decreased CD (up to 4° CA).Yilmaz et al. [46] obtained similar results with a 29° CA increase in CD compared to dieselfuel-only mode at a 50 Nm load.It should be noted that there is a discrepancy in how the SOC is interpreted by these authors.Dhole et al. [62] define the SOC as the minimum point in cumulative heat release, whereas Yilmaz et al. [46] define it as the point where the maximum rate of pressure rise occurs.
Results differ between low-and high-powered engines, as the majority of the consulted studies indicate that CD decreases at all load rates with low-powered engines.Das et al. [53] and Juknelevicius et al. [64], over a range of loads, as well as Pinto et al. [22] at medium load and Tutak et al. [24] at full load, reported that hydrogen leads to a lower CD.Das et al. [53] reported that hydrogen forms a homogeneous mixture with air, leading to an increased combustion rate and greater fuel burned in the premixed phase.Furthermore, Pinto et al. [22] analyzed the CA in both premixed and diffusive phases and found that the addition of hydrogen resulted in shorter CA values (5.77° and 16.87° CA with hydrogen compared to 6.29° and 20.37° CA without hydrogen in the premixed and diffusive phases, respectively).They argue that hydrogen leads to higher combustion rates due to the greater flame velocity, resulting in quicker and more efficient mixing of fuels.In contrast, the study conducted by Subramanian and Thangavel [56] found that the addition of hydrogen led to a higher CD.The authors suggested that introducing hydrogen reduces the concentration of air available for combustion, thus slowing down the diffusion combustion process.This stands in stark contrast to the reasoning provided by other authors.

Maximum In-Cylinder Pressure
The quantity of fuel burned during premixed combustion determines the maximum in-cylinder pressure in CI engines.A richer fuel-air mixture results in a higher maximum pressure.Research on CI engine performance relies on pressure variations in the operating cycle.The maximum in-cylinder pressure increases with the engine load due to higher temperatures at higher engine loads [60].Table 7 displays significant recent studies investigating maximum in-cylinder performance with the application of H2.For commercial engines in conventional vehicles, studies demonstrate that introducing H2 to the fuel's composition causes a rise in the maximum in-cylinder pressure.Cernat et al. [65] indicated that hydrogen's high burning velocity and calorific value result in increased heat generation in the initial combustion stages.It correspondingly contributes to an enhanced rate of maximum pressure in the premixing.Yilmaz and Gumus [51] reported that hydrogen combustion is practically instantaneous due to its high burning velocity and high diffusivity.It results in a more homogeneous fuel-air mixture.As a result, the maximum in-cylinder pressure increased with the addition of hydrogen.In contrast, Barrios et al. [49] did not observe a clear trend and suggested that the uncontrolled EGR system may have had an effect on the combustion process.
For lower-powered engines, most authors observed a rise in the maximum in-cylinder pressure, which was explained similarly to studies with more powerful engines.However, Das et al. [53] brought attention to a reduction in the maximum in-cylinder pressure when hydrogen was at its maximum level of supply (10 L/min) compared to the case with a 7 L/min supply.They argued that hydrogen with a higher concentration consumes more oxygen, giving less oxygen for the complete combustion of diesel fuel.Even so, the maximum pressure in the hydrogen mode is higher than that in the diesel fuel mode.However, as reported by Subramanian and Thangavel [56], the absence of oxygen in the combustion process leads to a decrease in the maximum in-cylinder pressure.This confirms previous findings by Das et al. [53] but also applies to all levels of H2 supplementation.
The high diffusivity of hydrogen produces a homogeneous air-hydrogen mixture.This mixture, along with hydrogen's high calorific value, lead to a higher heat release rate in the premix phase.The resulting increase leads to a higher maximum cylinder pressure, which grows with higher loads and increased hydrogen supply.An increase in load leads to a higher amount of fuel present in the chamber.As the amount of H2 in the fuel mix is higher than that of the diesel fuel, the maximum pressure increases in parallel.This observed increase in pressure has been consistent across engines of all sizes.

