A Review of Compressed Natural Gas as a Viable Alternative Fuel for Passenger Vehicle, Spark Ignition, Four-Stroke Engines

Transcription

1 A Review of Compressed Natural Gas as a Viable Alternative Fuel for Passenger Vehicle, Spark Ignition, Four-Stroke Engines by Kevin DeVos A Project Submitted to the Graduate Faculty of Rensselaer Polytechnic Institute in Partial Fulfillment of the Requirements for the degree of MASTER OF ENGINEERING Major Subject: MECHANICAL ENGINEERING Approved: Professor Ernesto Gutierrez-Miravete, Project Adviser Rensselaer Polytechnic Institute Hartford, Connecticut December, 2014

2 Copyright 2014 by Kevin DeVos All Rights Reserved ii

3 CONTENTS LIST OF TABLES... v LIST OF FIGURES... vi DEFINITIONS... vii ACRONYMS... viii NOMENCLATURE... ix KEYWORDS... x ACKNOWLEDGMENT... xi ABSTRACT... xii 1. Introduction Theory Suitability of Fuels Natural Gas Background Combustion Process Compression Ratio Air/Fuel Ratio Flame Speed and Combustion Chamber Turbulence Fuel Injection Ignition Timing Engine Temperature Methodology Previous Research Emissions of CNG Well to Tank Emissions Tank to Wheel Emissions Review of Consumer Ownership Summary and Discussion iii

4 4.1.1 Emissions Results Practical Realities of Natural Gas Vehicles Initial Cost Fuel Storage and Range Fuel Cost Fueling Stations Maintenance Fuel Safety Overall Vehicle Costs Conclusion References Works Cited Additional References Consulted Appendices iv

5 LIST OF TABLES Table 1: Vehicle Well to Wheels Emissions Table 2: Purchase price comparison of gasoline and CNG fueled vehicles based on a survey of Connecticut automobile dealers Table 3: Vehicle Ownership Costs v

6 LIST OF FIGURES Figure 1: A CNG vehicle and refueling station Figure 2: The four cycles of the Otto Cycle Figure 3: Hydrocarbon strings Figure 4: Thermodynamic cycle Figure 5: Emissions at Different Equivalence Ratios Figure 6: Vehicle Emissions Figure 7: CNG fuel tanks in a CNG vehicle Figure 8: Honda Civic Ownership Cost Figure 9: Ford Transit Ownership Cost Figure 10: Ford F-250 Ownership Cost Figure 11: GMC Sierra Ownership Cost vi

7 DEFINITIONS Breakeven Point Equivalence ratio (φ) Internal Combustion Engine Knock The point at which an option with a higher purchase price and lower operating costs achieves parity with the lower priced option. The ratio of the actual fuel/air ratio with the stoichiometric fuel/air ratio. An engine in which fuel is burned inside an enclosed volume and where the combusted fuel directly acts upon a piston in order to produce mechanical work. The autoignition of a portion of the air/fuel charge in front of the flame front created by the spark plug. Knock generally occurs at high cylinder temperatures and pressures. Each fuel has a characteristic resistance to knock which is described by its research octane number. Octane Number The measure of a fuel s resistance to preignition. A higher octane rating indicates that a fuel may be compressed to greater pressures before igniting. Preignition Stoichiometric ratio The autoignition of the air/fuel charge in a spark ignition engine before the spark plug fires. Preignition is often also known as dieseling because of its similarity to the operation of compression ignition (diesel) engines. The ratio of air to fuel which theoretically results in complete combustion with no leftover reactants. vii

8 ACRONYMS The following is a list of acronyms and abbreviations that are used throughout this paper. Acronym CNG HC ICE NO x Definition Compressed Natural Gas Hydrocarbons Internal Combustion Engine Nitrogen Oxides viii

9 NOMENCLATURE The following is a list of nomenclature used throughout this paper: Symbol Description Unit CH 4 Chemical formula of methane moles CO Chemical formula of carbon monoxide moles CO 2 Chemical formula of carbon dioxide moles H 2 O Chemical formula of water moles N 2 Chemical formula of diatomic nitrogen moles O 2 Chemical formula of diatomic oxygen moles ix

10 KEYWORDS Natural Gas, Compressed Natural Gas, CNG, Gasoline, Alternative Fuels, Automobiles, Engines, Combustion, Emissions, Well-to-Wheels, Carbon Dioxide, NOx, Greenhouse Gases, Operating Costs x

11 ACKNOWLEDGMENT I would like to thank my parents for their support whenever I encountered setbacks, my friends for their presence during our mutual suffering, and my girlfriend for her patience and understanding whenever I would disappear to go work on my project. xi

12 ABSTRACT This project reviews previous research to assess compressed natural gas (CNG) fueled vehicles and evaluate their suitability as alternatives to traditional gasoline-powered passenger vehicles. First, the combustion process and overall efficiency of a CNG fourstroke engine is discussed and compared to a typical gasoline four-stroke engine. Environmental benefits are explored, including reduced vehicle particle emissions and greenhouse gas emissions. Finally, practical concerns of using CNG as a fuel source, such as access to fueling stations and dangers of vehicular accidents, are presented. xii

