HCNG Fueled Spark-Ignition (SI) Engine with Its Eﬀects on Performance and MARK Emissions ⁎ Hayder A

Renewable and Sustainable Energy Reviews 82 (2018) 324–342

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Renewable and Sustainable Energy Reviews

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HCNG fueled spark-ignition (SI) engine with its eﬀects on performance and MARK emissions ⁎ Hayder A. Alrazena,b, , K.A. Ahmada a Department of Aerospace Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia b Technical Institute/Qurna, Southern Technical University, Qurna, Basra, Iraq

ARTICLE INFO ABSTRACT

Keywords: The usage of natural gas in internal combustion engines involves various difficulties, like weak lean-burn ability, Diesel low flame speed and ignitability, which demand deep studies for its usage in IC engines. For compressed natural Natural gas gas (CNG) in SI engines, there is an engine efficiency sacrifice at low loads and high levels of hydrocarbon (HC) Hydrogen and carbon monoxide (CO) emissions which cannot be solved without using after-treatment equipment. This Alternative fuel equipment, however, is very expensive. Therefore, an additional fuel can enhance the characteristics of the HCNG combustion of natural gas, which can be added in the intake charge. Hydrogen is an effective gas for enhancing Performance fl Emissions the ame rate regarding combustion in an SI-CNG engine, in addition to increasing engine stability. Small amounts of hydrogen improve performance and reduce exhaust emissions. Thus, a number of investigators have carried out research studies on SI engines with different ratios of HCNG. This paper is comprehensive overview

of CNG, H2 and HCNG blends. The main topics discussed consist of the combustion fundamentals of natural gas, hydrogen and hydrogen-natural gas mixture. Natural gas and hydrogen usage as fuels and their characteristics have been analysed. The storage of hydrogen and HCNG is still challenging researchers and, therefore, their storage has been taken into consideration. Moreover, a comprehensive review has been performed of HCNG blends in order to understand the eﬀect of hydrogen enriched CNG on performance and the emissions of SI engines. The combustion characteristics of HCNG engines are strongly dependent on the conditions of the engine. The air-fuel ratio, the time of injection, the compression ratio and speed play a major role in blending HCNG in an SI engine and have been discussed in this article.

1. Introduction (marine, air, road, etc.) encompasses 33% of the USA's emission; of which more than one third are from road transport. Power stations are The world is facing various issues in the 21st century. Some of the responsible for 41% of the emissions in the USA, agriculture and in- most important of these are the decreasing availability of low-cost dustry emit 16% and the remaining 10% is emitted by other sources sources of fuel, the growth of the population of the world, and the [3]. growth of energy demand in all sectors of industry. Fossil fuels, like Employing NG as a substitute fuel is one of the solutions that has coal, diesel, and gasoline, are depleting at a fast pace. Moreover, they been accepted and has spread throughout the world. NG, whose main pollute the environment and thus are not regarded as sustainable and constituent is methane, provides great environmental and economic permanent solutions to the world's energy requirements [1]. Further- advantages like lower emissions, along with enhanced availability and more, any shortage in these types of energy sources might result in the efficiency. The use of natural gas in internal combustion engines in- fluctuation of oil prices and is regarded as a threat to the energy se- volves various difficulties like methane's ignitability, poor lean-burn curity of the world and the global economy [2]. With the global in- capability and low flame speed [4]. Based on past research studies, crease of fossil fuel usage, the quality of local air deteriorates and the hydrogen is a green fuel that can be used in vehicles as an alternative to amount of greenhouse gas (GHG) emission increases. Transportation conventional fuel [5,6]. Compared to compression ignition (CI)

Abbreviations: A/F, Air fuel ratio; CAD, Crank angle degree; TDC, Top dead center; ATDC, After top dead center; SI, Spark ignition engines; CI, compression ignition; IC, Internal combustion; CNG, Compressed natural gas; NGVs, Natural gas vehicles; LPG, Liquefied petroleum gas; CO, Carbon monoxide; CO2, Carbon dioxide; HC, Hydrocarbons; PM, Particulate matter; NOx, Nitrogen oxides; BTE, Brake thermal efficiency; LHV, Lower heat value; BMEP, Brake mean effective pressure; HRR, Heat Release Rate; SOC, Start of combustion; CR, Compression ratio; CA, Crank angle; EGR, Exhaust gas recirculation; λ, Exceed air (1/4 air-fuel ratio); η, Molar fraction of methane ⁎ Corresponding author at: Technical Institute/Qurna, Southern Technical University, Qurna, Basra, Iraq. E-mail address: [email protected] (H.A. Alrazen). http://dx.doi.org/10.1016/j.rser.2017.09.035 Received 4 July 2016; Received in revised form 14 June 2017; Accepted 14 September 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved. H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Table 1 Air pollutants, their sources and health eﬀects [36].

Pollutants Source/formation Human health eﬀects

Carbon monoxide (CO) IC engine, industries and waste incineration When inhaled CO enters the bloodstream and disrupts the supply of oxygen to the body's tissues. Hydrocarbon (HC) Evaporative emissions from vehicle fuel system, or in exhaust Exposure can cause headaches or nausea, while some compounds may cause cancer. emissions PM and soot IC engine (e.g., cars and trucks) and Industry Long term exposure may cause to lung cancer, heart disease, and asthma attacks. engines, SI engines are more appropriate for adopting hydrogen since Some of them focused on the effect of HCNG on diesel engines only. In hydrogen's auto ignition temperature is rather high (around 858 K [7]) the present work, there is firstly a discussion of the necessity for al- [8–12]. Hydrogen has various combustion characteristics that are un- ternative fuels with regards to the current state of pollution and fossil ique and beneficial in emission performance and engine efficiency fuel reserves. How natural gas is implemented in IC engines and its [13,14]. economic aspects are then discussed. Hydrogen fuels in an IC engine, its Mixing hydrogen with natural gas is one of the most effective ways production and its storage are presented in a sub-section of this paper. of handling this issue since the burning velocity of the mixture is sub- In the main section of the article, using hydrogen and natural gas blends stantially high, the ignition energy is low, and the lean-burn capability in an SI engine are considered, as well as the effect of HCNG on the is satisfactory. There is a rise of interest in the research on pollutant engine performance, combustion and emission characteristics. Lastly, emissions and the performance of the hydrogen-enriched, CNG-fueled, the overall conclusions and future recommendations are presented. conventional internal combustion engine [13,15,16]. These studies indicate that mixing hydrogen with natural gas enhances the combustion stability and reduces the hydrocarbon emissions; however, it results in 2. The usage of alternative fuels in an SI engine the production of more nitrogen oxides (NOx). The impact of different CNG-hydrogen blend combustions on the combustion traits in direct- Harmful emissions (like PM, soot, NOx, HC and CO) are emitted by injection SI engines has also been examined by different researchers fossil fueled (like LPG, CNG, diesel, and gasoline) vehicles. These [13,15,17,18]. These studies have proposed an optimum hydrogen emissions eventually become part of the environment and pollute the volumetric fraction CNG-hydrogen blend that results in a balanced earth's atmosphere. During combustion, CO and CO2 are emitted from compromise in the emission and performance of the engine. It is ex- the carbon that existed in the fuel. CO2 and water vapour are formed as pected that natural gas combustion with hydrogen will enhance the a result of a perfect combustion which is illustrated by Eq. (1). How- lean-burn traits and reduce the engine emissions (particularly CO, HC, ever, in the actual combustion, other species (like N2O, NO2, NO, HC, and CO2); however, the main concern is the possibility of a rise in NOx CO) are generated, as shown in Eq. (2). These pollutants are generated emissions [19]. The addition of hydrogen enhances the process of by various sources; however, transportation vehicles contribute the combustion with the added possibility of developing engines that have most. Human health is affected by the increase of pollutants in the a lower environmental effect and higher performance. Hydrogen is environment [36]. Table 1 illustrates various pollutants and their entirely carbon-free and can be produced with relative ease; these are health impact. characteristics that make it a good alternative choice to conventional fuel. But, there are some disadvantages in using pure hydrogen, like the CHxy++ aO(22 3.76 N ) → bCO 22 + cHO +× a 3.76 N 2 (1) low calorific value per unit volume, the high adiabatic flame temperature, and pre-ignition phenomena caused by contact with the re- C H++ a(3.76) O N → bCO + cH O + dCO + eC H ++ fNO gN O sidual gas or the hot spots resulting from lower ignition energy [20–22]. xy 22 22 xy 2

Employing NG/hydrogen mixtures which contain H2 provides an op- +×aN3.76 2 (2) portunity to exploit the positive aspects of using hydrogen without the considerable alteration of the natural gas engines that already exist The level of pollution in urban areas can be controlled through the [23]. Moreover, employing hydrogen as an element that complements use of hybrid vehicles, battery-operated vehicles, and fuel change. Zero natural gas results in the extension of the lean-burn limit to the hy- emission is produced by battery-operated vehicles on the road. drogen's extended ﬂammability range. Lean-burn ability decreases the However, a number of factors contribute to the limited use of electric knock coincidence (a serious condition which is threatening to spark vehicles, e.g. the short travel range, the lower battery life, and the lower ignition (SI) engine's safe performance), enhances the thermal eﬃ- ratio of power to weight. Hybrid technology can be regarded as inter- ciency, and decreases the combustion temperature and the emission of mediate to fuel change and battery mode. It needs a battery and a

NOx [24,25]. Hydrogen's anti-knock characteristic enables it to enhance traditional power train, resulting in higher maintenance and capital the compression ratio (CR), which results in the further enhancement of expense. After observing road transportation vehicles’ future needs, fuel thermal efficiency [25]. Adding hydrogen can also result in a significant change seems like a more convenient solution [36]. Alternatively, decrease of IMEP (the coefficient of the indicated mean effective pres- biofuels, hydrogen and CNG can fuel IC engines. High emissions, such sure variation) and reduces the duration of combustion leading to as nitric oxides, carbon dioxide, hydrocarbons, and carbon monoxide, better thermal efficiency and a decrease in fuel consumption [26,27]. are produced from engines operated using petroleum, and this problem In this article, past studies examining hydrogen-enriched CNG as a is still a challenge to researchers [45]. Alternative fuel is one of the new fuel used in a reciprocating spark-ignited (SI) piston engine have been technologies used to reduce emissions in CI engines. Natural gas, pro- reviewed in detail. There are several review papers on this subject. pane, and methanol have been used since the year 2000 as alternative Several reviews have discussed the effect of natural gas on SI engines fuels for vehicles [6]. Due to their clean burning nature and their ever- and CI engines in dual-fueling [28–34]. The effect of hydrogen on SI increasing usage in the automobile industry, alternative fuels might be engines and on CI dual fuel engines was reviewed by several others termed as future fuels. Hydrogen, natural gas and mixtures of natural [1,34–41]. For hydrogen-enriched CNG, a few papers [26,42–44] con- gas and hydrogen have been taken into consideration as alternative tained short discussions about its influence on the performance and fuels in recent years in order to reduce the pollution from vehicles emissions of the engine, with their main focus being on other topics. [14,46].

