Effects of Water Injection in Diesel Engine Emission Treatment System—A Review in the Light of EURO 7

Abstract

Water in the engine/combustion chamber is not a novel phenomenon. Even humidity has a major effect on internal combustion engine emissions and can thus be considered the first invisibly present emission technology. With modern techniques, the problematic aspects of water, such as corrosion and lubrication issues, seem to disappear, and the benefits of water’s effect in combustion may also be enhanced in the context of EURO 7. The current study examines the literature on the effects of water on diesel combustion in chronological sequence, focusing on changes over the last three decades. Then it analyzes and re-evaluates the water effect in the current technology and the forthcoming Euro 7 regulatory context, comparing the conclusions with current automotive applications and mobility trends, in order to show the possible benefits and prospective research avenues in this sector. Techniques introducing water to combustion could be a major approach in terms of the EURO 7 retrofit mandate, as well as a feasible technique for concurrent nitrogen oxides and particulate reduction.

1. Introduction

Air is an essential medium in our existence that has undergone significant changes over millions of years. Its current components are responsible for the earth’s environment and ecosystem as we know it. These periodic changes seem to be disrupted by the vast amount of pollutants, emitted into the atmosphere by human activities. Wherever they occur—in homes, thermal power plants, internal combustion engines, or even in the human, animal or plant species themselves—combustion processes (oxidation) are the primary cause of both natural and human-induced air pollution.

Many of these have been present in the atmosphere for millions of years in smaller or larger quantities. Nitrogen oxide (NO_x) is a precursor to the formation of ground-level ozone, which has formed the ozone shield essential for life. But it is also responsible for acid rain and smog as a result of human activity. In the transportation sector, diesel engines continue to be the main source of NO_x emissions [1,2].

The destructive power of aerosol particles in the air (in addition to their carcinogenic and other harmful effects on the respiratory organs of living beings [3,4,5]) is illustrated by the catastrophe 65 million years ago in which 80% of living organisms died out. The onset of winter, caused by particulate matter in the air, had a devastating effect on living organisms.

In our time, exposure to pollutants is not as drastic, but it has been steady over the long term. Efforts to reduce them are now half a century old. In modern societies, the concept of emission reduction has become embedded, and a representative area of this is vehicle emissions. Current studies have shown that as the accumulation mode particle diameter decreases, both diffusive and inertial particles exhibit a greater tendency to deposit in the lower respiratory airways. Furthermore, their exhalation and clearance from the respiratory system can be progressively hindered [3,6].

The real ratio of natural and human activity-related emissions of each hazardous component is an important subject to address. A key feature of man-made sources is that they tend to be spatially concentrated—located in large cities or industrial sites—so that pollutants are released into the air over a very limited area and are only harmlessly diluted far outside the urban boundary (transmission). Natural sources can influence the usual concentration of contaminants in the troposphere. This overall concentration of contaminants is what humans and other living organisms will eventually inhale (immission) [7]. Natural sources include, for example, volcanic activity, forest fires, and sea salt aerosols, all of which can produce particles, nitrogen, and sulfur oxides. Available information suggests that globally 25% of urban ambient air pollution from particulate mass size distribution larger than 2.5 µm (PM_2.5) is caused by traffic, 15% by industrial combustion, 20% by domestic fuel combustion, 22% by unspecified anthropogenic sources, and 18% by natural dust and salt. But the proportion of natural dust and salt in certain areas in the world can reach up to 50% [8]. To meet the air quality standards set out in EU legislation [9], since 2008, the Air Quality Directive has taken natural sources into account in the emission limits for each country [10,11].

As a result of efforts made in the last 50-year long history of automotive emission aftertreatment dating back to California Air Research Board (CARB), nowadays emissions have become so low that focus has begun to dissipate between other high emission parts of the vehicles like brake particles and the microplastic emissions of the tires [12,13,14,15]. In addition, current toxicological research shows that such non-exhaust pollutants (brake wear and tire dust) have toxic properties that are similar or sometimes even more severe on a unit mass basis than, for example, diesel engine soot [16]. Even after half a century of development in emission aftertreatment, we see that even today, significant reductions (~50% or more) can still be achieved within a period of a decade or two [17,18,19].

Land transportation is the most visible and tangible form of transportation, with internal combustion engines being the most well-known propulsion technology at the moment. The diesel engine is the most widespread technology for non-road, heavy/medium duty applications [20,21]. After the millennium, the popularity of diesel engines in personal automobiles has also grown. However, after the 2016 diesel scandal, things have changed, and the transportation industry is currently going through a paradigm shift. With the introduction of EURO 7, the goal is to convert the entire fleet to zero-emission vehicles (ZEVs), and analysts predict that the main market share for passenger cars could be taken by battery electric vehicles (BEVs) within this decade [22]. With carbon-neutral e-fuels, created using renewable energy, the problem of internal combustion engines can be solved even after the introduction of zero carbon dioxide (CO₂) emissions in 2035 [23].

Up to now, the transportation, which typically based on internal combustion engines, is expected to lead in the coming decades to a parallel proliferation of different technologies for urban/short-range, long-range, freight, etc. Some of the new technologies (electric propulsion, fuel cells, etc.) and some of the existing but less widespread technologies (CNG, LNG, other alternative fuels, additives, including water emulsions, injection, variable compression ratio—(VCR)) [24,25] have a strong lack of technological soundness. It is expected that as the knowledge of these new propulsion technologies increases, the emissions of each technology over its entire ‘life cycle’ will become more accurately quantifiable and, as a result, more amenable to appropriate regulation. In light of this progress, the shares of new technologies in transport are expected to undergo a major shift towards the middle of the century. The forthcoming EURO 7 legislation, which will be stricter than ever before, is designed to deal with this, and to set technology-independent emission limits.

New developments in EURO 7 suggest that the planned ban on internal combustion vehicles by 2035 may be broken [26]. This would be in line with the essentially powertrain- neutral approach of EURO 7 and the European Commission’s stance that the EU does not wish to favor certain technologies.

Beyond all general considerations, the use of internal combustion engines—at least for heavy-duty and off-road applications—will be around for decades to come because of the advantages they offer in this area, such as fuel storage, weight, and range. Regulations are tightening for on road (EURO 7) and non-road (NRMM) vehicles (Tier 4, Stage V) in many parts of the world. They are also increasingly focused on ensuring that as many as possible of the vehicles with low or no emission standards can be retrofitted to reach decreasing emissions. The vast majority of vehicles worldwide fall into this category. Retrofitting should be as simple and cost-effective as possible. One such option could be the retrofitting using water as the substance intentionally supplied into the combustion chamber.