Effect of Hydrogen as a Dual Fuel on Emission Characteristics
The complete combustion of a hydrocarbon and the oxygen in the air produces CO2 and H2O.However, in a CI engine, combustion is incomplete, resulting in the formation of additional products for various reasons [67,68].

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The chemical reactions are not able to complete at high engine speeds, leading to the generation of CO and HC.

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Elevated temperatures in the combustion chamber, under conditions of excess air, facilitate air oxidation, leading to the formation of NOx.

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Situations with insufficient oxygen and excess fuel result in incomplete evaporation and carbonization of some fuel, causing the formation of PM.

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Fuel impurities can also produce SOx.
In the following sections, the results of CO2, CO, HC, soot, and NOx are shown.

CO2 Emissions
CO2 is a colorless and odorless gas that results from the complete oxidation of carbon found in carbon-based fuels.It is crucial to control the emission of this gas, as it is a GHG contributing to climate change.The amount of CO2 emitted, and the type of fuel used, are interrelated.Table 8 displays some of the most important recent studies that examine CO2 emissions with the addition of H2.All consulted authors, irrespective of engine size, have observed an identical trend: a rise in CO2 emissions with an increase in engine load and a decrease with growing HES.The majority of authors state that using H2, a carbon-free fuel, instead of diesel fuel helps in reducing emissions.In a study by Castro et al. [36] concerning a four-cylinder engine, the decrease in CO2 emissions matched the reduction in diesel fuel consumption.For example, at a 60% HES level, the reduction in CO2 was 45.7%, while the decrease in diesel fuel consumption was 43.9%.Comparable findings were presented in the studies by Cernat et al. [65] and Subramanian and Thangavel [56].Additionally, Rocha et al. [23] established a correlation between CO2, CO, HC, and PM, contending that a reduction in the levels of the latter three (which will be discussed below) results in an increase in CO2 emissions.The study found that the decrease in CO2 emissions was less than the decrease in carbon provided by diesel fuel, suggesting that a higher amount of carbon from CO, HC, and PM was converted to CO2.

CO Emissions
CO is a colorless and odorless gas resulting from incomplete combustion of carbonbased fuels.In CIEs, where the fuel-air mixture is usually lean, CO oxidation to CO2 is insufficient due to inadequate mixing of the fuel before ignition, leading to the presence of fuel-rich and fuel-lean areas [24,69,70].The lack of sufficient O2 in the fuel-rich area prevents complete oxidation to form CO2. In fuel-lean conditions, incomplete oxidation occurs due to lower temperatures and combustion rates.This particular compound can be oxidized in the atmosphere to form CO2 through a reversible chemical reaction (Equation (4) [71]) and may be indirectly classified as a GHG.Table 9 displays recent notable studies that investigate CO emissions in the presence of H2.
(4) For commercial engines intended for use in conventional vehicles, it has been found that increasing loads lead to a rise in CO emissions.Although most authors agree, Sharma et al. [55] noted a decrease in these emissions with increased loads.Their argument was that in-cylinder temperature rises, which can hasten CO oxidation to CO2.Similarly to CO2, all authors point out that the lack of carbon atoms in hydrogen results in a decrease in CO emissions, despite differences in loading.Nag et al. [54] found that at 1500 rpm and 25% load, 10% and 30% HES resulted in a 29.1% and 46.8% decrease in CO emissions, respectively.Moreover, Rocha et al. [23] reported that the maximum reduction in diesel fuel consumption was 15.8% (10% HES), whereas CO reduction was 63.4%.This indicates a faster combustion reaction.Nevertheless, there is a divergence in this trend when it comes to single-cylinder engines.

