A Review of Compressed Natural Gas as a Viable Alternative Fuel for
Spark Ignition, Four-Stroke Engines in Passenger Vehicles
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 Sudhangshu Bose, Project Adviser
_________________________________________
Professor Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
December, 2014
© Copyright 2014
by
Kevin DeVos
All Rights Reserved
ii
CONTENTS
LIST OF TABLES.......................................................................................................................... v
LIST OF FIGURES ....................................................................................................................... vi
DEFINITIONS ............................................................................................................................. vii
ACRONYMS .............................................................................................................................. viii
NOMENCLATURE ...................................................................................................................... ix
ACKNOWLEDGMENT ................................................................................................................ x
ABSTRACT ................................................................................................................................... 1
1. Introduction .............................................................................................................................. 2
1.1
Theory............................................................................................................................ 2
1.2
Suitability of Fuels......................................................................................................... 3
1.2.1
Chemical Properties of Natural Gas ................................................................. 4
2. Background .............................................................................................................................. 5
2.1
Combustion Process ...................................................................................................... 5
2.1.1
Compression Ratio ........................................................................................... 7
2.1.2
Air/Fuel Ratio ................................................................................................... 7
2.1.3
Flame Speed and Combustion Chamber Turbulence ....................................... 8
2.1.4
Fuel Injection .................................................................................................... 9
2.1.5
Ignition Timing ................................................................................................. 9
2.1.6
Engine Temperature ....................................................................................... 10
3. Methodology .......................................................................................................................... 11
4. Results and Discussion........................................................................................................... 17
4.1
4.2
Emissions of CNG ....................................................................................................... 12
4.1.1
Well to Tank Emissions.................................................................................. 12
4.1.2
Tank to Wheel Emissions ............................................................................... 12
4.1.3
Emissions Results ........................................................................................... 17
Practical Realities of Natural Gas Vehicles ................................................................. 18
4.2.1
Initial Cost ...................................................................................................... 18
iii
4.3
4.2.2
Fuel Storage and Range .................................................................................. 19
4.2.3
Fuel Cost ......................................................................................................... 19
4.2.4
Fueling Stations .............................................................................................. 20
4.2.5
Maintenance.................................................................................................... 20
4.2.6
Fuel Safety ...................................................................................................... 20
Overall Vehicle Costs .................................................................................................. 21
5. Conclusion ............................................................................................................................. 25
6. References .............................................................................................................................. 26
6.1
Works Cited ................................................................................................................. 26
6.2
Additional References Consulted ................................................................................ 27
7. Appendices ............................................................................................................................. 28
iv
LIST OF TABLES
Table X: Purchase Price Comparison of Gasoline and CNG fueled vehicles
Table XX: Overall Vehicle Ownership Cost
v
LIST OF FIGURES
Figure 1:
vi
DEFINITIONS
Internal Combustion
Engine
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.
Stoichiometric ratio
The ratio of air to fuel which theoretically results in
complete combustion with no leftover reactants.
Equivalence ratio (φ)
The ratio of the actual fuel/air ratio with the stoichiometric
fuel/air ratio.
Knock
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.
Preignition
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.
Octane Number
vii
ACRONYMS
The following is a list of acronyms and abbreviations that are used throughout this paper.
Acronym
Definition
CNG
Compressed Natural Gas
ICE
Internal Combustion Engine
BMEP
Brake Mean Effective Pressure
viii
NOMENCLATURE
The following is a list of nomenclature used throughout this paper:
Symbol
Description
ix
Unit
ACKNOWLEDGMENT
I would like to thank
x
ABSTRACT
This project assesses compressed natural gas (CNG) fueled vehicles and evaluates their
suitability as alternatives to traditional gasoline-powered passenger vehicles. First, the
combustion process and overall efficiency of a CNG four-stroke 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.
1
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 [7]. 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 fuel due to its relative
abundance and cleaner burning properties.
This project will analyze 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.
1.1 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, and exhaust. At the start of the inlet stroke, the piston is
2
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, 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.
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
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 CH4. 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 (C2H6) is formed. Longer strings such as propane (C3H8),
butane (C4H10), and octane (C8H18), are formed when more carbon atoms bond together.
See Figure (1) below. The gasoline that is used by most passenger vehicles is a blend of
hydrocarbons, usually ranging from four carbon strings (C4H10) to twelve carbon strings
(C12H26).
3
Figure 1: 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.
1.2.1
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 [Korakianitis]. 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 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.