325 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Table 2 From then onwards, with technological advancements, natural gas ve- Properties of hydrogen, methane, propane and gasoline at NTP [48–51]. hicles entered the transportation market of various regions or countries and exited it at various times. NGVs with dedicated CNG engines ori- Properties Hydrogen Methane Propane Gasoline ginate from Italy. The initial gas vehicles that utilised a gas container Thermodynamic properties were manufactured in 1936 in Italy [53]. However, the period with Chemical formula H2 CH4 C3H8 C7.6 H16.2 marked notable activity is the 1970s, when CNG was found to be a Molecular weight (a.m.u.) 2.016 16.04 44.09 107.0 stable and cheap fuel following the oil crisis. Density at NTP (kg/m3) 0.0824 0.643 1.767 730 LHV (MJ/kg) 120 50 46.3 44.8 LHV (MJ/m3) 9.9 32.6 81.2 32,704

Heat of combustion (MJ/kgair) 3.48 2.90 3.14 3.05 3.2. CNG economics Specific heat (Cp) at NTP gas 14.89 2.22 – 1.62 (MJ/kg K) Providing an affordable source of energy is among the main ad- Specific heat ratio (γ) at NTP 1.383 1.308 – 1.05 gas vantages of CNG. The low-cost CNG provides a glimmer of hope amid Diffusion co-efficient in air at 0.61 0.16 0.1 0.005 continuous consumption of costly fuel like gasoline and diesel. Even NTP (cm2/s) though the main reason for the use of CNG in road transportation, -3 – Viscosity of gas at NTP (10 g/ 0.0875 0.110 0.052 particularly in big cities, was its emission control and environmental cm s) Combustion properties aspects, today, with the sharp increase in oil prices, the economic ad- Flammability limits (vol% in 4–75 4.3–15 2.2–9.3 1.4–7.6 vantage of CNG usage is becoming more evident and attracts the at- air) tention of new users [54]. In most of the countries in the world, CNG Flammability limits 0.1–7.1 ––0.7–3.8 costs much less than diesel and gasoline (per gallon equivalent) even Minimum ignition energy (mJ) 0.02 0.29 0.3 0.24 – when taking the compression costs into account. Thus, despite having a Auto ignition temperature (°C) 585 540 495 228 470 ffi Flame velocity (m/s) 2.65–3.25 0.37–0.45 0.42–0.48 0.37–0.43 lower thermal e ciency than gasoline and diesel, CNG usage as a Quenching distance (mm) 0.64 2.1 2.0 2.0 transportation fuel has substantial economic advantages. Natural gas Stoichiometric composition in 29.53 9.48 4.03 1.76 needs to be slightly processed from the production field to the vehicle to air (vol%) become appropriate for transportation fuel usage, whereas gasoline and Stoichiometric air-fuel ratio 34.12 17.23 14.75 14.7 (mass bacis) diesel need to be separated from crude oil and undergo an intricate Adiabatic flame temperature 2318 2148 2267 2470 refining process. Moreover, CNG resource is more evenly distributed with air (K) throughout the world in comparison to oil and it has lower vulnerability to price fluctuation [55]. The natural gas price advantage over gasoline and diesel has been regarded as the most significant factor in attracting 3. Natural gas consumers and encouraging the change from traditional fuel to CNG [56–60]. 3.1. CNG Used as a fuel Retail prices in the top 15 CNG user countries during the 2011–2012 fiscal year have been compared and illustrated in US Dollars in Table 3. Natural gas vehicle usage is the same as is employed for heating and It can be seen that the pumping price of CNG is 50% lower than the cooking in the domestic sector [47]. To produce CNG, natural gas diesel and gasoline price on average in the majority of countries with – (which is mostly made up of methane CH4) is compressed to below 1% successful NGV penetration. The rapid pace with which the numbers of of the volume it occupies at standard atmospheric pressure. A rigid CNG vehicles have increased in the past decade, particularly in the – – container with 200 248 bar (2900 3600 psi) pressure is used for Asia-Pacific area, is mostly due to the lower price of CNG compared to storing and distributing it; this is often in the form of a metallic cy- gasoline and diesel [28]. The economic metrics for running a vehicle on linder. The physiochemical properties of gasoline, propane and hy- CNG, as opposed to operating it on diesel or petrol, have been calcu- drogen are compared with those of CNG (CH4)inTable 2. lated based on the 2011–2012 fiscal year's average global fuel price. The number of natural gas vehicles in the world is increasing at a The results of these calculations are illustrated in Fig. 3 and Table 4. fast pace, which is why consistent information on their quantity is not Based on the reports of the US Department of Energy's Alternative available. But, based on the latest authentic sources, Iran is (currently) Fuel Comparison for Jan-Mar 2011, CNG was one-third less expensive the leader in NGVs with as many as 4.07 million [28,52]. China follows in comparison to gasoline. According to the U.S. Energy Information Iran closely with 3.99 million NGVs. Based on Fig. 1, the NGV popu- Agency's reports, CNG costs 42% less on average in comparison to lation has rapidly increased globally, with a 24% annual growth rate. diesel (on an energy equivalent basis); it is expected to reach 50% by fi The Latin American and Asia-Paci c regions were the biggest con- 2035. In a similar vein, the Republic Services, which is the USA's second tributors (see Fig. 2). This trend is expected to continue until 2030 with largest waste management services company, has managed to reduce its a 3.7% annual growth rate, and non-OECD countries are expected to fuel cost by 50% with the effective deployment of CNG across various contribute the greatest portion of the growth. Currently, over 18 million fleets [61]. In recent years, the US Department of Energy has carried natural-gas vehicles are functioning in over 86 countries worldwide, out a survey on the subject of “alternative transportation fuels” and has with the majority being concentrated in Iran, China, Pakistan, Argen- discovered that using CNG, instead of conventional gasoline as the fuel tina, India, Brazil, Italy and Colombia [52]. Most (93%) of the CNG of the transportation fleet, will result in a 50% cost reduction [28].A vehicles are commercial vehicles and light-duty cars. The number of comparative assessment of the Washington Metropolitan Area Transit CNG refueling stations in the world is 26,677 [28]. Authority (WMATA) operated transit buses’ emissions was conducted fi Early in 1930, CNG was used as a vehicular fuel in Italy for the rst by the USA's NREL (National Renewable Energy competition) in 2004. time [53]. However, notable activity began in the 1970s when natural The results of the study showed that besides the CNG buses’ emission gas was regarded as a potential fuel as a result of the oil crisis after- benefits, CNG buses had remarkable fuel economy outputs in compar- math. The CNG vehicle market attracted further attention after the rise ison to diesel buses [62]. Clear strategies have been explicitly estab- in oil prices in the late 1970s and early 1980s. But the years up until lished to ban the usage of diesel in city buses (for instance in India) or to 2000 have been a challenging period for CNG to be established as a preserve CNG's cost benefits compared to diesel (for instance in Paki- vehicle fuel [28]. However, with the sharp increase in oil prices after stan) in the areas within which the government plans to replace diesel the 2000s, the CNG vehicle market became more attractive as CNG with CNG [63–69]. received the chance to prove itself as the cleanest and cheapest fuel.

326 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 1. Worldwide NGVs growth [28].

Fig. 2. Worldwide NGVs growth by region [31].

Fig. 3. Cost advantage of CNG fuel over gasoline and diesel [28]. Table 3 Retail fuel prices (US $) in top 15 CNG user countries [28]. Table 4 Rank Country Gasoline Diesel CNG per liter CNG per liter Cost comparison of CNG vs other fuel [28]. gasoline Diesel equivalent equivalent Description CNG Gasoline Diesel 1 Iran 0.42 0.17 0.30 0.34 Vehicle type Bus Bus Bus 2 Pakistan 1.02 0.79 0.72 0.80 km travelled per annum per vehicle 80,000 80,000 80,000 3 Argentina 1.44 1.44 0.33 0.39 Total annual consumption of fuel in liters (consider 36,184 39,400 32,000 4 Brazil 1.72 1.11 0.92 1.05 unit of ‘Nm3’ in case of CNG) 5 China 1.05 0.98 0.56 0.63 Retail fuel price per liter US $ (consider unit of 0.52 1.02 0.92 6 India 1.38 0.85 0.60 0.69 ‘Nm3’ in case of CNG) 7 Italy 2.03 1.85 0.85 0.95 Annual fuel cost (US $) 18,816 40,188 29,440 8 Colombia 1.31 0.96 0.80 0.92 % Fuel cost saving CNG vs gasoline 113% 9 Uzbekistan 1.03 0.98 0.30 0.34 % Fuel cost saving CNG vs diesel 57% 10 Thailand 1.25 1.06 0.27 0.32 11 Bolivia 0.83 0.66 0.30 0.29 12 USA 1.02 1.12 0.60 0.68 13 Armenia 1.31 1.19 0.49 0.56 like carbon dioxide (CO2) and molecular nitrogen (N2) [14]. Table 5 14 Bangladesh 0.79 0.56 0.27 0.29 shows the average natural gas composition in different countries. NG 15 Egypt 0.33 0.20 0.07 0.09 also contains other hydrocarbon species and sulphur compounds. The Average 1.13 0.93 0.49 0.56 level of these species is greatly influenced by the time of year, the geographical source, and the treatment used during the transportation – 3.3. Natural gas composition and production stages [32,70 73]. Thus, it can be said that NG does not have a narrow range of traits nor does it refer to a single kind of fuel. Natural gas is composed of deferent hydrocarbon molecules. The Fig. 4 shows the common composition of NG in percentage terms. composition of commercial natural-gas differs from 85% methane to In comparison to traditional liquid fuels like gasoline and diesel, NG 96%. In addition, NG is comprised of heavier hydrocarbons like ethane can be described as a clean-burning fuel. Its high level of octane makes it an appropriate choice for engines whose compression ratios are (C2H6), propane (C3H8) and butane (C4H10) along with inert diluents

327 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Table 5 Average natural gas composition in diﬀerent countries.

