Fuel consumption – European tire label

gasoline

fuel consumption

In the previous post we talked about tire noise, we have deepened the causes and some solutions to reduce it.

Today we talk about the second mark in the European tire label, in particular how tires affect fuel consumption; this parameter is related with tire rolling resistance.

What is the origin of this resistance? Which parameters affect it? Let’s have a closer look.

Hysteresis

Let’s go immediately to the central point: when the tire touches the ground (enters the contact patch), it is deformed by the reaction force of the ground. When it leaves the contact patch not all the energy received is released. This behavior is named as hysteresis of the material.

Hysteresis and fuel consumption

How can we imagine this behavior? What does this concept, maybe still conceptually empty, means? The secret is the viscoelastic origin of the material. Let’s imagine the tire compound as a dish of spaghetti; these are glued each other in some contact points. When an external force is applied to spaghetti, these tend to stretch, but being glued each other a total stretch is not allowed, so the external energy is stored as elastic one.

If gluing points were perfect, when external force is released all the elastic energy would be released too; but in the reality this in not like that because every spaghetto slides respect others and a part of energy is dissipated as heat. This is the reason why the material is called visco-elastic.

So when this dish of spaghetti touches the ground, part of energy is stored and then released, another part is lost as heat. Due to this effect the pressure distribution on the contact patch is not a symmetrical parabola, but the peak is slightly translated onward respect to the wheel rotation axis. The resultant vertical force generates a moment opposite to the driving torque.

Rolling resistance and fuel consumption

The main parts of the tire where energy dissipation is concentrated are:

– Thread: about 70%;
– Sidewalls: about 15%;
– Beads: about 15%.

In order to give some additional information, compared the global vehicle motion resistance, the rolling one has the following influence:

– 20% on highway;
– 25% on backroad;
– 30% on city road;

Parameters that influence fuel consumption

We can distinguish two categories: intrinsic parameters and the ones that can be controlled by the driver.

First group

In the first group of course we can find the compound “recipe”, that with its ingredients represented by natural rubber, styrene, butadiene, carbon black, silica, sulfur and so on, defines the hysteresis level for the finished product, also function of temperature and frequency of road unevenness.

Another important parameter is the thread thickness and void ratio that is related to tire grooves dimensions: the bigger are the channels, the greater are tread blocks deformations, so the more energy is dissipated as heat.

The tire diameter is another influent parameter because the bigger is it, the smaller is the tire bending on the contact patch at leading edge, having as consequence less deformation.

It is not possible work with these parameters, the only thing that we can do is look carefully the European tire label. In order to reduce fuel consumption we can buy “green” tires; working with some data we can consider that if we use a green tire instead “black” ones, we can reduce up to 30% of rolling resistance with a real fuel consumption up to 6%. These numbers are not the truth, but are useful to understand better the phenomena.

Second group

In this group we can find tire pressure, because if it is constantly maintained in the optimal range allows reducing fuel consumption.

Vertical load increase also the rolling resistance, at the same tire pressure. So if for example we take our car for a holiday with our family, and we put on it 2 bicycles, a canoe and the car is full of baggage the fuel consumption is higher also because the tire rolling resistance is higher.

High speed increases rolling resistance because tire is affected by strong waves and vibrations. In addition to being a low efficiency condition, it is also dangerous for the integrity of the tire.

How to reduce fuel consumption

reduce fuel consumption

Going directly to the target, in order to reduce fuel consumption we should read carefully the European tire label when we buy a new tire set and take care of its state, having a periodical inspection in order to check tire pressure and wear, also for safety reasons.

A higher class on the label may mean a tire with a lower hysteresis, but if fuel consumption is reduced, on the other side also grip is reduced, because it depends on the hysteresis too.

The higher class may mean a compound with less percentage of fillers as carbon black, but also in this case in the other side tire wear gets worse.

As usual the optimum is a compromise between what we need.

Tire noise – European tire label

Tire noise - European tire label

Buying a new tire set you may have seen a label like this.