13 1. Introduction Starting during the industrial revolution in the late 1700s, the chemical energy in various fuel sources has been harnessed to produce motion. The invention of the combustion engine, first developed in the form of a steam engine, allowed vehicles to be propelled by stored onboard fuel instead of by an external mechanism, such as being pulled by a horse. The internal combustion engine was a further improvement on this principal. It increased the efficiency and greatly decreased the required size of the engine for a given power output. These engines were generally small enough that they could be attached to wheeled vehicles and used as the basis of practical transportation. These early contraptions would quickly evolve to become the automobiles that we know today. There are approximately 1 billion passenger vehicles in the world and about 25% of those are in the United States [1]. The vast majority of these vehicles are spark ignition, gasoline-fueled vehicles. Recently, environmental and political pressures have highlighted the need for alternative fuels that are more readily available and the combustion of which causes less environmental pollution. Specifically, the corporate average fuel economy (CAFE) standards that are being imposed upon manufacturers of automobiles are forcing them to greatly increase the fuel economy of their vehicle fleets. Compressed natural gas (CNG) has emerged as a possible vehicle fuel due to its relative abundance and cleaner burning properties. This project reviews prior work and investigates the viability of using CNG for personal vehicles. This will include a comparison of vehicle emissions with conventional automobiles and a discussion of real-world issues involved in operating an alternative fuel vehicle. 1

14 Figure 1: A CNG vehicle and refueling station [2] 1.1 Theory The Otto cycle is the basis of the four cycle spark ignition engine. Within each cylinder of a four cycle engine, a piston sliding inside the cylinder draws fuel in, ignites it, harnesses the power of the fuel as it burns, and then exhausts it from the cylinder. Each of the four cycles correspond to a single stroke of the piston and can be described as inlet, compression, combustion (or power), and exhaust and are illustrated in Figure 2 below. At the start of the inlet stroke, the piston is at the top of the cylinder. It slides down the cylinder and at the same time, a valve in the top of the cylinder opens, allowing a charge of air and fuel to enter. Once the piston reaches the bottom of its travel, the valve closes. The piston then slides back up the cylinder bore, compressing the air and fuel mixture. When the piston nears the top of its stroke, the air and fuel mixture is ignited by a spark plug. The air/fuel mixture burns, which causes it to expand. The expanding gas mixture then drives the piston downward. When the combustion process has been completed and the piston reaches the bottom of its travel, 2

15 an exhaust valve opens. The piston moves up, pushing the exhaust gases out of the cylinder. The cycle is then repeated with a new charge of air and fuel. Figure 2: The four cycles of the Otto Cycle [3] 1.2 Suitability of Fuels The cycle described above is not specific to a type of fuel. Many different materials can be used in such an engine to produce power. Due to the practicalities of producing, storing, transporting, and selling fuels, some types of fuels are favored for commercial use. The attributes of a good fuel include being inexpensive, energy dense, stable at room temperature, abundant, and clean burning. One family of fuels stands out as having many of these properties: saturated hydrocarbons. Carbon atoms can form a total of four bonds with other atoms. Saturated 3

16 hydrocarbons are molecules composed of strings of carbon atoms with all open bond sites being taken by hydrogen atoms. The simplest hydrocarbon is methane. It consists of one carbon atom bonded to four hydrogen atoms, designated CH 4. More complex molecules can be formed when carbon atoms bond to each other as well as hydrogen atoms. When two carbon atoms bond to each other and then each bonds with three hydrogen atoms, ethane (C 2 H 6 ) is formed. Longer strings such as propane (C 3 H 8 ), butane (C 4 H 10 ), and octane (C 8 H 18 ), are formed when more carbon atoms bond together. See Figure 3 below. The gasoline that is used by most passenger vehicles is a blend of hydrocarbons, usually ranging from four carbon strings (C 4 H 10 ) to twelve carbon strings (C 12 H 26 ). Figure 3: Hydrocarbon strings Energy is stored in the chemical bonds between atoms and can be released to produce work through the chemical reaction of oxidation. Oxidation is normally accomplished by means of combustion Natural Gas Natural gas is, as its name implies, a mixture of volatile gases that occur naturally. Natural gas is approximately 92% methane. The remaining 8% is made up of higher level hydrocarbons as well as some contaminants [4]. Natural gas is formed from the decomposition of plant and animal matter. Small organisms, called methanogens, break down organic matter and emit methane as a byproduct. Deposits of natural gas are found underground, commonly in proximity to other petroleum products. Natural gas is extracted by drilling down to deposits and capturing the gas released. The recent boom 4

17 in natural gas production is a result of two new extraction technologies: hydraulic fracturing (fracking) and horizontal drilling. Hydraulic fracturing is the process of pumping a mixture of water and other chemicals down into a natural gas deposit in order to displace the gas and force it to the surface. Horizontal drilling is a refinement on traditional drilling methods that allows the drill shaft to travel underground horizontally. These two technologies have recently made it economically feasible to access large quantities of domestic natural gas which were previously too difficult to mine. 5

