Vehicles
can be "fast filled" in five to six minutes using compressed
gas stored in cascades of natural gas cylinders or fueled overnight
on a "timed fill" basis in about five to eight hours. Many private
fleet fueling stations use a combination of fast fill and timed
fill.
Emissions
Natural
gas is the cleanest burning alternative fuel. Exhaust emissions
from NGVs are much lower than those from equivalent gasoline-powered
vehicles. For instance, NGV emissions of carbon monoxide are
approximately 70 percent lower, non-methane organic gas emissions
are 89 percent lower, and oxides of nitrogen emissions are
87 percent lower. In addition to these reductions in pollutants,
NGVs also emit significantly lower amounts of greenhouse gases
and toxins than do gasoline vehicles.
Dedicated
NGVs produce little or no evaporative emissions during fueling
and use. For gasoline vehicles, evaporative and fueling emissions
account for at least 50 percent of a vehicle's total hydrocarbon
emissions. Dedicated NGVs also can reduce carbon dioxide exhaust
emissions by almost 20 percent.
Exposure
to the levels of suspended fine particulate matter found in
many U.S. cities has been shown to increase the risk of respiratory
illness. Diesel exhaust is under review as a hazardous air
pollutant. Natural gas engines produce only tiny amounts of
this matter.
Greenhouse
Gases
Per unit
of energy, natural gas contains less carbon than any other
fossil fuel, and thus produces lower CO2 emissions per vehicle
mile traveled. While natural gas vehicles (NGVs) do emit methane,
another principle greenhouse gas, any slight increase in methane
emissions would be more than offset by a substantial reduction
in CO2 emissions compared to other fuels.
NGVs
also emit very low levels of carbon monoxide (approximately
70 percent lower than a comparable gasoline vehicle) and volatile
organic compounds. Although these two pollutants are not themselves
greenhouse gases, they play an important role in helping to
break down methane and some other greenhouse gases in the
atmosphere, and thus increase the global rate of methane decomposition.
Safety
Vehicles
that run on clean burning natural gas are as safe as vehicles
operating on traditional fuels such as gasoline. In fact,
many school transportation managers choose natural gas to
power their school buses because compressed natural gas, unlike
gasoline, dissipates into the atmosphere in the event of an
accident. Gasoline pools on the ground creating a fire hazard.
In the
US a survey was taken of more than 8,000 vehicles that cumulatively
traveled approximately 278 million miles from 1987-1990. The
survey found that the injury rate for NGVs per vehicular mile
traveled (VMT) was 37 percent lower than the rate for gasoline-powered
fleet vehicles and 34 percent lower than the entire population
of registered gasoline vehicles. In addition to the lower
injury rate, no deaths were recorded for the NGVs in the survey.
In contrast the deaths associated with the gasoline fleet
vehicles surveyed came to 1.28 deaths per 100 million VMT.
The US national average was 2.2 deaths per 100 million VMT
for all U.S. gasoline vehicles.
There
are two fundamental reasons for this excellent NGV safety
record: the structural integrity of the NGV fuel system and
the physical qualities of natural gas as a fuel.
The fuel
storage cylinders used in NGVs are much stronger than gasoline
fuel tanks. The design of NGV cylinders are subjected to a
number of federally required "severe abuse" tests, such as
heat and pressure extremes, gunfire, collisions and fires.
While
fuel storage cylinders are stronger than gasoline fuel tanks,
the composite material used to encase the tanks are fundamentally
more susceptible to physical damage than metals under abusive
conditions. For this reason, composite materials on NGV cylinders
must always be properly handled and protected. Incidents involving
natural gas cylinder ruptures revealed that some form of chemical
attack or physical damage to the composite overwrap on the
cylinder was involved.
NGV fuel
systems are "sealed," which prevents any spills or evaporative
losses. Even if a leak were to occur in an NGV fuel system,
the natural gas would dissipate into the atmosphere because
it is lighter than air.
Natural
gas has a high ignition temperature, about 650 degress C,
compared with about 350 degrees C for gasoline. It also has
a narrow range of flammability; that is, in concentrations
in air below about 5 percent and above about 15 percent, natural
gas will not burn. The high ignition temperature and limited
flammability range make accidental ignition or combustion
of natural gas unlikely.
Natural
gas is not toxic or corrosive and will not contaminate ground
water. Natural gas combustion produces no significant aldehydes
or other air toxins, which are a concern in gasoline and some
other alternative fuels.
The natural
gas delivery system also has an excellent — and proven
— safety record. According to statistics from the U.S.
Department of Transportation, the 1.9 million Km natural gas
transmission and distribution system is the safest way to
transport energy in the United States.
NGVs
use the same energy that has safely and comfortably heated
homes and cooked meals for more than 100 years.
Job
Creation
According
to a US National Defense Council Foundation report, a significant
number of jobs would be created to meet the projected demand
for NGVs and fueling stations generated by government requirements
and environmental concerns.
Domestic
Abundance
Natural
gas is a domestic, readily available fuel. According to the
U.S. Department of Energy, a 65 year supply of natural gas
is available in the United States using existing technology.
Five million NGVs would use less than three percent of current
annual natural gas consumption.
Nearly
100 percent of all natural gas used in the United States is
produced in North America. Nearly 90 percent is produced in
the U.S. and almost all the remainder in Canada. Based on
the use of traditional and nontraditional exploration and
production technologies, it is estimated that the United States
has a 200-year supply of natural gas.
More NGV statistics
What is a
bi-fuel vehicle?
A bi-fuel
vehicle can run on either natural gas or gasoline. Many are
designed to switch automatically to gasoline when the natural
gas fuel tank reaches empty. These vehicles get the same or
slightly fewer miles per equivalent gallon of natural gas
as do vehicles using gasoline only.
What is
a dual-fuel vehicle?
A vehicle
that runs either on diesel fuel only, or diesel fuel and natural
gas simultaneously. In a dual-fuel vehicle, the combustion
of the diesel fuel serves to ignite the natural gas.
