CNG CARS SAFETY

How Safe are Natural Gas Vehicles?


Natural gas is an environmentally clean, plentiful, low-cost, domestically produced fuel that is used in
more than 70,000,000 homes and businesses throughout the US and as a transportation fuel for
motor vehicles. But is it a safe vehicle fuel?
Any fuel, including those used in motor vehicles, can be dangerous if handled improperly. Fuels
contain energy, which is released when the fuel is ignited. Gasoline is a potentially dangerous fuel, but,
by understanding how to handle it, we have learned to use it safely. The same is true of natural gas.
Natural gas safely generates our electricity, heats our homes and cooks our meals. But, like gasoline,
natural gas must be understood and respected in order to be used safely.
Natural gas is fuel that occurs naturally and requires very little processing before use. Chemically it
normally consists of over 90% methane with smaller amounts of ethane, propane, butane, carbon
dioxide and other trace gases. Pure methane has a very high octane rating (120-130) and the high
methane content of natural gas in turn gives natural gas this same attribute. The low carbon content of
methane also allows for the potential of low carbon and green house gas emissions
As with all vehicle fuels, natural gas can be used safely if the unique properties of the fuel are
understood and common sense procedures are followed. In fact, natural gas has safety advantages
compared to gasoline and diesel: it is non-toxic, and has no potential for ground or water
contamination in the event of a fuel release. Natural gas is lighter than air and dissipates rapidly when
released. An odorant is added to provide a distinctive and intentionally disagreeable smell that is easy
to recognize. The odor is detectable at one-fifth of the gas’ lower flammability limit.
Natural gas vehicles have an excellent safety record for two primary reasons: the properties of the
fuel itself and the integrity of the natural gas vehicle and its fuel delivery system.
Natural gas has a very limited range of flammability – it will not burn in concentrations below about 5%
or above about 15% when mixed with air. Gasoline and diesel burn at much lower concentrations and
ignite at lower temperatures. Although it takes very little energy to ignite a flammable mixture of air
and natural gas, gasoline, or diesel, natural gas burns at a somewhat lower temperature.
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Source: Murphy, Michael J., Properties of Alternative Fuels, Federal Transit Administration, 1994
From the gas field to the vehicle’s engine, natural gas requires very little processing to make it suitable
for use as a fuel. Gasoline and diesel must be processed from crude oil in large and complex oil
refineries. After water vapor, sulfur and heavy hydrocarbons are removed, natural gas flows by pipeline
(the safest way to transport energy) directly to the fueling station where it is compressed for use.
Alternatively it may be used as Liquefied Natural Gas (LNG) by being cooled to cryogenic temperatures
on site or at a central facility and delivered by truck. Gasoline and diesel are delivered to fueling
stations by tank trucks over the highway.
At a compressed natural gas fueling station the gas is compressed before being provided to vehicles
at 3000 to 3600 pounds per square inch (psi). Stations can deliver a “fast fill” to vehicles in minutes
or, using a “slow fill” strategy, in a few hours to overnight.
Although the use of high storage pressures might appear dangerous, compression, storage and fueling
of natural gas vehicles meet stringent industry and government safety standards. Remember that highpressure
gases are used safely every day in industrial and medical applications.
Natural gas powered vehicles are designed and built to be safe both in normal operation and in
accidents. New OEM natural gas vehicles are subjected to the same federal government crash tests
as other vehicles. OEM natural gas vehicle fuel systems must meet Federal Motor Vehicle Safety
Standards 303 and 304. Natural gas cylinders are much thicker and stronger than gasoline or diesel
tanks. Cylinders are designed not to rupture when fully fueled over six times a day, 365 days a year,
far beyond what they will see in service. Industry standards test them far beyond normal
environmental and service damage risks. Cylinders must even withstand a bonfire test and
penetration by a 30-caliber bullet without rupture! The cylinders are designed for a specific lifetime
from 15 up to 25 years and are required to be inspected every 3 years or 36,000 miles.