4
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 2 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 2: Thermodynamic cycle
In the combustion process which is used by automobile internal combustion engines, the
hydrocarbons undergo oxidation. The hydrocarbons react with oxygen forming carbon
dioxide (CO2) and water (H20). The chemical reaction formula for complete combustion
can be found in [8][2] and is reproduced below:
CH4 + 2 O2  CO2 + 2 H2O + energy
5
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
[Korakianitis]:
CH4 + 2O2 + 7.546N2 CO2 + 2H2O + 7.546N2 + 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. [EPA NOx] These compounds are collectively known as nitrogen oxides, often
shortened to NOx. The combustion process primarily forms nitric oxide (NO). The
introduction of NOx 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,
6
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.
2.1.1
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 Reference [6], 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.
2.1.2
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 nitrogen oxide (NOx)
production. The emission of HC and CO will be reduced. [Korakianitis] In addition to
higher compression ratios, Reference [1] discusses how operating natural gas engines at
an air/fuel ratio leaner than stoichiometric can result in acceptable power with lower
emissions.
7
Figure XX: Emissions at Different Equivalence Ratios
Figure XX: Emission trends with air/fuel ratio
2.1.3
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. 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
8
turbulence allows for a wider range of air/fuel ratios to be used, specifically
compensating for poor combustion during lean operation.[Cho]
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 [Korakianitis].
2.1.5
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). 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
9
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 increases the likelihood
of knock as cylinder temperatures are increased before compression is completed.
Continued operation with knocking can cause permanent engine damage.
2.1.6
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 NOx as discussed in Reference [3].
10
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 85-93 for gasoline. This allows for greater compression ratios as
described in [Korakianitis]. 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. CO2 emissions 20% lower for CNG engines
compared to equivalent gasoline engines [ref 7 in cho].
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. [cho, ref 11,14 in cho]
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. [ref 11 in cho] The higher compression ratios at which CNG
engines operate generate higher temperatures. This results in the emissions being
dominated by the production of NOx. [EPA NOx] Exhaust gas recirculation is the
traditional method to cool the air/fuel charge in order to reduce NOx. It has the added
benefit of reducing the amount of unburnt fuel in the exhaust. Reference [5] discusses
one other approach to keeping the NOx production at low levels: the addition of
supplementary hydrogen as a secondary fuel.
11
Perhaps the best avenue for further development of CNG engines is in lean and
ultra-lean burn technology. In [cho 3 and cho 4] 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
3.2.1
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 [Waller]. 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 150-1500 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.
3.2.2
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 (NOx), carbon monoxide (CO), and unburned
hydrocarbons (HC) as unintended byproducts.
12
Carbon dioxide (CO2) is a problematic emission. 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 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 NOx, 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. [EPA NOx] It impacts the air quality and can exacerbate breathing
difficulties. NOx is formed when combustion occurs at high temperatures and the
nitrogen gas, N2, present in the air dissociates and bonds with oxygen. CNG engines
typically form more NOx than gasoline engines when operated at the same equivalence
13
ratio. To combat this, CNG engines are run at leaner equivalence ratios. This results in
acceptable NOx 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. [Heywood] 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
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
14
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
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 [Polk]. 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 0.565 ft3 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
15
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.
16
4. Results and Discussion
4.1.1
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
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 2030% 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.
17
4.2 Practical Realities of Natural Gas Vehicles
4.2.1
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
Gasoline Vehicle Factory CNG Vehicle
Honda Civic Cost
$18,500
$26,500
Ford Transit
$29,500
$44,500
Ford F-250
$42,500
$55,000
GMC Sierra 2500HD
$43,000
$53,000
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
18
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
[korak], 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.
4.2.2
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.
Most gasoline automobiles sold today have gasoline storage tank capacities of 12-30
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 [cng cylinder design]. 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.
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.
4.2.3
Fuel Cost
At the time of this publication, gasoline is approximately $3.70/gallon in the United
States. Natural gas is approximately $2.17 for 1 GGE.
19
4.2.4
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 [US department of energy], compared to over 120,000
gas stations. [US Census Bureau]
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.
4.2.6
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
20
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:
Table 3: Vehicle Ownership Costs
Honda Civic
CNG Honda
Civic
Ford Transit
CNG Ford
Transit
Ford F-250
CNG Ford F-250
GMC Sierra
2500HD
CNG GMC Sierra
2500HD
Payback
period
(miles)
Purchase
Cost
Combined
MPG
Fuel Cost
Cost per
mile
$18,500
33
$3.70
$0.11
$26,500
31
$2.17
$0.07
$29,500
25
$3.70
$0.15
$44,500
23.75
$2.17
$0.09
264,870
$42,500
$55,000
15
14.25
$3.70
$2.17
$0.25
$0.15
132,435
$43,000
14
$3.70
$0.26
$53,000
13.3
$2.17
$0.16
189,928
98,885
Also presented are figures showing the ownership costs as a function of miles driven.