What is a dedicated
vehicle?
A dedicated
NGV runs on natural gas only. Dedicated NGVs can be gasoline-fueled
vehicles that have been converted to run on natural gas. Most
dedicated NGVs, however, are produced by original equipment
manufacturers, such as Ford, American Honda and General Motors,
in the light duty market and a host of truck and bus manufacturers
in the medium and heavy duty market.
Internationally, most major vehicle manufacturers have prototype,
demonstration or production NGV's. For instance, Japanese
NGV's here.
What is a light-duty
vehicle?
According
to the Environmental Protection Agency, a light-duty vehicle
is any vehicle weighing 3860 gross vehicle weight (GVW) or
less. In California, vehicles weighing less than 2725 Kg gvw
are classified as light-duty vehicles.
What is a heavy-duty
vehicle?
According
to the Environmental Protection Agency, a heavy-duty vehicle
is any vehicle weighing 3860 Kg gross vehicle weight (GVW)
or more. In California, vehicles weighing more than 6360 Kg
gvw are classified as heavy-duty vehicles.
Where are NGVs
used now?
Approximately
100,000 NGVs are on U.S. roads today. NGVs have a long-established
record in Europe, Canada, New Zealand and Australia, as well.
Italy has been using natural gas as a vehicular fuel since
the 1940s, with more than 350,000 NGVs. In Canada, nearly
20,000 NGVs operate with a network of 220 public fueling stations.
Argentina has 680,000 NGVs, and Russia has more than 30,000.
Worldwide, nearly two million NGVs are in use, in countries
now including Uzbekistan, Venezuela, Mexico, the Philippines
and Indonesia. Click here
for detailed statistics.
Growing
interest in NGVs led to the formation of the International
Association for Natural Gas Vehicles in 1986, which now
has more than 500 members from over 30 countries, and to the
establishment in 1994 of the European
Natural Gas Vehicle Association, with more than 120 members
from 20 countries.
How much do NGVs
cost?
Currently
all alternative fuelled vehicles have a price premium
over traditional fuelled vehicles (unless manufacturers have
special promotional prices that they subsidise).
With
more vehicles coming on to the market, certain economies of
scale will be achieved. The price of an NGV varies depending
upon whether it is a petrol vehicle converted to run on natural
gas or a factory-built vehicle. Different size vehicles also
vary in price.
Typical
sedans are less expensive and trucks, which require more storage
cylinders, are more expensive. NGV conversion equipment can
be purchased for about US$4000 and installed by the fleet
owner, who can receive training from the conversion companies
or conversion kit manufacturers. Alternatively, NGV specialists
can do the conversion, which adds about 25% to the vehicle
cost. Larger vehicles require more fuel storage cylinders,
and the price can increase depending how many cylinders and
the type installed. Light duty NGVs from the factory can range
in price from US$ 1000-6000 over the price of a traditional
fuelled vehicle.
Heavy
duty engines, trucks and buses typically cost US$30,000- 50,000
more than standard diesel engines and vehicles. Much of the
added cost can be attributed to the number of cylinders required
to obtain the desired range of the vehicle.
However,
natural gas costs significantly less than gasoline and diesel.
For many fleet customers, the up-front costs can be recovered
over the life of the vehicle.
Where can an
NGV be fueled with natural gas?
A growing
number of public fueling stations are available in some countries.
In the US more than 1,200 natural gas fueling stations operate
in 46 states and the District of Columbia, and more than half
of these are open or available to the public. Oil companies
such as UNOCAL and Shell are involved in public NGV fueling
stations. Many utilities provide compressor station equipment
or compressed natural gas for on-site fueling of large customer
fleets
NGVs
also can be fueled from a small dispenser directly connected
to a home or business natural gas line. This is commonly known
as a Vehicle Refuelling Appliance (VRA). The dispenser is
operated by a small electrically driven compressor.
Click
here for more information on refuelling station numbers
in various countries.
What dedicated
NGVs are being manufactured now?
All major
US car, truck and bus manufacturers have built dedicated prototype
NGVs. Many NGVs are directly available from the original equipment
manufacturers.
Bus manufacturers
like Blue Bird and Orion Bus Industries sells buses designed
to run on natural gas. Major diesel engine manufacturers,
such as Caterpillar, Cummins, Detroit Diesel, Mack and Deere
Power Systems, are developing or producing heavy-duty natural
gas engines for a wide variety of vehicular applications.
Forty-two manufacturers produce more than 93 varieties of
natural gas vehicles, engines and chassis, varying from light-duty
passenger vehicles to school buses and forklifts.
Japanese
Auto Development
In other
countries most vehicle manufacturers have an NGV programme.
Can current
NGV technology keep pace with the advances in the auto industry?
Recent
advances in NGV technology will keep the industry on track,
with the most advanced technologies coming from major automotive
manufacturers. The NGV industry is intently focused on new
research and development in areas of infrastructure, vehicle
and engine technology, and reductions in the emissions of
NGVs.
NGV conversion
mechanicals are compatible with throttle body and multiport
fuel-injected engines. Closed-loop, computer-compatible conversion
kits are now being developed and marketed. These will improve
bi-fuel NGV performance and further reduce their already-lower
emissions.
How do NGVs work?
The only
major difference between a gasoline vehicle and an NGV is
the fuel system. Natural gas is compressed to between 3,000
and 3,600 pounds per square inch (200 bar) and is stored on
board the vehicle in cylinders installed in the rear, undercarriage,
or on the roof. When natural gas is required by the engine,
it leaves the cylinders, passes through a master manual shut-off
valve and travels through a high-pressure fuel regulator located
in the engine compartment. The natural gas is injected at
atmospheric pressure through a specially designed natural
gas mixer where it is properly mixed with air. Natural gas
then flows into the engine's combustion chamber and is ignited
to create the power required to drive the vehicle. Special
solenoid-operated valves prevent the gas from entering the
engine when it is shut off.
How will NGVs
help the U.S. meet environmental and energy-security laws?