It is just a summary of informations about the characteristics of the tire we are buying, in terms of fuel consumption, wet grip and sound emission.
This label is subjected to European regulations, and tire manufacturer must declare  tire class on three fields, defined behalf standard tests.
The difference between the last and the first class can be summarized as follows:

  • Fuel consumption: tests define tire rolling resistance coefficient. From G class to A the consumption is reduced by 7,5%;
  • Wet grip: From G class to A the braking distance is reduced by 30%;
  • Sound emission: one black wave corresponds to a silent tire, 3 waves indicate a noise one, and there is always the value in dB.

Let’s explore the three categories, starting with the sound emission.

Tire noise

Who have driven an electric car, will have noticed that when the car stops there is no sound…is very difficult understand if it is on or off.
The “sound” changes when the speed increase, the driver is able to listen the aerodynamic and tire noise.
Why tires are noisy? Which are the main reasons?

Sidewall vibrations

Tire noise from sidewalls pumping

During wheel rotation, the tire portion that touch the ground entering the contact patch causes tire deformation.
One of the main parts involved in this deformation process are the sidewalls, due to their lower stiffness compared to the other parts of the tire.
When this region leaves the contact patch, the corresponding portion of the sidewall return to its undeformed shape.
This cyclic deformation create pressure waves in the air with a variable frequency in the range from 500 Hz and 800 Hz.

Horn effect

Tire noise horn effect

Let’s look the tire from the side, as if it were bidimensional. When the tire region enters the contact patch, pressure waves are generated and their propagation is toward the vehicle running direction, and due to the shape of the cavity between the tire and the ground, like a horn, the amplitude of wave pressure is increased.
The same effect is produced when the tire leaves the contact patch.

Tread pumping

Every tread has is own sculpture, created mainly to improve handling on every type of ground, dry, slippery or to drain in the best way the water behalf circumferential and lateral channels.
When the tire region touches the ground, tread blocks are deformed and as consequence also grooves. The air that fills the grooves is pushed out the tire and noise is generated. In proximity to the trailing edge of the contact patch the tire returns to its original shape and the air comes back to fill the grooves.
The amplitude of this phenomena is a function of:

Grooves dimensions, the bigger are, the more is the noise;
– The angle between lateral grooves and vehicle running direction, proportional to the noise generated;
Vehicle speed, also proportional to the noise generated.

Cavity noise

Tire noise - cavity

When the tire is radially deformed pressure waves propagate also inside the tire, in the area between the tread and the wheel rim, and noise is generated.

Stick-slip

Tread blocks in contact with the road are subjected to cyclic stick and slip. In some conditions these vibrations enter in the hearing frequency range, so we can hear the typical tire screech. The frequency of these vibrations depends on blocks dimensions and stiffness.

Tire noise is also influenced by wheel torque, not as source itself but as a parameter that can modify the amplitude of the phenomena; the greater is the torque, the noisier is the tire.
Furthermore, it is influenced proportional to inflation pressure. For low wheel torque the trend can be reversed, so a tire with higher inflation pressure can be less noisy.

Tire noise reduction

Tire noise reduction

In order to reduce the noise produced by tread is useful optimize grooves dimensions and its angles, finding a compromise between noise and the need to have a good grip, also on wet.
It is possible also create a circumferential offset between internal and external tread blocks.
The internal noise can be reduced using foams (in the following image Contisilent) to damp carcass vibrations; it can be mounted also on the rim internal channel. The disadvantage of the first option is the increased moment of inertia of the wheel, so it needs more power to be accelerated or slowed, but foams are very light.

Another option to reduce sidewalls noise is change their stiffness using a different compound. Anyway, each of these solutions is a compromise between low noise level, a good grip and a low fuel consumption.

TPMS

TPMS

TPMS is the acronym of Tire Pressure Monitoring System, and nowadays is widely used in production cars. How does this system work? How many types exist?

Tires pressure and handling

Before talking about the system we need to understand why is important monitoring tire pressure. The answer seems to be simple, the reason is security. But from which phenomena does security depend?