Hydrocarbons
HC emissions are organic compounds with a bluish color and a pungent odor that typically result from incomplete diesel fuel combustion.In situations of low loads, fuel does not completely ignite due to a lean fuel-air mixture.Additionally, over-fueling can contribute to HC emissions, as there are areas in the combustion chamber with an overrich fuel-air mixture, resulting in incomplete combustion.Table 10 displays some of the most significant recent studies examining HC emissions with the inclusion of H2.For commercial vehicle engines, it has been found that the use of H2 results in a reduction in HC emissions.This decline is due to the lack of carbon atoms in hydrogen, as previously mentioned.The majority of studies note a trend towards a greater reduction of HC emissions with an increase in the quantity of hydrogen.This correlation also holds true for CO2 and CO.However, Yilmaz and Gumus [51] reported higher HC emissions for a hydrogen supply of 40 L/min compared to 20 L/min.They argued that the increase may be attributed to a lack of oxygen during diffusion combustion, as the instantaneous combustion of hydrogen consumes most of the available oxygen.This trend was further confirmed for BTE.
Most studies indicate a downward trend for lower-powered engines.Kanth et al. [21] and Rocha et al. [23] explained that hydrogen's higher flame speed leads to better combustion efficiency, resulting in HC reduction.However, contradictory results were observed by Sharma and Dhar [55], who reported increasing HC emissions for lower HES concentrations.They stated that very low levels of HES affected the competition between OH and H radicals during the high-temperature phase of combustion.Subramanian and Thangavel [56] hypothesized that higher HC emissions for lower HES may be attributed to decreased air excess and lower combustion temperature, which increase the likelihood of incomplete combustion.Tutak et al.'s research [24] demonstrated that HC emissions displayed an upward trend with supplementation below 12% HES and decreased with higher HES supplementations.Nonetheless, emissions stayed below 100 ppm, an amount considered negligible.It is worth mentioning that the study denotes an equipment measuring accuracy of ±12 ppm.

Soot/PM/Smoke
Soot formation in CIEs is attributed to the presence of heterogeneous mixtures of air and fuel.The degree of homogeneity of the mixture can be increased to significantly reduce soot emissions.Soot formation typically occurs in areas with oxygen defects in the combustion chamber, where fuel oxidation is not allowed [39].Lubricating oils can also be a contributing factor to soot emissions [72].Table 11 presents some of the recent studies exploring soot emissions with the incorporation of H2.All consulted authors, regardless of engine size, demonstrated a consistent pattern: an increase in soot/PM with increasing load and a decrease with increasing HES.Most authors reported that hydrogen's high diffusivity facilitates the formation of a more uniform mixture, consequently providing fewer locations where pilot fuel cannot oxidize.In addition, they advocate for swapping fossil fuels with carbon-free alternatives, such as hydrogen.

NOx Emissions
NOx refers to gaseous binary chemical compounds that form when nitrogen and oxygen combine.Emissions of NOx from exhaust primarily contain NO and NO2 in a 95-5 ratio, respectively [73].N2 and O2 do not react at normal temperature and pressure (NTP), but reaction occurs in the combustion chamber under the necessary conditions.In an ICE, three mechanisms contribute to NOx formation, namely, thermal NOx, prompt NOx, and fuel NOx [74].The Zeldovich mechanism is the principal NOx formation route, brought about by the high combustion chamber temperature, where N2 from the air is oxidized (described by Equation ( 5)) [23,25].The Arrhenius law [75,76] confirms that NO formation rises exponentially with temperature.This chemical reaction strongly relies on the local combustion temperature, which is influenced by fuel-air distribution, O2 concentration, pressure, and temperature of the intake charge, as well as the effective combustion volume [77].Table 12 shows some of the most relevant recent studies in which soot emissions, with the addition of H2, are studied.
Table 12.Results of NOx emissions with the addition of H2.
Ref.  Studies on commercial engines for conventional vehicles do not yield conclusive results.Barrios et al.'s [49] work showed an unclear trend.NOx emissions decreased with H2 addition at a 30% load but increased at a 45% load.The authors argued that H2 addition could lead to an increase in NOx emissions due to a rise in gas temperature.However, in cases of low HES (25% in that study), this effect is mitigated as a result of exhaust gas mixture dilution from hydrogen-oxidation-induced water formation.Furthermore, it is worth noting that the EGR system was not controlled in that study, which could potentially affect combustion.At medium load, Cernat et al. [65] found a comparable pattern, showing a decrease in NOx emissions with hydrogen substitution levels up to 7.5% HES.For higher HES levels, NOx emissions rose, but they remained lower than those of diesel fuel.It is worth noting that H2 substitution levels, as seen in the study by Barrios et al., remained relatively low, not exceeding 21% HES.However, Wu et al. [52] clearly found an increase in NOx emissions with the addition of H2, regardless of HES and engine load.This is due to the higher combustion rate of hydrogen compared to diesel fuel.