The Clean
Air Act Amendments (CAAA), signed into law in November 1990,
contain numerous provisions that affect vehicles — the
primary source of air pollution in many urban areas. The law
includes new emission standards for passenger cars, trucks
and buses, as well as off-road engines and vehicles. There
also are new requirements for fuels and inspection and maintenance
(I/M) requirements for the nation's most polluted areas.
Although
the law does not require automotive manufacturers to produce
alternative-fuel vehicles, the number of NGVs in use is increasing
measurably because of the law's tougher emission standards.
The introduction of more expensive emission reduction technologies
on gasoline vehicles is expected to make NGVs more economic,
since they generally are expected to meet the new standards
with little or no major modifications. Laws that focus on
vehicle emissions reduction have served to educate millions
of Americans on the importance of controlling motor vehicle
pollution. Many air-quality officials are now looking for
ways to increase the number of NGVs in their states.
The Clean
Fuel Fleet (CFF) Program, perhaps more than any other CAAA
program, is cited as a program that will lead to greater use
of alternative fuel vehicles (AFVs). Starting in model year
1999 (October 1998), publicly and privately owned fleets must
acquire clean-fuel vehicles (CFVs) when they replace their
vehicles. The program initially requires that 30 percent of
new light duty vehicles must be CFVs. Fifty percent of newly
acquired medium- and heavy-duty vehicles (i.e., 8,500 - 26,000
gross vehicle weight) must be CFVs.
Congress,
intended to cover 22 nonattainment areas. Fourteen of these
areas have asked the U.S. Environmental Protection Agency
(EPA) to allow them to adopt other measures, as allowed under
federal law as long as the program each adopts provides equivalent
or greater emission reductions.
The current
list of areas that have adopted regulations for the CFF program
includes Atlanta, Ga.; Baton Rouge, La.; Chicago, Ill.; Denver-Boulder,
Colo.; Gary, Ind.; Milwaukee-Racine, Wis.; North Carolina;
Virginia and Washington, D.C.
So far,
only NGVs have been certified to the emission standards required
for CFVs. Vehicles certified to the low-emission vehicle (LEV)
or better standards must be used to satisfy fleet purchase
requirements. Currently, the only vehicles to be certified
under the EPA LEV program are NGVs.
What is the CMAQ?
The Congestion
Mitigation Air Quality (CMAQ) program established by the Intermodal
Surface Transportation Efficiency Act of 1991 allocates money
to states for use in their transportation/air-quality plans.
A primary focus of CMAQ funding is investment in air-quality
improvements, and NGV projects have successfully received
these funds.
What is the
US NGV industry's strategy?
NGVs
are ideal for fleet operations. The industry is concentrating
on high fuel-use commercial fleets, such as transit buses,
airport shuttles and taxis, refuse haulers and over-the-road
trucks. NGVs of all types are on the road now, an indication
that the industry has moved beyond the developmental stage
into commercialization and expanded applications.
Many
states view the emerging alternative-fuels industry as an
economic development opportunity. These states support combining
the use of incentives and the implementation of Environmental
Protection Act fleet regulations to make the AFV industry
sustainable. The Department of Energy's Clean Cities program
is currently operating in 56 areas across the United States.
More than 1,200 stakeholders have signed agreements to increase
the use of AFVs in their localities. Natural gas is the only
alternative fuel that has a presence in all Clean Cities locations
except Hawaii.
What about the
vehicle's power?
Gasoline
vehicles converted to natural gas are subject to a small power
loss when running on natural gas; however, vehicles designed
specifically to run on natural gas will have no loss of power
and may even have greater power and efficiency. Natural gas
has a 130 octane rating, compared with 87 to 96 octane rating
of gasoline.
How can I find
out more about NGVs?
In the
US contact the marketing or NGV department of your local natural
gas utility or the Natural Gas Vehicle Coalition at (703)
527-3022.
In other
countries contact IANGV members
for the best sources of information.
The 1997
Position Paper is an authoritative review of the current
technology and industry.
How much
energy does Compressed Natural gas release compared to petrol?
A direct
answer to your question is that the energy content of natural
gas (NG) is about 47 MJ/kg or 40 MJ/m3. (gross heating value).
The values for a typical petrol are 60 MJ/kg and 44 MJ/litre.
Another comparison on an energy basis is that 1 kg of NG is
equivalent to about 1.33 litre of petrol or 1.22 litre of
diesel. Or on a volume basis 1 m3 of NG is equivalent to about
1.10 L of petrol or 1.0 L of diesel. When making comparisons
you may also need to take into account the relative energy
efficiency of the engines that use the various fuels. Generally
engines that are designed for natural gas fuel are slightly
more efficient than a similar petrol engine (because they
can run at a higher compression ratio). The NG and diesel
engines of similar size will have a much the same thermal
efficiency.
What is the miles
per gallon equivalant of natural gas compared to gasoline?
If the
NGV is an original Equipment Manufacturer (OEM) model, it
will have been designed to make the most of the excellent
properties of Natural Gas - eg it will have a higher compression
ratio than the petrol model and different ignition timing
- and you could expect to see an improvement in performance
and fuel consumption on an energy basis. This might be about
5% or more. Of course you might then drive faster, and not
have any advantage. If the car has been converted from gasoline
to NG and you can choose to run on either fuel (ie a bi-fuel
vehicle) then it is not possible to make the most of the higher
octane rating of the NG. In this case the change in fuel consumption
will depend very much on the vehicle and engine design and
on the conversion equipment used and how it is tuned. In this
case you might expect an increase of possibly 5% in consumption.
However the tune may be optimised to a particular power and
speed range and if you can hit this you might get a small
improvement. There may be more scope to achieve this on a
high capacity engine with reserves of power. On a smaller
engine there may be a noticeable drop in power and your comsumption
could increase if you try to match the old on-road performance.
What is Liquefied
Natural Gas (LNG)?
LNG is
natural gas that has been liquefied by reducing its temperature
to -260 degrees Fahrenheit at atmospheric pressure. In volume
at standard conditions, it occupies 1/600 that of natural
gas as a vapor.