No. Composition Formula Mole Mole Mole Mole Mole Mole Mole %%%% %%%

1 Methane CH4 92.69 82.2 94 94.42 80.00 76.4 99.5

2 Nitrogen N2 2.18 7.7 0 0.44 0 1.8 0

3 Carbon CO2 0.52 0.2 1 0.57 19.05 13.4 0 Dioxide

4 Ethane C2H6 3.43 6.1 4 2.29 0 6.3 0.3

5 Propane C3H8 0.71 2.4 1 0.03 0 2.1 0

6 Isobutene I-C4H10 0.12 1 0 0 0 0 0

7 n-butane N-C4H10 0.15 0.4 0 0.25 0 0 0

8 n-pentane C-C5H12 0.09 0 0 0 0 0 0

9 Hexane C6H14 0.11 0 0 0 0 0 0

10 Others H2O0002000

11 oxygen O2 0 0 0 0 0.95 0 0 Sum 100 100 100 100 100 100 99.8 Country Nigeria [74] India [75] U.A.E. [76] Malaysia [77] China [78] Thailand [79] Norway [80]

discovered hydrogen in 1766 in England and Lavoisier named the atom [88]. Hydrogen is one of the crucial elements of water and more than 60% of the surface of the earth is covered in it. Hydrogen can be found in hydrocarbons and fossil fuels (like propane, natural gas and methanol), and in biomass, organic compounds, and water [46,89]. However, hydrogen in its elemental form is not naturally available. In normal pressure and temperature conditions, hydrogen is a ﬂammable gas that is tasteless, odourless, and colourless.

4.1. Hydrogen production

Various sources and methods can be used for manufacturing hydrogen gas. Due to the lightness of the hydrogen molecules, the grav- itational forces of the Earth have difficulty in retaining hydrogen gas within the atmosphere. This is why hydrogen can only be found in Fig. 4. Typical natural gas composition by volume. compound with other elements, e.g. several types of organic material, various types of hydride, hydrocarbons, and water. Thus, to use hy- relatively high. The self-ignition temperature of NG is high, which drogen as a fuel, it has to be manufactured and produced from other therefore enables combustion needing an intense energy source like materials that contain hydrogen like biomass, water, fossil fuels or pilot liquid fuel, a spark plug or a glow plug. It can be mixed with air at other biological sources. Hydrogen has been manufactured and em- a fast pace and creates a homogenous air fuel mixture inside the en- ployed for industrial usage for many years. Currently, approximately gine's cylinder for efficient combustion and significant decrease of 90% (45 billion kg) of hydrogen is produced from fossil fuels [90–93]. harmful emissions [31,47,81]. Some of the available studies in the lit- Direct production of hydrogen from fossil fuels is possible through erature have examined the use of NG in CI and SI engines. By making a using processes such as coal gasification (CG), partial oxidation (POX), comparison between the emissions of diesel and NG engines, it was thermo-cracking (TC), and methane reforming (SMR). Thermochemical discovered that the emission of particulate matter (PM), nitrogen oxides and biochemical processes are the main ways of manufacturing hy- (NOx), and hydrocarbon (THC) from SI engines fueled with NG is drogen from biomass. Moreover, hydrogen can be manufactured substantially less than from the engines fueled with diesel. Moreover, through photo-biological processes, water thermolysis (WT) (also based on the results of another research study, SI engines fueled with known as thermochemical water splitting), photoelectrolysis (PHE) or NG have the potential to successfully reduce their emission of non- photolysis (also known as photocatalytic water splitting or photoelec- methane hydrocarbon, carbon monoxide (CO), CO2 and NOx whereas tron chemical), and dissociating water with electrolysis (HE) [1]. gasoline engines lack this capability [82–86]. The number of NG ve- The storage and production of hydrogen gas have been issues which hicles is consistently growing, with old vehicles being converted to prevent adoption of hydrogen thus far. Currently processes used to natural gas using an engine modification. Around 15.2 million vehicles produce hydrogen require high energy typically coming from conven- are powered by NG throughout the world. Natural gas vehicles (NGVs) tional fuels, negating the environmental benefits of the otherwise pol- that are fueled with CNG are suitable for high-mileage, centrally-fueled lution-free fuel and carbon-free. vehicles that work in a limited region [87]. The physiochemical characteristics of gasoline, propane and hydrogen are compared with those 4.2. Hydrogen storage of CNG (CH4)inTable 2. Prior to any potential usage, the issue of a cheap system for storing 4. Hydrogen hydrogen, which is both efficient and safe, remains a major problem. Moreover, risks of leaks or explosions and expensive tanks are prevent Hydrogen (H) is the lightest of all the elements with an atomic on face of storing hydrogen as liquid or compressed gas because they weight of 1.00794 a.m.u. and an atomic number of Z = 1. The name require extremely high pressures. It would be regrettable if this problem hydrogen was made by combining two Greek words - “hydro” and were to hinder the creation of a clean, renewable energy system, which “geinomai” - with the former meaning ‘water’ and the latter meaning ‘to is necessary for the safe survival of animals, plants and human beings in bring forth’ as in “the element that brings forth water”. Cavendish the long-run [94]. At 293 K temperature and 20 MPa pressure, gaseous

328 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Table 6 Comparison of on-board hydrogen storage [92].

Fuel Total Energy (MJ) Fuel Weight (kg) Tank Weight (kg) Total Fuel System Weight (kg) Volume (gal.)

5 gal. gasoline 662 14 6.4 20.4 5 Liquid hydrogen (20 k) 662 4.7 18.6 23.3 47

1.2 wt% H2 stored in FeTi metal hydride 662 4.7 549.3 554 50 Compressed hydrogen (207–690 bar) 662 4.7 63.6–86.3 68.3–91 108–60 hydrogen has a density which is five-times lower than the density of 5.2. Onboard HCNG storage liquid hydrogen [95–103]. Therefore, compared to liquid hydrogen, a tank with approximately 5.5 times greater volume is required [94]. Storing hydrogen in high-strength steel tanks involves the issue of Different onboard options of hydrogen storage have been summarised hydrogen embrittlement, where the crystal structure of the steel is pe- in terms of the weight requirements in Table 6 (It has to be noted that netrated by the hydrogen atoms resulting in elasticity deterioration 5 gal. of the gasoline was used as a reference. With this amount, a ve- [118]. With the increase of hydrogen's partial pressure and steel alloy's hicle can drive up to 300 miles distance). strength yield, this condition becomes more notable. High-carbon steels are especially susceptible. Therefore in high-strength steel tanks, which are generally employed in the low-cost storage of CNG at the 250 bar 5. Using of CNG and H2 blend in SI engine standard pressure on board vehicles, a high density of hydrogen must 5.1. CNG/hydrogen mixtures not be permitted. Even though hydrogen embrittlement can be achieved with HCNG, the hydrogen partial pressure is lower than that of pure Hydrogen can be employed as a fuel in various ways; it can be hydrogen. For instance, the hydrogen partial pressure will be only 7.5 employed as an only fuel in the presence of air or as an addition in a in a 30% HCNG mixture in a 250-bar storage tank. HCNG-converted mixture of hydrocarbon. If hydrogen is to be introduced commercially, F150 trucks were demonstrated in a long-term exhibition carried out by fi it can be employed in hydrocarbon mixtures at low density and with a the Arizona Public Service Company. OEM bre-reinforced 4130 steel minimum need for adaptations and adjustments in IC engines was used as the storage tank. A destructive test was conducted on these fi [104–111]. The enrichment of hydrogen in fuel made of hydrocarbons tanks after they nished 1 year of service in which no trace of hydrogen is encouraged as a strategy to boost the performance, along with en- embrittlement could be found [119,120]. If the objective is to com- hancing the emissions, of combustion engines. Similarly, a number of pletely remove the risk of hydrogen embrittlement, aluminum and composite construction tanks must be alternatively employed since they researchers have tested the performance of SI engines using H2-CNG are not susceptible to hydrogen embrittlement. Composite tanks also [112,114,115] and H2-gasoline fuels [112,113]. Moreover, these investigators have reported that hydrogen is a good choice for enhancing have the advantage of being lighter; however, their cost is almost twice performance in SI engines. This is due to the increase in the ratio of the as much as their high-strength steel counterparts. With the current hydrocarbon in the fuel and this reduces the duration of combustion. developments in materials and the increasing volumes across the hy- This is because hydrogen has a high flame speed compared to other drogen and CNG markets, their price is decreasing. Based on successful fi fuels [116,117]. eld experiences, it is possible for all other parts - like engine fuel Even though NG mainly consists of methane, the precise composi- systems, regulators, valves, sensors, and plumbing - to be identical to tion differs regionally and seasonably. Instead of NG, pure methane the generally used standard of CNG 250 bar [120]. (with 99.5% purity) was employed to ensure comparability. Three variations of premixed-fuels were employed which are methane with 5.3. Effects of HCNG on the combustion of SI engines 25 vol% hydrogen, CH4 with 15 vol% H2, and CH4. Methane-hydrogen mixtures, and the stoichiometric and calorific properties are illustrated 5.3.1. In cylinder pressure in Fig. 5. The LHV per volume of stoichiometrically mixed fresh gas is In cylinder pressure has been measured with some parameters by quantified by the heating value of the stoichiometric intake mixture Luigi et al. [23]. Their study was tested with two blends of NG/H with [18]. The higher is this amount, the greater is the chemical energy that 2 respect to the NG case. For H , a shorter combustion results in a higher can be delivered to the PFI engine that is naturally aspirated in WOT 2 pressure rise (Bar/CAD). The H content will lead to a decreased in- circumstances. Pure hydrogen's stoichiometric intake mixture heating 2 cubation time and main combustion duration. For NG/H 20%, the value is reduced by approximately 6% in comparison to pure methane. 2 main combustion was up to 5% shorter. Moreover, for NG/H 40%, it By comparison, with the addition of 15 or 25 vol% H to CH , the 2 2 4 was about 10–15% shorter. As such, for NG/H 40%, a 30–40% higher heating value of the stoichiometric intake mixture reduces by just 0.3% 2 maximum rate of pressure rise was measured; and with NG/H 20%, and 0.6%, respectively [18]. This creates the expected result, which is 2 there was a 5–15% higher rise. It is evident that there is a peak pressure that the WOT torque is not significantly affected by the addition of increase and it switches from NG to NG/H 40% as a result of the partly relatively small amounts of hydrogen. 2 faster heat release [23]. Hora and Agarwal [119] carried out work at different BMEP for baseline CNG and HCNG mixtures. Their outcomes showed that an increase in BMEP will result in a peak cylinder pressure increase. The peak cylinder pressure will be increased by a higher fuel quantity generated at a higher engine load. The crank angle position matches the peak pressure for a given fuel. It shifts towards TDC, as a result of the decreased combustion duration with an increase in BMEP. This improved fuel use, therefore, leads to a BTE increase. With an increase in the hydrogen percentage in the HCNG, the peak cylinder pressure shifts towards TDC at a given BMEP. This is as a result of the higher flame speeds with HCNG, which makes the combustion duration shorter and results in the comparatively earlier start of combustion fi Fig. 5. Calori c and stoichiometric properties of methane-hydrogen mixtures [18]. (SOC). At the same BMEP, a rather higher peak cylinder pressure is

