Most Common Energy Harvesting Mechanism Used in TPMS Sensors (RFID, Piezo, EM, etc.)

What is the most used “energy harvesting mechanism” in Tire Pressure Monitoring Systems (TPMS)? (e.g, RFID, Piezoelectric, Electromagnetic, Thermoelectric, Other…).I am wondering what kind of transducer fits the most in such a sensor system.
Thanks

very beneficial

All the systems I have seen have batteries in the sensors.  Presumably these would be small watch type batteries or coin cells.  I could see a possibility for a rotating cap on the sensor that had a magnet in and induced a voltage in a coil on the "stationary" part of the sensor, but this would only work when the wheel was in motion.  You could use a supercapacitor for energy storage but your space is extremely limited.  And you'd have to wring the last bit of power saving out of the sensor system - but I don't think you have to transmit too often, maybe once every 5 - 30 seconds?

Like David, the only ones I am familiar with use batteries- and to make things worse, you have to pull the tire off the rim to get to the battery, AND the battery is encapsulated in a block with the sensor, so they want you to throw the whole thing away and replace it with a new one every 5 to 7 years.  An energy harvesting system would make a lot more sense.  I'll bet some manufacturers are doing that- but I am not aware of any.
David- you mentioned limited space- In the case of my Ford Ranger pickup truck the sensor is mounted on a belt that is strapped around the rim..  It is entirely inside the wheel, so there is tons of space available.  The sensor only needs to work when the vehicle is in motion, so I would think a magnet on the stationary support and a coil inside the wheel would be doable.-Rick   

Thanks guys, that's very interesting. I never had such a system in my cars, but I see that we can buy it online and mount it onto the valve (one CR1632 battery for each sensor, battery life up to 24 months).But Rick, if a battery would last 5-7 years, it is probably more then the life of the sensors...so why should a manufacturer bother to insert an energy harvester? I heard of a TPMS without batteries, that is feeding the sensors with energy from RF reader, but I didn't see it online.-Yair

All the ones I have seen advertised here (Australia) have the sensors mounted on the valves on the wheels, in place of the valve cap.  They all say that spare sensor batteries are available.  I didn't see any estimates of battery life though.  A year or so would be ok, coin cells are not expensive.

The sensor should last for a log time.  The battery is the only thing that limits the life to a few years. FYI- Many of the built-in TPMS systems only turn on an icon on the dash if one of the ties is low- but it doesn't tell you which tire or how low the pressure is. For my old Jeep, I installed a Nonda ZUS Tire Safety Monitoring System.  The sensors mount on the valve stems and have easily replaceable coin call batteries. This system is a little more expensive than some, but it integrates with other stuff I already had.  It indicates the pressure AND temperature of each tire.-Rick