What are
the benefits of LNG?
LNG's
numerous benefits are leading to a growing appreciation of
its potential as a transportation fuel for heavy-duty vehicles.
These benefits include:
- Higher
energy density. Since it's a liquid, a greater volume of
LNG can be stored in a smaller space. Especially onboard
a vehicle, getting the greatest possible range and lowest
weight are important considerations.
- Speed
of fueling. Large vehicles can often be filled in four to
six minutes, and fuel composition can be determined with
a high degree of accuracy since most LNG produced for vehicles
is now in the 99+ percent range for methane. This control
over composition results in a more finely tuned fuel system
and engine, which leads to optimization of engine performance
and thus greater fuel economy and lower emissions.
- Deliver
and availability. LNG is frequently transported in trailer
trucks that hold up to 44,000 litres, in small tank trucks
and trailers, railcars, barges, and 30 million-gallon LNG
ships. LNG trailer trucks are often used to deliver LNG
to refueling stations, much like diesel or gasoline delivery.
- Ongoing
research promises to lower the cost of LNG fueling facilities,
produce lighter fuel tanks and increase engine efficiencies.
Where is
LNG produced?
LNG can
be produced in about half of the almost 90 LNG storage locations
in the United States and Canada operated by local gas utilities.
In addition, several cryogenic natural gas extraction plants
in the gas-producing states now produce LNG as a sidestream.
Large liquefaction plants are being built specifically to
produce LNG for fuel, and there are now about 70 liquefaction
facilities in the United States, compared with 170 gasoline
refineries.
Cylinders
in Accidents
A pressurized
gas cylinder is probably the strongest component on the vehicle.
Vehicles that totally destroyed in collisions show the only
discernible component being the intact gas cylinder. It is
unlikely that cylinders will rupture due to collision impact.
Regarding
the danger of fire from leaking cylinders, all we have is
the experience to date that indicates that such an event is
unlikely to occur. In North America there was a problem with
leaking type 4 designs from a particular manufacturer, but
there has never been an ensuing fire. The risk of fire from
leaking cylinders must be low since there are well over a
million CNG vehicle installations worldwide that have not
experienced such problems.
It is
worth pointing out that natural gas is lighter than air and
in the unlikely event of a leak from piping or container the
gas will dissipate upwards quite quickly. In the case of petrol
and LPG the vapour given off is heavier than air and will
tend to pool near the ground. This is where there is a strong
risk of some ignition source. In general terms diesel ranks
high in terms of safety, but most people rank Naturak Gas
next.
NGV at High
Altitude
There
is a problem with the standard mechanical petrol carburettor
when driving at higher altitudes, where the air density is
lowered, and that is that the engine runs progressively more
rich. So that the power falls off both because the engine
is breathing less oxygen (because of the decreasing air density
with altitude) and also because a venturi actuated carburettor
will run richer as the air density decreases. A conversion
to natural gas fuelling, using a typical mechanical carburettor
with a venturi metering system will suffer the same problems
and so in this respect you will be no worse or better off.
But recall that the power of a natural gas engine also falls
off by about 12 to 14% because the gas occupies about 12%
of the intake volume and so you have less air or oxygen (and
of course the liquid fuel does not suffer this problem).
On the
other hand there is a possibility of using an electronic natural
gas metering system operated by an oxygen sensor which will
maintain a constant fuel/air ratio with altitude and this
would solve the enrichment problem, but not the 12% loss.
SO if you're driving is limited in terms of getting over the
mountain quickly it might be better to stick to petrol - although
if you convert to gas you can still switch back to petrol
when hitting the high mountain passes.
CNG Fueling
Speed and Range
A slow
fill gets more gas into the tank than does a fast fill. The
reason is that as the gas builds up the pressure in the tank
it is in effect compressing the gas that is already there
- and this causes a rise in temperature, which in turn lowers
the density of the gas. As the tank cools the pressure will
fall. If you use the slow fill approach there is time for
the tank to come to equilibrium with the ambient air temperature
and the result is higher density and a more complete fill.
During
a fast fill, at the point where the gas enters the cylinder
(going from high pressure to to a lower pressure) it is expanding
and chilling. At the far end of the cylinder the gas is compressing
and heating. This temperature difference is observed for about
5 seconds until an equilibrium is reached and the temperature
of the gas within the cylinder rises uniformly as it is compressed.
Both the cylinder and the gas will be relatively warm at the
end of a fast fill. As the cylinder and gas cools down to
the ambient temperature the pressure correspondingly decreases.
What are the
factors which affect the fuel efficiency of CNG?
In the
first place let us list the energy content of the fuels you
mention. Using units of MJ per kilogram, the net heating values
of petrol, diesel, LPG and NG are about 45, 43, 46, and 44;
the net heating value does not include the heat in the water
vapour of the combustion products. If you look up the gross
heating values - which do include this, the values are different
(higher). So the differences between the fuels are not large.
But the values will also vary quite a lot depending on the
composition of the fuel - particularly for NG and LPG.
We now
need to consider the way in which different engines use the
fuels - in particular the efficiency. The engine thermal efficiency
is a function of many different factors but perhaps the most
important one is the engine compression ratio. The higher
the compression ratio the higher is the theoretical and also
the actual efficiency. The maximum compression ratio (CR)
different fuels can tolerate in fact defines the efficiency.
Since diesel used in a compression ignition engine can operate
at say 14:1 the diesel will be expected to have the highest
efficiency - lets say 40% as an upper limit. The next highest
efficiency in the fuels comes from CNG, which can operate
at say 12:1. with an efficiency of say 35%. It is possible
to run an engine on CNG at 14:1 but that is the very upper
limit. We would not expect to be able to run petrol and LPG
engines at much more than 9:1 and an efficiency of about 30%.
These efficiencies are the upper limits and at full load -
in normal operation they will be lower than the values quoted,
but in much the same proportion. The main reason for the differences
is the variation in limiting CR for the different fuels. So
here is a starting point for a discussion of the differences
in efficiency.
