Evaluation 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
The original of the complete thesis is on file
In the Rensselaer Polytechnic Institute Library
Approved:
_________________________________________
Professor Sudhangshu Bose, Project Adviser
Professor Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
December, 2014
i
© 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
Background .................................................................................................................... 2
2. Theory ...................................................................................................................................... 3
2.1
Otto Cycle ...................................................................................................................... 3
2.2
Suitability of Fuels.......................................................................................................... 3
2.2.1
Chemical Properties of Natural Gas ................................................................. 4
3. Methodology ............................................................................................................................ 5
3.1
Combustion Process ...................................................................................................... 5
3.1.1
Exhaust Emissions ............................................................................................. 6
4. Improving Internal Combustion ............................................................................................... 8
4.1.1
Compression Ratio ............................................................................................ 8
4.1.2
Air/Fuel Ratio .................................................................................................... 8
4.1.3
Combustion Chamber Turbulence .................................................................... 9
4.1.4
Fuel Injection .................................................................................................... 9
4.1.5
Ignition Timing .................................................................................................. 9
4.1.6
Engine Temperature ......................................................................................... 9
5. Well to Wheel Emissions of CNG ........................................................................................... 11
5.1
Extraction ..................................................................................................................... 11
6. Results and Discussion ........................................................................................................... 12
iii
6.1
ASDF ............................................................................................................................. 12
7. Practical Realities of Natural Gas Vehicles ............................................................................. 13
7.2
7.1.1
Initial Cost ....................................................................................................... 13
7.1.2
Fuel Storage and Range .................................................................................. 13
7.1.3
Fuel Cost ......................................................................................................... 14
7.1.4
Fueling Stations .............................................................................................. 14
7.1.5
Maintenance ................................................................................................... 14
7.1.6
Fuel Safety ...................................................................................................... 14
Overall Vehicle Costs ................................................................................................... 14
8. Conclusion .............................................................................................................................. 15
9. References.............................................................................................................................. 16
9.1
Works Cited ................................................................................................................. 16
9.2
Additional References Consulted ................................................................................ 16
10. Appendices ............................................................................................................................. 17
iv
LIST OF TABLES
Table 1:
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 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, specific fuel consumption, 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
1.1 Background
With the beginning of the industrial revolution in the late 1700s, work has progressed to
convert the chemical energy in various fuel sources to 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 is forcing them to greatly increase
the fuel economy of their vehicle fleets. Compressed natural gas 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
theoretical efficiency study, a comparison of vehicle emissions with conventional automobiles,
and a discussion of pragmatic realities of operating an alternative energy vehicle.
2
2. Theory
2.1 Otto Cycle
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. 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 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 combusts which causes it to expand. The expanding gas mixture then drives the piston
downward. When the combustion 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.
2.2 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 combustion.
2.2.1
Chemical Properties of Natural Gas
Natural gas is, as its name implies, a mixture of volatile gases that occur naturally. Natural gas
is approximately 95% methane. The remaining 5% is made up of higher level hydrocarbons as
well as some contaminants. [Citation needed] 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.
4
3. Methodology
3.1 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 is given below.
CH4 + 2 O2 -> CO2 + 2 H2O + energy
[8][2]
However, the combustion process 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:
CH4 + 2O2 + 7.546N2 -> CO2 + 2H2O + 7.546N2 + energy
[Korakianitis]
Air/Fuel Ratio and how it plays a role in lean operation
5
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 Nitric oxide and other nitrogen oxides are responsible for much of
the smog production in cities.
3.1.1
Exhaust 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.
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 fuel in this regard because each carbon has formed bonds with
four hydrogen atoms. This 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
6
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 is also an unwelcome product. It reacts with sunlight to form ozone and
contributes to the smog found in many large cities. It impacts the air quality and can
exacerbate breathing difficulties.
Carbon monoxide is highly toxic to humans.
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.