18 2. Background 2.1 Combustion Process Before being able to compare various engine designs, we must first gain a basic understanding of the parameters that describe engine performance. The basic thermodynamic cycle for the Otto cycle is shown in Figure 4 below. It is composed of two adiabatic/isentropic steps (compression and expansion) and two isochoric processes (combustion and exhaust). Examination of the figure, shows many of the basic principles pursued in engine research and design. For example, it can be seen that the net work is a function of the change in volume and that the greater the compression ratio, the larger the amount of work that can theoretically be achieved. Figure 4: Thermodynamic cycle [5] In the combustion process which is used by automobile internal combustion engines, the hydrocarbons undergo oxidation. The hydrocarbons react with oxygen forming carbon dioxide (CO 2 ) and water (H 2 0). The chemical reaction formula for complete combustion can be found in [4] and is reproduced below: CH4 + 2 O2 CO2 + 2 H2O + energy 6

19 However, the combustion process for automobiles does not occur with pure oxygen but with air. That means that there is a large portion of the incoming charge which is composed of nitrogen. The above equation should then be rewritten as follows [4]: CH4 + 2O N 2 CO2 + 2H2O N 2 + energy The nitrogen is largely nonreactive in the combustion process. However, at temperatures above 1,300 C (2,370 F) the nitrogen will begin to form compounds with oxygen [6]. These compounds are collectively known as nitrogen oxides, often shortened to NO x. The combustion process primarily forms nitric oxide (NO). The introduction of NO x deviates from the stoichiometric equation. Because the amount of nitric oxide is dependent on the temperature at which the reaction occurs and the air fuel ratio, it cannot be analytically predicted like the quantities of products in the stoichiometric equation above. Nitric oxide and other nitrogen oxides are responsible for much of the smog production in cities. The equation above assumes that the reactants of air and fuel are present in exactly the ratio needed to complete the oxidation of methane. This ratio is rarely achieved in practice. The air/fuel ratio can affect the parameters of combustion such as combustion temperature, resultant emissions, and, at extreme ratios, the drivability of an engine. An overabundance of fuel for the amount of oxygen present is known as a rich mixture or running rich. The opposite situation, excess oxygen present for the amount of fuel, is a lean mixture or lean operation. For automotive applications this air/fuel ratio is often manipulated to allow the engine to produce more power or better fuel economy as the situation and load on the engine fluctuates. Most current research focuses on lean operation as it is usually associated with better fuel economy with only slightly reduced power. Lean operation increases the combustion temperature and therefore the levels of some emissions. The ultimate goal of engine design is to improve engine efficiency, and thereby fuel economy, while reducing both harmful emissions to the environment and purchase cost, 7

20 all while not sacrificing drivability or durability. As such, these goals can be at odds with each other. These characteristics are all interdependent, so for each characteristic which is improved upon, another may be adversely affected. The main engine parameters which are varied to affect the above characteristics are operating temperature, compression ratio, air/fuel ratio, combustion chamber turbulence, indirect vs. direct injection, and engine ignition timing Compression Ratio It is known that the thermal efficiency of an engine increases with the compression ratio. As previously discussed, increasing the compression ratio allows the pressure in the cylinder to be higher and therefore more work to be done by the expanding exhaust gases. Zheng, in [7], experimentally determined that increasing the compression ratio of a CNG fueled engine from the ~9:1 typical of gasoline powered engines to ~12:1 allowed for increased efficiency without significantly increasing unwanted emissions Air/Fuel Ratio The ratio of air to fuel is a measure of the mass of air in the cylinder compared to the mass of fuel. The ratio for gasoline which will theoretically result in complete combustion is approximately 14.7 to 1. Natural gas requires a larger mass of air for the stoichiometric ratio at 17.2 to 1. However, for most situations, the optimal air/fuel ratio is not the stoichiometric ratio. Running rich will generally result in slight power gains and lower exhaust temperatures. However, fuel consumption will increase, as will unburned hydrocarbon and carbon monoxide emissions. Running lean will generally result in less power, higher exhaust temperatures, and increased NO x production. The emission of HC and CO will be reduced [4]. In addition to higher compression ratios, Haeng Muk Cho [8] discusses how operating natural gas engines at an air/fuel ratio leaner than stoichiometric can result in acceptable power with lower emissions. 8

21 Figure 5: Emissions at Different Equivalence Ratios [9] Flame Speed and Combustion Chamber Turbulence The speed at which the fuel charge in a cylinder is burned plays a role on the resulting products of combustion. A slow burn results in lower peak cylinder temperatures [10]. This is of primary concern because lower cylinder temperatures mean lower peak pressures and less resultant work done on the cylinder piston. In addition, slower burn times mean that complete combustion may not occur before the mixture is exhausted from the cylinder. This results in wasted fuel and unburned hydrocarbon emissions. The speed of combustion is dependent on the fuel chosen, the air/fuel ratio, and the gas mixing which occurs in the cylinder. If natural gas is used as fuel, increasing the turbulence allows for a wider range of air/fuel ratios to be used, specifically compensating for poor combustion during lean operation [8]. 9