Water in combustion can also solve one of the major problems of internal combustion engines, the trade-off between nitrogen oxides and particulate matter. High temperature, oxygen-rich conditions, and increased combustion duration can ensure the proper combustion of hydrocarbons (HC) [27] and, as a result, also lower particle emissions. However, these conditions also contribute to the generation of NO_x emissions. HC, particle number (PN), and NO_x emissions generally show opposite trends, with one increasing and the other decreasing [28,29,30,31,32]. Few techniques can decrease these emissions by allowing better mixture formation, like the water in the combustion and intake air swirl techniques, with pressure-swirl atomizer [24,32]. There are numerous options for introducing water into the combustion chamber, each with its own advantages and disadvantages, which will be presented individually later on. These techniques will be evaluated based on the new emission regulations and the related retrofitting considerations. However, it is also important to discuss the potential harmful effects of water, such as corrosion in the presence of sulfur in diesel fuel, the wear of engine components, and the appearance of liquid water in the combustion chamber, resulting from various water emulsion and injection techniques [33,34,35,36]. Additionally, as a new element, the effects of water on other components of the emission control system will be examined, such as exhaust gas recirculation (EGR) and after-treatment system parts in the exhaust. These will be considered from the point of view of emissions as well as other impacts, including temperature.

In real-world applications, after water is introduced into the engine combustion chamber, excess water exits with the exhaust fumes and passes through the entire aftertreatment system (ATS). This water in the exhaust system has effects on all component of the ATS. For this reason, the effect of water has been examined in the literature. There is currently no available information regarding the specific emissions that may be present at the tailpipe for different ATSs. Clarifying this aspect could be particularly relevant in the field of retrofitting; however, it remains an unexplored area of research.

Water in the combustion as a technique should be reevaluated for subsequent modifications, the cost, results, and benefits it can provide. It needs a significantly different approach compared to the accumulated materials written in the recent decades. Applications dealing with performance and/or emission reduction on raw engine emissions need to be excluded, as retrofitting is about real vehicles with different levels of existing emission treatment systems. This research concentrates on practical-based results and needs to be able to separate comparisons between theory and practice and their applications. Without the upcoming regulatory incentive, up until now, the literature on vehicle retrofitting with such different approaches has been very limited because of low interest.

2. Emission Regulations

As was recognized from the 1960s onwards, land transport is a major contributor to the increase in atmospheric pollutants that can cause severe climate change. From 1968, defined by CARB, soot emissions from diesel engines—previously subjective measurements used based on questionable correlations—were measured by smoke measurement equipment, thus providing an objective, quantifiable framework for the emissions of internal combustion engines. This is the origin of the emission legislation which, in response to health concerns, has introduced ever stricter emission limits for new diesel engines worldwide. Current emission levels around the world are forcing continuous improvement and system design, where advanced engine control must be used in parallel and combined with a variety of sophisticated exhaust aftertreatment technologies. Compliance with the current emission limits is also a societal requirement that no car manufacturer can ignore (diesel scandal). It is also an essential element in the design of internal combustion engines and represents a significant part of the cost of engine development.

2.1. On Road Regulations

As one of the major leading areas of emission regulations, the first standard dates back to 1992 in Europe, with the introduction of EURO 1.

Between each EURO standard, approximately 50% reduction can be seen both for particulates and nitrogen oxide emissions in case of passenger car (PC), light duty (LD), and also heavy duty (HD) vehicles, as shown on Figure 1 and Figure 2.

With the Euro 6 regulation, particle number (PN₂₃—Particle number size distribution larger than 23 nm) emission limits were introduced, and as a result, gradually particulate filters became unavoidable in every emission treatment strategy.

Also, the standards became much stricter along the evolution of measurement procedures and cycles. This became more pronounced from Euro 6 onwards (Figure 1 and Figure 2).

The base idea is to simulate the traffic conditions of the given era more and more precisely. As a result, from “Euro 6d temp”, real driving emissions (RDE) were also introduced, along with the instrumentation, portable emission measurement system (PEMS) which makes these measurements possible and force further efforts from car makers to comply with the regulation.

Depending on the exact emission limit values of particulate mass (PM) and NO_x, since the introduction of EURO 1, the tightening of emission levels is around 95%. This does not include the effect of the more and more severe test procedures and test cycles. In a study performed by Automotive Edge Computing Consortium (AECC) [37] with EURO 6b LD vehicles, the emissions of individual vehicles vary greatly depending on the different emission cycles they run, from New European Driving cycle (NEDC) to the latest RDE cycle. In case of PM and PN, there are not so big differences between the different cycles because of the diesel particulate filter effectiveness, but the NO_x results are varying highly up to 10–30 times higher if emissions are measured with RDE. We can expect a significantly higher raw emissions with these new cycles.

Although significant changes have already been made to the original wording of EURO 7, the main concept left untouched: it will be the strictest fuel and technology independent emission regulation. It will deal with not only emissions and vehicles but also components like brakes and tires. In addition to this, EURO 7 is also an energy and electrification legislation, defining battery durability and driving range also.

The latest iteration of the upcoming standard keeps the lowest EURO 6 numerical emission limits, but introducing several new items identified as environmental threats. The particle number size distribution larger than 10 nm (PN₁₀) nucleation size particles are responsible for severe health effects, known for the years 2000 onwards [38]. With available sophisticated instruments of the last half decade [39], more and more studies confirm this [40,41]. Although there is no research available in the direction of the effects of water on this particle size range, studies investigating the relation between humidity and PN₁₀ [42] suggest possible advantages with the usage of water in the combustion.

The ammonia (NH₃) emission limit for HD diesel engines was established in EURO VI; however, as of EURO 7, it is applied consistently to all powertrains. In addition to the usual diesel emissions, if we apply new additives to the vehicle emission reduction system, these new sources can cause the release of further harmful substances. Such additives can be introduced to the engine through fuel or can be injected into the exhaust gas. Ammonia is a good example of this, created by the urea injection introduced to the exhaust fumes. The ammonia gas produced is necessary for the operation of the Selective catalytic system (SCR). To reduce NO_x emissions, the SCR is frequently used. The water vapor, which is always present in flue gas, has garnered a lot of attention as it has a significant impact on the performance of SCR catalysts [43]. At the end of the exhaust system, the Ammonia Slip Catalyst (ASC) is responsible for the remaining NH₃ removal, where the water also can have catalyst effects. Both emission aftertreatment elements and their interactions with water will be checked in detail in the next chapter.

The European Commission’s 2022/0365 (COD) original EURO 7 proposal [44] was already described in the starting paragraphs: “Appropriate incentives could be put in place for older cars to be retrofitted to meet Euro 7 requirements for tailpipe emissions but also for tyre and brake emissions”. As a baseline of this ambition, studies and projects have been established before, including the Commission’s successful “2018 Horizon Prize on Engine Retrofit for Clean Air” project and the “Study on retrofit technologies’ potential to reduce emissions from passenger cars and light commercial vehicles” project, which started in 2020. To make this goal possible and realistic, easy and cost-effective solutions should be proposed to elongate vehicle life, reduce emissions, and possibly increase fuel efficiency. With the fuel efficiency reduction, the CO₂ emission will also decrease as it is a product of the perfect combustion of fuel, along with water. CO₂ also appears directly as a main emission reduction item in the EURO 7. In a study using Variable Compression Ratio (VCR) technology, a direct correlation was observed between reduced fuel consumption and decreased CO₂ emissions [25]. Introducing water into combustion (the different techniques are collectively referred to as—W.I.), with its low implementation costs, presents another potential solution for achieving these objectives. From these perspectives, W.I. will be further examined later in this material.

In Figure 3, we can see which countries are using EURO standards as a base of their emission regulations. These figures show that more than half of the world’s countries derive or align their emission reduction efforts to European standards. Regulations matching EURO 4-6/IV-VI are in effect throughout Australia, Oceania, Asia, South America, and Europe.