329 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

compared to gasoline. The profiles of pure methane and the 5% hydrogen appear to be identical. The maximum decrease in the peak pressure at 7000 rpm was 2.6 bars. This shows only 4% of the peak pressure in the base methane engine [15]. Studies by Ceper et al. [123] and Kahraman et al. [124] show that the highest peak pressure values are acquired when 30% (by volume) of hydrogen is added to the mixture of methane-air. The focus of their studies was on examining the pollutant emissions when hydrogen fractions (by volume) were added. Friction, the gases flowing to the exhaust and the heat transfer to the cylinder walls caused the loss of a significant part of the energy released from the combustion of the blends. The major part increases the pressure to perform the work of use. The energy content is reduced when hydrogen gas is added to methane or gasoline as a result of its low energy content. This decrease results in the reduction of the peak pressure [125]. Moreover, the presence of the hydrogen fractions causes the fix spark timing (at the MBT) to move from its place. The combustion is accelerated due to the presence of the methane-air or gasoline-air mixture. Moreover, the related crank angle for the highest pressure is advanced as a result of the presence of the methane-air or gasoline-air mixture. In this situation, the expected highest pressure occurs in a larger cylinder volume place, which causes the pressure to disappear and decreases its peak [124]. Meanwhile Lim et al. [25] reported that the excess air ratio affected the boost pressure. Also they reported that at the same compression ratio (CR), no pressure distinction could be found. However, when the CR was altered to 11.5 with the same excess air ratio and fuel, the pressure reduced. This reduction in the boost pressure can be due to improved thermal effectiveness. The boost pressure is directly matched with the throttle valve's opening. Since for a CR of 11.5 with the same power output, less air and fuel were required, the throttle value was opened with a CR of 10.5 [25]. An increase in CR results in the decrease in the volumetric efficiency [126,127]. The decrease in the boost pressure can be controlled and abated for the purpose of extending the Fig. 6. Peak cylinder pressure vs. engine speed and equivalence ratio [15]. (b) Methane- hydrogen (a) Gasoline-hydrogen. limit of lean operation with a low pumping function. As such, the excess boost capability from the improved thermal efficiency can cause the alteration in the surplus air ratio that displayed the better efficiency delivered by HCNG [121]. On the other hand, if the hydrogen content with an 11.5 CR [25]. In the meantime, the boost pressure can be in HCNG increases, the knock's tendency would increase due to an in- limited by the reduction of exhaust gas temperature under the full load crease in the rate of burning. Untypical combustion features, such as situation [128]. The decrease in the exhaust gas energy linked to the knocking, backfire or pre-ignition, can be caused by the higher hy- rise in the TE can cause the limitation of the boost pressure. drogen content of the test fuel. Moreover, the engine efficiency will be reduced due to the higher hydrogen content of the test fuel. However, 5.3.2. Heat release rate in the fuel-air mixture, an optimum hydrogen content would bring The difference in the heat release rate is affected by blending hy- about higher thermal efficiency in comparison to CNG. This is caused drogen with natural gas during combustion. Tangöz et al. [129] in- by hydrogen's lower ignition energy need, which decreases the ignition vestigated the alteration in the heat release rate with the increase of the delay as well. In the case of higher speeds, loads and compression ra- compression ratio and the hydrogen fraction in the blends. The increase tios, the influence of hydrogen knocking would be influential. Ac- in hydrogen and CR leads to the advance of the heat release's rate. cording to Flekiewicz et al. [122] knocking happens when adding hy- According to their outcomes, the highest rates of heat release of 84, drogen beyond 40%. 88.19, 85.18 and 78.06 J/0 CA are obtained at 17.0, 15.3, 13.4 and Many researchers have challenged themselves with solving the 11.6 CA, as well as ATDC at λ = 1.01 for 9.6 CR. Moreover, the values problem of knocking and the undesired characteristics of the engine. of 89.56, 102.06, 90.69 and 80.36 J/0 CA are achieved at 13.4, 13.4, Kamil and Rahman [15] performed a research study to determine the 11.3 and 9.8 CA, as well as ATDC at λ = 1.0 for 12.5 CR for CNG, profiles of in cylinder pressure with engine speeds under the stoichio- HCNG5, HCNG10 and HCNG20, respectively [129]. This is due to the metric condition for various hydrogen-gasoline and hydrogen-methane fact that an increase in the CR and adding hydrogen can cause the fractions. For gasoline-hydrogen blends, the different peak cylinder blend's burning velocity to be increased, which makes the phase of pressures with engine speed are illustrated in Fig. 6(a). The reduction in ROHRMAX level with the reach of the top dead center [130–132].In the peak pressure of the cylinder is noticed. This reduction is not de- this context, Hora and Agarwal [123] also examined HRR under dif- sired because of the inherent dependence of the acquired work on the ferent ratios of HCNG fuels with various BMEPs. As shown in Fig. 7, pressure inside the cylinder. When 20% hydrogen is added, there is a they reported that higher engine loads inducted a higher fuel quantity. decrease of 4.6 bars at 5000 rpm. In the engine speed area, the max- It will burn quickly and this results in higher HRR. An increase in the imum cylinder pressure's trend caused by methane-hydrogen combus- engine load can cause the HRR curve peak to move towards TDC. For tion is shown in Fig. 6(b). Based on this figure, when hydrogen fractions HCNG, the heat release rate was higher. This was due to the higher are added, the peak cylinder pressure reduces. However, compared to flame speed caused by adding hydrogen. For 6.18 bar BMEP, the the decrease of gasoline-hydrogen blends, this reduction is small [15]. highest HRR acquired for CNG, 10HCNG, 20HCNG and 30HCNG were This is due to the fact that hydrogen, as well as methane, are gaseous 59.3 J/°CA, 69.2 J/°CA, 72.4 J/°CA and 77.7 J/°CA, with the matching fuels and the distinction among their energy densities is very small crank angle places for the HRR peak being at 8.5°, 5°, 2.5° and 0.5°

330 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 8. Combustion durations of a hypothetical fuel mass fraction burned curve [18].