H2 Enrichment and Mixing Process
When discussing lower-powered engines, there are several important factors to consider.Sharma and Dhar [55], as well as Subramanian and Thangavel [56], supported previous findings of a slight decrease in NOx emissions.They stated that it was due to a low level of H2 substitution and its dilutive effect on oxygen displacement in the mixture at low and medium loads.However, both studies revealed an increase in NOx emissions at higher loads, overall, with higher HES.Bakar et al. [37] presented a detailed overview of the impact of H2 at various loads.They found that at low loads NOx emissions were linked to the combustion zone of pilot diffusion, leading to very high temperatures and extended reaction times.The addition of H2 reduced the quantity of pilot fuels, thus decreasing high-temperature combustion zones and resulting in a decline in NOx emissions.At medium loads, NOx emissions showed consistency.At high loads, the diesel fuel portion led to more efficient combustion.It increased in-cylinder pressure and, hence, NOx emissions.However, other studies have indicated an increase in NOx emissions independent of the load level and hydrogen substitution.This is argued to be due to the temperature increase in the combustion chamber.

Techniques for Emission Control and Enhancement of HES
The utilization of H2 as an alternative fuel presents a potential solution to the reduction of pollutant emissions from CIEs, as discussed in the previous section.Nevertheless, its usage adversely affects NOx emissions.As such, it is crucial to implement strategies to combat these emissions.Presented below are three feasible alternatives to decrease pollutant emissions associated with H2 infusion, some of which are already implemented in traditional engines.These techniques, simultaneously, enable an increase in HES.In the following sections, both experimental and numerical studies, in which the model is validated with experimental data, have been considered.