In the US, “Tier 3” and “2007 Heavy Duty Highway Rule” are the corresponding regulations for PC/LD and HD vehicles. In general, these regulations are more stringent than EU standards for important local air quality pollutants, especially NO_x. Canada has brought its vehicle and fuel standards into line with those of the United States; for example, LD vehicles are currently subject to Tier 2 restrictions, while HD vehicles operating on Canadian roads are covered by the same 2007 Heavy Duty Highway Rule.

In Africa, where regulation is already in place (8 out of 54 countries), EURO 2-3/II-III standards are mostly applied to the sale of new vehicles [46]. Other countries on the continent are also moving towards setting emission reduction targets.

These figures allow us to estimate that these countries’ used automobile stocks are mostly EURO 4-5/IV-V or even lower in the case of Australia, South America, and Russia. Africa’s vehicle stock is anticipated to be lowest in the world, although in the case of used automobiles, there should be a greater number of vehicles with the EURO 1-2/I-II standard or even without emission management systems [47].

With the current emissions regulations, new car sales are around 72.5 million worldwide in 2023 [48]. By contrast, the number of used cars in the world is estimated at 1.5 billion. The volume of used cars between Euro 3-5/III-V is expected to be the largest, for which the greatest results could be achieved globally from retrofitting point of view and for which modifications could also be solved economically.

2.2. Non-Road Regulations

European non-road regulations deal with mobile machinery used for agricultural, forestry, harvesting, piste machines, construction machines, stable engines, gardening machines, and other vehicles whose primary working area is not the public roadways. The first implementation of this regulation in Europe is dated back to 1999.

For the purpose of regulating non-road vehicles, they employ a broad range of classifications, ranging from several hundred cubic centimeters to marine vessel engines, or based on engine power. These classifications change with the different stages of the standard. In Figure 4, a 60–70 kW engine power output level was chosen for comparison purposes.

The engine gaseous emissions and PM are the components that are regulated. PN control restrictions were introduced for non-road vehicles in the most recent EU Stage V. Diesel particulate filter (DPF) application became required as a result. Between each stage, approximately a 45% reduction can be observed for both particulate and nitrogen oxide emissions. Since the introduction of Stage I, depending on the exact emission limit values of PM and NO_x, the tightening of emission levels is 98% and 95%, respectively. Here, we can also expect higher emissions with the new emission cycles and procedures. In the US, the regulation of non-road engines was somewhat similar to the EU regulations, but starting from the Tier 3/4 and the corresponding Stage III/IV, they tend to drift apart. In other parts of the world, Tier/Stage 1–3 standards have typically been adopted.

As most of these machines are not registered, it is difficult to quantify their volume, but their emissions should be definitely higher than those ofon-road vehicles [49,50]. In order to control these emissions, various emission inventories are being developed, and some already exist. These inventories—depending on, for example, exhaust emission data, fuel mix, geographic features, and other factors—estimate the emissions across a chosen area. These data help to monitor the impact of emission reduction measures [49]. With such emission inventories existing in several European countries, we can compare the countries NRMM and on road emissions. From this, we can see that in case of NO_x, the emissions of NRMM is around 1.5–3.5 times higher. The same value for PM emissions ranges from 2.3–11.9, with an average of 6.6 [51]. This shows the clear potential of retrofitting in this area, even if we cannot clearly estimate the ratio in case of less technologically developed areas.

In addition to these, on global scale, for non-road machinery, the fuel specification is much less strict. For on-road vehicles—since using catalytic converters (EURO 1/I)—the essential requirement is to lower Sulfur content in fuel. Most of the world currently uses ultra-low Sulfur fuels for on-road vehicles. In contrast, in many places, the fuel requirement for NRMM equipment is much less stringent. Sulfur content causes catalyst poisoning through cerium oxide (CeO₂) deactivation [52]. With high sulfur content in fuel, retrofitting with catalytic converters is not feasible.

Non-road machinery includes industrial construction machines, shipping vessels, and other high value applications where the investment in retrofitting has much less impact compared to the cost of operation. Some of these machines have much higher lifespan compared to PC vehicles, where the depreciation is much less, and retrofitting can be a cost-effective solution.

3. Water in the Combustion

The three main parts of the internal combustion engine emission control system are (Figure 5):

• Pre-engine processes (different fuel/fuels introduced into the engine);
• Processes implemented in the engine (EGR, optimized engine control);
• Post-engine systems (catalysts, filters, after-treatment systems) [53].

W.I., implemented as a pre-engine process, introduces a significant factor influencing the entire ATS. The injected water, while it has a primary effect on combustion, will inevitably impact both the in-engine processes (EGR) and the post-engine components. Excess water, after its effects on combustion, will be carried through the exhaust system, potentially affecting the performance and efficiency of downstream components. This necessitates a comprehensive understanding of the water’s impact on the ATS, particularly in the context of retrofitting existing vehicles with W.I. technology.

In an efficient emission control system, the individual parts of this whole assembly should work together to get the designed parameters. Since retrofitting is the goal, we need to look at all the expected and possible emission management systems from EURO 1/I onwards. For each of these components, like diesel oxidation catalyst (DOC), DPF, or SCR, we should examine and evaluate the impact of W.I. on them. When retrofitting, we are adding the W.I. system into an already existing emission system, so we need to look at the issue in depth.

3.1. Water in Combustion, Past and Present

The technique of W.I. is not new, having been used since the 1940s, originally by the German Air Force during World War II as a performance-enhancing device on a number of its fighter aircraft [54].

However, the literature shows that until the 1980s, the emission reduction potential of W.I. was not researched. It came to the fore afterwards, but typically in the form of water-ethanol/methanol emulsion injection. Mostly simulations and studies with pilot engines have been carried out on this topic. In the 2000s, at the state of the art, the use of W.I. was considered a less promising technique, based on a consideration of the potential risks and the results that could be achieved. The main reason for this is that there have always been methods that performed better in removing a single specific dangerous emission component. The fact that this is practically the only significant technique capable of simultaneously removing particles and nitrogen oxides during diesel combustion has always been overshadowed. Nowadays, the problems addressing W.I. can be solved and technically eliminated. The sulfur content of previously used fuels has been reduced with the ultra-low sulfur fuels, so the risk of sulfuric acid and sulfurous acid formation is much lower or negligible. Furthermore, corrosion problems can also be avoided by the appropriate use of advanced materials, the increased use of plastic parts, and the sophisticated engine management [24,55,56].

In the non-road sector, we can see samples of practical uses of W.I. for diesel engines in maritime applications [27,57]. There are no viable substitutes for the prevalent diesel engines in the commercial industry, particularly when traveling by sea [58].

In general, the emission regulations are much more stringent in the on-road sector. In today’s sophisticated diesel ATS, there is a lack of knowledge about the effect of W.I. on the parts and on the whole ATS.

