Self-inflating tire technology
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Joined: Jun 2010
24-11-2010, 11:27 AM

Self-inflating tire technology

SIT system description

The SIT system is based on highly reliable and proven peristaltic pump principles. It uses
the weight and motion of the vehicle to inflate the tire as needed, sourcing air from the
outside atmosphere. The whole system consists of only two components – a tube chamber
functioning as a peristaltic pump for the tire and a pressure management device to control
the inflation.

The peristaltic tubing is located longitudinally between the rim and the tire wall and copies
almost the whole perimeter of the rim. Normal tire deformation caused by the weight of the
vehicle creates a closure of the tubing at its lowest point. As the tire moves against the
road this closure pushes the air contained inside the tubing into the tire and simultaneously
it pulls outside air back into the tubing. As a result, the tire is inflated with the contents of
the tubing with each wheel revolution until it reaches its desired pressure.

Tests conducted on a regular passenger car wheel have proven that the forces between
the deformed tire wall and the rim are sufficient to generate significantly higher pressure
than what is needed for tire inflation.

SIT peristaltic tubing solution

The peristaltic tubing can be implemented into the tire wall in the following way. As there
already exists room between a regular tire and rim where the reinforced tire wall is
periodically pressed against the rim due to the weight of the vehicle, the tubing can be
created as a crevice in the tire side wall. When tire and rim are assembled together, the
crevice is sealed by the pressure between them (which is sometimes more than ten times
higher than the required tire pressure) and therefore it becomes a sealed tubing inside the
tire wall. This could be accomplished by a simple design change in the tire side wall,
which, simplified, could be achieved by a modification of the mould in the normal tire
production process. The crevice in the tire wall and its sealing during the wheel assembly
is shown in figures 3 and 4.

It is also possible to create the tubing outside the tire wall as a separate unit placed
between the rim and the tire. In that case the tire would just lean on the tubing and enclose
it by its deformation, as shown in figure 2. Such a setup was used during the prototype

Therefore, SIT can be either a part of the tire or a part of the rim. Upcoming road testing of

a pre -production prototype will show which one of the two options will be more suitable for
the introduction on the market.

SIT pressure management solution

The above-described inflation system would pump up the tire with every wheel revolution
regardless of whether the tire is under-inflated or not. Therefore a managing system has
been designed to turn the inflation ON in case the tire is under-pressured, and turn it OFF
once the desired pressure inside the tire has been reached.

In case of a properly inflated tire the managing system connects both intake and outlet of
the tubing with the inside space of the tire. Therefore the air only circulates between the
tubing and the tire with each rotation of the wheel while the pressures remain almost
equal. In addition to the intake opening into the tire there is another opening from outside
of the tire, equipped with a check valve. This setup of properly inflated tire is shown in
figure 5, where the internal air circulation is indicated by the red arrow.

When the tire pressure falls below its optimal level the pressure management device
closes the intake from the tire and instantly creates a vacuum at the intake part of the
tubing. This vacuum opens the check valve and the tube starts pulling air from the outside
atmosphere into the tubing and subsequently into the tire. Once the correct tire pressure is
reached, the pressure management device again opens the intake of the tubing from the
tire, the tubing and tire pressures equalize and the check valve closes down. This setup of
under-inflated tire is shown in figure 6, where the stream of inflation air from the outside
atmosphere is indicated by the green arrow.

In summary, the SIT consists of the tubing with a single opening (1) at its one end leading
into the tire and two openings at its other end, one of which (2) leads back into the tire and
is equipped with the pressure management device, and the other intake opening (3),
equipped with the check valve, leads outside the tire.

The pressure management device in its simplest form is a container of compressed air
equipped with a membrane. This device is located inside the tire and is therefore
surrounded by the air pressure of the tire. The pressure inside this container is pre-set to
any desired tire pressure depending on the type of the vehicle the tire is intended for. The
membrane is located against the intake opening (3) of the tubing from the tire. As the tire
pressure falls below its desired level (the pre-set pressure of the container), the air inside
the pressure management device will expand and push the membrane towards the
opening (3) and close it. Resulting vacuum in the tubing starts pulling air from the outside
atmosphere and the tire gets inflated. Once the tire pressure reaches its optimal level
equal to the pressure of the management device, the membrane is drawn out of the
opening (3) and the inflation stops. As the pressure management device is always
surrounded by the pressure of the tire, there is no big demand on its robustness – due to
its design, the pressures inside it and outside it will be most of its lifetime equal, and
occasionally slightly lower in its surrounding (the inside of the tire) when the tire is under-
pressured until it gets properly inflated again.

Beside its simplicity, where the whole system consists of only the tubing, check valve and
pressure management device, there is another great advantage of this setup. Statistics
show, that in order to solve under-inflation caused purely by regular tire leakages, the SIT
has to inflate the tire only in every 3000th revolution; or in other words, it would be in
operation for 3km out of every 10,000km drive. It means that it would not inflate the tire for
the remaining 9,997km. Therefore, most of the time there would be only inside air
circulation between the tubing and the tire back and forth with equal pressures between
them, which further increases the durability of the system.

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