As far
as fuel energy comparisons go (and this does not take into
account the different engine efficiencies), 1 kg of NG is
equivalent to about 1.33 litres of petrol or 1.22 litres of
diesel - but of course occupies a greater volume. Or 1 cubic
metre of NG at atmospheric pressure is equivalent to 1.10
litres of petrol and 1.00 litres of diesel.
Note
that there are a lot of other factors that we have not considered
- for example the diesel engine will be much heavier than
the other engines, and the gaseous fuels will need pressure
vessels to contain them. Having established how much energy
you get from the different fuels and how efficiently the different
engines can use the fuels, you will be able find out how much
they cost and then work out a cost per km or mile. In many
countries CNG will come out as best value and that diesel
will be next, followed by LPG and then petrol. But prices
do vary a great deal. Incidentally if you have a petrol engine
that has been converted to use NG you will not achieve the
high efficiency quoted above because the compression ratio
will be fixed at the level required for petrol - you will
only get the benefit of higher efficiency in an OEM.
What are the
economics of small vehicle conversion?
In very
general terms the smaller the vehicle the longer is the payback
period for the cost of conversion. This is because the fuel
consumption - and therefore savings - for the smaller vehicles
is lower, and at the same time the cost of conversion does
not go down much with vehicle size. The cost of the fuel control
system stays much the same and the price of a smaller storage
cylinder will not be much lower (and in a small vehicle it
is more difficult to find a space for the cylinder).
It may
be difficult to justify the conversion on an economic basis,
but this does depend on annual mileage. Do a rough sum on
the basis of saving half (or whatever the price differential
is in your area) of your fuel costs in a year. You should
expect better than a three year payback on the basis of economics.
Environmental
benefits and lower maintenance costs will be small.
Check
where you would be able to find space for the storage cylinder
which must be sized to meet commuter needs. And of course
allow something for the satisafaction of knowing that you
are lowering your output of hydrocarbon emissions.
What are the
safety issues with gaseous fuels?
First
of all the safety regulations for all fuels - whether liquid
or gaseous - will generally ensure that the risk of a fire
under normal operating conditions is very small indeed. So
it is generally in the event of a crash or equipment failur
that a hazard will occur. As with most fuels the main fire
hazard comes from leakage - either during refuelling operations
or during operation of the equipment, a vehicle crash etc.
In any
of these situations there needs to be all of three requirements
before there is the potential for a fire or an explosion.
First the leakage of the fuel, second a situation where a
mixing of the fuel with air gives a mixture in the flammable
range and third a source of ignition. Most gases have an oderant
added so that leakage can be detected by people in the vicinity.
Once a
leakage occurs and a source of ignition is present - say a
spark or a naked flame of sufficient energy - there must still
be a mixture of the gas in the flammable range. The likelyhood
of this flammable mixture occurring is less for natural gas
(NG) than for LPG since the NG is ligher than air and ends
to float away. LPG vapour is heavier than air and tends to
form 'pools' near the ground. It is generally accepted that
the various automotive fuels range in safety from diesel (safest)
to LPG as the most hazardous, with alcohol fuels, methane
and gasoline lying in the missle of the range. But in all
cases it needs an equipment failure or an accident to set
up the conditions for a fire. The safety measures include
a strict adherence to the regulations for installation and
operation of the equipment and the use of care and common
sense.
Is driving
around with cylinders full of gas under pressure dangerous?
Thick-walled
reinforced aluminum cylinders, steel cylinders or 100% composite
materials are used to store compressed natural gas as a vehicle
fuel. These cylinders are manufactured and tested in compliance
with strict regulations, and have withstood severe abuse testing
under conditions far more stringent than tanks designed for
storing gasoline. Natural gas vehicles submitted to test crashes
up to 52 miles per hour, which have been totally destroyed,
but show little or no damage to the compressed gas cylinders.
Bonfire and dynamite tests push cylinders to temperature and
pressures exceeding specified limits showing that compressed
natural gas cylinders are durable and safe. Of course, as
with all fuel systems, these cylinders are not indestructible
and should be inspected periodically to ensure that no surface
damage has occurred.
Does the size
and added weight of the cylinders inhibit conversion of vehicles
to use natural gas?
All the
alternative fuels -- natural gas, LPG, electricity and alcohols
-- suffer from problems associated with fuel storage size
and weight. For NGVs, the size of the cylinder is a factor
in the conversion process. Installation of the cylinders in
autos with severely restrictive truck space does inhibit conversion.
Added weight also is a factor, particularly where vehicle
carriage weight is a concern such as on transit buses and
refuse lorries. (In some countries the vehicle tax is based
upon weight, and this is an added disadvantage.) But there
are many options to installing cylinders and the industry
is gaining valuable field experience that is leading to improved
placement of cylinders in vehicles. Development of 'cylinder
packs' to fit beneath the vehicle also have resulted in improvements
of the on-board storage system.
Conversion
of large automobiles, speciality vans, trucks, forklifts and
many other vehicles are not hampered because of cylinder size.
LNG also is used to increase the fuel storage capacity on
a vehicle.
In which countries
are natural gas vehicles popular?
Natural
gas as a vehicle fuel has a long and established record in
Europe, Canada, New Zealand, Australia, and in the U.S.A.
Other countries are recognizing the benefits of NGV's, and
plan to expand the use of NGV.
In Europe,
Italy has been using natural gas as a vehicle fuel since the
1920's and has about 370,000 NGV's. The Italians have a network
of 280 filling stations to support their use of compressed
natural gas (CNG). Russia has about 75,000 NGV's and a fuelling
network of some 250 stations. Outside of these countries,
there are now several thousand NGV's in Europe and a slowly
growing fuelling station infrastructure.
Argentina
has 700,000 NGV's - the largest fleet in the world - and is
converting more than 3,000 vehicles a month and has over 500
fuelling stations in operation or under development. Venezuela
has a national NGV programme and will be installing 60 fuelling
stations and converting vehicles.