3.2. Water Effect on NO_x Emission

Water clearly has a significant effect on combustion processes in internal combustion engines. Diesel engines operate with excess air during the combustion. Through humidity, air drags a higher amount of water into the combustion chamber of the diesel engine, compared to an Otto cycle engine. Water can already have significant effect and can follow through this. A study was performed by wide scale of humidity conditions [42] on a EURO V, but no EGR type engine, to see clearly just the water effects. With such a relatively low amount of water, we can see the decrease in PN₂₃ and NO_x emissions (Figure 6).

In case of humidity, the main physical NO_x reduction mechanism is dilution effect and specific heat effect (or Thermal effect). Water has higher specific heat capacity (2.03 J/(g K)) compared to the mixed air (1.01 J/(g K)) and fuel charge (2.09 J/(g K)). With the higher mass of air/fuel mixture with higher specific heat, the combustion temperatures are lowered remarkably [59,60,61,62]. Another significant physical mechanism is the diluting effect of water, which is also related to the higher mass of air/fuel mixture. It reduces the concentration of the fuel and the oxygen in the cylinder charge and thus the energy from combustion [62,63,64]. The NO_x production of diffuse flames is also reduced due to a decrease in the total combustion duration. This is because accelerated mixture formation and increased ignition delays lead to increased premixed combustion [65,66,67,68]. When the engine is running at higher loads, W.I. exhibits greater NO_x reductions [69,70,71]. In the premixed combustion, with the decreasing temperature and in lean conditions, the prompt NO (Fenimore) reactions (Reactions (1)–(6)) [72] are suppressed, and the significance of thermal (Zeldovich) mechanism is decreases.

CH + N₂ ↔ HCN + N

(1)

C + N₂ ↔ CN + N

(2)

HCN + O ↔ NCO + H

(3)

NCO + H ↔ NH + CO

(4)

NH + H ↔ N + H₂

(5)

N + OH ↔ NO + H

(6)

In contrast, the nitrous oxide (N₂O) intermediate mechanism reaction is kinetically favorable (Reactions (7)–(9)) [27,73]:

O + N₂ ↔ N₂O

(7)

H + N₂O ↔ NO + NH

(8)

O + N₂O ↔ NO + NO

(9)

Overall, a decrease in nitrogen oxide emissions can be seen with the decreasing proportion of the main thermal mechanism.

After the start of the burning process with premixed combustion, increased pressure compresses and warms the unburned portion of the charge, shortening the time it takes to ignite. In such cases, with an overall lean air fuel ratio greater than 25:1, the rate of fuel/air mixing determines combustion until all of the injected fuel has been consumed, which includes continuous atomization, vaporization, fuel vapor-air mixing, and combustion [74]. Temperatures in this inhomogeneous combustion can reach 1600 °C or more. In such circumstances (non-premixed combustion), the thermal mechanism becomes advantageous in terms of reaction kinetics (Reaction (10)–(12)) [75,76]:

O + N₂ ↔ NO + N

(10)

N + O₂ ↔ NO + O

(11)

N + OH ↔ NO + H

(12)

With artificially increased water amount in a liquid form of water, the phase change heat sink effect appears (or, in some literature, charge cooling effect), and the dilution effect becomes more pronounced. The enthalpy changes of the vaporization of the water (2256 J/g) is around 8 times higher compared to the diesel fuel (277 J/g). When water is injected in liquid form, its specific heat capacity (4.18 J/(g K)) is double compared to its gas form (2.03 J/(g K)). These values show the scale of the heat absorption effect of the phase change besides specific heat. The values are for room conditions, which may differ under cylinder conditions. According to a study performed by Neshat in 2019 [71], the thermal effect of the specific heat has a 3–4 times higher impact on the NO_x formation compared to the dilution effect.

Heat-induced dissociation of water (H₂O) can occur during combustion, and the products of this process might participate in the combustion [77]. There are two main reaction paths (direct and indirect) of these products under the typical circumstances of combustion. These reactions are collectively known as the chemical effects of water [78]. Combustion starts with the formation of H radical from the typical C₁₂–C₂₀ hydrocarbons of the diesel fuel, as shown in Reaction (13). At high temperatures, the formed hydrogen (H) radical reacts with oxygen and leads to a hydroxyl (OH) and to an O radical (Reaction (14)) [71,79].

C_nH_2n+2 ↔ C_nH_2n+1 + H

(13)

H + O₂ ↔ OH + O

(14)

Water then reacts and consumes O radicals (Reaction (15)), which are initiating the thermal NO production in the post-flame region (Reaction (10)). In this way, NO formation will be reduced. This reaction is the direct chemical effect of water [77,78].

H₂O + O ↔ OH + OH

(15)

As a result, OH radical production increases, and these radicals are good oxidizing agents, so the main carbon monoxide (CO) Reaction (16) is utilized [80,81].

CO + OH ↔ CO₂ + H

(16)

This reaction increases the in-cylinder temperature and, as a result, suppresses the HC formation [82,83]. This can be one of the mechanisms by which water can decrease nitric oxide (NO) and HC emissions, in parallel against the well documented trade-off between these emissions. However, with increasing amounts of water injected, as a strong polyatomic collider, water’s indirect three body reactions are strengthened, resulting in more hydroperoxyl (HO₂) radicals (Reaction (17)). These radicals are finally consumed to form H₂O along with oxygen (O₂) at the termination step of combustion (Reaction (18)) [71,78,84].

H + O₂ (+H₂O) ↔ HO₂ (+H₂O)

(17)

HO₂ + OH ↔ O₂ + H₂O

(18)

Some researchers distinguish further effects, such as thermal throttling, which is separated from the dilution effect. In addition to the increase in oxygen concentration with the addition of water (dilution effect), the mass flow rate of the intake air also increases with an increase in density, known as thermal throttling. With the addition of water into the intake charge, volumetric efficiency also increases [85,86]. Apart from thermal throttling and dilution effect, the temperature difference of the inlet charge also has a significant effect on the temperature of the cylinder gas at the end of the compression stroke and during the combustion process (inlet temperature effect). Also, this calculation does not account for the phase change effect, considering only the effect of the incoming charge temperature [87].

In case of the raw diesel engine emissions, the thermal and the dilution effects are the most characteristic in the NO_x removal efficiency. One of the primary variables affecting thermal NO_x yields is the oxygen availability, which is generally high in diesel engine. In diesel engine, the burning is typically inhomogeneous and mixing-controlled, so the air fuel mix can be rich locally, which is responsible for the NO_x emission. The duration of burning is also one of the most important variables here, along with the temperature of the burning. While chemical effects are important, they generally have a minor influence on nitrogen oxides due to the many pathways involved working against each other. NO_x emissions might vary under different loads of the engine, even under the identical testing conditions. Results are highly dependent on the testing setup. In general, each percentage increase in the water to fuel ratio results in a half percent reduction in NO_x [35,69,88].

3.3. Effects of Water on Particulates

Particulates in the internal combustion engine are produced by the agglomeration of different particles, which can be solid, such as:

• carbon;
• ash from fuel and lubricating oil;
• metal particles from engine wear.
• In the emission, liquid particulate matter (soluble organic fraction (SOF)) also exists in the form of:

  ○

  unburnt or partially burnt hydrocarbon;

  ○

  water;

  ○

  sulfates (which is negligible in modern ultra-low Sulfur diesel (ULSD))

The many components make the particles very complex. The substances listed above usually adsorb onto a carbon central core to form agglomerates [89,90]. Soot particle formation, a complex phenomenon occurring during combustion, can be broadly categorized into four distinct stages: nucleation, surface growth/coagulation, agglomeration, and oxidation. These stages represent the progressive evolution of soot particles, beginning with the formation of nascent nuclei and culminating in their eventual oxidation and destruction [91].