Starting from a reference condition, the contact patch of tire with an excessive inflation pressure is reduced. In this condition main effects are a reduced grip and an irregular tire wear, more in the center and less on the external surface. Due to the increased tire vertical stiffness the ride comfort is worse. Furthermore, the steering wheel is “lighter” due to the reduced self-aligning moment of the tire, i.e. the moment created by the tire lateral force that in normal conditions is in opposition to the steering moment.

At the opposite a too low tire pressure leads anyway to a reduced contact patch and an irregular tire wear, more on the outer and less in the center. Vertical stiffness is reduced because vertical stiffness is lower, but at the same time strain amplitude is increased and more heat is generated inside the tire, increasing the possibility to damage.

In both cases tire cornering stiffness changes, in this last case decreases, so tire tends to generate less lateral force.

Tire deflation

Pressure can be lost mainly in two ways:

  • Tire damage or valve failure: this two cases lead to a fast deflation, and are the most dangerous;
  • Air diffusion: it consists in air leakage through tire compound due to its permeability. It is a slow process, but for this reason car manufacturers advice to check tire pressure regularly. In order to reduce air diffusion, tire manufacturers introduce on the inner part of the compound a special layer, called inner layer, with a low permeability.

Types of TPMS

There are two types of systems: direct and indirect. The main differences are the measurement accuracy and the cost, higher for the first one.

 1. Direct system

Is a stand-alone system. Is composed by pressure (and sometime also temperature) sensors mounted instead of normal inflation valves that send the informations as a signal to one or more antennas. The last ones send the signal to the Electronic Control Unit and are processed.

TPMS valve

This kind of TPMS is able to measure not only id the tire pressure is too high or too low, but indicates which wheel is affected to the problem and shows the values of pressure and temperature for every tire.

The software used to process the signals must take in account all the different conditions in which tire pressure varies but falls in the normal range, for example the increased or reduced vertical load on the tire  due to load transfer on braking or turning.

In order to have always the correct reference pressure, the system bust be initialized correctly. When tire pressure is changed manually, or when tires are changed, the reset button must be used. From this moment learning phase starts: sensors start to record the pressure, using that values as reference. Being a very sensitive system it should be initialized correctly to avoid false alarms.

Among advantages there are of course measurement accuracy and the possibility to read pressure and temperature values.

Some weaknesses are the higher cost of the system than the indirect one due to necessity to design a stand-alone system; the possibility of failure of one or more sensor and the presence of a battery with its own life cycle.

 2. Indirect system

This TPMS does not need additional components but are used speed sensors already mounted on the wheels and used by ABS system.

This is the first advantage, i.e. a cheaper system because no additional components must be installed, programmed and calibrated. The disadvantage can be found on the title of the paragraph: because it is an indirect system, tire pressure is not measured but estimated…even better the difference between a reverence value and a limit one is estimated.

How does it work?

Let’s imagine that we are driving our car with all four wheel with the same size on a straight line, If tires have the same pressure the linear speed of each of them, calculated as product between angular velocity $\omega$ and rolling radius, must be equal to the other ones and to the vehicle one. If one tire is deflated, to reach the same linear speed of the vehicle it must increase its $\omega$. The system measure this change due to the reduced rolling radius and a warning appears on the dashboard.

As usual the real principle is not simple as explained. The dimension of rolling radius i modified by many factors, some of which are:

  • Longitudinal and lateral load transfers;
  • A different load distribution on the four wheels, for example because we are living for the holidays and our car is full of bags;
  • Just driving on a turn, because the four wheels have a different angular speed;
  • On high speed, because the tire is deformed on a radial way;
  • Different wear between one tire and one other.

All this factors must be evaluated by ECU.

Every time that we change tires, also this system must be correctly initialized at the new reference pressures.

One more disadvantage is that the warning message is very generic, no value is displayed, nor in which wheel there is the problem. Just a warning light is displayed, like in the following image.

TPMS warning light

Some indirect TPMS are able to estimate the pressure reduction caused by air diffusion. Through frequency analysis of belt oscillations, linked to inflation pressure, are able to notice the problem and a warning alarm is displayed.

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