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Tyres are an important factor in passenger vehicle energy use. Globally, improvements in tyre energy efficiency that lie well within existing capabilities could reduce fuel consumption by 3 to 5% across existing passenger vehicle fleets. The result would be a global reduction in greenhouse gas emissions of more than 100 million metric tons annually. These benefits can be achieved at relatively low cost through the design and sale of more efficient tyres which can be produced without sacrificing safety or other important design attributes.
For many years, nitrogen inflation of tyres has been common for tyres fitted on race cars and Tour de France bike tyres as well as aircraft, trucks and vehicles used in mining and other industrial applications. Even the moon buggy had nitrogen in its tyres. Nitrogen is also used in the tyres of vehicles that operate in hazardous areas, such as mines, to reduce the risk of fire. It is commonly used in off-highway vehicles where the tyres operate at their maximum load and are highly stressed.
Nitrogen is a colourless, odourless, tasteless, and non-toxic gas that forms about 78% of the earth's atmosphere. It is non-combustible, non-flammable, non-corrosive and environmentally friendly in pure form. It is the biggest molecule gas, so it is the slowest gas to migrate through a tyre and as a result there is very little loss of pressure. In fact, it maintains pressure up to four times longer than ordinary air and does not attack or oxidise the rubber of the tyre from the inside. In addition, it doesn't hold heat and tyres therefore run cooler.
Experiment between nitrogen inflation and air inflation
Figure 1 - Comparison of tyre wear per mile for nitrogen and compressed air inflated tyres. (Tyre nitrogen filling system, Dept of Mechanical Engineering, Clemson, South Carolina)
A study at Clemson University in South Carolina found that nitrogen’s lower permeability allowed the tyre to maintain correct pressure much longer, thereby decreasing the surface area of the tyre subjected to friction. Rolling resistance of tyres inflated with nitrogen was found to be 70% lower than those filled with compressed air. The table below, taken from the study, shows a comparison of wear per mile for front and rear tyres inflated with nitrogen and compressed air.
The net effect is to decrease tread wear thereby extending the useful life of the tyre. This effect is more dramatic when looking at fleet vehicles due to the greater distances travelled and longer operation times. Based on the results of four independent studies, tyre life in fleet vehicles was extended by an average of 57.5%. Passenger vehicle tyres have been shown to last 31% longer when using nitrogen inflation.
Inflation pressure has a significant impact on a vehicle’s fuel economy. In a study by Energy and Environmental Analysis, Inc., tyres proved to be directly responsible for about 15 to 27% of typical fuel consumption. For every 1 psi drop in inflation pressure, fuel economy decreased by 0.3%. In a field survey conducted by the NHTSA, it was found that 85% of all vehicles had at least one underinflated tyre and 29% had at least one tyre seriously underinflated (by >8 psi). The overall impact of this is the loss of as much as 13 billion gallons of fuel a year.
The key to nitrogen tyre inflation
Proper tyre inflation is an important factor for achieving safety, environmental and economic benefits. Standards requiring the installation of tyre pressure monitoring systems (TPMSs) as a safety and efficiency measure are an effective strategy aided by the global spread of the technology. Consumer education programs and automotive service “check and inflate” programs are complementary options that will continue to provide benefits even in areas where TPMSs are implemented. Both types of programs will improve safety and will reduce the number of tyres that wear out early because they are underinflated. Pure nitrogen might leak more slowly through the liner, but the condition and pressure of nitrogen filled tyres still needs to be checked regularly.
Advantages of nitrogen filled tyres
The key benefits are a slower rate of pressure loss and a cooler running temperature.
Nitrogen escapes through the tyre’s inner liner or tube at a slower rate than regular compressed air. For example, it might take up to six months to lose 0.14kpa with nitrogen compared to just one month with compressed air.
As tyres heat up the inflation pressure increases, which in turn reduces the size of the tyre’s footprint - the area that has contact with the road. The tyre then loses grip because of this smaller footprint. The cooler the tyres run the better they will grip the road.
Nitrogen also leads to reduced corrosion - unlike air there is no moisture in pure nitrogen.
Disadvantages of nitrogen filled tyres
Maintenance – optimal tyre efficiency requires proper maintenance, particularly the maintenance of correct tyre inflation which affects rolling resistance.
Availability and affordability of nitrogen tyre inflation in South Africa
There are several methods available to help maintain pressure; tyre additives, automatic tyre pressure maintenance systems, TPMS systems…but all involve a large initial investment per vehicle, and none have the proven performance record of nitrogen. A simple TPMS system for a tractor/trailer will cost about R14 000 in 2019, an automatic pressure maintenance system even more. But none of these can alleviate the inherent problems with compressed air; oxidation, permeation, and contaminants.
Changing to nitrogen involves removing all the air which is already in the tyres and then re- inflating them with purified compressed nitrogen. There is sometimes a once-off charge per tyre - once filled with nitrogen any future top-ups would also have to be with nitrogen if any advantages are to be realised. The increased savings realised from nitrogen filled tyres will more than cover the initial cost of converting to nitrogen.