Farming

Friday 25 July 2014

Release the pressure and watch your diesel fuel bills plummet

Published 04/12/2012|05:00

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By lowering air pressure in their tractor tyres, farmers could make significant fuel savings in addition to reducing the soil damage that their tractors are causing on soft ground.

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That's the advice from tyre specialists Michelin, who demonstrate how it can be done on a YouTube video accessible on the Michelin channel and available at the link http://tinyurl.com/9ubxqcp ?

The video experiment features two Fendt 724 tractors, both fitted with Michelin's new Ultraflex XeoBib tyres.

One set is inflated to conventional pressures of 21 psi front and 23 psi rear, while the other is inflated to very low pressures of nine psi front and 10 psi rear.

By lowering air pressure, the video shows how the design of the more flexible sidewalls enlarges the contact patch to create shallower ruts, reduce wheel slippage, deliver greater speed and cut down on soil damage.

In a parallel demonstration, viewers can also see how significant fuel savings in the order of 20pc can be made by using these pressures.

As seen in the pictures, fuel use was accurately measured in both scenarios in the experiment through the use of vertical glass columns which showed how much diesel was burnt for each run.

Using the tractor running on conventional pressures, there was 9cm of fuel left in the glass column after the experiment.

Just three tyre lugs were in contact with the ground at any one point as the tyre pressures were quite high.

Wheel slippage was estimated at 30pc.

During the second part of the experiment using the tractor running on lower tyre pressures of nine psi front and 10 psi rear, four tyre lugs were observed to be on the ground at any given point.

This reduced wheel slippage to about 15pc, leaving shallower ruts and generally causing less damage to the soil.

But the real eye-opener came when fuel use was measured when running at these lower tyre pressures.

This time, some 29cm of fuel was left in the glass column; the equivalent of a 23pc fuel saving.

Based upon these figures, with a consumption of 25 litres per hour for a tractor used for 400 hours per year and green diesel price of 95c per litre, the fuel saving achieved could be as much as €2,000 per annum.

I have to admit, when I first heard that lower pressures could result in fuel savings I was very sceptical. It seemed counter-intuitive.

I had always thought that lower pressures would lead to increased rolling resistance and, therefore, if anything you would see marginally increased fuel usage from running at lower pressures.

But according to Dermot Forristal, senior researcher and machinery expert at Teagasc, there is a scientific basis to Michelin's claim.

"As long as we are talking about a fieldwork situation, then, yes, Michelin is correct in saying that lower tyre pressure can increase contact area and reduce field rolling resistance [and therefore fuel use]," Dermot explained.

"The fuel savings they are talking about are attained through reducing rolling resistance. Rolling resistance can be simply thought of as the effort to pull the loaded wheel.

"It is important not to confuse this with a road situation, where tyre flex is the only contributor to rolling resistance and so, for a given tyre, reducing inflation pressure would increase rolling resistance, as the deforming tyre absorbs more energy.

"But with agricultural tyres in fields, especially tillage fields as in this particular experiment, rolling resistance is dominated by soil deformation – in effect, tyre sinkage.

"Tyre carcass rolling resistance is miniscule in this situation, as all the energy is used deforming the soil. Even small increases in wheel sinkage will significantly increase rolling resistance.

"Consequently, in a field situation, anything that can be done to reduce sinkage will reduce rolling resistance and therefore fuel consumption as well.

"Reducing tyre pressure marginally increases the tyre flexing rolling resistance component, but it will reduce the sinkage rolling resistance component by a much greater extent on a cultivated soil, so you have a net reduction in power and fuel requirements."

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