In raw emissions of the diesel engine, nanoparticles with diameter lower than 50 nm significantly appear only during low load conditions, when oil consumption is typically highest (predicts that the nanoparticles are of ash origin). With increased load, more fuel is injected, and the emitted number of particles increases. Mostly accumulation mode particles (~50 nm–1000 nm) can be found here, with diameters below 100 nm (ultrafine particles), and typically the size of these particles decreases with increased load. This is because particles have declining residence time to agglomerate and adsorb volatile materials on their central carbonaceous core. Above 100 nm, we can find the soot particles and the agglomerations created along the exhaust pipe [92,93].

A study using only DPF with low and medium load engine conditions shows two magnitudes of PN emission (~99%) reduction in the region of nanoparticles. It maintains at least 90% reduction above (10–50 nm) and even below the 10 nm range [94].

W.I. decreases the combustion flame temperature by the dilution effect of water, in a way that the heat of combustion is released into the increased amount of gas. Under higher loads of the engine, water’s main thermal effect still decreases temperature. This leads to an increase in unburnt and partially burnt HC, which is responsible for soot particles and larger particle agglomerates. This can be only reduced by the mechanism described earlier, where the oxidizing OH radicals can suppress the HC formation at lower water quantities [95]. It is the unique effect of water that can elude the well-documented NO_x PN/HC trade-off in internal combustion engine (ICE) combustion. A moderated PN emission decrease can be seen even with increased air humidity (Figure 7). The air flow rate remains approximately unchanged at higher loads with W.I., so while NO_x is reduced, PM emissions also remain moderated [96].

With water introduced to the engine, an increase in liquid in the raw exhaust cannot be avoided. This can make a significant difference during measuring PM compared to PN. In the case of PN measurement, the sample is typically treated at 350 °C, using either a catalytic stripper or an evaporation tube, and the SOF and any liquid components are deliberately removed. In this way, only solid particles are counted on the instrument. During PM measurement, liquid particulates are also included. In case of W.I., this brings a lot of uncertainty into the evaluation of the results.

Considering the current aftertreatment systems, after a typical Euro 5 system with DOC and DPF, at least 96% of the particles are filtered out on number base—mainly in the ultrafine region—with the lowest filtration for the liquid PM, which is predominantly water PM. DPF filters out the solid particulates, while the DOC removes the SOF in HC form [86,97]. In such ATS, only NO_x remains, and the question becomes how to remove further, with synchronization of EGR and W.I.

3.4. Effects of Water on Engine Efficiency

At low and medium engine loads, the addition of water enriches the in-cylinder mixture by temperature reduction, which leads to greater engine output (up to 5%) at lower water injection levels (1–3%). With introduction of higher water content, the dilution effect causes the decrease of the possible reactions, leading to a drop in the cumulative heat release and engine performance [35,71].

Most of the publications report increased cylinder wall heat transfer, which can affect negatively the engine’s global efficiency [96,98,99]. Changes in the brake thermal efficiency (BTE) can be caused by the increase in exhaust enthalpy as well. Because of the W.I., the temperature of exhaust gases decreases to a large extent. However, the total enthalpy may not change significantly because of the increased exhaust mass flow resulting from the additional water. However, the increased exhaust mass flow with increasing water/fuel ratios can be used in a turbocharger [98,100].

Depending on studies performed, at high load condition, W.I. results in a decrease in brake specific fuel consumption (BSFC) [101,102,103]. At low loads or with a high amount of water, typically BSFC penalty can be seen [99,103].

There are many contradictory studies in this area, probably because the results are often engine specific and can depend on measurement devices, transient or stationary emission cycles and conditions, or W.I. systems.

3.5. Wear and Corrosion Effects of Water

One problem that always arises with W.I. is the effect of water on the oil film. It is therefore important that the cylinder wall is not exposed to water droplets, because water in its liquid state has a detrimental effect on the lubricant film [35]. Once the water evaporates, it can no longer affect the lubricant oil film.

The most commonly studied and well-known type of wear caused by water occurs in the fuel system. In the case of high-sulfur fuel, corrosion can already occur in the fuel tank for NRMM and marine applications, leading to significant damage in the injector by rust particles. In addition to corrosion, water also affects the longevity of injectors by degrading the lubricating properties of diesel fuel. If the used injectors and systems not specifically designed for W.I. but instead utilize the engine’s own injectors, as with emulsion technology, additional wear may occur in the injector. Experiment conducted in real vehicles have shown that the emulsion system can result in serious wear within the vehicle’s own injection system due to metal oxidation, cavitation, and reduced lubricity [104]. The cavitation effect can arise in both direct injection and fumigation scenarios. Oxidation can occur in all three main W.I. cases under acidic conditions, depending on the fuel used [33,34]. Injector systems specifically designed for W.I.—regardless of the type—are much more resistant to corrosion and lower lubricating properties. A study conducted with intake fumigation did not reveal irreversible changes in the engine’s own injector [34]. Corrosion behaviors have been widely researched in marine and some non-road applications where fuel properties are not advanced. The cylinder liner and piston ring are also typical locations for corrosion. It is believed that the major factors influencing corrosion are cylinder liner temperature, charge air humidity, and fuel sulfur content. Studies have shown that the condensation of sulfuric acid is linearly correlated with both the fuel sulfur content and the total mass of water vapor [105,106]

In the intake system, the incorporation of plastics and advanced materials, coupled with a reduced sulfur content, indicates that the likelihood of corrosion is minimal. Conversely, corrosion emerges as a significant issue in the exhaust system and its associated emission after-treatment components. The condensation of water vapor from combustion gases and ambient moisture can occur on the cooler surfaces of the exhaust system. A study investigating corrosion within the metal housings of emission control systems exposed to 10% water vapor content revealed that sulfide formation is likely a contributing factor to the corrosion processes affecting stainless steel samples [107]. The acidity of the condensate is a critical determinant of uniform corrosion, as evidenced by an increase in corrosion current density in acidic environments. The presence of sulfate ions suggests the potential formation of highly concentrated sulfuric acid, which can facilitate uniform corrosion [108]. Nonetheless, there remains a paucity of information regarding the impacts of water on the entirety of the emission control system, highlighting the necessity for further investigation on this area.

In summary, the issues of corrosion and wear can be mitigated in the following ways today:

• improved material qualities, the use of plastics,
• precise injection systems, high injection pressures,
• the possible use of ULSD fuels and, on the other hand, the negligible proportion of engine conditions that favor Sulfurous and Sulfuric acid formation [36].

3.6. Types of W.I.

The following methods are used to introduce water and its various emulsions into the engine combustion chamber:

• Intake side fumigation (port water injection):
  • water,
  • steam,
  • (emulsions).
• Direct water injection:
  • With separate injectors:

    ○

    water,

    ○

    (emulsions).

  • With the own injectors of the engine:

    ○

    emulsions.