22 2.1.4 Fuel Injection There are two main methods of introducing fuel into the combustion chamber: indirect and direct injection. Indirect injection mixes the fuel and air in the intake manifold before the intake valve opens. Once the intake valve opens, the premixed charge enters the cylinder together. For direct injection, the air passes through the intake manifold and enters the cylinder without being mixed with the fuel. Once the air has entered the cylinder, fuel is injected directly into the cylinder and mixes with the air. The type of fuel injection is especially important for gaseous fuels when being used in naturally aspirated engines. Normally aspirated engines can pull a fixed amount of gaseous volume into the cylinder with every intake stroke. With indirect injection, the fuel comprises a portion of that volume. Liquid fuels do not significantly change the volume of air in the intake charge. Gaseous fuels, on the other hand, do take up a large volume of the intake charge. For a fixed intake volume, decreasing the amount of air in the intake charge effectively decreases the overall mass of the intake charge. This means that there is less energy in a given intake and thus less energy will be released with the combustion event. This decrease in volumetric efficiency can be overcome by either forced induction (such as turbo or supercharging) or by direct injection of the fuel. Without using one or both of these methods, natural gas vehicles suffer a reduction in power as discussed in [4] Ignition Timing Ignition timing refers to the point in the cycle at which the spark plug fires. It is most often given as a degree measurement comparing the crankshaft s rotational angle to its angle when the piston is at the top of the compression stroke. Common ignition timings are 10 before top dead center (BTDC) [8]. This allows the fuel/air mixture to combust and reach peak cylinder pressure at approximately 10 after top dead center (ATDC). This allows the expansion stroke to do the maximum amount of useful work. The ignition timing is varied to account for changes in fuel, air/fuel mixture, and load. Increasing the spark advance allows slower burning fuels more time to complete combustion, thereby decreasing unburned fuel. Spark advance greater than required 10

23 increases the likelihood of knock as cylinder temperatures are increased before compression is completed. Continued operation with knocking can cause permanent engine damage Engine Temperature The temperature at which an engine operates can affect the output power as well as the resultant emissions. It can also cause wear or failure of engine parts if the temperature increases above certain material limits. The temperature is affected by the air/fuel ratio, the ignition timing, the compression ratio, and the type of fuel. High temperatures are generally the result of leaner air/fuel mixtures, higher compression ratios, and earlier ignition timings. Each of the above parameters has a general effect on engine performance. Each parameter affects the others. For example, lean operation and high compression ratios can result in high combustion temperatures. That leads to the formation of large amounts of NO x as discussed in Reference [4]. 11

24 3. Methodology 3.1 Previous Research A review of previously published data was performed. The results were analyzed to highlight trends. When being used as an internal combustion engine fuel, natural gas has a number of advantages over gasoline. Natural gas has an octane number of 130 compared to octane numbers of about for gasoline. This allows for greater compression ratios as described in [4]. This allows for more work to be extracted and greater efficiencies. CNG combustion results in less carbon dioxide and greatly reduces the amount of non-methane hydrocarbon emissions. CO 2 emissions are 20% lower for CNG engines compared to equivalent gasoline engines [11]. Unfortunately, a number of factors conspire to reduce the theoretical efficiency gains and even result in lower power. The gaseous nature of the fuel introduces additional pumping losses and reduced volumetric efficiency. The fuel either takes up a greater percentage of the intake charge volume, thereby reducing the overall amount of fuel to be burned, or requires additional work to be added in order to compress the gas for direct injection. This yields an engine with less overall power than a comparable displacement gasoline engine [12, 13]. CNG burns slower than gasoline which results in more unburned fuel. Additionally, it does not provide the cooling effect that gasoline does when liquid fuel is vaporized in the combustion chamber [13]. The higher compression ratios at which CNG engines operate generate higher temperatures. This results in the emissions being dominated by the production of NO x [6]. Exhaust gas recirculation is the traditional method to cool the air/fuel charge in order to reduce NO x. It has the added benefit of reducing the amount of unburnt fuel in the exhaust. Reference [10] discusses one other approach to keeping the NO x production at low levels: the addition of supplementary hydrogen as a secondary fuel. 12

25 Perhaps the best avenue for further development of CNG engines is in lean and ultra-lean burn technology. In [14, 15] high compression, lean burn, and EGR were used to get performance comparable to gasoline engines from CNG engines at significantly reduced emission levels. 3.2 Emissions of CNG Well to Tank Emissions Often, the focus of vehicle emissions is solely on the emissions coming from the operation of the vehicle. While these emissions do normally dominate the total lifecycle emissions, the emissions from fuel extraction and transport are non-negligible. The traditional extraction process is typically around 99% efficient which means that approximately 1% of all natural gas produced is vented to the atmosphere [16]. In addition, hydraulic fracturing increases these emissions due to the need for hydraulic pumps. The motors which run these pumps are typically driven by gasoline engines which use as much as 4% of the retrieved energy content. Once extracted, the natural gas is transported via pipeline to refueling stations. CNG is transported at pressures ranging from psig. The pumps needed to pressurize the pipelines consume an additional energy quantity of approximately 4.5% of the total transported energy. Finally, once at the refueling station, the gas must be pressurized to approximately 4000 psig so that it can refuel vehicles with onboard storage of 3600 psig. This compression takes about 8% of the total energy content and emits it to the atmosphere Tank to Wheel Emissions The reaction products are effectively byproducts that must be created in order to get the product that we seek: namely energy in the form of heat. The oxidation reaction for hydrocarbons necessarily results in carbon dioxide and water vapor. In addition, most engines produce nitrogen oxides (NO x ), carbon monoxide (CO), and unburned hydrocarbons (HC) as unintended byproducts. 13