Exhaust Gas Recirculation
The high temperatures attained in the combustion chamber cause reactions between atmospheric N2 and other components, producing NOx that is emitted into the atmosphere.To mitigate this, EGR is a highly precise system for reintroducing exhaust gases back into the intake manifold, thus modifying the incoming mixture's composition.This approach is widely utilized in contemporary CIEs.Studies investigating the advantages and disadvantages of this technology date back to the 1970s.According to the literature, the EGR rate refers to the alteration in air mass flow upon the introduction of EGR, and it can be computed using Equation (6).
Table 13 presents notable recent studies that employed H2 in conjunction with EGR to decrease NOx emissions.The table specifies the EGR rate, the amount of H2 utilized, and key outcomes of these investigations.All consulted authors, irrespective of engine size, demonstrated a consistent pattern of the effectiveness of EGR in reducing NOx emissions.According to the work of Wu et al. [79] in a four-cylinder engine, at a 40% engine load, NOx emissions were 900 ppm in dieselfuel-only mode.Nevertheless, adding H2 resulted in increased NOx emissions (nearly up to 1400 ppm, with the maximum amount of injected H2 8 L/min).At an EGR rate of 20% for this H2 injection, NOx emissions decreased to 283 ppm, resulting in a 68% reduction compared to the base case with diesel fuel.According to Vijayaragavan et al. [81], at full load in a single-cylinder engine, NOx emissions were 5.09 g/kWh in diesel-fuel-only mode.However, the addition of H2 increased NOx emissions to 6.8 g/kWh.At an EGR rate of 30%, NOx emissions dropped to 4.86 g/kWh, which means a 10% decline compared to the base case with diesel fuel.However, in certain scenarios, the use of EGR proved insufficient in minimizing emissions below the baseline, as demonstrated by Vimalananth et al. [66].In that study, the initial emission level of 9.5 g/kWh without H2 remained unaltered despite the application of the maximum EGR rate (10%).
The authors reported that raising the EGR rate lowers NOx emissions due to dilutive and capacitive effects.Gnanamoorthi et al. [80] further explained that recirculated inert gases decreased O2 levels and served as a heat sink.This resulted in a delayed combustion process until the expansion stroke.Subsequently, it caused lower combustion temperatures and, in turn, led to decreased NOx formation.In addition to reducing NOx emissions, EGR has been shown to decrease knocking through dilution effects [82,83].As a result, EGR allowed for increased high-end torque without compromising system performance.
However, there are also drawbacks.Several authors have reported a reduction in engine performance due to the dilution effect.Regarding BTE, Chintala et al. [84] claimed that it decreased with EGR as a result of this effect.However, H2 enrichment brought BTE back to levels similar to diesel fuel's base case.Nag et al. [54,83] reported that a slight enhancement in BTE was achieved for a 5% EGR rate at low loads.They attributed it to the recirculation of unburned fuel into the combustion chamber.However, at higher loads, the lack of unburnt fuel decreased the BTE, with this effect becoming more dominant as the load increased.Like H2 enrichment of fresh air in the intake manifold, intake of recirculated gases displaced fresh air mass, reducing VE [54,81,82].De Serio et al. [85] emphasized that dilutive and capacitive effects negatively impacted the combustion rate and resulted in reduced peak pressure values.Jafarmadar and Nemati [86] stated that a lack of oxygen caused a delay in ignition as EGR rates increased.Additionally, the mixture ignited with less intensity due to insufficient oxygen levels.Vimalananth et al. [66] reported a decrease in the heat release rate peak, delayed ignition, and a slight rise in the diffusion combustion peak's area under the curve with EGR addition.They attributed it to an increase in the mixture-specific heat value, which lowered the overall combustion temperature and led to a higher CD.
The use of EGR with H2 has proven to be an effective technique for reducing NOx emissions.Nevertheless, relevant studies have shown that the dilution effect causes an increase in other pollutant emissions [50,54,66,[79][80][81][82]87,88].CO emissions increase with the EGR rate due to the O2-deficient environment in the combustion chamber, thus lowering the in-cylinder temperature and reaction speed.This leads to a retardation of the oxidation reaction, resulting in higher production of CO [54].The EGR rate also causes CO2 emissions to increase because of additional CO2 that is recirculated to the combustion chamber [54,79,87].The decrease in oxygen results in a rise of HC and PM emissions, as restricted O2 access leads to incomplete combustion, ultimately causing an increase in these emissions.