• Emulsions can be:

• injection of water and fuel together (as non-stable emulsions),
• stable emulsions as substitute fuel injection [61,96,99,100,109,110,111,112].

In general, in ICE, the emulsion system can be defined as an emulsion of water or other secondary immiscible liquid dispersed in a diesel, gasoline, or other HC containing fluid with specific additives and surfactants to stabilize the system. The usage of water-fuel emulsion in diesel engines is more common because of its higher combustion pressure and temperature, so the effects can be used up more efficiently [113].

If we use the engine’s own injector, emulsion system needs less modification in the vehicle, compared to all other W.I. systems. However, it can be further optimized with engine control modifications. The limitation of this system is given with the injector of the engine and with its delivery capacity. If we just replace the normal fuel in the engine with the emulsion, the engine cannot reach its original performance, and there are also limitations on water content to keep engine proper operation. In some studies, water content was checked up to 40–45%, but the optimum value is around 10–20%. At that point, a BSFC decrease can be observed also [88,114]. With this system, there is roughly a 2–3% reduction in NO_x for each percentage of water [115,116]. In these cases, it is already ensured that the water cools the flame zone directly, rather than cooling the whole combustion chamber. This can have a positive effect on the thermal efficiency. Water suddenly vaporizes upon injection, and as a result of the 1600-fold increase in volume (micro-explosions phenomenon), the mixing of the charge is improved, resulting in decreased soot formation [117,118,119].

With installation of bigger injectors, we can improve the system’s performance also (emission and engine output work), but from that point, it requires significant modifications. There is also a possible drawback that the ratio of water in fuel is given, it cannot be adjusted to fit operation circumstances. With this technique, the previously discussed benefits of water effects—like thermal throttling and inlet temperature effect—remain unrealized. Using the engine’s original injector or even bigger injectors, the wall wetting risk is smallest compared to all the other W.I. systems.

One of the main problems with this technique is sedimentation and separation of components. The requirements for emulsion stability are a wide temperature tolerance and to keep the ingredients in emulsion as long as possible. However, degradation is inevitable over time. One possibility to solve this problem is to keep the components in different reservoirs and just readily mix it right before the injection. But in this case, obviously further infrastructure needs to be built up. Furthermore, surfactants in the emulsions should be burnt without soot and contain as little Sulfur, nitrogen and aromatic hydrocarbons as possible [114] to decrease particulates.

Direct water injection injects water directly into the combustion chamber using an independent injection system. This allows for free selection of injectors for the application, making it possible to inject large quantities of water. This means less trade-offs between performance and emissions, with parameters that can be varied over a wide range. Compared to emulsions, the water/fuel ratio is also freely variable. This is useful for transient, variable conditions, allowing further optimization of the settings for each operating condition. However, with the right design, we can also take advantage of the fact that we only inject specifically into the flame at the right time, so that the wall heat transfer remains low and the formation of soot and nitrogen oxides can remain low. Careful planning and execution of the injector placement, injection timings, optimized water injection quantities are the most important to avoid oil film and engine damage. Because of these circumstances, direct injection is more like the area of original equipment manufacturers (OEM), rather than retrofitting. All the related modifications involving high costs and expertise in various fields—like engine electronics, mechanics, and the separate tank and fuel system—are necessary. Similar to emulsions, water effects—like thermal throttling and inlet temperature effect—remain here also unrealized.

Intake manifold injection or fumigation has the benefit of being one of the most cost-effective investments, as well as being reasonably simple to modify and install on any diesel engine. Figure 8 shows an example design used in a test bench.

Furthermore, it is the most durable system with a low failure rate and enables the system to be turned off at any moment. The engine can operate even after the water has run out. The water-fuel ratio may also be adjusted in real time, but in comparison to other methods like timing or spatial coordinates, it offers the least control over injection settings.

Because of this, publications consider the risk of wall wetting to be highest with this technique [61,121], and wall heat transmission may also increase as a result. Compared to emulsions or direct injection, studies show lower utilization of injected water amount per volume NO_x reduced [122]. On the emission reduction side, this can be offset by the mass flow rate increase (thermal throttling) and the inlet temperature effect.

Considering all of these factors, intake fumigation is the best match in the area of the retrofitting. We can inject in different positions, but W.I. has the same magnitude of emission reduction in general, if we compare it to other emission reduction techniques. Also considering the pros and cons, in the few cases where W.I. has been fitted in series cars (Saab 900, BMW M4 GTS or Oldsmobile Jetfire [123,124,125])—and even if it has almost always been for performance—it has always been intake water fumigation. It has also been used almost exclusively for emission control in marine applications [27,57]. Additional water tank construction is required, but the distilled water required is available at all service stations without the need for additional infrastructure.

Nowadays commercially available W.I. systems are almost without exception intake fumigation systems. They are typically used for performance enhancement, but since the 2000s, we have seen an increasing number of these systems being promoted for emission reduction and fuel economy. Basically, intake port water injection is a low-pressure injection with up to 20 bar [110,124], but lately we can see applications with up to 350 bar improving engine efficiency further [123,126].

3.7. Effects of Water on Exhaust Gas Recirculation

The most basic nitrogen oxide removal technology, known as EGR, is found in virtually all cars with emission control systems. Depending on engine load, it recirculates a certain percentage of exhaust gases back into the engine cylinder, to reduce NO_x emissions. First applications appeared as early as in the 1970s. This is why, in the case of retrofitting with W.I., EGR is the first and essential technique to consider. It is most important to not just check and compare the effects on the raw diesel engine emission, but to assess the possible interactions between EGR and W.I. The physical method of lowering temperature in EGRs, known as the air/fuel dilution effect, is similar to W.I. This is the most significant NO_x removal effect of the EGR. EGR also shares further effects as W.I., such as thermal choking, inlet temperature effects, chemical effects, but the extent and in some cases the direction of the effects are different. These differences are summarized in

Table 1. Comparison of the investigated effects of EGR and water introduced to combustion (W.I.): (“-x”—lowest, “xxx”—highest effect) [70,71,87,95,96,127,128,129].

	EGR	W.I.
NOx removal	xxx	xxx
Particulates removal	-x	-
Dilution effect	xxx	xx
Thermal effect	-	xxx
Chemical effect	x	x
Thermal throttling	-x	x
Inlet temp. effect	-x	x

.

When air surplus is high like in case of diesel combustion, and load conditions are low, EGR can lower NO_x emissions without significantly raising PM emissions. Practically, under these circumstances, EGR appears to be superior to W.I. as it does not need a second liquid (water).

EGR is unable to considerably reduce NO_x emissions at higher loads, due to further decrease of air fuel ratio, resulting in significant increase of PM emissions [96]. Additionally, dilution effect significantly decreases the maximum performance, and efficiency, leading to degraded combustion and increased pumping losses [130,131]. For these reasons, during high power vehicle operating situations, EGR rates are often lowered or shut off entirely [1,132,133].

Intake fumigation can reduce NO_x emissions under high load operating conditions without significantly increasing PM emissions, as the dilution effect is in smaller proportion in case of W.I. than the other effects, and the air flow rate remains relatively constant. At high loads, the main thermal effect still successfully suppresses the cylinder temperature with much lower degradation of the other engine parameters. Under these conditions, the main thermal effect is also effectively supported by the effect of density increase (thermal throttling) and inlet temperature effect [86,87,134].