21
Honda Civic Ownership Cost
$50,000.00
$40,000.00
$30,000.00
$20,000.00
$10,000.00
$0.00
Civic Gasoline
Civic CNG
Figure XX: 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
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 was cheaper to have purchased the gasoline powered
car, despite the higher price of fuel.
Ford Transit Ownership Cost
$70,000.00
$60,000.00
$50,000.00
$40,000.00
$30,000.00
$20,000.00
$10,000.00
$0.00
Transit Gasoline
Transit CNG
Figure XX: Ford Transit Ownership Cost
22
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.
Ford F-250 Ownership Cost
$90,000.00
$80,000.00
$70,000.00
$60,000.00
$50,000.00
$40,000.00
$30,000.00
$20,000.00
$10,000.00
$0.00
F-250 Gasoline
F-250 CNG
Figure XX: 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.
23
GMC Sierra Ownership Cost
$100,000.00
$90,000.00
$80,000.00
$70,000.00
$60,000.00
$50,000.00
$40,000.00
$30,000.00
$20,000.00
$10,000.00
$0.00
Sierra Gasoline
Sierra CNG
Figure XX: 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.
24
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.
25
6. References
6.1 Works Cited
[1] Haeng Muk Cho, Bang-Quan He. “Spark ignition natural gas engines – A review”.
Energy Conversion and Management 48, 2007.
[2] Heywood, John B. Internal Combustion Engine Fundamentals. McGraw Hill,
1988. Print.
[3] Korakianitis, T., Namasivayam, A. M., Crookes, R. J. “Natural-gas fueled sparkignition (SI) and compression ignition (CI) engine performance and
emissions”. Progress in Energy and Combustion Science 37, 2011.
[4] Norton, Robert L. Design of Machinery – An Introduction to the Synthesis and
Analysis of Mechanisms and Machines. 3rd ed. McGraw Hill, 2004. Print.
[5] Tunestal, P., Christensen, M., Einewall, P., Andersson, T., and Johansson, B.
“Hydrogen Addition For Improved Lean Burn Capability of Slow and Fast
Burning Natural Gas Combustion Chambers”. Society of Automotive
Engineers 2002-01-2686, 2002.
[6] Zheng, J. J., Wang, J. H., Wang, B., and Huang, Z. H. “Effect of the compression
ratio on the performance and combustion of a natural-gas direct-injection
engine”. IMechE: Vol. 223 Part D: J. Automobile Engineering, 2009.
[7] Ward’s Automotive Group. “Vehicles in Operation by Country”. Penton Media
Inc. 2011.
[8] http://www.elmhurst.edu/~chm/vchembook/511natgascombust.html
26
[9] Sobiesiak, A., and S. Zhang. "2003-01-3091 The First And Second Law Analysis
Of Spark Ignition Engine Fuelled With Compressed Natural Gas." Sae Sp 1809
(2003): 25-42. {CHO 3}
[10] Johnson, Caley. “Business Case for Compressed Natural Gas in Municipal
Fleets”. NREL/TP-7A2-47919. June 2010.