No matter what the fuel, fueling stations, indoor parking structures and repair garages must be built to
ensure high levels of safety. Again recognizing the unique requirements for facilities handling natural
gas and natural gas vehicles will differ from those for gasoline or diesel vehicles. For example, leaking
diesel and gasoline form puddles on the floor. Natural gas normally rises toward the ceiling and
disperses. Therefore the danger of fire would be greatest near the floor for liquid fuels and near the
ceiling for natural gas.
Data collected over time has demonstrated natural gas vehicles to be safe in actual operation. Based
on a survey of 8,331 natural gas utility, school, municipal and business fleet vehicles (NGVs) that
traveled
178.3 million miles:
• The NGV fleet vehicle injury rate was 37% lower than the gasoline fleet vehicle rate.
Property Natural Gas Gasoline Diesel
Flammability Limits (volume % in air) 5-15 1.4-7.6 0.6-5.5
Autoignition Temperature (°F) 842 572 446
Minimum Ignition Energy in Air (10-6 BTU) 0.27 0.23 0.23
Peak Flame Temperature (°F) 3423 3591 3729
• There were no fatalities compared with 1.28 deaths per 100 million miles for gasoline fleet
vehicles The collision rate for NGV fleet vehicles was 31% lower than the rate for gasoline fleet vehicles
• The fleet of 8,331 NGVs was involved in seven fire incidents, only one of which was directly
attributable to failure of the natural gas fuel system.
Natural gas vehicles were first commercialized after World War II in Italy. There are now over twelve
million in use worldwide. Natural gas vehicles have been used in the US since the early 1970s, with
over 120,000 in use today. Yet there has been only one fatality in the US involving a NGV in all that
time and it was attributed to human error.
Even more important than statistics is the confidence that natural gas vehicle users feel. One in five
new transit buses are natural gas powered, with about 12,000 transit buses and community shuttles in
service. Police in Rocky Hill, CT report “the safety record of the (NGV) cars has been excellent.” The
Department of Energy states that “after rigorous testing… (the King County, Washington police) found
their… (Compressed Natural Gas) cars to be as safe and reliable as conventional vehicles”
Not only are transit agencies and police using natural gas vehicles, more and more school buses are
now powered by natural gas. The Department of Energy worked with a major school bus supplier to
develop “an ultra-safe and low-emission” natural gas powered school bus. School buses from that
manufacturer and others, using natural gas engines , are in widespread use today with over 3,000 in
service.
How do natural gas vehicles behave in crashes? The strength of the natural gas cylinders and fuel
system generally avoids any leakage or fire. For example an accident involving a CNG-powered pickup…
proved to be a testimonial to the safety of CNG tanks. As reported in the May 1995 edition of
Automotive Fleet:
When the 1992 CNG pick-up was broadsided in Midland, Texas, the most vulnerable part of the
fueling system bore the brunt of the hit. While the force drove an imprint of the tank safety valve into
the side of the truck, the CNG tanks did not rupture, and driver Jimmy Oden walked away.
And in a tragic 1998 accident, a stopped bi-fueled Honda (a vehicle which could run on either natural
gas or gasoline) was impacted by another vehicle moving at nearly 100 mph and a fire fed by gasoline
broke out. The 50-liter natural gas fuel tank was intact and remained secured in its support brackets.
(Reported in a June 1998 BC Gas press release).
Nationwide Insurance, in looking at the safety of natural gas buses in a fleet, concluded as long ago as
1992 that “…the natural gas powered vehicles will be the safest vehicles in your fleet and (we) have no
reservations about insuring them.” (Palmer, Pat, Nationwide Insurance, letter to Kenneth E. Bauman
Bus, Inc., September 10, 1992)
In summary, technical data, appropriate safety regulations and years of experience show natural gas
vehicles to be as safe as, or safer than, conventionally fueled vehicles.