Deflation and punctures - A TPMS debate

By David Shaw, ERJ editor

One of the debates going on within the TPMS (tyre pressure monitoring systems) working party due to meet next week in Geneva, is the difference between pressure loss due to diffusion and pressure loss due to punctures.

It is an entirely false debate, created to allow car makers to save money at the expense of safety and fuel economy.

Everyone agrees that tyres lose pressure over time, as the air within them gradually seeps out through the material of the tyre. This pressure loss is called diffusion. It takes weeks or months for pressure lost in this way to become dangerous.

There are a number of other ways for a tyre to lose pressure. In some cases the valve leaks. In other cases, the tyre strikes the kerb or other obstruction, or a sudden manouevre causes the seal between the tyre and the rim to temporarily break. This allows a pulse of air to escape.

The best-known cause, however, is a puncture. Punctures can be slow or fast. Often they are caused by nails or other debris lying on the highway. If the nail remains in the tyre, plugging the hole, then the tyre might retain plenty of pressure for an hour, a day, or even a week.

If the nail falls out after penetrating the tyre, then the tyre can easily lose all pressure within a few minutes. This is still plenty of time to bring the vehicle to a controlled stop in safety, provided the driver is made aware of the danger. This indeed is the main safety-related purpose of a TPM system.

All parties at the GRRF group have agreed that when a single tyre is affected by a puncture, the TPMS system should detect it and alert the driver within 10 minutes. The evidence shows that, for most situations, this is enough time to allow the driver to slow down and stop somewhere safe before the tyre loses so much air that the vehicle becomes uncontrollable at speed.

The debate starts when two or more tyres are affected. Among the car makers, this is seen as either a rare event, or a situation that develops over weeks and months, and thus does not require a fast reaction time.

Among the tyre makers, however, it is seen as something which affects over half the vehicles on the road at any given time. Thus, say the tyre makers, the situation for multiple tyres is no different to that for a single tyre and the system should alert a driver within a few minutes.

The car makers, however, have persuaded the tyre makers to accept a time limit of 30 minutes for this special situation. Tyre makers reluctantly agreed to this, but continue to believe that it is a dangerous compromise forced on them by the car makers.

Strangely, this compromise time limit of 30 minutes agreed in meetings, emerged in the draft document as either 30 minutes or 60 minutes, with the decision to be made at the upcoming meeting in Geneva. Tyre makers are adamant that 30 minutes is already too long and that any further compromise would seriously affect both safety and fuel consumption -- and hence CO₂ emissions.

Car makers, however, would like the extra time because their favoured low-cost solution (indirect pressure monitoring) finds it hard to detect pressure loss in multiple tyres. Thus, the indirect systems needs more time to alert the driver to a potentially dangerous situation.

The nature of indirect systems is to compare the rolling diameter of one tyre against the other three on a car. When one tyre rotates more times per km than the other three, the system interprets that as a pressure loss and alerts the driver. If two tyres lose pressure at the same rate, the system becomes a bit confused and if if all four tyres lose pressure, then many indirect systems can find it almost impossible to detect the problem.

These systems are cheap to implement because sensors already exist on the vehicle (in the anti-lock braking systems) which detect wheel rotation rates.

Even so-called 2nd generation indirect systems which use characteristic vibration patterns within the tyre to monitor pressure, cannot guarantee to detect these pressure losses in a short time period. 

The alternative -- direct systems -- require a pressure sensor within each wheel, and more hardware to transmit that information back to a central control system. The difference in cost is about euro 20 per vehicle. The benefit, however, is that there is an absolute reading of pressure. The driver can see if the pressure is 2.4 bar or 2.6 bar from the comfort of the driving seat. With indirect systems, there is no such information; only a warning, driven by sophisticated computer algorithms, that one or more tyres might be under-inflated.

The vehicle makers' argument is that diffusion losses are separate from punctures. Diffusion losses, they say, take place over weeks and months. Meanwhile the data shows that any given tyre will get one puncture every 100 000 km or so, which means very few drivers will ever get two punctures simultaneously.

The tyre makers argue that these pressure losses are not independent. The evidence shows that the vast majority of drivers ignore tyre pressure. For millions of European drivers, their tyre pressures are only checked during the annual service, or when changing from Winter to Summer tyres (or Summer to Winter).

Many cars on the road have tyres under-inflated by around 10 - 15 percent on all four tyres through diffusion losses. These figures are strongly supported by extensive real-world surveys of vehicles on Europe's roads today.

The current TPMS specification requires an alert to be given only when the pressure loss reaches 20 percent.

If one of these under-inflated tyres then suffers a puncture, the current draft of the specification gives the TPMS system between 30 and 60 minutes to alert the driver. This, say the tyre makers, is unacceptable. The system should react to a puncture within the 10 minute time frame, even when the other tyres are partially under-inflated.

There is plenty of evidence (see links below) to show that direct systems bring specific, tangible benefits to inflation pressure, while indirect systems do not. The car industry has so far failed to produce any evidence to show that even cars fitted with 2nd-generation indirect systems are better at maintaining inflation pressures than cars with no TPMS system at all.

The tyre industry therefore wishes to ensure that the UNECE specification does not permit systems which are intrinsically unsafe to be allowed on European roads.  It is not, they say a debate between the philosophy of direct -v- indirect, but about the capability of the systems to deliver fuel saving and improved safety. If  an indirect system can be designed which is safe, accurate and effective, then they would welcome the cost advantaqes, but to date, that is not the case. 

In a further safety compromise, the car makers say the limit should not be 20 percent -- a number agreed in earlier meetings --but there should be an extra allowance to compensate for inaccurate pressure readings. This would reduce the sensitivity and accuracy of the pressure reading, and thus increase overall CO₂ emissions across Europe.

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Tire Pressure Maintenance--A Statistical Investigation from NHTSA

TPMS TF Conclusions V04.pdf