stoichiometric situations is not slower than H2-enriched CH4, while the effect of H2 at surplus 1.3 air ratio is considered to be quite strong. The early combustion phases are affected strongly by hydrogen's direct injection into port-injected methane through changing the injection direction and time. When the injection is put towards the spark plug, particularly at higher surplus air ratios, the maximum effect can be seen. In these situations, later injection results in shorter combustion durations with the exception of EOI at 50° CA before TDCF in which the mixing time is expected to be too short [18]. A minor dependence on the injection timing and direction is shown in the combustion of phases 2 and 3. This is due to the fact that the influences of stratification and turbulence have already degraded. Phase 3 demonstrates that the influence of adding hydrogen is inclined to decrease when using late injection. Methane direct injection's behaviour into premixed methane is considered to be interesting. As demonstrated by various authors, the

direct injection of 9% (i.e., energy, mass, volume) of CH4 is capable of accelerating the combustion to the same degree as by adding H2. Adding methane and hydrogen affect the early phase of combustion significantly. A Phase 1 prolongation can be seen by adding hydrogen, which is through setting the injection timing from 70° CA to 50° CA EOI Fig. 7. Variations in HRR for HCNG mixtures [121]. before TDCF. This behaviour cannot be seen when methane is added [18]. Therefore, it can be concluded that the best method of accel- ATDC respectively [121]. The HRR trends, which were perceived at erating the early combustion phase is through delaying the direct in- various hydrogen contents and BMEPs at broad open bottle, were much jection of CH4 towards the spark plug. the same as found by Moreno et al. [133]. This was noted at various speeds and equivalence ratios, together with the results of Xu et al. 5.3.3. In cylinder temperature [130], at a part throttle (10%) engine state. According to Wang et al. The influence of cylinder peak temperature on the creation of un- [134] and Mariani et al. [135], the HRR will be increased by enhancing desired emission, particularly NOx, and on the losses of heat transfer is the hydrogen fraction in HCNG with direct HCNG injection technology, the basis of its importance [136,137]. Kamil and Rahman [15] con- in comparison with the showed port HCNG injection. The findings of ducted a comparison between gasoline and gasoline-hydrogen, as well different studies have shown that the burning speed (heat release rate) as between methane and methane-hydrogen, in SI engines, in order to of HCNG can be increased through adding hydrogen to CNG, which examine the effect of peak temperature under different engine speeds. leads to a shorter combustion duration. Therefore, earlier in the ex- The peak cylinder temperature's variation for the considered hydrogen pansion stroke, a large fraction of chemical energy was released as heat fractions is shown in Fig. 9. Fig. 9(a) shows the trends of the gasoline- and this was lower after burning occurred. hydrogen blends. The peak temperature is increased through adding 5% Combustion stability corresponds to combustion duration. The cycle hydrogen. The reports show an increase of 34 °C with this fraction at by cycle differences are decreased by shorter combustion phases and 6000 rpm. The peak temperature will not be increased through adding shorter ignition delay. Biffiger and Soltic [18] stated that the combus- more hydrogen (10%). Nonetheless, the peak temperature is still more tion event can be divided into three phases, which are shown in Fig. 8. than that of the base gasoline engine (the difference is now 28 °C at Phase one is the period between 5% of the fuel mass burnt and ignition. 6000 rpm). The peak temperature will be back and become identical to Phase two refers to the period from 5% to 50% of the fuel mass burnt. that of the base engine when a 15% fraction is added. Therefore, the Phase three refers to the period from 50% to 90% of the fuel mass temperature will be reduced to below that of the base engine with any burnt. Biffiger and Soltic [18] determined that the turbulent kinetic further enrichment. A temperature decrease to 25 °C at 6000 rpm is in energy and the charge stratification at the ignition's moment are in- order to go with the 20% fraction. This behaviour continues for the creased by direct injection. Based on the HR analysis, the direct injec- whole speed range reviewed [15]. Similar trends can be observed for tion direction and injection time affect the early phase of combustion, methane-hydrogen blends which are illustrated in Fig. 9(b). The peak whereas fuel composition and global stoichiometry affect mainly the temperature will be increased by the hydrogen's adiabatic flame tem- late stage of combustion (phase 3). Methane's combustion in perature. The quick flame speed of hydrogen plays a part in increasing

331 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 10. Peak combustion temperature in K for both CNG and HCNG idle operations in various EARs with spark ignition timing sweep [139]. Fig. 9. Peak cylinder temperature vs. engine speed and equivalence ratio [15]. (a) Ga- soline-hydrogen (b) Methane-hydrogen. adding hydrogen, brake thermal efficiencies are enhanced slightly. This also varies easily with the surplus air ratio, which shows that adding the peak temperature as well. This is because the enriched mixture is hydrogen caused the engine lean burn ability to be improved effectively capable of burning much faster than pure gasoline or methane, which [13]. The engine performance correlative surplus air-fuel ratio (λ) to- signifies that the combustion is increased after adding hydrogen [138]. gether with the exhaust gas temperature (EGT) at various loads (BMEP) Lee et al. [139] also made a comparison of peak in-cylinder com- for 4 different structures of HCNG mixtures was examined by Hora and bustion temperatures between CNG and HCNG idle operations. It is Agarwal [121]. The overall chemical energy's conversion efficiency of shown very clearly in Fig. 10 that the peak temperature was kept at less fuel into mechanical energy, which is accessible at the engine shaft, is than 1500 K for both cases. This was due to idle's near-zero load con- displayed in Fig. 11. In comparison with HCNG mixtures, CNG dis- dition and the larger portion of residual gas in the cylinder. Despite the played a lower brake thermal efficiency. An increase in the hydrogen general perception that the highest combustion temperature of H en- 2 content in HCNG leads to an increase in BTE at all loads as a result of riched fuel is increased by the higher adiabatic temperature of H , the 2 the higher combustion stability and higher combustion efficiency. final level of the peak combustion temperature in HCNG operations was 25.7% BTE was shown by CNG at 5.3 bar BMEP, whereas 27.2%, 28.1% lower than in CNG operations [80,139,140]. This is due to the fact that and 28.5% BTE values were shown by 10HCNG, 20HCNG and 30HCNG the amount of fuel provided for a stable idle was much lower in the respectively. Likewise, BTE values of 28%, 28.6%, 29.3% and 29.7% HCNG engine operation in comparison with CNG. A decrease in the in- were shown by CNG, 10HCNG, 20HCNG and 30HCNG respectively at cylinder pressure throughout the combustion procedure was brought about by less fuel and finally the lower peak combustion temperature was induced. Much the same trend occurred when the spark ignition timing was delayed towards TDC for each air/fuel ratio. The peak temperature was increased due to the enhanced fuel consumption rate close to TDC. The increase in EAR for each timing spark ignition caused the combustion temperature to be decreased due to the increased influence of surplus air dilution. Therefore, for both fuels, the maximum peak combustion temperature occurs at the lower right corner [139].

5.4. Eﬀects of HCNG on performance of SI engine

Engine thermal eﬃciency is important for assessing an engine's economics and its general performance. Optimising the fuel properties ﬃ or the combustion system can lead to its improvement. Moreover, after Fig. 11. Variation in thermal e ciency with engine load [121].

332 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 12. Evolution of the thermal eﬃciency as a function of λ and the fuel composition at Fig. 13. Evolution of the engine brake torque as a function of λ and the fuel composition full load, 4200 rpm and optimum spark advance [141]. at full load, 3400 rpm and optimum spark advance [141].

6.18 bar BMEP [121]. Based on the findings, an increase in BMEP led to the consumption of a greater fuel quantity for the purpose of producing a higher power output. This reduced the excess air–fuel ratio (λ) and caused the fuel-air mixture to become quite rich (Fig. 11). Similar power can be generated at lower BMEP using a leaner mixture of HCNG-air in comparison with the CNG-air mixture, whereas a richer mixture of fuel-air was needed at a higher BMEP. This showed that the CNG operation's lean limit increases as a result of adding hydrogen to the CNG. The engine is going to operate very near to the stoichiometric condition (λ 1.01, 1.02, 1.01 and 1.0 for CNG, 10HCNG, 20HCNG and 30HCNG respectively) at 5.3 bar BMEP [121]. The ratio between the fuel heating power and the effective power, as studied by Diéguez et al. [141], corresponds to the thermal efficiency as shown in Fig. 12. They conducted their test at 4300 rpm, optimum spark advance and full load. Based on their findings, when the air-to- fuel ratio is enhanced up to 2.5, the efficiency decreases from 34–35% to 28–30% for λ values of 1.6–2.0. Their results also showed that an increase in the methane content of the fuel causes a trend towards lower efficiencies. The effect of the operating conditions on the me- ffi chanical e ciency can be described concerning the combustion tem- fl ff Fig. 14. Evolution of the adiabatic ame temperature as a function of the air-to-fuel ratio perature's e ect for the engine torque [13,15,121,141]. An increase in (λ) and the methane molar fraction (η) of the fuel [141]. the methane content of the fuel causes the combustion temperature to decrease. Therefore, the maximum efficiencies are achieved when pure of λ [141]. From another point of view, combusting methane at the hydrogen is utilised. Likewise, as the fuel mixture comes to be leaner, stoichiometric condition (λ = 1) will cause no problems. As such, from the combustion temperature reduces. Therefore, as λ increases, the the engine performance perspective, adding methane to hydrogen has mechanical efficiencies decrease. the positive influence of enabling a more fuel rich operation, which Engine thermal efficiency can be improved by an increase in the leads to an increase in the engine torque [141]. HCNG mixtures have a mixture burning speed [142]. The hydrogen-CH4 mixture has an ex- higher brake thermal efficiency compared to CNG. Bauer and Forest tended flame limit and faster burning velocity compared to CH4, due to [113] showed much the same trend at various BMEP and HCNG blends the large flame speed of hydrogen which is five times larger than that of at lower engine speeds (700 rpm and 900 rpm) and equivalence ratios. CH4, and also the wide flammability range of hydrogen which is much Specific fuel consumption (BSFC) refers to the ratio of the mass fuel greater than CH4, which can lead to more complete burning and a consumption to the break power [8,143]. According to Açıkgöz et al. shorter burning duration [13]. Concerning the hydrogen methane [13], as the surplus air ratio, a decrease in the BSFC and an increase in mixtures, the fuel composition at full load, 3400 rpm, the brake torque the BTE were noticed through an enhancement of the hydrogen fraction as a function of λ and the optimum spark advance are shown in Fig. 13. in the fuel blends of more than 20%. Adding hydrogen was not useful As can be seen, the brake torque changes with fuel composition to a for improving the efficiency when the excess air ratio was less than 1.4 certain extent. However, it obviously reduces as the air-to-fuel ratio [13]. As shown in Fig. 15, the influences of spark timing together with enhances. This behaviour can be explained concerning the influence of the air/fuel ratio on the rate of fuel consumption for CNG and HCNG combustion temperature as it is much the same as the adiabatic flame idle operations were examined by Lee et al. [139]. In their study, HCNG temperature's evolution, as shown in Fig. 14. Diéguez et al. [141] ex- was compared with CNG. They found that the utilisation of HCNG had a amined the brake torque connected to the gases’ power cycle in the lower rate of fuel consumption for all λ and spark timing conditions. A cylinder, which in turn is based on the engine speed, spark advance, more than 25% reduction in energy consumption was obtained load, and fuel nature. Hydrogen combustion carries the risks of back- throughout HCNG operations [139]. Low cyclic variation and closer-to firing and knocking, which are an impedance to operation at low values

333 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 16. Fuel consumption in MJ/km over NEDC and ARTEMIS driving cycles [135].

the reasons behind this. But that said, the values of BSFC decrease after adding hydrogen to CNG above λ = 1.1 out of HCNG20 for 9.6 CR [129]. Mariani et al. [135] tested different lower heating values and mixing fuels of HCNG and reported that (as Fig. 16 shows) the influence of the fuel on engine efficiency can be determined, taking into account the fuel consumption in MJ/km. There were no significant variations among HCNG15 and CNG, whereas a decrease has been seen when fueling the engine with HCNG30. The SFC that was achieved with this blend was 2.33 MJ/km for the NEDC. This was 6% lower than CNG. For urban, road and motorway, the SFC values were 3.41, 2.04 and 1.86 MJ/km for the Artemis cycles. They were 6%, 3% and 7% lower than CNG [135]. Faster combustion move up through adding hydrogen is the reason behind the enhancement of the engine's average efficiency for HCNG30 with respect to natural gas. The air-fuel ratio's estimation was enabled by exhaust gas analysis. This caused close to stoichiometric values for the whole of the conditions of operating.