Water Injection
Another approach to mitigating high temperatures in the combustion chamber is through WI [89].Inlet WI is considered the simpler option.There are several techniques documented in the literature, including multipoint WI in the intake pipes, near the inlet valves, or single-point injection upstream or downstream of the compressor [90].The primary benefit of this method is its simple and easy integration into new or existing engines, which is similar to the intake manifold H2 injection discussed in the previous section.In DWI, water is delivered directly into the combustion chamber using a dedicated injector with an electronic control that provides precise measurements of water volumes [91].The integration of additional components to the existing engine system and redesigning the fuel supply system provide added complexity to this technique, unlike inlet WI.H2 direct injection, which was discussed in the previous section, is one requirement for achieving this.In practice, the first two forms are more commonly used.Water diesel fuel emulsion (DWE) is an emulsion of water in standard diesel fuel that includes specific additives and surfactants for system stabilization [92].Emulsion fuels have several deficiencies that hinder their widespread practical use.Additionally, the highly advanced and well-developed infrastructure required to implement this blend leads to excessive costs.Fuel's physical properties also change, which can significantly impact fuel injection system performance [91].Consequently, practical application of emulsion fuels is still under investigation.
Numerous studies have investigated the impact of WI on CIEs.However, few have explored this technique in conjunction with H2 in dual-fuel settings.Table 14 presents a comprehensive list of relevant studies that utilize different WI approaches, both experimentally and numerically.This table outlines the specific WI technique employed, the amount of H2 supplied, and important findings.All consulted authors, regardless of engine size or whether the test was numerical or experimental, indicated that WI is an effective strategy for reducing NOx emissions.For instance, Serrano et al. [47] measured emissions of approximately 850 ppm by adding 0.3 kg/h of H2 to a four-cylinder engine while keeping diesel fuel constant.By adding 16 kg/h of WI into the intake manifold, emissions were reduced to approximately 240 ppm, representing about a 70% reduction.By increasing the amount of hydrogen to 0.9 kg/h and using 48 kg/h of WI, emissions remained below the base case.However, more power was generated due to the increased amount of hydrogen.Emissions were approximately 690 ppm.In a single-cylinder engine at full load, Chintala and Subramanian [95] reported NOx emissions of 8.67 g/kWh, with 20% HES.Emissions were reduced to 8.24, 6.62, and 5.47 g/kWh with 130, 200, and 270 g/kWh of water injection, respectively, resulting in a reduction of up to 37%.
The authors noted that WI reduced the combustion chamber temperature and, therefore, lowered NOx emissions.Chintala and Subramanian [95] explained that WI also decreased O2 concentration.Additionally, the introduction of water during suction stroke caused an increase in the mixture-specific heat, resulting in an overall decrease in temperature.Serrano et al. [94] demonstrated that WI resulted in thermal mechanism domination and minimum NOx emissions.Moreover, they indicated that such emissions asymptotically decreased to a minimum.Thus, they suggested an upper limit of 48 kg/h of WI, beyond which there was no substantial effect on NOx emissions.
Moreover, WI can extend the knock limit and increase HES in ICEs.Karthic et al. [98] conducted a study in which they achieved an HES increase from 6% to 8.5% using WI in a single-cylinder engine.Similarly, Chintala and Subramanian [95] observed that HES significantly increased with the quantity of WI.Specifically, HES increased from 20% (without WI) to 32%, 36%, and 39% with 130, 200, and 270 g/kWh of WI, respectively.However, BTE decreased with an increase in WI for a specific HES.Chintala and Subramanian [97] found that BTE decreased up to 3.3% when comparing cases with and without WI.They argued that WI resulted in a reduction of the in-cylinder temperature.In addition, Karthic et al. [98] utilized WI to increase HES, which was limited due to knocking problems.Increasing the HES resulted in a higher BTE, thus reducing the impact of WI.Serrano et al. [100] argued that WI induced a cooling effect and an almost uniform water dispersion in the combustion chamber, thereby promoting increased VE.Similarly to EGR, replacing O2 with water led to a delayed ignition and decreased HRR [94,95,98,101].
The use of WI with H2 as an effective technique for reducing NOx emissions has been demonstrated.However, according to the literature, the use of WI results in increased CO and HC emissions compared to the dual-fuel operation mode.This is due to the low temperature in the combustion chamber favoring NOx reduction while reducing the oxidation of CO into CO2 [95,98,102].Chintala and Subramanian [95] argued that reducing the temperature decreased the flame speed.It led to cooling of the combustion chamber walls and incomplete combustion, thus increasing HC emissions.Taghavifar et al. [103] linked combustion chamber temperature reduction to an increase in PM emissions due to the reduced soot oxidation rate.This finding was previously found by Chintala and Subramanian [95].They argued that the use of WI could harm the fuel-air mixing process, ultimately resulting in a higher degree of fuel heterogeneity.