26 Figure 6: Vehicle Emissions [17] Carbon dioxide (CO 2 ) is a problematic emission. As shown in Figure 6 above, CO 2 emissions from vehicles have been increasing in most areas of the world. Research has shown that carbon dioxide is one of a number of gases responsible for the greenhouse effect which is believed to be responsible for climate change. However, the entire premise of using hydrocarbon combustion for energy use is based on breaking apart the bonds between carbon and hydrogen and forming carbon and hydrogen oxides. For the combustion of hydrocarbons, the creation of carbon dioxide cannot be prevented, but the ratio of carbon dioxide per amount of energy released can be minimized. By choosing fuels with the largest number of hydrogen atoms per each carbon atom, more energy can be released per molecule of carbon dioxide produced. As mentioned above, methane is the optimal hydrocarbon fuel in this regard because each carbon has formed bonds with 14

27 four hydrogen atoms. This is the maximum number of hydrogen bonds per carbon atom that carbon s chemical structure will allow. Water vapor is formed in large quantities from the combustion of hydrocarbons. Water vapor is classified as a greenhouse gas and as such can be linked to climate change. However, since the amount of water vapor in the atmosphere is largely determined by the vast quantity of water in the world s oceans, adding additional water vapor via combustion does not affect the overall concentration in the atmosphere. The concentration is regulated by evaporation and precipitation as part of the natural water cycle. The amount of unintended byproducts of NO x, CO, and HC are heavily dependent on the engine design, engine temperature, and operating air/fuel ratio. Since these byproducts are not necessary for achieving the desired result of combustion (converting the chemical energy of the fuel to kinetic energy) and they generally have a negative impact when released to the environment, they are typically the focus of research aiming to reduce emissions. Nitric oxide reacts with sunlight to form ozone and contributes to the smog found in many large cities [6]. It impacts the air quality and can exacerbate breathing difficulties. NO x is formed when combustion occurs at high temperatures and the nitrogen gas, N 2, present in the air dissociates and bonds with oxygen. CNG engines typically form more NO x than gasoline engines when operated at the same equivalence ratio. To combat this, CNG engines are run at leaner equivalence ratios. This results in acceptable NO x levels but slightly reduced power. Carbon monoxide is highly toxic to humans. CO forms when there is insufficient oxygen present for complete combustion and so forms predominantly at rich air/fuel ratios [18]. CNG engines are run at lean conditions to keep combustion temperatures low and therefore, have very low emissions of CO. Unburnt hydrocarbons result in two negative impacts. They represent fuel which is wasted and is not contributing to goal of extracting energy. They also often have adverse impacts on the environment when released. High HC emissions occur at rich 15

28 and very lean air/fuel ratios. At very lean ratios, the flame speed of natural gas is too low to support complete combustion in the short time required by the movement of the piston. At rich ratios, insufficient oxygen is present to allow complete combustion. CNG engines are operated at lean conditions so the main consideration is to ensure that combustion completes in the appropriate time period. Piston and combustion chamber designs which cause highly turbulent flow increase the flame speed and ensure that combustion is completed in the required time period. 3.3 Review of Consumer Ownership For most consumers, the choice of a vehicle is dependent on appearance and status, operating experience, and ownership cost. CNG engines can be fitted to automobiles of every type with no impact on the appearance of the vehicle. In this respect, CNG vehicles are indistinguishable from their gasoline powered brethren. The operating experience of a vehicle is a combination of vehicle performance (acceleration, fuel economy, range) and the actions needed to keep the vehicle operational (refueling, servicing). The effect fuel type has on these attributes will be discussed in the results section below. The ownership cost of a vehicle is influenced by several factors: initial purchase cost, operating costs such as fuel and administrative fees, and maintenance. As part of the evaluation of using CNG as an alternative fuel, the cost of ownership will be compared with a conventional gasoline powered vehicle. For the purposes of the evaluation of CNG fueled vehicles, it will be assumed that only costs stemming directly from the fuel choice change as a result of the power source. Administrative costs such as vehicle insurance and registration are applicable to all vehicle types. Maintenance costs are somewhat affected by the fuel types used but each fuel type has some benefits and some drawbacks, so general trends are difficult to predict. Natural gas burns cleaner than gasoline reducing the amount of wear-causing particles on moving parts. The higher compression ratios in CNG engines would tend to increase the stresses on the engine, increasing wear. Due to the ambiguous trends and the fact that the differences in 16