Fig. 15. Fuel consumption rate in kw for both CNG and HCNG idle operations in various 5.5. Effects of HCNG on the emissions of an SI engine EARs with spark ignition timing sweep [139]. 5.5.1. Hydrocarbon emissions (HC) constant volume combustion were the reasons for these improvements. Incomplete combustion can lead to the production of unburned fl The ame propagation speed of the HCNG/air mixture was much faster hydrocarbon. Due to a big valve overlap angle, CH4 is the source of THC in comparison with the speed of the in-cylinder volume change through (Total Hydrocarbon) emission for the HCNG engine. Moreover, hydro- changing the position at idle and as a result it comes to be closer to carbon emission can be caused by both misfire and abnormal com- constant volume combustion due to adding H2 [144]. Based on the bustion. The wall quenching influence and incomplete combustion are findings, the fuel consumption rate reached the lowest level at roughly the main sources of HC emission for the combustion procedures [147]. λ = 1.1 (for CNG) and between 1.1 and 1.2 (for HCNG) for each spark The unburned HC emission was decreased as a result of adding hy- ignition timing. For higher λ, when the flame propagation speed re- drogen, signifying improved combustion efficiency [13]. The decrease duces, the combustion duration increases, which eliminates the effi- of heat transfer as a result of the fast burn speed can be eliminated ciency advantages of lean operation and increases the fuel consumption through the higher in-cylinder temperature, together with the shorter rate. As shown in Fig. 15, the fuel consumption rate will decrease when quenching distance induced by adding hydrogen, can be the reasons the spark ignition timing increases [139]. This decrease will continue behind this phenomenon [143,148]. Lima et al. [25] did a comparison till idle stability significantly worsens. between CNG and HCNG30 with two compression ratios to test the ff The compression ratio and the air-fuel ratio also a ected the BSFC emissions of THC and CH4. As shown in Fig. 17, the emission of THC with different HCNG mixtures. Tangöz et al. [129] found that the values with HCNG30 and CR of 11.5 reduced in comparison with 10.5 CR and of BSFC decrease when the value of CR increases from 9.6 to 12.5. Next, HCNG30 in the lean combustion condition. The THC emissions with a the values of BSFC are enhanced at 15 of CR. Besides, the lowest values high temperature of combustion are reduced by the high efficiency of BSFC are acquired at low λ for pure CNG rather than HCNG blends combustion. From the other point of view, CR in the CNG case did not and all CR. For example, the lowest values of 207, 210, 215 and 247 g/ affect the THC emission. Fig. 18 shows that the increment in CR at the λ λ λ λ kW h are achieved at = 1.1, = 1.15, = 1.22 and = 1.27, same air ratio leads to a decrease of CH4 emission with HCNG30. CNG sustained by CNG, HCNG5, HCNG10 and HCNG20 for 15 CR, respec- shows the same trend. Nevertheless, the decrease in the CH4 emission tively [129]. The poor lean-burn ability of CNG together with the low was greater than that of the THC emissions with 11.5 CR [25]. Hu et al. fl ame speed caused the CNG to be considered as a preferable fuel at the [149] found much the same result in lean premixed CH4-H2-air flames stoichiometric state [145]. Moreover, HCNG is regarded as the desir- at enhanced temperatures and pressures. The results of Hu et al. [149] fl able fuel at lean mixtures as a result of hydrogen's high ame speed and were similar to this for lean premixed CH4-H2-air flames at high tem- high flame temperature [26,145,146]. The observation of the author peratures and pressures. Based on their findings, the dominant chain shows that an increase of the hydrogen fraction in natural gas for CRs of branching reaction was (R38), whereas the chain recombination reac- 15 and 12.5 leads to an increase in BSFC. The combined influences of tion in the combustion of HCNG was (R52) and (R35). volumetric efficiency and the reduction of the volumetric heat value are H+↔+OOOH2 (R38)

334 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

occurred at 2 CAD, BTDC for λ = 1, 8 CAD, BTDC for λ = 1.5 and among these values for 1.1 ≤ λ ≤ 1.4. However, since spark timing was set back toward TDC in HCNG operations, there was a high possibility of oxidising the HC released from the crevice volume throughout the early step of the expansion procedure. This happened due to the smaller quenching zone and shorter combustion duration. Therefore, when λ was less than 1.3, the emission of THC continued to decrease [139].

5.5.2. Carbon monoxide emissions (CO) Carbon monoxide is formed as a result of the fuel's incomplete burning in the engine. Incomplete burning happens when the combustion cannot be completed as a result of insufficient oxygen [16,150,151]. Based on the studies of Gharehghani et al. [154], with the decrease of the equivalence ratio, the CO emission is reduced and then it began to increase rapidly after a minimum value as a result of a misfire in the huge lean mixture. The addition of hydrogen did not have Fig. 17. THC in relation to excess air ratio for each fuel/CR [25]. a significant effect on the formation of CO before the misfiring limitation for pure CNG (Ф ~ 0.6); however after this, adding hydrogen resulted in a greater decrease of CO, and on the basis of estimated outcomes, misfiring was delayed [152]. The engine's operating range could be extended to that of a leaner mixture with no impact on the CO concentration through enhancing the percentage of hydrogen. The differences in the engine's CO emissions at various engine speeds and different fuel mixtures were investigated by Açıkgöz et al. [13]. The entirety of the fuel cannot be burned by the engine even though there is excess air in the cylinder to complete the combustion. The hydrogen fueled engine was expected to have zero CO emission. A small amount of CO could still be found in the results. This resulted from the burning of the lubricating oil film inside the engine cylinder. CO emission diminishes with the increase of the engine speed. The addition of hydrogen increases the CO emission when the excess air ratio is almost stoichiometric; however it decreases when hydrogen is added under

lean conditions. The oxidation reaction of CO into CO2 is stimulated by the enhanced in-cylinder temperature after the addition of hydrogen [13]. The in-cylinder CO mass fraction distribution for diﬀerent hy-

Fig. 18. CH4 emission in relation to excess air ratio for each fuel/CR [25]. drogen fractions with an equivalence ratio of 0.625 at EVO is shown in Fig. 19. The fuel's incomplete combustion within the combustion chamber results in CO emission. The incomplete combustion is highly H++OH22O ↔ HOHO 22 + (R35) dependent on the blend's combustion temperature. Gharehghani et al. [152] believed that, based on the observation, the CO emissions pro- H ++↔+CH34 (M) CH (M) (R52) duced at 30% and 50% hydrogen fractions were insignificant in com- Based on the findings of the research, the recombination reactions parison to pure natural gas fuel combustion. are considered to be pressure sensitive and the reactions of chain Diéguez et al. [141] accomplished a research study which tested the branching are temperature-sensitive. The division of CH4 into various air-fuel ratio at full load, 3400 rpm engine speed and optimum spark radicals is promoted by the increase in the combustion temperature. As advance as shown in Fig. 20. They reported that there is a high increase such, the decrease in CH4 emission when CR is increased can be asso- in the emission of CO with λ. For instance, for η = 0.10 (10 vol% ciated with the chain branching reaction, which a high combustion methane) the CO emissions at λ = 2.5 (3.6 g/kW h) are almost 7 times temperature activates, instead of an enhancement in the pressure [25]. higher than when λ = 1.6 (0.5 g/kW h) [141]. This unexpected out- Lee et al. [139] studied in idle operations and stated that the re- come can be explained by the fact that despite the decrease of the air/ sidual gas fraction can be described as high and the combustion tem- fuel mixture's carbon content with the increase of λ, a greater decrease perature as low that lead to the production of a high level of THC in the power is produced which results in the increase of specific emissions which means that it is not easy to fulfill the emission rules. As emissions. This is in line with the findings of Moreno et al. [153] where such, each operation's THC emission against spark timing and air/fuel a similar trend was found in a naturally aspirated two-cylinder SI en- ratio are compared in their study. For HCNG engine operations, the gine, which was fueled with methane and hydrogen mixtures at full overall level of THC emissions was much lower than that of CNG op- load, in its specificCO2 emissions. Diéguez et al. [141] also reported erations. This was due to the low carbon content in HCNG together with that the specific CO emissions for η = 0.20 under stoichiometric con- the outstanding combustion features of H2.Atfirst, the emission of THC ditions and at λ = 2 are almost coincident (about 2.2 g/kW h) and 3 reduced with an increase in λ (until λ almost = 1.1), then it increased times higher than at λ = 1.6 (about 0.8 g/kW h). Based on this, stoi- for both fuels. THC emissions were enhanced for CNG as well as HCNG chiometric conditions are unfavourable for minimum carbon monoxide operations with an increase in the spark ignition timing. This enhanced emissions in this situation because there is not enough oxygen avail- the in-cylinder pressure; therefore, there was an increase in the charge able. Therefore, the level of CO emission is notably lower than that density captured in the crevice volume. An increase in THC levels is generated at the intermediate λ value of 1.6, compared with λ = 1 or 2 caused by high spark retardation in CNG operation due to bulk [141]. Hora and Agarwal [121] examined CO emissions with BMEP. quenching close by the wall throughout the expansion procedures. They stated that with the increase of BMEP, the CO emission decreases Therefore, the spark timing for the lowest level of THC emissions since the higher peak cylinder temperature at higher BMEP is