Effect of Compression Rate Reduction
The last strategy examined in this study is the reduction of the CR.The CR refers to the proportion of cylinder volume and its headspace at the bottom of the piston stroke compared to the headspace volume at TDC [104].Normally, the CIE CR ranges from 15:1 to 20:1.The CR directly affects combustion in two ways [105].Firstly, it impacts the thermodynamic cycle.In fact, pressure and temperature at the end of compression are influenced by the CR.A higher CR increases both parameters.Combustion chamber geometry is also significantly affected by the CR, often resulting in a narrower aspect ratio with higher ratios.While some studies analyze the impact of the CR on CIEs in the context of WI, few have studied the influence of H2.Moreover, most significant studies are those including biofuels.Table 15 displays relevant studies that integrate H2 with various WI approaches, including experimental and numerical ones.This table presents the selected CR, supplied H2, and essential findings.Regardless of engine size or testing methodology, all consulted authors indicated that reducing the CR is effective in decreasing NOx emissions.Recently, authors of this review conducted a study on a four-cylinder car engine confirming this reduction [93].At 1500 rpm and 62% HES, the results showed that for a standard engine CR (17.5:1),NOx emissions were 903 ppm.Modifying the CR to 16.3:1, 15.5:1, 14.5:1, and 13.5:1 resulted in 744, 677, 653, and 510 ppm of NOx emissions, respectively.A numerical study conducted by Sharma and Dhar [107] using a single-cylinder engine observed similar results to those of other authors during experimental developments.At 1500 rpm, full load, and 55%, the authors found NOx emissions of 25.5, 23.5, and 14.5 g/kWh for CRs of 19.5:1, 16.5:1, and 14.5:1, respectively.In both studies, a maximum reduction of 43% in NOx emissions was achieved.Similarly to WI, CR reduction resulted in a decrease in the maximum temperature within the combustion chamber, ultimately leading to a reduction in NOx emissions [97,99,102,[106][107][108][109][110].
Moreover, reducing the CR may improve knock limit extension while increasing HES.Chintala and Subramanian [102] found that decreasing the CR from 19.5:1 to 15.4:1 resulted in increased knock limits at 19%, 59%, and 63% HES, respectively.Similarly, Sharma and Dhar [107] reported an increase in knock limits from 20% to 45% HES as the CR decreased from 19.5:1 to 14.5:1.However, it should be noted that decreasing the CR may also lead to a decrease in BTE.Rosha et al. [106] found that BTE decreased from 36.1% to 34.4% and 32% for 16:1, 17:1, and 18:1 CRs, respectively.The authors argued that increasing the CR improved the BTE by enhancing the in-cylinder pressure and temperature.Additionally, in-cylinder temperature reduction caused a longer ID and CD.
In terms of emissions, the advantage of reducing NOx emissions with CR reduction comes with some disadvantages.Lowering the combustion chamber temperature through CR reduction leads to slower and incomplete combustion, resulting in increased HC and soot emissions.As a consequence, the oxidation rate of CO into CO2 is also reduced [106,107,109].
These methods for decreasing NOx emissions may be compared and utilized in unison.In a study utilizing solely diesel fuel, Serrano et al. [100] compared the implementation of EGR with WI in the intake manifold.The authors stated that the use of WI was more effective than EGR at all engine speeds and under all engine loads.Additionally, Chintala and Subramanian [97] reported a reduction in NOx when testing on a singlecylinder engine.They found that HES was limited to 18.8% in a conventional dual-fuel setup.However, it increased to 66.5% with the use of WI at 480 g/kWh.Moreover, it further increased to 79% with WI at 340 g/kWh (the selected optimum quantity) and a CR reduction from 19.5:1 to 16.5:1.Raju and Masimalai [99] conducted a numerical study on a single-cylinder engine, confirming previous studies on enhancing HES and reducing knocking and NOx emissions.Additionally, authors of this review undertook a study on a four-cylinder commercial engine and found an almost linear relationship between the CR and NOx [93].As a result, decreasing the CR could promote NOx reduction.Moreover, the use of WI further supports this reduction, as evidenced from Figure 3.

Conclusions
This review provides an overview of the utilization of H2 and diesel fuel in dual-fuel mode for a sustainable energy solution for diesel engines.The subsequent conclusions are grounded in the outcomes obtained within the literature.

