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X5 E70 (2007 - 2013)
E70 BMW X5 produced between 2007 and 2013. Discuss the E70 X5 with other BMW owners here.

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Old 06-20-2012, 07:08 AM
Pourboire Pourboire is offline
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Location: Scottsdale, AZ
 
Join Date: Jun 2012
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Mein Auto: 2013 X5 35i
Since air contains 78% nitrogen, it has been argued that pure nitrogen has little, if any, advantage over air. However, this argument assumes that the compressed air in our tires is free of moisture, but it is not, as evidenced by the water and oil that must be drained from a compressor's tank. This is of particular importance because as we put heat into a tire, the moisture in the compressed air will cause the air in the tire to expand at a faster rate than a dry gas. Furthermore, water vapor causes the air to expand in a non-linear fashion based on the amount of vapor present, which is usually unknown. Since processed nitrogen is a dry gas, there is no moisture. So, while inflation pressure will still increase with tire temperature, the absence of moisture means there will be less increase in
pressure and the increase will be consistent.

The graph below illustrates the difference in expansion characteristics between nitrogen and moist air. Notice the nitrogen pressure increases at a slower rate than that of the compressed air and that the increase is linear. Expansion of the compressed air, on the other hand, increases at a notably faster rate and, as previously mentioned, in a non-linear manner due to the presence of water vapor. The degree to which it is non-linear is dependent upon a number of factors including the ambient humidity when the compressor was last run and the ambient temperature when the tire was inflated. As a result, moist compressor air will increase the hot inflation pressure of your tires at a rate which is essentially unknown, making your car's handling less predictable. When referring to the graph, note that the “Temperature” axis is referring to the tire’s internal air temperature. Remember that the gas in the tire is heated by not only the friction of the tire sliding against the surface of the track, but also from the
brakes heating the wheel. Flexing of the tire sidewall makes a contribution also. The amount the air heats up depends on how hard we use the tires and the brakes.

Graph has been uploaded







In order to gain a sense of how much temperature increase we might expect, we can use the Ideal Gas Law which states that PV = nRT. In this equation, P = pressure, V = volume, n = the amount of the gas, R = the gas constant (for the appropriate gas), and T = temperature. With a little manipulation, we can state this equation as P/T=nR/V. If we assume that n,R, and V are constant, then we can set the equations for both cold inflation and hot inflation equal to each other to get P1/T1=P2/T2. The temperature must be in degrees Kelvin. If we assume a cold inflation pressure of 20 PSI, an air temperature of 70 °F (294.3 K), and a hot inflation pressure of 24 PSI, solving for T2 we find that the temperature of the air has increased to 176 °F (353.1 K)!

Another frequently mentioned advantage is the fact that nitrogen permeates through the sidewall of a tire more slowly than air. Although the difference is small and requires a substantial length of time to create a significant difference, it is to our benefit that it leaks out more slowly. However, since tire pressures are set frequently, this phenomenon alone does not warrant the use of nitrogen.



Some proponents of nitrogen in passenger car tires point out that oxygen in the air will, given sufficient time, oxidize the rubber causing it to break down. However, tire manufacturers formulate the rubber compound to slow this attack. Hopefully, a set of tires will be worn out from racing long before they suffer significant deterioration from atmospheric exposure. However, because nitrogen is a dry inert gas, it slows internal degradation of both the rubber tire and the metal wheel. This is of particular significance for cars where the tires are infrequently replaced.

Since few people have unlimited racing budgets, cost is another facet of this decision. A common size of nitrogen cylinder (Ř9.25 inches x 60 inches) rents for approximately $70 for six months. That's relatively inexpensive compared to the cost of a compressor which may cost several hundreds of dollars and requires either gasoline or electricity to operate. Another plus to using nitrogen is that it eliminates the need for air compressor maintenance and you probably already have enough pieces of equipment to maintain.
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