If we consider these together, it can be realized that these two techniques can complement each other superbly across the full power range of the internal combustion engine. Practically, we can see that the commercially available water fumigation systems are optimized for medium to high loads, which means that with the easily applicable engine intake port water injection, we can reach further emission reduction without even modifying the engine control. However, the full potential can be reached with engine control modifications, where the operation of EGR and W.I. can be synchronized.

3.8. Effects of Excess Water after Combustion on the Internal Combustion Engine’s Emission Aftertreatment System

Attempts to reduce emissions by controlling the engine began in the late 1960s, but this also resulted in a significant reduction in engine power. In the 1970s, many companies invested a lot of effort in introducing new processes that created new opportunities to reduce air pollution. As a result of these findings, a number of low-cost solutions have been developed that have shown the ability to drastically reduce these dangerous emissions without compromising vehicle performance [42].

Water is constantly present in aftertreatment systems as a result of air humidity and combustion processes. Reaction 19 shows the complete combustion:

C_xH₂ + (x + y/4) O₂ → x CO₂ + (y/2) H₂O

(19)

Incomplete combustion occurs when there is not enough air or oxygen to completely burn the fuel. During the process, carbon and hydrogen are oxidized, producing water, carbon and carbon monoxide. There is a significant excess of air in diesel engine operation, which limits the amount of carbon monoxide produced [135].

W.I. has basically two main effects on emission aftertreatment systems in relation to the inlet combustion products:

• reduced exhaust gas temperature,
• increased number of water and its related compounds, radicals after combustion.

3.8.1. Effect on DOC

In diesel engines, oxidation catalysts are essential for the oxidation of CO and hydrocarbons. Although originally designed to minimize gas emissions, they have also been shown to be effective in reducing particulate emissions, which can be as high as 20–50% (total particulate mass) [136].

Water plays a critical role in the Pd catalyst-based DOC reactions, especially at low temperatures, found under cold start conditions. In exhaust gases, where water is always present, the main mechanisms of CO and hydrocarbon oxidation involve the reaction of CO with water or a water-related compound such as OH, with oxygen playing only a secondary role [137]. The Pd catalyst produces additional intermediates, such as carboxyl species and bicarbonate/bidentate carbonate, resulting in enhanced CO and hydrocarbon absorption. “Light off” temperature (50% catalyst conversion efficiency) is reduced in the presence of water due to the interaction with water and OH species. In the case of Pt-based catalyst, water competed with CO adsorption on the active sites of the Pt-based catalyst and inhibited CO oxidation in the presence of water.

The conversion of NO to nitrogen dioxide (NO₂), is one of the key functions of the DOC in contemporary diesel aftertreatment systems, because NO₂ gas is required to sustain the operation of diesel particulate filters and SCR catalysts are used for NO_x reduction. With Pt-catalyst, water and NO₂ act as a hydrocarbon oxidant, where the hydrocarbons occupy Pt active sites. With that, NO oxidation to NO₂ is also suppressed. Therefore, in case of excess water in the emission system, Pd containing diesel oxidation catalyst is preferred [138,139].

3.8.2. Effect of DPF

Diesel particulate filters consist of a honeycomb-shaped ceramic monolith, where the opposite side of each cell is always closed. Thus, filtration takes place in the pores of the cell wall. Silicon carbide (SiC) and Cordierite (2MgO-2Al₂O₃-5SiO₂) are the two most commonly used raw materials for the ceramic body of the particulate filter. Engine particulate emissions can be eliminated 99.9% of the time by using modern Cordierite or SiC particulate filters [135].

Soot continuously accumulates in the particulate filter while the engine is running. From time to time, we need to remove it from the filter to avoid performance loss and consumption increase by the increasing back pressure as an effect of soot build up [82]. This is called regeneration, and the soot oxidation process is dominated by NO₂ as it is the primary oxidant compared to oxygen. In fact, the soot-NO₂ reaction takes place at low temperatures, while the soot-O₂ reaction is inefficient. It has also been discovered that soot oxidation proceeds faster in the presence of both NO₂ and O₂ than in the presence of O₂ or pure NO₂ alone, indicating the need for molecular oxygen presence in the soot-NO₂ process. Temperature facilitates soot oxidation by NO₂ and NO₂/O₂. At lower temperatures, the addition of water to the gas further increases the rate of NO₂-assisted oxidation. However, there is no direct involvement of water’s oxygen atoms, but water allows the formation of nitric and nitrous acid which generates acidic surface sites on the carbonaceous material that can decompose to CO₂ at relatively low temperatures. The beneficial effect of water also requires the presence of NO₂ [140,141].

The oxidation temperature can be reduced even further by using a catalyst, which can be a fuel additive such as CeO₂, iron oxide (FeO), or other compounds with superior oxygen storage capacity (OSC), or by using catalyst support filters containing a catalyst material (platinum group material—PGM) added to the ceramic substrate.

The main regeneration mechanisms are passive and active regeneration. The passive technique regenerates PM at a lower temperature by using a catalyst to reduce the activation energy for oxidation and combustion of the particulates. In the active system, the energy required for soot regeneration is obtained by burning excess fuel with post injection or by an additional injector upstream of the particulate filter.

As mentioned above, water can enhance the passive regeneration with increased oxidation in lower temperatures. This can result in fuel savings in the system with lower backpressures and extended periods between active regenerations.

3.8.3. Effect of SCR

In the 1970s, SCR systems were first used for NO_x management in mobile applications such as marine engines. Since the mid-1990s, a number of research projects have been carried out to modify SCR technology in diesel truck and passenger car engines. Since the mid-2000s, urea-SCR technology has been gradually introduced to the onshore mobile diesel engine market. Its advantages include no secondary pollution of the environment, low reaction temperature and high conversion efficiency. Along with NO_x removal (up to 75–90% efficiency) and solid particle emissions reduction of an extra 20–30%, it helps to significantly reduce hydrocarbon emissions by 80% [136]. Ammonia (NH₄), a highly toxic gas required for selective catalytic reduction, would be too dangerous to store in a car. To solve this problem, in mobility applications urea is used which thermally decomposes (>132 °C) to ammonia at the point of use by injection into hot exhaust gases. In the SCR system, a number of chemical reactions involving NO and NO₂ take place, resulting in NO_x being reduced to elemental nitrogen. Since the NO:NO₂ ratio in exhaust gases is roughly 90% and 10% respectively, the reaction using NO is known as “normal SCR” reaction (Reaction (20)). Reaction (21) is the so-called “slow SCR” reaction, in which NO₂ reacts. Reaction (22) is called “fast SCR” because it occurs at lower temperatures in the presence of both NO and NO₂. This reaction is responsible for the enhancement of low temperature SCR by NO₂ [142,143].

4 NH₃ + 4 NO + O₂ → 4 N₂ + 6 H₂O

(20)

4 NH₃ + 2 NO₂ + O₂ → 3 N₂ + 6 H₂O

(21)

4 NH₃ + 2 NO + 2 NO₂ → 4 N₂ + 6 H₂O

(22)

Most flue gases, including diesel exhaust, typically have low NO₂ concentrations. In order to improve NO_x conversion at low temperatures, NO₂ levels in diesel SCR systems are often deliberately increased. At low temperatures platinum-based catalyst are used, for higher temperatures around 250 to 500 °C, Zeolite and Vanadium oxide (V₂O₅) catalysts are the typical applications [144,145,146,147].