29 maintenance costs are generally not significant, it was assumed that there is no difference between CNG and gasoline maintenance costs. Therefore, only the initial purchase price and fuel costs will be discussed. The cost of ownership of a vehicle takes into account all the costs over the lifetime of the vehicle. In order to compare the cost of ownership, the lifetime of the vehicle must first be determined. Research firm R. L. Polk tracks vehicle registrations and found that the average age of registered vehicles in 2013 was 11.4 years old [19]. The EPA assumes that the average vehicle drives 15,000 miles per year. Taking those averages as the life of a vehicle results in a total lifetime mileage of 171,000 miles. Alternative fuels are commonly sold by the gallon of gasoline equivalent (GGE). The amount of the alternative fuel is selected to contain the same energy content as one gallon of gasoline. One gallon of gasoline has approximately 114,000 BTUs of heat energy. One GGE of CNG is 5.66 lbs. which takes up about ft 3 at 3600 psig. Current production CNG vehicles are generally 5-10% less efficient than their gasoline counterparts. The national average for CNG for automotive use is $2.17/GGE while gasoline is $3.70/gallon. The fuel cost is a function of how efficient the engine is and how far the car is driven. CNG vehicles have a higher purchase price but a lower fuel cost than gasoline vehicles. After some number of miles travelled, the lower fuel cost result in a lower ownership cost for the CNG vehicle. The cost per mile can be calculated and then used in conjunction with the purchase price to determine the breakeven point. Cost per mile = Purchase Price + Fuel Cost Fuel Economy Lifetime Mileage If the breakeven point is less than the average vehicle lifetime, the CNG vehicle is the better financial choice. 17

30 4. Summary and Discussion Emissions Results Many studies have been done to attempt to quantify the amount of greenhouse gas emissions produced by various modes of transportation. Each study has its own set of assumptions which results in occasionally contradictory conclusions. The general consensus of studies is presented below in Table 1. It has been noted that the consumers shopping for CNG vehicles are often cross shopping other alternative energy vehicles. Therefore, when comparing emissions, in addition to presenting the emissions of a comparable gasoline vehicle, the emissions from an electric vehicle are presented. Table 1: Vehicle Well to Wheels Emissions [20] Emissions Gasoline CNG Electric From Fuel Production and Distribution 100 g/mile equivalent 50 g/mile equivalent 125 g/mile equivalent From Vehicle Operation 325 g/mile 250 g/mile 0 g/mile Total 425 g/mile equivalent 300 g/mile equivalent 125 g/mile equivalent CNG vehicles currently in production have been found to produce approximately 20-30% fewer greenhouse gas emissions compared to electric vehicles. Electric vehicles produce significantly fewer emissions still. However, the production of the batteries needed for electrical vehicles produces far more emissions than the production of either gasoline or CNG vehicles. Those emissions are outside of the scope of this work. 18

31 4.2 Practical Realities of Natural Gas Vehicles Initial Cost The initial purchase cost of a CNG fueled vehicle is greater than a comparable gasoline-fueled vehicle. A comparison of the prices of different vehicles is presented in Table 2 below. The initial purchase of a natural gas vehicle is very constrained compared to the broader gasoline-fueled vehicle market. The vehicles for sale are typically those sold to fleet operators rather than consumers. In the United States, there are only a handful of choices of CNG vehicles offered from automobile manufacturers. The only passenger car is the Honda Civic. There are a few larger pick-up trucks and vans such as the Ford F-250, F-350, and Transit van, Chevy Silverado 2500, GMC Sierra 2500HD, and Savana van. Even assuming that customers are shopping for one of the above vehicles, most dealers do not have them available for sale. A survey of Connecticut dealerships was met with initial confusion and then apologetic response. The vehicles must be special ordered and are such low volume sales that most dealerships are unsure how to do so. Table 2: Purchase price comparison of gasoline and CNG fueled vehicles based on a survey of Connecticut automobile dealers Gasoline Vehicle Factory CNG Vehicle Honda Civic $18,500 $26,500 Ford Transit $29,500 $44,500 Ford F-250 $42,500 $55,000 GMC Sierra 2500HD $43,000 $53,000 19

32 Converting a traditional gasoline vehicle to run on natural gas is not technically complicated. A separate fuel system can be fitted and CNG injected into the same cylinder port. However, many of the efficiency increases rely on changing the engine internals. Therefore, a complete engine redesign is required to realize the theoretical performance achievable by CNG. Additional modifications are needed for the drivetrains of natural gas vehicles. The power vs. speed characteristic, as seen in [4], shows that for any engine, there is an optimal gear ratio which should be used. Converting a gasoline powered car is considered a poor method for using CNG as a fuel Fuel Storage and Range CNG has a lower energy content per unit volume than that of gasoline. The fact that natural gas exists at stable temperature in gas form puts it at a disadvantage when it comes to storing energy. CNG must be kept in fully sealed storage containers and must be pressurized to thousands of pounds per square inch to even approach the energy density of gasoline. Figure 7: CNG fuel tanks in a CNG vehicle [21] 20

