|
Columnists
Sustainability
and Sustainable Development
Larry
Olson, PhD Professor ASU
Polytechnic
Larry
Olson, PhD., Professor, Arizona State University Environmental
Technology Management Program. Dr. Olson holds a Ph.D. in Chemistry from
the University of Pennsylvania, and is an environmental chemist with
interests in remediation technologies and international environmental
management. He can be reached at 480-727-1499 or by email at Larry.Olson@asu.edu.
Nitrogen
For Your Tires: Is it Just Hot Air?
April
/ May 2009
It’s hard these days to avoid
the advertising blitz for using nitrogen (at $5 or more per charge) to
fill your tires rather than just compressed air. Among the benefits
claimed are:
* better control of tire
pressures because the loss rate is less for nitrogen than air
* lower rolling resistance
for tires resulting in better gas mileage and treadwear
* lower running temperatures
and fewer blowouts due to tire failure.
First of all, I’m not a car
guy and claim no particular expertise in this area. But from a chemical
standpoint, is there any basis for these claims?
Dry air is composed of about
78.1% N 2,
20.9% O2,
almost 1% Ar, and various other gases in ppm quantities. So air used to
fill your tires is already mostly nitrogen and even the nitrogen gas
available at tire centers is not pure. Typically it may be between 94-99%
N2.
The molecular weight of O 2
(32 g/mole) is higher than that of N2
(28 g/mole) and that means the N2
molecules are moving faster than O2
by a factor of √32/28 = 1.07. If there was a small
hole in your tire or in the seal between the rim and tire, then nitrogen
molecules would be expected to leak out more quickly.
But diffusion through the walls
of the tire is a different matter. Here the size of the molecules is
important and N 2
is actually a slightly larger molecule (4.1 Å long and 3.0 Å wide) than
O2
(3.9 Å long and 2.8 Å wide). This may seem counterintuitive since there
are more electrons around O than N, but there are also more protons. Since
all the valence electrons in O are about the same distance from the
positively charged nucleus, they don’t shield each other very well and
so an electron in O feels a stronger positive charge making oxygen atoms
smaller than N atoms.
Tires lose pressure
continuously since rubber is permeable to gas molecules as well as from
losses through the tire/wheel/valve interface. There are a lot of
anecdotal claims about nitrogen’s performance vs compressed air, but one
of the best controlled studies was led by James D. MacIsaac Jr. of the
National Highway Traffic Safety Administration (http://www.regulations.gov/fdmspublic/ContentViewer?objectId=0900006480739e82&disposition=
attachment&contentType=ppt8). He looked at 17 different tire types and
measured the Inflation Pressure Loss Rate (IPLR) using ASTM standard
F1112-06 which measures the static loss of pressure over time under
controlled temperature and pressure conditions. Pressure transducers able
to measure to 0.25 psi and accurate to ± 1% were used. He found that the
average IPLR for nitrogen was only 66% of that for air and that the
benefits applied to all tire types and inflation pressures. A similar, but
less rigorous, test by Consumer Reports was conducted with 31 different
tire types (http://blogs.consumerreports.org/cars/2007/10/tires-nitrogen-.html).
One tire of each type was filled to 30 psi with 95% N 2
and another was filled with air. After 1 year, the tires filled with air
lost on average 3.5 psi and those with nitrogen lost 2.2 psi – also a
difference of 66%. So it does appear there is evidence that the IPLR for
nitrogen is less than that for air.
MacIsaac found no direct effect
on rolling resistance for tires filled with N 2.
Any improvement in mileage or treadwear for a nitrogen filled tire was
therefore likely due to lower fluctuations in tire pressure. So the lesson
is that you may need to check your air filled tires more frequently than
if they were nitrogen filled, but performance for properly inflated air
tires is equivalent to nitrogen.
They also tested tires filled
with a 50:50 mixture of oxygen and nitrogen and oven aged for 5 weeks at
65 oC.
These tires showed significant deleterious effects, but there were no
differences between tires filled with nitrogen or compressed air. The
claim that air accelerates tire degradation was not borne out, at least in
this test.
GM (http://www.gminsidenews.com/forums/f53/what-gm-says-about-nitrogen-tires-2005-already-51446/)
acknowledges that pure nitrogen should reduce oxidation of tire
components, but since only a small amount of oxygen is necessary for
oxidation, the commercially available nitrogen that may be only 95% N 2
might not provide any real benefit in this area. Interestingly, the NHTSA
study found that tires inflated with almost pure nitrogen (>97%)
actually showed diffusion of oxygen into the tire after 90 days.
Thus, even if you start with nitrogen, ultimately there will be some
oxygen present inside your tires.
So what is the verdict? All tires, no matter
what gas is used to inflate them, lose pressure over time, but the loss
rate for nitrogen is only about 2/3 that for air filled tires. If you don’t
keep a close check on your tire pressure, there can be some advantage for
using nitrogen. But in normal driving you can derive essentially the same
benefit, for less cost, from simply maintaining proper air pressure.
2008/1234
|
