Although looked down upon, and looking very technologically backward, the tires are actually one of the most critical elements in a car (old cars did have tires, but did not have airbags, for instance). They are one of the most important factors in driving on various surfaces (dry or icy) and are vital system components when trying to prevent accidents. Incorrectly inflated tires are known to have contributed to a significant percentage of road accidents. Even when not producing any straightforward accident, low pressure tires continuously harm the environment by increasing the carbon dioxide emissions of the car (and correcting it is completely costless and easy, as long as you know the car has a problem). To assist the driver in this respect, tire pressure monitoring systems (TPMS) are employed in many premium cars today.
So significant are the effects tire pressure has on the overall behavior of the car that word goes about of making TPMS (Tire Pressure Monitoring System Solution) mandatory for new cars sold in Europe as early as 2012. The final decision in this respect will be taken during the first half of this year (2009) being driven also by environmental friendly groups which argue that incorrect inflated tires generate CO2 through the additional fuel consumption determined by an increased rolling distance and resistance of the tire.
There are currently only a handful of suppliers in the automotive market offering major carmakers customized TPMS (out of which at least one, Continental, is directly affected by the recession which influences the entire industry). Together they have sold a significant number of such systems in 2008 (in tens of millions) and they all realize that they compete for a market in which demand could explode, should pressure monitoring be required by legislation.
Already such systems are mandatory in the US starting with the first month of 2008. According to the law, a pressure drop of 25% is considered acceptable, but not anything less. The people working on European legislation are aware that this value is too high for Europe, where higher speed limits are in place (especially on German motorways). The final figure that will be in place for Europe will probably be below 15% to account for these high speed limits. Under such circumstances, monitoring the pressure in an easy manner is critical, as tires are known to lose pressure just by the diffusion of gas molecules in the rubber, or through leaks caused by faulty valves.
There are extensive studies conducted both by various European Safety Authorities or by TPMS supplier companies (obviously in a bid to convince the carmakers that what they sell is needed) showing various figures of accidents caused by incorrect tire pressure (in France, for instance, these studies claim that almost 10% of road accidents could have been averted by having the tires correctly inflated).
Not only safety is a major concern driving this technology into cars, but also the effect on the surrounding environment is an important consideration. Research has shown that tires with a pressure deficit of over (but close to) 15% would lead to increased resistance to rolling and thus to an increased fuel consumption of about 5%. This becomes an important criterion in a market where major players advertise their cars as having small carbon footprint and where car taxes paid by the end user of the car might be determined by the CO2 emissions of the vehicle they own.
Although TPMS sounds very technologically advanced (and it is, because it evolved), the idea and implementation have been around since the mid 1980s (introduced by Porsche); they only started to be offered as standard equipment during the very end of last century. Today most carmakers offer this feature on their car either as an option or as a standard, on more expensive cars. Many of them do it, though, just to keep up with the competition, as not all of them are convinced of the reliability and accuracy of the measurements.
This is especially true regarding indirect TPMS. These are systems consisting mostly of software applications and using elements in already existing car systems (like for instance ABS speed sensors, which are installed as standard or ESP system elements). The cost advantage of such systems is obvious, as once developed, software does not cost anything. What earns the nomination of “indirect” for this method is the fact that the tire air pressure is actually computed from other variables measured outside the tire. Older versions of indirect systems would only be able to tell when one of the tires would be under-inflated taking into account the increased rotational speed of that tire, which would be determined by the slightly smaller diameter of the tire itself. To overcome this disadvantage newer versions have been developed, capable of detecting decreased pressure in more than one tire, by making a vibration analysis of all four wheels, or by analyzing the load shift during acceleration/deceleration. Indirect TPMS also has the disadvantage of requiring a “system reset” at a good moment in time. The driver is actually required to mark (either through the push of a button or through dashboard controls) the moment in time when all wheels are correctly inflated. If the driver does that when one of the tires has a slightly decreased pressure, this will negatively influence the correct functioning of the system. Indirect TPMS is the main method used with run-flat tires (which are designed to resist puncture and allow the car to be driven a good number of kilometers like that) due to the nature of these tires not being tolerant to internal unwanted elements (like the embedded chip of a direct TMPS, for instance).
But the accuracy of such methods lags behind that of the direct TPMS. These are sensibly more expensive and are rather to be found in more expensive cars, too. They are built around embedded sensors mounted inside the tire and are more reliable and more accurate.
So much about the advantages of the direct method! The disadvantage is not only the increased cost of the system itself, but also the carmakers’ cost of assembling them inside the tire. Continental, one of the suppliers of such systems is at a clear advantage compared against its competitors, as it is also a major supplier of tires, being, therefore able to offer a complete solution to its customers (tire + TPMS). The information about all four tires (sometimes five, when the spare is included) is sent by the electronics around the sensor (and inside the tire itself) through a wireless method and is then displayed on the instrument cluster under board computer control. As a bonus, direct TPMS systems also monitor the temperature inside a tire and can account that for every tire individually.
Besides the cost, the main disadvantage of direct TPMS is the battery required to power each sensor inside the tire; not only does it have a limited life (sometimes you have to change the tire because the TPMS battery has depleted!) batteries do not perform well in various harsh climatic environments (do not forget, the main battery in a car is still lead acid battery, but you cannot put one like that inside the tire). To overcome this advantage, some companies invest in research for getting battery-less TPMS systems on the market. One option at the moment is a method based on the Surface Acoustic Wave technology (also known as SAW); another option would be to use electromagnetic cross-coupling to eliminate the batteries of a conventional system. Also some theorists look into using energy harvesting devices for the same purpose.
The advantages of the direct method are prone to make it the method of the future, as designers are confident in the expanded possibilities of the embedded sensor chip. The current economic climate is the only thing still driving the indirect method, as the solution can cost as low as less than half price from the direct method. Its accuracy makes it not only undesired by car manufacturers, but also illegal in the US only (with a few exceptions).
Browsing the history of TPMS, one can conclude they were rather slow to come into cars due to the reluctance of end users to pay for it, due to the split investment in both existing methods (direct and indirect) which ended up making both of them not as cheap as they could have been, and due to the unreliability of the early versions. They will, however, be on the cars in the future due to legal reasons, safety reasons and environmental reasons.
Many semiconductor manufacturers (for example Freescale) support design of such systems providing almost off-the shelf solutions for designers.
Read the Italian version: Metodi per controllare la pressione degli pneumatici
If you want to know more about Freescale products, please submit your request to Arrow Italy using this form.