33 Most gasoline automobiles sold today have gasoline storage tank capacities of gallons. The capacity is chosen to deliver a range between 300 and 500 miles. For a CNG fueled automobile to replace a gasoline version, it must also have a range of approximately 300 miles. Assuming that the efficiency of the engine has already been optimized, the pressure and storage capacity dictate the range of the vehicle. The pressure of most CNG filling stations is about 3600 psig [22]. In order to achieve a range of 300 miles, a CNG fueled vehicle needs to have a fuel storage capacity of 6-10 cubic feet. This is about three times the volume of a comparable gasoline tank and means that there is less passenger and cargo volume as shown in Figure 7 above. The tanks required to store the CNG also need to be capable of withstanding approximately 3600 psig. In addition to withstanding the internal pressure, tanks must resist degradation due to environmental conditions (heat, water, road salt, oils) and mechanical damage in the event of an accident Fuel Cost As of July 2014, gasoline is approximately $3.70/gallon in the northeast United States. Natural gas is approximately $2.17 for 1 GGE [23] Fueling Stations CNG fueling stations require additional infrastructure compared to gasoline stations. Large compressors and their associated cooling systems are required to compress the natural gas. This makes the initial construction costs of such stations greater than gasoline stations. CNG stations also suffer from the chicken or the egg scenario. There is low demand for the stations because consumers have not embraced CNG vehicles for personal use. Consumers are hesitant to purchase CNG vehicles because there are insufficient refueling stations available. Currently, there are less than 1000 public CNG stations nationally [24], compared to over 120,000 gas stations [25]. 21

34 4.2.5 Maintenance Natural gas burns cleaner than gasoline. This means less carbon fouling in the combustion chamber. Fewer particulates will be deposited in the lubricating oil which will result in less wear between parts experiencing relative motion. Conversely, the higher compression ratios used by CNG engines result in higher stresses on the moving parts within the engine. Overall, the maintenance for CNG engines will be comparable or slightly less frequent than equivalent gasoline engines. As most dealerships do not sell CNG vehicles, they tend not to be prepared to handle the special requirements of the high pressure fuel systems in CNG automobiles. In addition, most mechanics are not familiar with the systems nor their effects on engine internal parts. This will make diagnosing problems difficult. Maintaining CNG vehicles will likely be more expensive and more of a hassle due primarily to the scarcity of qualified mechanics Fuel Safety Natural gas does have a number of safety benefits over gasoline. Since it is a gas instead of a liquid, it has high dispersal rates. Natural gas is lighter than air so it will tend to rise and disperse into the atmosphere rather than pooling on the ground like gasoline. Natural gas is non-toxic to people or animals. The only respiratory risk of natural gas is that it may displace the oxygen needed for breathing if a leak occurs in a confined space. Even then, since it is lighter than air, it will displace the oxygen at the highest point in the space first meaning that the lowest points will still have breathable air. 4.3 Overall Vehicle Costs Table 3 below details the ownership costs of a gasoline powered automobile and its CNG equivalent: 22

35 Purchase Cost Table 3: Vehicle Ownership Costs Combined MPG Fuel Cost Cost per mile Payback period (miles) Honda Civic $18, $3.70 $0.11 CNG Honda Civic $26, $2.17 $ ,928 Ford Transit $29, $3.70 $0.15 CNG Ford Transit $44, $2.17 $ ,870 Ford F-250 $42, $3.70 $0.25 CNG Ford F-250 $55, $2.17 $ ,435 GMC Sierra 2500HD CNG GMC Sierra 2500HD $43, $3.70 $0.26 $53, $2.17 $ ,885 Also presented are figures showing the ownership costs as a function of miles driven. $45, $40, $35, $30, $25, $20, $15, $10, $5, $0.00 Honda Civic Ownership Cost Miles Driven Civic Gasoline Civic CNG Figure 8: Honda Civic Ownership Cost This figure shows the overall ownership cost for the gasoline and CNG powered Honda Civics. The first data point for each vehicle corresponds to the initial purchase price of the vehicle. The slope of the lines corresponds to the operating cost of the vehicle, which is a function of the fuel cost and miles travelled. The gasoline vehicle has a lower purchase price but a higher operating cost. Over the life of the vehicle, the higher 23

36 operating cost of the gasoline vehicle brings the overall ownership cost closer to the CNG vehicle, but still maintains a slight margin at the end of vehicle life (171,000 miles). At the end of vehicle life, it is cheaper to have purchased the gasoline powered car, despite the higher price of fuel. $70,000 $60,000 $50,000 $40,000 $30,000 $20,000 $10,000 $0 Ford Transit Ownership Cost Miles Driven Transit Gasoline Transit CNG Figure 9: Ford Transit Ownership Cost This figure shows a similar trend of ownership costs between the gasoline and CNG powered Ford Transit vans. The large price premium for a CNG powered Transit coupled with relatively high fuel economy results in a breakeven point beyond the average vehicle life expectancy. 24

37 $90,000 $80,000 $70,000 $60,000 $50,000 $40,000 $30,000 $20,000 $10,000 $0 Ford F-250 Ownership Cost Miles Driven F-250 Gasoline F-250 CNG Figure 10: Ford F-250 Ownership Cost This figure shows a similar trend with a notable difference from the Civic and Transit above. The price premium for the initial purchase of the CNG vehicle is similar in magnitude but is a smaller percentage of the purchase price. The F-250 is also significantly less efficient than the previous two vehicle models which means that the fuel cost plays a larger role in the overall ownership costs. This results in a breakeven point before the end of the vehicle s useful life. After that point, the CNG vehicle becomes the more cost effective purchase. 25