Canada
has about 36,000 vehicles converted to natural gas, and the
government-supported NGV programme has created a number of
incentives. The Canadian government provides cash incentives
for fleets to convert their vehicles, and hopes to use CNG
for 10% of the entire country's future vehicle fuel requirements.
In the
U.S.A. there are now about 68,000 vehicles fuelled on natural
gas. Natural gas vehicles have been in use there since the
late 1960s, but comparative prices with gasoline and state-of-the-art
technologies are only now making natural gas economically
and technologically competitive with gasoline vehicles. There
are about 1,200 private and public refuelling stations.
How much
do fuelling stations cost to purchase and install?
Installation
of a compressor station to refuel the vehicles is an additional
expense, if public refuelling is unavailable. Some utilities
are providing compressor station equipment free, or are making
special arrangements with fleet operators to provide the natural
gas in the form it is used - compressed. Depending upon the
design of the service station of vehicles to be refuelled,
and fuel storage requirements, compressors and related equipment
can cost from US $5,000 - 10,000 (for small compressors) to
US $400,000 or more for stations capable of serving hundreds
of vehicles. Bus fuelling stations, where 3 minute quick fill
is required for large numbers of vehicles can cost US $1 million
or more.
For normal
fleet vehicles, however, as a general rule you can expect
to spend US $1000 - 2000 per vehicle to install a fuelling
station.
How well do bi-fuel
natural gas/petrol vehicles perform?
In terms
of kilometres(km) per litre, a light duty NGV will get about
the same km/hr equivalent litre of natural gas as it does
on petrol. The range of each vehicle will depend, therefore,
on a vehicle's performance (km/litre fuel), and the number
of fuel storage tanks on board.
In terms
of power, bi-fuel natural gas vehicles lose about ten to eleven
percent because the natural gas displaces oxygen in the engine's
combustion chamber. The reduced power is less noticeable in
larger capacity engines, although four cylinder engines perform
successfully at high and low altitudes at all temperature
extremes. In terms of acceleration, the natural gas octane
rating of 130 helps ensure performance that is close to that
of a normal petrol vehicle.
In heavy
duty engines, performance is slightly improved when running
on natural gas due to the high octane fuel in higher compression
engines, however, in heavy duty natural gas engines that are
designed to run on an even fuel/air ratio (i.e. stoichiometric)
engines lose some thermal efficiency. This translates into
fuel consumption that, in some engines, has been as much as
25% higher than the diesel counterpart. But new approaches
using lean burn (less fuel/more air) or high pressure
fuel injection are helping to improve the fuel performance
of these larger engines. As with diesel engine technology
developments, heavy duty natural gas engines continue to improve
as technology becomes more refined.
What kind of
markets are strongest for NGVs? Do we expect every commuter
car to be running on natural gas?
In Europe
vehicle fleets operated by industry, national and local governments
are the strongest candidates to use natural gas.
About
10 million vehicles across Europe could, right now, be economically
retrofitted with natural gas equipment. Another 40 million
vehicles in fleets could be converted successfully. Because
the network of public natural gas fuelling stations is not
yet as well developed as in Italy, widespread use of natural
gas in privately owned, individual vehicles is something that
will be more possible in the not-so-distant future. Major
automotive manufacturers such as BMW, Volvo, and Ford are
producing factory-built NGV's, and this should help expand
stronger growth within the NGV sector.
How to
improve combustion of natural gas in the engine.
It would
be difficult to try. to 'improve' the combustion of natural
gas (NG) with an additive; it burns very well on its own,
when mixed with the right amount of air. I assume you mean
to 'control the speed of combustion' because natural gas burns
very well when, like any other combustible fuel, it is fully
mixed with air in the right ratio (stoichiometric ratio).
This ratio varies with gas composition but is about 10 to
1 (air to fuel or A/F ratio) for a typical natural gas. If
you wanted to extend the range over which it will burn you
might try to mix in some hydrogen which burns over a very
wide A/F ratio but this would not really make sense.
On the
other hand you may want to try to increase the speed with
which it burns, particularly if it is used in a lean burn
situation - which slows down the burning speed. In this case
there is a variety of techniques to be used. Generally it
may be possible to have a richer mix close to the point of
ignition - the spark plug - and have a leaner mix more remote
from the plug; a stratified charge you might say. In general
as the mixture is leaner the flame speed goes down and in
this case you may be left with unburnt gas in the more remote
parts of the cylinder. So the system requires a lot of research
and development to optimise. Lean burn operation can increase
the thermal efficiency of the engine and in some cases with
special combustion chamber design a lambda value of 1.5 (implying
an excess of air of 50%) can be achieved with significant
gains in thermal efficiency of the engine.
So, generally
speaking, the flame speed depends on the A/F ratio, the temperature
and the turbulence in the cylinder and the cylinder shape,
and it is best to play around with these parameters. Or in
a larger engine put in two spark plugs.
Can methane
from a digestor/reactor be compressed to become a fuel suitable
for the same uses as LNG?
There
is a number of fleets around the world that have run or are
running - on biogas. One of the earliest was in Christchurch,
New Zealand - and this used methane from the city sewage processing
plant. There is now a significant number of fleets operating
on biogas, with examples in France, Sweden, USA etc. It is
certainly a very 'green' approach to part of our energy and
pollution problems. It was estimated in 1993 by the company
Solagro in France that the potential in that country for annual
biogas production was between 2.5 and 5 million tonnes of
petrol equivalent. The main problem that must be faced is
that raw biogas contains about 50% CO2 and quite a few other
gaseous contaminants that may foul up mechanical plant or
be corrosive. A light water scrubbing process can remove most
of these nasties except the CO2, but while it is possible
to run stationary engines on gas than contains 50% CO2 this
is obviously going to lower the volumetric efficiency of the
engine and cause other problems. For transport use it is undesirable
to try to store a gas that contains so much CO2 that is of
no value. So it is generally removed; this is usually done
in a high pressure water scrubbing process - after which there
remains a gas with 95% methane, or better, content. The scrubbing
process may be followed by a molecular sieve clean up for
the small amount of remaining impurities. The gas in turn
needs to be dried to a low dew point to prevent corrosion
in the storage cylinders. There is a lot more info available
in the literature.