•
The use of H2 presents two obstacles that must be considered: knocking and backfiring.Too much H2 increases the probability of knocking.Fortunately, backfiring can be remedied by proper injection of H2 into the engine.Thus, choosing the optimal method for mixing air with H2 is crucial.• Among H2 injection techniques (carburation, manifold, port, and direct injection), port injection provides superior outcomes for enhancing engine performance and reducing emissions.Nonetheless, direct injection shows potential as the superior option that may address the issue of backfiring.However, given the nonexistence of commercial H2 injection solutions, it remains impractical at present.

•
It is unclear how H2 affects brake thermal efficiency (BTE) in terms of efficiency.While some studies have reported improved combustion and increased BTE with the addition of H2, others have found incomplete combustion due to H2.This issue, combined with higher thermal losses, may lead to lower BTE.Additionally, the introduction of H2 into the combustion chamber tends to displace a significant amount of air, resulting in a decrease in volumetric efficiency (VE).

•
The inclusion of H2 in the combustion chamber, mixed with air, means that more energy is needed to increase the temperature of the mixture.This gas property justifies the increase in ignition delay (ID), as more time is required for the mixture's ignition to occur.Furthermore, hydrogen accelerates both premixed and diffusive phases, resulting in a faster reaction compared to a reaction without hydrogen, which leads to a lower combustion duration (CD) and a higher maximum in-cylinder pressure.• In terms of emissions, CO2.CO, HC, and soot decrease with the addition of H2.H2 is a carbon-free fuel with a significant influence on the reduction of these emissions.In addition, it seems that this fuel's properties favor more complete combustion.

•
The introduction of hydrogen in the chamber results in an increase in pressure rise rates and increased combustion temperatures.This tends to favor the Zeldovich mechanism for NOx formation.Consequently, the addition of H2 may need further NOx reduction.

•
Exhaust gas recirculation (EGR), water injection (WI), and compression ratio (CR) reduction are three effective NOx reduction strategies.These approaches reduce combustion chamber temperature and, thus, NOx emissions up to 70%.Furthermore, these techniques increase the knocking limit, which in turn increases the hydrogen energy share (HES).

•
These techniques have a negative impact on both performance and emissions.They reduce BTE and increase ID and CD, thus leading to a decrease in the maximum incylinder pressure.Furthermore, combustion with these techniques promotes incomplete combustion.This, in turn, increases the emission levels of HC and soot, while reducing the oxidation rate of CO into CO2.Despite this, the levels achieved may be lower than those observed during diesel fuel operation mode.

Figure 1 .
Figure 1.Number of publications and citations per year about the use of H2 in combustion ignition engines under dual-fuel mode.

Figure 2 .
Figure 2. Schematic drawing of the different H2 (orange) and water (blue) injection techniques.

Table 2 .
Summary of the types of tests.

Table 3 .
Results of brake thermal efficiency (BTE) with the addition of H2.

Table 4 .
Results of volumetric efficiency (VE) with the addition of H2.

Table 5 .
Results of ignition delay (ID) with the addition of H2.

Table 6 .
Results of combustion duration (CD) with the addition of H2.

Table 7 .
Results of in-cylinder pressure with the addition of H2.

Table 8 .
Results of CO2 emissions with the addition of H2.

Table 9 .
Results of CO emissions with the addition of H2.

Table 10 .
Results of HC emissions with the addition of H2.

H2 Enrichment and Mixing Process Type of Test Engine Specifications Key Findings (HC=) Data Directly Reflected in the Document (HC≈) Approximately Measured Data in the Document's Figures
HES: hydrogen energy share, ML: medium load, NL: nominal load.

Table 11 .
Results of soot emissions with the addition of H2.

Table 13 .
Results of the reduction of NOx with the addition of H2 and EGR.

Table 14 .
Results of the addition of H2 and WI.

Table 15 .
Results of the addition of H2 and CR reduction.