6.2 Additional References Consulted
27
7. Appendices
Miles driven
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
55000
60000
65000
70000
75000
80000
85000
90000
95000
100000
105000
110000
115000
120000
125000
130000
135000
140000
145000
150000
155000
160000
165000
170000
175000
180000
185000
190000
Civic
Gasoline
=$B$2+A13*$E$2
=$B$2+A14*$E$2
=$B$2+A15*$E$2
=$B$2+A16*$E$2
=$B$2+A17*$E$2
=$B$2+A18*$E$2
=$B$2+A19*$E$2
=$B$2+A20*$E$2
=$B$2+A21*$E$2
=$B$2+A22*$E$2
=$B$2+A23*$E$2
=$B$2+A24*$E$2
=$B$2+A25*$E$2
=$B$2+A26*$E$2
=$B$2+A27*$E$2
=$B$2+A28*$E$2
=$B$2+A29*$E$2
=$B$2+A30*$E$2
=$B$2+A31*$E$2
=$B$2+A32*$E$2
=$B$2+A33*$E$2
=$B$2+A34*$E$2
=$B$2+A35*$E$2
=$B$2+A36*$E$2
=$B$2+A37*$E$2
=$B$2+A38*$E$2
=$B$2+A39*$E$2
=$B$2+A40*$E$2
=$B$2+A41*$E$2
=$B$2+A42*$E$2
=$B$2+A43*$E$2
=$B$2+A44*$E$2
=$B$2+A45*$E$2
=$B$2+A46*$E$2
=$B$2+A47*$E$2
=$B$2+A48*$E$2
=$B$2+A49*$E$2
=$B$2+A50*$E$2
28
Civic
CNG
=$B$3+A13*$E$3
=$B$3+A14*$E$3
=$B$3+A15*$E$3
=$B$3+A16*$E$3
=$B$3+A17*$E$3
=$B$3+A18*$E$3
=$B$3+A19*$E$3
=$B$3+A20*$E$3
=$B$3+A21*$E$3
=$B$3+A22*$E$3
=$B$3+A23*$E$3
=$B$3+A24*$E$3
=$B$3+A25*$E$3
=$B$3+A26*$E$3
=$B$3+A27*$E$3
=$B$3+A28*$E$3
=$B$3+A29*$E$3
=$B$3+A30*$E$3
=$B$3+A31*$E$3
=$B$3+A32*$E$3
=$B$3+A33*$E$3
=$B$3+A34*$E$3
=$B$3+A35*$E$3
=$B$3+A36*$E$3
=$B$3+A37*$E$3
=$B$3+A38*$E$3
=$B$3+A39*$E$3
=$B$3+A40*$E$3
=$B$3+A41*$E$3
=$B$3+A42*$E$3
=$B$3+A43*$E$3
=$B$3+A44*$E$3
=$B$3+A45*$E$3
=$B$3+A46*$E$3
=$B$3+A47*$E$3
=$B$3+A48*$E$3
=$B$3+A49*$E$3
=$B$3+A50*$E$3
Transit
Gasoline
=$B$4+A13*$E$4
=$B$4+A14*$E$4
=$B$4+A15*$E$4
=$B$4+A16*$E$4
=$B$4+A17*$E$4
=$B$4+A18*$E$4
=$B$4+A19*$E$4
=$B$4+A20*$E$4
=$B$4+A21*$E$4
=$B$4+A22*$E$4
=$B$4+A23*$E$4
=$B$4+A24*$E$4
=$B$4+A25*$E$4
=$B$4+A26*$E$4
=$B$4+A27*$E$4
=$B$4+A28*$E$4
=$B$4+A29*$E$4
=$B$4+A30*$E$4
=$B$4+A31*$E$4
=$B$4+A32*$E$4
=$B$4+A33*$E$4
=$B$4+A34*$E$4
=$B$4+A35*$E$4
=$B$4+A36*$E$4
=$B$4+A37*$E$4
=$B$4+A38*$E$4
=$B$4+A39*$E$4
=$B$4+A40*$E$4
=$B$4+A41*$E$4
=$B$4+A42*$E$4
=$B$4+A43*$E$4
=$B$4+A44*$E$4
=$B$4+A45*$E$4
=$B$4+A46*$E$4
=$B$4+A47*$E$4
=$B$4+A48*$E$4
=$B$4+A49*$E$4
=$B$4+A50*$E$4
=$B$4+A51*$E$4
=$B$4+A52*$E$4
=$B$4+A53*$E$4
Transit
CNG
=$B$5+A13*$E$5
=$B$5+A14*$E$5
=$B$5+A15*$E$5
=$B$5+A16*$E$5
=$B$5+A17*$E$5
=$B$5+A18*$E$5
=$B$5+A19*$E$5
=$B$5+A20*$E$5
=$B$5+A21*$E$5
=$B$5+A22*$E$5
=$B$5+A23*$E$5