What are
the power requirements of CNG compressors?
Power
requirements depend on the size and type of the compressor
- the larger ones will be more efficient. The power demand
decreases as the inlet (mains) pressure increases. Some rough
figures are inlet pressure 2 bar, specific power (SP) in kW
hours per cubic meter will be 0.32 for a small compressor,
0.28 for a large compressor. Inlet 5 bar SP 0.26, 0.22 (for
small & large) Inlet 10 bar, 0.22, 0.18. Inlet 15bar, 0.18,
0.14.
Can biogas
be used as a source of compressed gas for vehicles?
There
is no doubt that methane is an excellent fuel and that very
low emissions can be achieved with it. The IANGV is dedicated
to furthering its use. So we all agree that it would be good
to see the wider use of natural gas (NG) as a fuel, from the
point of view of lower emissions, better resource use and
higher engine efficiency. But there are many other factors
to be taken into account. First the storage issue: this requires
that gas be compressed to high pressures (or liquefied) for
storage - this requires a network of costly refuelling stations.
And the on-vehicle storage requirements are a significant
weight and cost factor. It is clear that the early development
of networks of stations are best concentrated on areas where
heavy duty vehicles - buses and other service vehicles - are
operating.
We certainly
support the use of biogas, which in its original form will
contain a high proportion of CO2 which must be removed if
storage costs are to be minimised and high engine performance
is to be maintained. When it is cleaned up (eg sulphur content
reduced) and the CO2 is removed, it has properties that are
more or less identical to NG. But the processing is costly.
All these problems can be overcome, but require careful planning.
To all intents and purposes the problems involved in the use
of biogas and NG are identical once this processing has been
done - and we are dealing with a gas with a content of methane
of 90 to 95% or more.
Because
of the high octane rating of methane the engine efficiency
will - with a purpose designed engine - be significantly higher
than for petrol - and about the same as for diesel.
And the
emissions can be improved by a small margin - but of course
an exhaust catalyst must be used. I would note here that there
is no magic way in which these improvements are to be made
- the fuel control system must provide for very precise fuel/air
mixture control and the catalyst must be matched to the needs
of the methane fuel. In this way, since the fuel now is made
up largely of a single and relatively simple chemical compound
(CH4), the emissions are likely to be, for the same degree
of sophistication of equipment, marginally better. But note
that the emissions of NOx are largely dependent on the combustion
temperature. And since the methane fuel can use a higher compression
ratio than petrol (and thus achieve a better thermal efficiency),
it will also have a higher combustion temperature than petrol
and therefore higher NOx (before the catalyst). There are
many difficult and complicated trade-offs to be made to achieve
the best balance. It is by no means a simple problem and is
worth some detailed study.
What are the
conversion factors for natural gas?
Methane
values are fixed and can be found in any chemical handbook,
while the composition of natural gas varies with source and
time so try to find a local value. Approximately, 1kg NG is
equivalent to about 1.33 litres (L) of petrol or 1.22 L diesel.
And 1 cu metre of NG is equivalent to about 1.10 L of petrol
or 1.00 L diesel.
What is the power
and range of a heavy duty (HD)NGV?
Typical
HD diesel city buses run about 300 km per day and consume
40 to 60 litres per 100 km. Thus diesel tanks of 250 to 300
L provide adequate supply for two days light duty (LD) and
one day HD operation. The total weight of the on-board installation
would be about 300 kg. When using CNG the same types of operation
would result in a fuel consumption of 45 to 65 kg of gas per
100 km. To achieve the same range as the diesel vehicle the
gas quantity would be about 300 kg with a margin of safety
(remember there may not be many compressor stations available
in case of running out of fuel). With standard steel 120 litre
cylinders each weighing about 120 kg and each holding about
30 kg gas when fully charged - a total 10 cylinders is required,
resulting in a total mounted weight of about 1.4 tonnes including
fittings etc.
In regard
to power loss, this depends very much on how the engine is
converted to gas. Since the process involves considerable
modification if the diesel engine is converted to spark ignition
with a lower compression ratio (perhaps 10 or 12 to 1) the
efficiency may be slightly lower than the diesel and there
may be a small power loss - or even gain. This is because
the gas /air mixture with spark ignition can be at stoichiometric.
This means that all the air is used to burn gas. In the case
of the diesel engine with fuel injection the overall mixture
is very lean in order to ensure comlete combustion. Quite
a complicated picture. There is a great deal of this type
of information in the IANGV Position Paper. The Paper covers
10 Chapters in 220 pages. They are still some available to
individuals or companies that join the IANGV. See the web
site on how to join IANGV.
Does an NGV
engine run hotter than a gasoline engine?
There
are several different factors at work that determine the exhaust
temperature of an NG engine as compared to a similar engine
running on petrol. NG engines that are converted from petrol
operation do generally run with hotter exhausts.
There
are several reasons for this - one of the main ones being
that with petrol there is a cooling effect as it evaporates
in the induction system and in the cylinder. This does not
happen with gas. Also a gas mixture tends to burn more slowly
than petrol and so may still be burning when it is exhausting
through the valves.
There
are several things that must be taken care of when doing a
conversion - all important. The cooling system must be in
good order and clean on the water side and the radiator clean
on the air side. The engine ignition timing must be correctly
adjusted (usually advanced) for gas - which tends to burn
more slowly and you have to avoid gas still burning as it
passes through the exhaust system. These are some of the important
issues.
What are the
reasons for engine knock? How do you compare fuels?