7
4. Improving Internal Combustion
As the pressure to find an abundant, clean burning fuel has increased, significant research into
natural gas fueled engines has been performed. Many of the engine parameters
The ultimate goal is to improve engine efficiency, and thereby fuel economy, while reducing
both harmful emissions to the environment and purchase cost, all while not sacrificing
drivability or durability. Obviously, these goals are often at odds with each other. These
characteristics are all interdependent so for each characteristic which is improved upon,
another may be adversely affected. Therein lies the challenge.
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.
4.1.1
Compression Ratio
It is known that the thermal efficiency of an engine increases with the compression ratio. As
was discussed above, 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 from ~9:1 for
gasoline powered engines to ~12:1 allowed for increased efficiency without significantly
increasing unwanted emissions.
4.1.2
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. Increasing the ratio of fuel (decreasing the air/fuel ratio) is known as running rich.
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. Decreasing the ratio of fuel (increasing the air/fuel ratio) is called running lean.
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.
Figure XX: Emission trends with air/fuel ratio
8
4.1.3
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. Assuming that natural gas has been selected as the fuel,
increasing the turbulence allows for a wider range of air/fuel ratios to be used, specifically
compensating for poor combustion at during lean operation.[Cho]
4.1.4
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].
4.1.5
Ignition Timing
4.1.6
Engine Temperature
9
Each of the above parameters have 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].
Reference [5] discusses one approach to keep the NOx production at low levels: the addition of
supplementary hydrogen as a secondary fuel.
10
5. Well to Wheel Emissions of CNG
5.1 Extraction
The extraction process of natural gas
See Waller.
11
6. Results and Discussion
6.1 ASDF
CNG has a lower energy content per unit volume than gasoline, but CNG fueled engines can
operate at higher compression ratios and therefore higher efficiencies. The tradeoffs involved
will be contrasted.
The stoichiometric equations for combustion indicate the resulting exhaust gases that will
make up the vehicle emissions. The predicted emissions will be compared to test data.
Additional lifecycle greenhouse gas emission differences will be discussed.
The practical concerns to be investigated include refueling, fuel concerns during accidents,
higher initial cost, and less range due to storage tank size.
Asdf
12
7. Practical Realities of Natural Gas Vehicles
7.1.1
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 X 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 more typically those sold to fleet
operators 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 you 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 X: Purchase Price Comparison of Gasoline and CNG fueled vehicles
Gasoline Vehicle
Factory CNG Vehicle
Aftermarket
Conversion
Honda Civic Cost
$18,500
$26,500
$25,000
Ford Transit
$29,500
$44,500
$39,500
Ford F-250
$42,500
$55,000
$55,000
GMC Sierra 2500HD
$43,000
$53,000
$55,500
7.1.2
Fuel Storage and Range
CNG has a lower energy content per unit volume than that of gasoline. The fact that natural
gas is just that, a gas, 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 [citation needed]. In order to achieve a range of 300 miles, a CNG fueled
vehicle needs to have a fuel storage capacity of XX cubic feet.
13
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.
7.1.3
Fuel Cost
Diesel is approximately $4/gallon. Gasoline is approximately $3.50/gallon. Natural gas is
approximately $XX per cubic foot or $XX for 1 gallon gasoline equivalent.
7.1.4
Fueling Stations
7.1.5
Maintenance
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 not their effects on engine internal parts. This will
make diagnosing problems difficult.
7.1.6
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 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.
7.2 Overall Vehicle Costs
Table XX below details the ownership costs of a gasoline powered automobile and its CNG
equivalent:
Table XX: Overall Vehicle Ownership Cost
14
8. Conclusion
With all the potential benefits outlined in section 4, it seems that a strong case could be made
for the adoption of CNG vehicles. That optimism is tempered slightly by the results of the
emissions review in section 5. The real blow to 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. 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.
15
9. References
9.1 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 spark-ignition
(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
[9] Johnson, Caley. “Business Case for Compressed Natural Gas in Municipal Fleets”.
NREL/TP-7A2-47919. June 2010.
9.2 Additional References Consulted
16
10.Appendices
10.1
17