38 $100,000 $90,000 $80,000 $70,000 $60,000 $50,000 $40,000 $30,000 $20,000 $10,000 $0 GMC Sierra Ownership Cost Miles Driven Sierra Gasoline Sierra CNG Figure 11: GMC Sierra Ownership Cost This figure makes the case for purchasing the CNG version of the Sierra most convincingly. The small price premium and large fuel bill, bring about a breakeven point approximately halfway through the vehicle s life. The CNG vehicle is the better choice financially even if the vehicle is driven relatively sparingly. 26

39 5. Conclusion CNG has many potential benefits for the adoption of CNG vehicles. That optimism is tempered slightly by the results of the emissions. The problem with CNG adoption comes with the practical realities facing the owner of a CNG vehicle. The operating costs of a CNG engine are not reduced enough to offset the greatly increased acquisition price. Somewhat intuitively, CNG makes the most sense in applications where fuel costs are the dominate cost in overall ownership. Therefore, large vehicles tend to benefit most from CNG power and have the earliest payback period. In addition, adoption of CNG as a fuel comes with a number of extra annoyances such as decreased overall range, dramatically reduced number of refueling locations, difficulty and reduced selection when purchasing the vehicle, and similar difficulty when servicing the vehicle. CNG vehicles have begun operation in commercial fleets as municipal vehicles and for larger vehicles such as transit buses, school buses, and garbage trucks. In larger fleet applications, the lower fuel costs can offset the initial purchase price due to bulk purchases and shared refueling and maintenance facilities. For the individual consumer, CNG vehicles have not achieved the required market penetration to make financial sense at this time. In time, the production of CNG vehicles may develop to the point where the price premium over gasoline vehicles is reduced. The continued development of lean burn technologies will increase the efficiency and range of vehicles. Together with the reduced emission potential of CNG vehicles, gasoline vehicles may soon have even more competition. 27

40 6. References 6.1 Works Cited [1] Ward s Automotive Group, 2011, Vehicles in Operation by Country, Penton Media Inc. [2] Voelcker, J., 2011, 2012 Honda Civic Natural Gas Sedan Priced at $26,905, Cleanest Combustion Car Sold, from date last accessed 11/28/2014. [3] Federal Aviation Administration, 2014, Helicopter Engine Systems, from &preview=true, date last accessed 11/28/2014. [4] Korakianitis, T., Namasivayam, A. M. and Crookes, R. J, 2011, Natural-gas fueled spark-ignition (SI) and compression ignition (CI) engine performance and emissions, Progress in Energy and Combustion Science, 37. [5] National Aeuronautics and Space Administration, Benson, T., 2014, Ideal Otto Cycle, from date last accessed 11/28/2014. [6] Clean Air Technology Center, 1999, Nitrogen Oxides (NOx), Why and How They Are Controlled, EPA-456/F R, Office of Air Quality Planning and Standards U.S. Environmental Protection Agency, Research Triangle Park, North Carolina [7] Zheng, J. J., Wang, J. H., Wang, B., and Huang, Z. H., 2009, Effect of the compression ratio on the performance and combustion of a natural-gas directinjection engine, IMechE Part D: J. Automobile Engineering, vol. 223, pp [8] Haeng Muk Cho, Bang-Quan He, 2007, Spark ignition natural gas engines A review, Energy Conversion and Management, vol. 48, pp

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42 [19] IHS Inc. and POLK, 2013, Polk Finds Average Age of Light Vehicles Continues to Rise, from _average_age_of_light_vehicles_continues_to_rise, date last accessed 11/28/2014. [20] TIAX LLC, 2007, Full Fuel Cycle Assessment: Well-to-Wheels Energy Inputs, Emissions, and Water Impacts, California Energy Commission consultant report CEC F, Cupertino, California. [21] Green Car Congress, 2012, 3M and Chesapeake Energy Corporation partner to develop and market new CNG tank technology for natural gas vehicles, from date last accessed 11/28/2014. [22] Gambone, L., 2005, CNG Cylinders 101, Powertech, Lawrenceville, GA. [23] Alternative Fuels Data Center, 2014, Fuel Prices, from date last accessed 11/28/2014. [24] Alternative Fuels Data Center, 2014, Natural Gas Fueling Station Locations, from date last accessed 11/28/2014. [25] U. S. Census Bureau, 2014, Gas Station Statistics, from date last accessed 11/28/ Additional References Consulted [1] Elgowainy, A., Rousseau, A., Wang, M., Ruth, M., Andress, D., Ward, J., Joseck, F., Nguyen, T. and Das, S., 2013, Cost of ownership and well-to-wheels carbon emissions/oil use of alternative fuels and advanced light-duty vehicle technologies, Energy for Sustainable Development, vol. 17, pp [2] Johnson, C., 2010, Business Case for Compressed Natural Gas in Municipal Fleets, National Renewable Energy Laboratory NREL/TP-7A

43 [3] Norton, Robert L., 2004, Design of Machinery An Introduction to the Synthesis and Analysis of Mechanisms and Machines. 3rd ed. McGraw Hill, Boston, MA. [4] Ophardt, C. E., 2003, Combustion of Fossil Fuels, from date last accessed 11/28/2014. [5] Varde, K. S., 2003, Fueling System Control and Exhaust Emissions from Natural Gas Fueled Engines, Journal of Scientific & Industrial Research, vol. 62, pp

44 7. Appendices Screen capture of ownership cost calculation 32