There
is a number of possible reasons for encountering engine knock
when operating on natural gas. One is operating in high ambient
temperature conditions. Others are incorrect ignition timing
and changes in gas composition. The issue of knock (detonation)
in engines is a very complicated one since its incidence depends
on the engine design and the operating conditions (eg temperature)
as well as the fuel composition and on the air/fuel ratio
with which the fuel is used. It is also an important one to
the NGV industry since natural gas with a high methane content
has a strong resistance to knock. We are all familiar with
the octane rating for petrol fuels. There is also a method
of rating gaseous fuels - the knock rating of methane is arbitrarily
taken as 100 and hydrogen as zero. A simplified description
of how other gaseous fuel samples are rated is as follows:
they are compared and given a Methane Number (MN) by runnning
them on a special test engine and raising the compression
ratio (CR) of the engine until knock just occurs. Then a mix
of methane and hydrogen that just gives knock at the same
CR is found. The percentage of methane in this test is the
methane number (MN) for the fuel under test. So, on this scale
of methane as 100 and hydrogen as 0, it is found that ethane
comes out at 44, propane as 32 and butane as 8. If you have
a lot of butane in the gas it will be more likely to result
in knock under adverse conditions (eg high temperatures).
Interestingly, sewage gas with 40% CO2 has a MN of 140. It
is also possible and convenient to rate a gaseous fuel on
the usual octane scale and give them an MON number. Now you
can compare methane (MON 122) with petrol directly. Here propane
comes out at MON 97 and hydrogen 63. Designers like to build
their engines with as high a CR as possible since as CR increases
so does the engine efficiency. When doing a conversion job
on an existing petrol vehicle you have little choice in the
compression ratio (CR) that is used. We have run converted
diesel engines at a compression ratio of 15 under controlled
conditions and not had any signs of knock; but this would
not be a practical choice. So in the case of any fuel it is
sometimes not the major constituents that determine the knock
rating but rather the presence of relatively small quantities
of other constituents: these may be impurities or additives.
For example you can take the situation with petrol where the
octane rating is controlled to a significant extent by the
removal of pro-knock compounds (or the addition of anti-knock
compounds) during refining. And small proportions of additives
can also be used to control the onset of knock. Tetraethyl
lead was extremely effective and widely used until it was
shown to have have negative health effects and to act as a
poison to exhaust catalysts. The design of the combustion
chamber shape will also have a significant bearing on the
sensitivity of an engine to knock.
What is the tax
on CNG in US?
As of
March 2000 (NGVC latest survey of CNG prices) in the US a
liter of regular grade gasoline cost on average 39.9 cents,
of which 11.1 cents was state (6.3) and federal (4.8) tax.
A leq of CNG cost 25.4 cents of which about 7.8 cents was
state (6.3) and federal (1.5) tax. Note that state tax on
CNG is an educated guess. Most states charge about the same
tax on CNG as they do on gasoline, and some charge less, but
we don't actually have a state-by-state breakdown. At the
present time, there are no proposals to change the federal
tax on CNG, although the Natural
Gas Vehicle Coalition are supporting efforts to provide
tax credits to the users of various alternative fuels.
What factors
affect the quantity of gas in a cylinder?
The quantity
of the CNG in the cylinder is depending on two factors. Pressure
and temperature. But there is a third factor which has an
effect on the quantity of CNG in the cylinder, the compressibility
factor. The compressebiity factor z is a correction factor
to the Boyle Gay-Lussac Law P.V=z .R.T. The z factor is depending
on gascomposition and temperature. As an example at 150 bar
the z factor is at 250 bar the z factor is -20 degrees C z
= 1.45 -20 degrees C z =1.18 0 degrees C z = 1.3 0 degrees
C z = 1.16 20 degrees C z = 1.2 20 degrees C z = 1.13 40 degrees
C z = 1.15 40 degrees C z = 1.09. This means that at an pressure
of 250 bar and a gas temperature of 20 degrees C the actual
content of the CNG cyclinder is 13 % more than you expect
from the Boyle Gay-Lussac law
What does the
conversion of a diesel engine to natural gas entail?
Click
here for diesel conversion information.
Where can emissions
information and data be found?
Emissions
data can be found in the IANGV
Emissions Report, NGV98
papers, NGV2000
papers, and the World
Bank papers.
Can older vehicles
be converted to run on natural gas?
Converting
cars 20 years old is like playing roulette. A car should be
in the best possible condition BEFORE making the conversion.
We used to have an expression in the early days of NGV conversions:
If you convert a dog (a bad car), it's going to bark just
like a dog AFTER the conversion as it did BEFORE the conversion.
So, it should be tuned, have hoses and belts replaced, checked
for body damage and rust (i.e. before mounting a CNG cylinder
to the body) etc.
The "best"
emission test (and easiest/cheapest) for such vehicles BEFORE
(and after) conversion is a three way tailpipe test. This
indicates CO, HC and NOx. After conversion the HC still will
be high due to methane output (which is not smog-forming or
toxic) BUT, there should be very dramatic reductions -- 85%
or more -- on CO and maybe a bit less on NOx. The beauty of
converting an older, non-catalytic converter car is that their
emissions are terrible. With CNG, the emissions improvement
normally is dramatic. (Hence in computer controlled petrol
vehicles that are converted, there is a lot less dramatic
reduction in emissions because they already are relatively
lower than a carburetted, un-catalyzed car.)
With
conversions on older vehicles, we always found it useful to
run them once a week (an hour will do) on petrol to avoid
all the rubber seals and gaskets from drying out. This also
can save wear and tear on valves and seats, which today are
of hardened steel to make up for unleaded gasoline as well
as 'dry' natural gas.
What is the effect
of variations in gas composition and engine performance?
With
regard to the different gas compositions that you have available,
the issues will depend on what the percentage differences
are for the various gases that make up the mix and also on
what your local regulations are with regard to emissions.
If the
gas compositions do not differ very much - say the differences
on methane content are in the range below 5%, then it is probable
that you will be able to tune for somewhere in between the
optimum for each and there will not be significant differences
in engine performance. However, there could be differences
in emissions levels. In some situations this may not be a
concern. In the case of countries that have stringent emisssions
regulations, it would be difficult to meet the regulations.
So it really depends very much on how large the variations
in gas are and what your emissions regulations allow.