Metal ion-exchanged zeolite with transition metal cations such as Fe and Cu has excellent nitrogen (N₂) selectivity and a wide operating temperature window. However, it is sensitive to hydrothermal deactivation at high temperatures where H₂O can damage the catalyst structure. Also, due to the condensation behavior of water vapor, which blocks catalyst channels, SCR shows decreased activity when significant amounts of water vapor are present. But catalyst’s channel activities are recovered rapidly after the water vapor is removed. Studies have shown that H₂O enhances the selectivity of N₂ by preventing the formation of N₂O [148,149].

The two primary SCR reaction mechanisms are Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R). The E-R mechanism dominates at high temperatures, while the L-H mechanism is utilized at low temperatures. High temperatures cause the E-R processes to speed up, because the presence of water encourages the production of hydroxyl groups, which in turn increases the SCR’s denitrification activity. However, because H₂O has a greater adsorption capacity than NH₃ at lower temperatures, when the temperature drops, so too does the denitrification effectiveness of SCR in the presence of water [43,83].

It can be concluded that in the emission system, water promotes the increase of NO₂, which can be positive on the DOC and the DPF. However, its positive effect on the SCR at low temperatures cannot be utilized as the water has inhibitory effect on the fast SCR reaction, which is using the NO₂. At moderately high temperatures, the water effect can also be positive for SCR.

4. Conclusions

This study aims to investigate the potential benefits of W.I. in light of current emission control technologies, contemporary emission control trends, and the latest emission regulations. While retrofitting is increasingly encouraged by the European Commission through retrofitting projects and is also addressed in EURO 7, the existing literature on W.I. in diesel vehicle emission control is limited. This lack of research highlights the need for a comprehensive analysis of W.I.’s potential to improve diesel vehicle emissions.

In real-world applications, after W.I. is introduced into the engine combustion chamber, excess water exits with the exhaust fumes and passes through the entire ATS. This water in the exhaust system affects all parts of the various ATSs. For this reason, the effect of water has been examined in the literature. There is currently no available information regarding the specific emissions that may be present at the tailpipe for different aftertreatment systems. Clarifying this aspect could be particularly relevant in the field of retrofitting, however, this remains an unexplored area of research. Numerical simulations are not applicable because of the high complexity involved in checking the full effect on ATS system.

Until now, without a regulatory incentive, the literature on vehicle retrofitting with these varied approaches has been very limited because of a lack of interest.

In light of the observations made in the literature, the following findings can be drawn:

• It can be concluded that much more emission reduction can be achieved through retrofitting than through further tightening of emission standards:

  ○

  Depending on the exact emission limit values of PM and NO_x, since the introduction of EURO 1/I and NRMM regulations, the tightening of emission levels has reached at least 95%.

  ○

  With the current emission regulations, new car sales were around 72.5 million worldwide in 2023. By contrast, the number of used cars in the world is estimated at 1.5 billion. The volume of used cars between Euro 3-5/III-IV is expected to be the largest, for which the greatest results could be achieved globally from a retrofitting point of view and for which modifications could also be solved economically.

  ○

  The fuel requirement for NRMM equipment is much less stringent. As most of these machines are not registered, it is difficult to quantify their volume, but their emissions are likely significantly higher than for on-road. Sulfur content causes catalyst poisoning through CeO₂ deactivation. In case of high sulfur content in fuel, the retrofitting with conventional catalytic converters is not feasible.

  ○

  Non-road machinery has much higher lifespan compared to PC vehicles, where the depreciation is much less and retrofitting can be a cost-effective solution.

• Based on the literature review, it can be concluded that W.I. can be used effectively in different ATSs, causing further emission reductions at the tailpipe:

  ○

  EGR—EGR is unable to considerably reduce NO_x emissions at higher loads and significantly decreases the maximum performance. For this reason, during high power vehicle operating situations, EGR rates are often lowered or shut off entirely. However, intake fumigation can reduce NO_x emissions under high load operating conditions without significantly increasing PM emissions, as the main thermal effect still successfully suppresses the cylinder temperature. Intake water fumigation would be the best match with EGR, for NO_x and particulate emission reduction across the entire operating range of the engine, with further emission reductions.

  ○

  DOC—Pd-coated DOC is preferable in the case of excess water in the emission system. At low temperatures, enhanced CO and hydrocarbon absorption, and reduced light off temperatures can be seen.

  ○

  DPF—Water can enhance passive regeneration with increased oxidation at lower temperatures. It can result in fuel savings in the system with lower backpressures and extended periods between active regenerations.

  ○

  SCR—During high loads, temperature continuously increases along the DOC, DPF and SCR, with the highest temperature at the SCR, where water has a positive effect on the SCR’s denitrification activity through the Eley-Rideal mechanism. However, because H₂O has a greater adsorption capacity than NH₃ at lower temperatures, when the temperature drops, so does the denitrification effectiveness of SCR in the presence of water.

• Conclusions on full emission system tailpipe emissions:

  ○

  In vehicles without emission treatment systems (even without EGR), intake fumigation can be a universal and a cost-effective retrofitting option.

  ○

  Adding intake fumigation to vehicles with EGR, the two techniques can complement each other effectively across the full power range of the internal combustion engine. This means that with easily applicable engine intake water fumigation, we can reach further emission reduction without even modifying the engine control.

  ○

  After a typical Euro 5/V system with DOC and DPF, at least 96% of the particles are filtered out on a number basis. DPF filters out the solid particulates, while DOC removes the SOF in HC form. In such ATS, only NO_x remains, which W.I. and EGR combined can further decrease.

  -

  No research is available on the effects of W.I. on the PN₁₀ particle size range. Studies with intake charge moisture content suggest that the use of water may have potential benefits in this area

  -

  From a practical point of view, an important achievement is that the cooling effect of W.I. does not depend on traffic conditions. This cooling effect typically comes from the high air flow of the moving vehicle, which decreases the temperature of the intake charge and the EGR cooler. W.I.’s cooling effect can continue to cool the charge air during city conditions, even when there is no significant airflow during idling or hard accelerations.

  -

  Due to downsizing, the load on internal combustion engines is increasing, which could make the use of intake water fumigation increasingly beneficial.

Future Directions

• At the end of the exhaust system, the Ammonia Slip Catalyst (ASC) is responsible for removing the remaining NH₃. The new EURO 7 establishes emission limits for NH₃. The effect of water in this context also requires further research.
• To verify findings mentioned above, intake water fumigation tests are planned with EURO 5/V aftertreatment systems, along with synchronized EGR and water fumigation. This will include measuring particles below 23 nanometers to provide information in relation to EURO 7.
• Meanwhile, investigating the potential corrosive effects in the planned EURO 5/V system, that may arise in individual components due to the presence of excess water could represent a significant contribution to the existing body of research.
• Consider integrating W.I. and pressure swirl techniques in future projects to further optimize emission reduction strategies.