Research and development of natural-gas fueled engines in Iran
Introduction
Global warming and energy crisis are among the most important issues that threaten the peaceful existence of the mankind. Their adverse effects have been much more obvious during the past century and no certain solution is introduced to confine their deleterious effects on the planet. There are three major forms of fossil fuels: coal, oil and natural gas (NG). During the nineteenth and early twentieth century coal was the only used fuel but was gradually replaced by oil right after World War II. The application of natural gas in the world goes back to the middle of twentieth century when, the share of natural gas has been progressively increased within the energy market. These three fossil fuels account for more than 85% of the world's primary energy as shown in Fig. 1 [1]. The consumption of fossil fuel is continuously rising due to the population growth as well as development of industries and transportation facilities. The world's total energy consumption has increased by about 38% over the last decade. In addition, global energy consumption grew by 2.5% in 2011 compared with 2010. These energy trends can be seen in Fig. 2. Increased energy demand requires increased fuel production, draining current fossil fuel reserve levels at a faster rate [2]. Fig. 3 depicts the world primary energy demand by fuel from 1980 to 2035 years. It is clear that gas overtakes coal before 2030 and meets one quarter of global energy demand by 2035. In addition, through 2030, gas demand grows by 2% per year, compared with just 1.2% per year for total energy [3]. According to the International Energy Outlook 2013 natural gas is the fastest-growing primary energy source in the future. Natural gas consumption is forecasted to be double between 2020 and 2040, with the most robust growth in demand expected among the developing nations [4].
Increasing fossil fuel prices and their deterioration to environment have led to the search for alternative fuels since past several decades. Natural gas is one of such fuels available in large quantities in many parts of world at attractive prices. Natural gas is expected to be the promising fuel for many countries in the future because it is a cleaner fuel than oil or coal and not as controversial as nuclear power. Natural gas combustion is clean and emits less CO2 compared to other fossil fuels, which makes it favorable for utilization in internal combustion engines (ICEs). Natural gas is used across all sectors including in industrial, residential, electricity generation, commercial, and transportation sectors. Natural gas vehicles (NGVs) are widely used in the Asia-Pacific region (especially Iran), Latin America, Europe, and North America due to increase in gasoline prices.
It was found that exhaust emissions of NGVs are much lower compared with gasoline or diesel engines. Natural gas can be stored in compressed form known as compressed natural gas (CNG), or at a low temperature (111 K) in liquid form, known as liquefied natural gas (LNG). It is unfeasible to use natural gas as a transportation fuel at ambient temperatures and pressures. Therefore, it must be either compressed or liquefied to increase its volumetric energy density [5].
The gigantic issues on energy depletion and environment deterioration brought by the vehicle industry are more remarkable in Iran, and they already have been the obstacles of the sustainable development of Iran. Since the mid of 1990s, Iranian vehicle industry has been developed greatly, and the continuous increase of vehicles led to the increases need for more petroleum. Meanwhile, the massive air emissions emitted from the vehicles are deteriorating Iran environment and changing the climate. For solving the urgent issues, searching environmental benign, plenteous supply, cost effective and reliable fuel for vehicles and utilizing the fuel in vehicles have become one utmost urgent task for Iran government. Among many and various alternative fuels, natural gas is the fuel which can satisfy the requirements well. As will be mentioned later, Iran has very rich fossil energy resources. Iran has the world's second largest reserves of natural gas and is one of the leading countries in utilization of natural gas in internal combustion engines. CNG projects in Iran were started in 1975 with the conversion of 1200 taxis and private passenger cars in Shiraz city as a pilot project. Since that time, CNG vehicles have achieved an appreciable share in the vehicle stock of Iran [6].
There have been some previous reviews regarding the natural gas application in internal combustion engines; however none of them extensively discussed and analyzed natural gas fuel and its application in internal combustion engines in Iran [7], [8], [9], [10], [11]. Due to the distinguished characteristics of natural gas, abundance of natural gas fuel and NGVs in Iran, investigating the performance and emissions characteristics of natural gas fueled engines become one of the utmost important research directions for Iranian researchers. The present paper reviews the background, prospects and challenges of natural gas fuel and natural gas fueled vehicles in Iran along with progress of the research works on natural gas fueled engines in Iran, and reports most of the achievements obtained by Iranian researchers during the process in order to make them known to the world. In the first part of the paper, the role of natural gas fuel in the world and Iran energy market as an alternative fuel for NGVs is discussed and then a summary of research works on natural gas implication in SI, CI, dual fuel and HCCI engines in Iran is presented. The results of this study provide a comprehensive insight for future researchers in Iran to address the oncoming research challenges regarding natural gas fueled ICEs.
Section snippets
Natural gas as a vehicular fuel
Natural gas is a hydrocarbon gas mixture consisting primarily of methane, with other hydrocarbons, carbon dioxide, nitrogen and hydrogen sulfide. A detailed typical composition of natural gas in Iran is given in Fig. 4 [12]. Table 1 lists the properties of natural gas, gasoline and diesel fuels [13]. Typical combustion properties of natural gas are given in Table 2 [14].
A natural gas vehicle is an alternative fuel vehicle that utilizes CNG or LNG as an environmentally friendly alternative fuel
Natural gas vehicles in Iran
Iran is located in southwest of Asia with about 1648,195 km2 area. The population of Iran has increased from 61.83 million in 1998 to 75.14 million in 2012 [23]. Iranian energy sector is largely depends on the crude oil and natural gas and about 99% of energy production in Iran is from oil & gas (Fig. 7) [24], [25]. Iran is ranked 10th among the most polluted countries and According to Fig. 8, Iran is among the top 10 GHG emitting countries in the world [26].
As mention in previous section,
Natural gas in spark-ignition engines
The earliest research on bi-fuel engine in Iran should be traced back to the middle of 1990s, when natural gas was employed as the secondary fuel in gasoline engine. Subsequently, the curtain of bi-fuel engine has been opened on the stage in Iran. The research on the bi-fuel NGVs has made a breakthrough during the recent decade, as shown in Table 3.
Dordaei et al. [34] experimentally investigated the pollutant emissions of a SI engine with CNG fuel. They studied the effects of various parameters
Dual fuel CI operation
The earliest researches on utilization of natural gas in dual fuel engine in Iran were carried out at the end of 1980s. However, the main researches were done since the beginning of 2000s, as listed in Table 4.
Pirouzpanah and Asadi [50] used a model to predict Dual-Fuel Diesel Engine (D.F.D.E) performance, based on limited-pressure diesel cycle. The model predicts D.F.D.E performance with LPG, and CNG gases. By increasing gas proportion in dual-fuel mode, indicated power and hence indicated
Summary and conclusions
From the above review, it can be concluded that natural gas is a domestic, abundant and promising alternative fuel for SI and CI engines in Iran. Iran is one of the most polluted countries and it has the world's first largest reserves of natural gas. Thus, it can be widely used in transport section in attractive prices. Iran has the world largest NGV fleet with 3.30 million vehicles and it is one of world's fast growing countries in terms of the highest number of CNG stations.
The present review
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