335 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 19. CO mass fractions for various hydrogen fractions for Ф = 0.625 at EVO [152].

limits, and CNG [155]. The flame propagation speed of hydrogen is high and its ignition energy is low, which makes it easier to work with leaner blends compared to natural gas. The lean air-fuel mixtures have a limit, which if reached, causes the combustion to become incomplete and unstable. The combustion speed decreases and the air-fuel mixture's ignition energy increases when operating close to this limit. This results in cycle-by-cycle variations which decreases the thermal efficiency. Navarro et al. [155] investigated the effect of hydrogen addition

and air/fuel ratio on CO2 emissions at various engine speeds as shown in Fig. 21. It can be observed from the ﬁgure that there is a decrease in

particular CO2 emissions when there is an increase in the crankshaft rotational speed, when there is an increase in the air/fuel ratio, and when the employed fuel mixtures have increasing hydrogen content

[155]. The reduction of speciﬁcCO2 emissions and the increase of H2 content in the fuel mixture can be caused by the progressive reduction of the carbon content and the combustion process’ advancement. Var-

ious aspects have to be considered in explaining this behaviour of CO2 emissions, which is observed with an increase in the air/fuel ratios.

Fig. 20. Evolution of the speciﬁc CO emissions as a function of λ and fuel composition at 5.5.4. Nitrogen oxides emissions (NOx) full load, 3400 rpm and optimum spark advance [141]. Coal burning creates nitrogen oxides (NOx), which are essentially nitrogen dioxide (NO2) or nitric oxide (NO), and are generally referred favourable for CO to CO2 oxidation. The CO emission of HCNG blends is to as NOx.NOx is created by ignition and is comprised of NO (90–95%) ’ less than baseline CNG since the HCNG mixtures C/H ratio is lower, along with lesser (5–10%) amounts of NO2 [46]. Nitric oxide's ar- and they have higher peak cylinder temperatures which is in favour of rangement in the burning zone results from two elements which are the CO oxidation. According to the study, a higher CO emission was observed at 6.18 bar BMEP because the blend in the combustion chamber was richer and lower levels of oxygen were available for combustion [121]. Ma et al. [154] observed similar trends for the discussed emission at various BMEP for wide open throttle from HCNG mixtures at various ignition timings and Xu et al. [130] observed it at part throttle conditions.

5.5.3. Carbon dioxide emissions (CO2) When the degree of hydrocarbon fuel's combustion has been completed, this is indicated by the engine exhaust's CO2 formation. Lower CO and higher CO2 indicate the eﬃcient utilisation of fuel and better combustion quality. HCNG has a low emission of CO2 because the C/H ratio decreases with the increase of the hydrogen content [33,121,150]. To investigate the impact of adding hydrogen and the eﬀect of the air/ fuel ratio on the emission of CO2, various mixtures of natural gas and hydrogen were examined including HCNG with hydrogen fractions Fig. 21. CO2 emissions as a function of the relative air/fuel ratio, fuel composition and from 10% to 90%, with an air/fuel ratio within reasonable operating crankshaft rotational speed [155].

336 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

prompt system (Fenimore system) and the thermal system (Zeldovich the case of premixed combustion vs NOx emissions. They reported that system). High-temperature ignition, i.e. the point at which the burning at higher excess air ratios, the NOx was decreased. Likewise, they re- temperature is above 1400 K, sets up the development of thermal NO. ported that when the excess air ratios are higher, there is a less notable With the increase of the burning temperature, the development rate of decrease in the split injection. Furthermore, in methane into methane the NO increases rapidly, and with the decrease of the burning tem- cases, there is an increase in the emission. This can be regarded as a perature, the development rate decreases in this component [151]. The better understanding of employing charge stratification. Since pre- brief development of NO occurs within the rich, low-temperature mature direct injection is more like the premixed combustion case, less burning zones where a reasonable amount of dynamic radicals can be NOx is shown at lean combustion while late rejection results in charge accessed [150,156,157]. Because of the dominant impact of tempera- stratification. The mixture that is nearer to the spark plug becomes ture, the formation of NOx is favoured by fuel-rich mixtures (with low heterogeneous in a stratified combustion and prompts a higher flame values of λ) [133,158–160]. However, by controlling the spark advance temperature and consequently a higher NOx emission [18]. (SA), the temperature and, as a result, the combustion temperature that Hora and Agarwal [121] tested the NOx emissions with different is reached within the cylinders, can be modified. Wang et al. [161] have BMEP and various HCNG mixtures. They reported that when the BMEP studied the impact ignition timing on the lean combustion limit in the is low and is at 2.98 bar, an insignificant difference in the formation of cases where natural/hydrogen gas is used as a fuel. NOx was found in the entirety of the tested fuels. This resulted from Diéguez et al. [141] performed a research study to examine the comparatively lower peak cylinder temperatures and the leaner fuel-air spark advance and molar fraction of methane (η) in the HCNG mixture mixtures which were outside the NOx formation window. According to as a function, with the specificNOx emissions at full load, λ of 1.6 and Wang et al. [135], less NOx is formed when HCNG is injected directly; speed of 3400 rpm. Spark advance increases the emission of particulate however, insignificant differences were reported due to the addition of

NOx, while the methane content decreases it significantly because of the hydrogen at BMEPs. But there was a change in this pattern at higher combustion temperature decrease. This is one of the positive impacts of loads when there was a higher formation of NOx. With an increase in the addition of methane to hydrogen fuel. As illustrated in Fig. 22, BMEP and the enhancement of the hydrogen fraction within HCNG when the spark advance is optimal, only the air-to-fuel ratio governs the mixtures, the brake specificNOx emission and raw emissions increase specific emission and it is fairly unaffected by fuel composition at a [121]. Based on the report of Diéguez et al. [141], who also reported at specific value of λ in the limits set by their study. In stoichiometric various engine speeds, the specificNOx emissions were found to in- conditions (λ = 1), the fuel mixture that has the maximum methane crease when the engine speed increased. For instance, at λ = 2, full content (η = 0.20) can only be combusted. The other fuels, like pure load, η = 0.2 and optimum spark advance, the emissions increased hydrogen, demonstrated a tendency to knock which prevented the use from 0.32 to 0.39 and finally 0.47 g/kW h when the engine speed in- of a λ value less than 1.6. In stoichiometric conditions, the emission of creases from 3400 to 4200 and finally 5000 rpm [141]. The turbulence

NOx was high and reached 3.05 g/kW h, despite the significantly low intensifies with the increase in the speed of the engine, which results in availability of oxygen. The emissions were near 1 g/kW h when λ = 1.6 higher NOx emissions and combustion temperatures, and enhanced was used and when the air-to-fuel was enhanced to 2–2.5, it decreased mixing. Despite this, the effect of the engine speed on the emission of to 0.3–0.4 g/kW h [141]. nitrogen oxides is significantly less notable than that of the spark ad-

HCNG mixtures have relatively higher NOx emissions, the formation vance or air-to-fuel ratio. of which is dependent on the oxygen content and the combustion Tangöz et al. [129] have examined the compression ratio (CR) in a temperature. This results from a comparatively higher in-cylinder diesel engine fueled by different mixture ratios of HCNG. The NOx temperature caused by the addition of hydrogen to CNG, which favours emissions at various CR and HCNG are illustrated in Fig. 23. Because of the formation of NOx. There was an increase in the flame-front tem- hydrogen's high flame speed and high flame temperature, increasing perature when the hydrogen fraction of the HCNG mixtures increased the hydrogen fraction of HCNG results in an increase of the mixture's [25]. Diluting the mixture with exhaust gases or excess air is a typical temperature as well as the residence time of air in the hot zone. Thus, method employed in decreasing the peak temperature of combustion adding hydrogen to CNG increases the NOx [145]. The maximum NOx and, consequently, the NOx emission. Biffiger and Soltic [18] accom- values obtained at λ = 1.15 for 9.6 CR fueled by CNG, HCNG5, plished work under various excess air ratios and injection strategies in HCNG10 and HCNG20 are 18, 19.8, 21.5 and 25.5 g/kW h respectively. In a similar fashion, the maximum values obtained at λ = 1.16 and = 1.21 for 12.5 and 15 CR fueled by the same fuels are 18.4, 19.1, 20.7 and 22.1 g/kW h and 18, 19.2, 21.7 and 25.8 g/kW h respectively. It can be observed from the figure that with an increase in the CR, the emission values reach maximum values at higher λ. Furthermore, these

NOx values are greater than the Euro VI standard (0.4 g/kWh) [162] for a gas engine. Thus, in order to reach the Euro VI standard, the NOx values need to be reduced. It can be noticed in the ﬁgure that with

excess air ratios, the emission values of NOx reach a maximum level and then decrease. The NOx emission values gained at λ of 0.96, 1.15 and 1.3 for 9.6, 12.5 and 15 CR fueled by HCNG10 are 4, 21.5 and 16.7 g/ kW h; 4, 20.5 and 17 g/kW h; 3.7, 21 and 18.6 g/kW h respectively [129]. As a matter of fact, the oxygen environment is close to lambda 1.1 and the combustion temperature is high in the cylinder. Thus it can be said that with the gradual lean out of the engine, there is a rapid

increase in the NOx emission which reaches a peak and then experiences a rapid decrease to a fairly small value. The high combustion

temperature is aided by the growth of the compression ratio. Thus, NOx emission worsens with an increase in the compression ratio [163]. The emission can be reduced through various methods, which include internal engine measures like various injections, exhaust after- ﬁ λ Fig. 22. Evolution of the speci cNOx emissions as a function of and fuel composition at treatment, and EGR or water treatment. On the basis of an engine op- full load, 2000 rpm and optimum spark advance [141]. eration strategy, either the conventional 3-way catalysts or the lean

337 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342

Fig. 23. The NOx emission values versus excess air ratio using diﬀerent compression ratios and HCNG blends [129].

