Tire and energy efficiency

Michal Sura [email protected]

On July 17 th 2021, European Commission adopted a package of proposals to make the EU's climate, energy, land use, transport and taxation policies fit for reducing net greenhouse gas emissions by at least 55% by 2030. China decided to cut its energy intensity by around 3% in 2021 to meet climate goals. Energy efficiency is an important topic for today’s transport. We are facing to global energy crisis because the shortage of energy across the world. It seems that energy will be not cheap in the next few years. Transport consumes around 19 percent of EU’s energy use and we have to make transport more energy efficient, if we want to reduce energy consumption and this step will help also to reduce greenhouses emissions. In the following analysis we will make focus on rolling resistance and see what would be possible reduce it.

It is known that any vehicle requires a propulsion energy to operate. To increase the propulsion efficiency of a vehicle there is necessary to understand why the energy losses occur. A part of the propulsion energy is lost due to rolling resistance. A rolling resistance is caused by a deformation cycle of the tire that creates the energy dissipation in a form of heat and contributes to overall energy loss. We will briefly discuss what rolling resistance is and how we can reduce it.

The earliest passenger vehicles had iron placed on wooden or iron wheels. In 1887, a Scottish inventor John Boyd Dunlop developed the first practical pneumatic or inflatable tyre for his son's tricycle (1). In 1891, Edouard Michelin was granted a patent for the first detachable tire with an inner tube for bicycles, making punctures easy to repair (2). Pneumatic tires were able to reduce shocks and vibrations and in late of 1890’s there appeared the first cars equipped with pneumatic tires (Figure 1.)?

Figure 1.

Rolling resistance

The rolling resistance is defined as a rolling friction between a road and a tire? without sliding (Figure 2). The rolling coefficient Crr depends on road roughness, road material, tire material, tire structure, tire temperature, tire pressure, tire tread pattern, etc.

Figure 2.

The rolling resistance Fr depends on the vehicle’s mass ??, the acceleration constant g, the rolling resistance coefficient Crr and the road grade α.?

Fr = Crr mgcos α

Since most of roads have small road grade α, only a few degrees, there is possible to approximate equation to a form:

? Fr = Crr mg?

Rolling friction

Rolling friction force is generated when a body rolls on a surface without sliding. When tyre is in a dead-stop position there is possible to a symmetric pressure distribution (Figure 3).

Figure 3.

A rubber tire consists of various polymers, they are formed in a tangled mass. Rubber polymers are tangled around each other and their neighbours, and when the material is stressed, the polymers unwin and dragging their chains across each other. This way is a friction created, which is then dissipated as heat. Rolling resistance of rubber tire is caused by a hysteresis. The energy used to deform the tire at the beginning of contact patch is greater than the energy recovered at the end of the contact patch (Figure 4). The energy loss is caused by the creation of heat in the rubber when the tire deforms.?

Figure 4.

The energy losses mean that in the rear part of the contact patch, which corresponds to the unloading part of the loading cycle, the pressure between the wheel rim and track is reduced because the tire? does not spring back because it is not perfectly elastic. The pressure distribution is no longer symmetrical and? the centre of pressure moves forward and let suppose it moves forward by distance e (Figure 5).

Figure 5.

Let’s think in terms of forces, instead of energies to find out a rolling resistance force. Let’s suppose that there is pulled a cart at steady speed with a load N (N=mg). When the wheel reaches the contact point, there is possible to see the resultant force Fc that passes trough the axle centerline. When we split resultant force Fc into its two components; a force that is equal to the load component N, but acts in opposite direction and a horizontal shear force Fr acting on a wheel through the contact patch. The horizontal shear force Fr that resists forward motion is the force that we are looking for and it is called rolling resistance force Fr (Figure 6).

Figure 6.

We prefer this kind explanation of rolling resistance force. It is obvious that a more flatten tire has a bigger contact patch and distance e increases (the pressure centre is moved further forward) what will increase the rolling resistance force. A properly inflated tire, on the other hand, has a smaller contact patch. The pressure pressure centre is not shifted as forward as in the case of an underinflated tire, which results into smaller rolling resistance force Fr.

Here is another definition of rolling resistance:

Rolling resistance is a result of energy loss in the tyre, which can be traced back to the deformation of the area of tyre contact and the damping properties of the rubber. These lead to the transformation of mechanical into thermal energy, contributing to warming of the tyre. (J?rnsen Reimpell Prof.Dipl.-Ing., ... Jürgen W. Betzler Prof.Dr.-Ing., in The Automotive Chassis (Second Edition), 2001)

When the vehicle is travelling at low speeds in a town rolling resistance has almost 50% share of total propulsion energy consumption. On rural roads, energy consumption is determined up to 40% by the rolling resistance, whereas at higher speeds the air drag is the determining factor. The Figure 7 shows a study carried out by VW on the Golf (3).

Figure 7.

From sixty to seventy percents of the rolling resistance is generated in the running tread and its level is mainly dependent on the rubber mixture. Low damping running tread mixtures improve the rolling resistance, but at the same time reduce the coefficient of friction on a wet road surface. It can be said that the ratio is approximately 1:1, which means a 10% reduction in the rolling resistance leads to a 10% longer braking distance on a wet road surface. The use of new combinations of materials in the running tread (use of silica) has led to partial reduction of the conflict between these aims (3).

Some features of tires which affect propulsion energy consumption?

Tire structure:

• the smaller mass of the tire the lower rolling resistance?
• the rubber compound used to produce the tire
• polymers (butadiene rubbers, natural rubbers, styrene-butadiene rubbers)
• fillers (soot and silica)
• vulcanizing agent (sulphur)
• the process of production of compound (duration, temperature, etc.)

Tire size:

• the lower tire height, the more rigid will be, lower rolling resistance
• the narrower tire has lower aerodynamic resistance

Tire wear:

• the more worn tire has lower rolling resistance, because its mass decreases

Tire pressure:

• low tire pressure causes large contact patch and increases rolling resistance
• a reduction in pressure by 0.3 bar means a 6% rise in rolling resistance
• a reduction in pressure by 1 bar means 30% rise in rolling resistance and increasing propulsion energy consumption

From 1 May 2021, the new EU rules on the energy labelling of on-road tires apply at consumer level (Figure 8). Updating the label first introduced for car and van tires in 2012, the new rules require that tires for buses and lorries must now be labelled. There is possible to see energy-efficiency of a tire. If we want to save fuel and emissions, we should use the most economical tires possible.

Figure 8.

References:

1, https://www.bicyclehistory.net/bicycle-inventor/john-boyd-dunlop/

2, https://www.automotivehalloffame.org/honoree/edouard-michelin/

3, https://www.sciencedirect.com/book/9780750650540/the-automotive-chassis