=$B$5+A24*$E$5
=$B$5+A25*$E$5
=$B$5+A26*$E$5
=$B$5+A27*$E$5
=$B$5+A28*$E$5
=$B$5+A29*$E$5
=$B$5+A30*$E$5
=$B$5+A31*$E$5
=$B$5+A32*$E$5
=$B$5+A33*$E$5
=$B$5+A34*$E$5
=$B$5+A35*$E$5
=$B$5+A36*$E$5
=$B$5+A37*$E$5
=$B$5+A38*$E$5
=$B$5+A39*$E$5
=$B$5+A40*$E$5
=$B$5+A41*$E$5
=$B$5+A42*$E$5
=$B$5+A43*$E$5
=$B$5+A44*$E$5
=$B$5+A45*$E$5
=$B$5+A46*$E$5
=$B$5+A47*$E$5
=$B$5+A48*$E$5
=$B$5+A49*$E$5
=$B$5+A50*$E$5
=$B$5+A51*$E$5
=$B$5+A52*$E$5
=$B$5+A53*$E$5
29
=$B$4+A54*$E$4
=$B$4+A55*$E$4
=$B$4+A56*$E$4
=$B$4+A57*$E$4
=$B$4+A58*$E$4
=$B$4+A59*$E$4
=$B$4+A60*$E$4
=$B$4+A61*$E$4
=$B$4+A62*$E$4
=$B$4+A63*$E$4
=$B$4+A64*$E$4
=$B$4+A65*$E$4
F-250
Gasoline
=$B$6+A13*$E$6
=$B$6+A14*$E$6
=$B$6+A15*$E$6
=$B$6+A16*$E$6
=$B$6+A17*$E$6
=$B$6+A18*$E$6
=$B$6+A19*$E$6
=$B$6+A20*$E$6
=$B$6+A21*$E$6
=$B$6+A22*$E$6
=$B$6+A23*$E$6
=$B$6+A24*$E$6
=$B$6+A25*$E$6
=$B$6+A26*$E$6
=$B$6+A27*$E$6
=$B$6+A28*$E$6
=$B$6+A29*$E$6
=$B$6+A30*$E$6
=$B$6+A31*$E$6
=$B$6+A32*$E$6
=$B$6+A33*$E$6
=$B$6+A34*$E$6
=$B$6+A35*$E$6
=$B$6+A36*$E$6
=$B$6+A37*$E$6
=$B$6+A38*$E$6
=$B$6+A39*$E$6
Sierra
Gasoline
=$B$5+A54*$E$5
=$B$5+A55*$E$5
=$B$5+A56*$E$5
=$B$5+A57*$E$5
=$B$5+A58*$E$5
=$B$5+A59*$E$5
=$B$5+A60*$E$5
=$B$5+A61*$E$5
=$B$5+A62*$E$5
=$B$5+A63*$E$5
=$B$5+A64*$E$5
=$B$5+A65*$E$5
F-250
CNG
=$B$7+A13*$E$7
=$B$7+A14*$E$7
=$B$7+A15*$E$7
=$B$7+A16*$E$7
=$B$7+A17*$E$7
=$B$7+A18*$E$7
=$B$7+A19*$E$7
=$B$7+A20*$E$7
=$B$7+A21*$E$7
=$B$7+A22*$E$7
=$B$7+A23*$E$7
=$B$7+A24*$E$7
=$B$7+A25*$E$7
=$B$7+A26*$E$7
=$B$7+A27*$E$7
=$B$7+A28*$E$7
=$B$7+A29*$E$7
=$B$7+A30*$E$7
=$B$7+A31*$E$7
=$B$7+A32*$E$7
=$B$7+A33*$E$7
=$B$7+A34*$E$7
=$B$7+A35*$E$7
=$B$7+A36*$E$7
=$B$7+A37*$E$7
=$B$7+A38*$E$7
=$B$7+A39*$E$7
Sierra
CNG
30
=$B$8+A13*$E$8
=$B$8+A14*$E$8
=$B$8+A15*$E$8
=$B$8+A16*$E$8
=$B$8+A17*$E$8
=$B$8+A18*$E$8
=$B$8+A19*$E$8
=$B$8+A20*$E$8
=$B$8+A21*$E$8
=$B$8+A22*$E$8
=$B$8+A23*$E$8
=$B$8+A24*$E$8
=$B$8+A25*$E$8
=$B$8+A26*$E$8
=$B$8+A27*$E$8
=$B$8+A28*$E$8
=$B$8+A29*$E$8
=$B$8+A30*$E$8
=$B$8+A31*$E$8
=$B$8+A32*$E$8
=$B$9+A13*$E$9
=$B$9+A14*$E$9
=$B$9+A15*$E$9
=$B$9+A16*$E$9
=$B$9+A17*$E$9
=$B$9+A18*$E$9
=$B$9+A19*$E$9
=$B$9+A20*$E$9
=$B$9+A21*$E$9
=$B$9+A22*$E$9
=$B$9+A23*$E$9
=$B$9+A24*$E$9
=$B$9+A25*$E$9
=$B$9+A26*$E$9
=$B$9+A27*$E$9
=$B$9+A28*$E$9
=$B$9+A29*$E$9
=$B$9+A30*$E$9
=$B$9+A31*$E$9
=$B$9+A32*$E$9
31