NOx must be used [46,164,165]. With regards to NOx, in spite of the 7. Conclusions promising outcomes such that the emission level was reached by 3-way catalysts, various studies have tried to elaborate on and investigate the High emissions were produced from the engine-operated petroleum lean NOx after treatment systems. This was due to the fact that for the and this problem is still a challenge to researchers. Natural gas, which actual operation of 3-way catalysts, particular stoichiometric conditions in turn generally includes methane, gives considerable monetary as were required which resulted in a great decrease of the engine's effi- well as environmental advantages, including much better productivity ciency in comparison to the lean mode. Methods that were employed in and availability, as well as lower emissions. However, the weak lean- decreasing the NOx emissions consisted of 3-way catalysts, selective burn ability, the low flame speed and the ignitability of methane de- catalytic reduction (SCR) and a lean NOx trap. There have also been mand deep studies for its usage in an IC engine. When using natural gas patents of concepts where the additional injection of the decreasing in SI engines, there is an engine efficiency sacrifice at low loads and a agent in the exhaust was prevented through altering the operation higher level of some of the emissions such as HC/CO, which cannot be procedure from lean to fuel-rich using EGR in purifying the lean NOx solved without using the after-treatment equipment. This equipment, trap [165–167]. Moreover, the measurements to decrease the engine's however, is expensive. Therefore, an additional fuel can enhance the internal nitrogen oxide emissions include reducing the temperature characteristics of combustion of NG - which could be used in the intake inside the cylinder using water injection and EGR [168]. Water injec- charge. H2 is an efficient gas for enhancing the flame rate regarding tion decreased the NOx emissions significantly and had little negative combustion in an SI-CNG engine, together with decreasing the COV and effect on the efficiency of the engine [169,170]. Even though water increasing engine stability. Tiny amounts of H2 improve performance injection is regarded as a suitable process for decreasing the NOx and reduce exhaust emissions. Thus, many investigators have per- emissions, its functional application is based on an effective way of formed research studies on SI engines with different ratios of HCNG. providing the liquid (for instance, through restoring and liquefying it Many studies have found that the combustion characteristics of HCNG from the engine exhaust) [171,172]. engine are strongly dependent on the conditions of the engine. The air- fuel ratio, the time of injection, the compression ratio and the speed play a major role when blending HCNG in an SI engine. Adding hy- 6. Challenges of HCNG utilisation drogen to natural gas increases cylinder pressures and temperatures. These increments were obtained due to enhancing the rate of heat re- To use HCNG as an engine fuel, hydrogen/natural gas ratio opti- lease (the burning speed) of HCNG, leading to a shortened combustion misation is the most challenging. This is due to abnormal combustion duration. For this, a large proportion of chemical energy was emitted behaviour, such as pre-ignition and knock, and backfire for imbalanced before the power stroke because of heat production and comparatively ratios of the two mixtures. This means that it is important to adjust the less after-burning happened. With stoichiometric operation, the com- spark timing and the fuel-air ratios to within the acceptable ranges. The bustion of natural gas is not significantly slower when compared with hydrogen percentage is directly proportional to the lean operation hydrogen-enriched CNG, although the effect associated with hydrogen limit, while at the same time it is inversely proportional to the max- from air-fuel (λ) of 1.3 is pretty strong. Additionally, the injection of imum brake torque (MBT). From here, a relationship among the three hydrogen into port-injected CNG affects the early combustion stages of (of hydrogen fraction, excess air ratio and ignition timing) can be im- development rather highly through diverse injection timing along with proved, the determination of which means solving the hurdle to finding position. The largest impact is usually noted if the injection can be the optimum ratios. pointed to approach the spark plug, particularly from higher excess air In order to specify whether or not a fuel is a suitable alternative fuel, percentages. In most of these conditions, shortened combustion dura- the most important parameter to be considered is the emission level, tions result from later injection. The authors have shown that the early which mostly refers to NO emissions. In the case of CNG, the excessive x combustion phases are accelerated when the method of direct injection hydrogen causes an enhancement in the NO levels while it reduces the x is delayed towards the spark plug. emissive hydrocarbons. Due to this contrast, the hydrogen ratio needs Using HCNG fuel confirms a reduced energy consumption rate for to be adjusted in such a way as to keep both the NO and hydrocarbon x all excess air and spark timing conditions. In particular, an greater than emissions as low as possible. Such a challenge is a huge limitation to 25% reduction in fuel consumption was obtained when using HCNG extending new-fuels applications on a broader scale. On the other hand, blends compared to CNG. This specific development was assigned to like other gaseous fuels, both hydrogen and natural gas are lighter than closer-to constant volume combustion along with minimal cyclic dif- air; they thus disperse quickly granted adequate ventilation, which is ferences. However, some authors observed that when the hydrogen problematic in the case of leakage. Last but not least, as hydrogen is fraction is increased in natural gas with different CRs, the BSFC is ad- more expensive than natural gas, producing HCNG will not be cost ef- vanced. This is a result of the combined consequences of volumetric fective.

338 H.A. Alrazen, K.A. Ahmad Renewable and Sustainable Energy Reviews 82 (2018) 324–342 effectiveness as well as the lowering of volumetric heat worth. powertrains and short range IC powertrains at larger scales such as Regarding brake thermal efficiency and power, CNG showed a lower airport shuttles and local delivery trucks. BTE compared to the HCNG blend. At all loads, the power and BTE were The second advantage is the overall cost effectiveness of HCNG. The enhanced when improving the hydrogen particles with HCNG, due to optimum H2 value to deploy the maximum usage from both the range the somewhat larger combustion efficiency and remarkable combustion and the emissions point of view is 30% H2. Utilising this value, the stability. The increment in the burning velocity of the mixture has an methane combustion will be close to complete, meaning that the after optimistic relation to improving the TE of the engine. Because the flame combustion exhausts are near to zero. This leads to not only a reduction speed regarding H2 is 5 times greater than that regarding CNG and the of after-treatment exhaust equipment costs, but also a reduction of inflammability selection of H2 is quite broad compared to CNG, the space and design complexity as well as weight considerations. Due to particular H2–CNG mixture can have a quicker ignition and also a de- this reduced amount of H2 the hydrogen energy content is equal to layed flame control compared to CNG, which can mean a quicker 11%, which makes it easier for renewable sources to meet the emerging burning and a fairly complete combustion. HCNG market demands. This is as the applications of fuel cell vehicles The level of THC emissions with regards to the HCNG engine has increase, and the hydrogen price lowers, from which HCNG could been reduced, compared to that involving the CNG engine, due to the benefit. lower carbon formation inside HCNG, along with the exceptional As for the challenges HCNG is facing, a number of topics can be combustion of hydrogen. As engine speed and loads increase, CO/CO2 discussed. In terms of GHG, another alternative for hydrogen could be emissions tend to minimise. However, CO emission improves along biogas. There is a possibility that by adding locally derived biogas to with hydrogen improvement if the air-fuel ratio is stoichiometric, but it CNG, its performance could improve in such a way as to be considered a reduces with the help of H2 beneath the lean-condition. The elevated in- better treatment over adding hydrogen. As for the engine's compat- cylinder temperature after the enrichment of hydrogen likewise is en- ibility, the engine should be designed in such a way as to operate on gaged to revitalise the particular oxidation reaction of CO into CO2.HC both HCNG and CNG as per necessity in the case of HCNG fueling and CO emissions are usually a direct result of the incomplete com- system breakdown. Although the HCNG-fueling station is much more bustion associated with the fuel inside the combustion chamber and are expensive than either of the CNG or hydrogen fueling stations, devel- also powerfully dependent on the mixture temperature. It is noticed oping an HCNG flex-fuel system is more affordable based on current that each of the 30% and 50% hydrogen fraction cases makes minimal price and availability as it will enable the combustion of any mixture of

CO/HC/CO2 emissions in comparison to natural gas combustion. any of the named gases even on a random basis. Such flexi-fuel NOx emissions, whose formation are determined by the combustion equipment will therefore make it possible to utilise hydrogen from all temperature and air formation, are comparatively larger for HCNG sources, thus expanding the hydrogen market. From the other side, any blends. This is on account of the comparatively larger temperatures in expansion of HCNG relies on the expansion of current international the cylinder due to the hydrogen supplement helping CNG, which often standards for receptacle and nozzle design for more HCNG blends. The meant desired NOx formation. The temperature of the flame front im- standards would consist of different mixing ratios of HCNG blends such proved with the addition of hydrogen in the HCNG blends. Also the air- as 20% and 30% or any other percent. Promoting such standards will fuel ratio, the loads and the engine speed affect NOx emissions with the motivate manufacturers to produce dispensing products to match the HCNG blend. It has been discovered that the particular NOx emissions requirements of HCNG engines. improve with changing engine speeds and loads. When the engine speed rises, the turbulence tends to be enhanced resulting in improved Acknowledgments mixing and higher combustion temperatures, in addition to NOx emissions. On the other hand, the impact of the engine speed on the NOx The authors gratefully acknowledge the Universiti Putra Malaysia emissions is much less noticeable as compared to that on the air-fuel which provided the facilities to complete this work. percentage or even the sparks timing. So we can see that as the engine tends to lean operation, NOx formation expands quickly until it reaches References its highest value, and afterwards it diminishes to a comparatively low value. 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