F1technical.net

How do you calculate the temperature of the air inside the tire? I have the initial pressure and temperature of the tire and the pressure after the run. Using the ideal gas equations gives me some ridiculous results. What are the equations for say nitrogen?

For example I start with 210kPa and 25°C and after the run the pressure is 290kPa. Ideal gas equations give me (25°+273°)*290kpa/210kpa=139°+273°
I think 139°C is too much because the peak rubber temperature is 120° and the average is 98°. I would expect air temperature close to the average rubber temp as it is the source of the heat.

I agree with your maths, if the final pressure is 290 then the final temp is ~130 C

I suppose the exterior surface of the rubber cools a bit in the wind.

Are you measuring the tyre surface temp? Don't forget that the heat has to conduct through the rubber + belts, as well as experiencing convection and radiation away from the surface the tyre.

I have the surface temps from the entire lap. It gets to 120° inside the turn so I don't think wind plays a role. If parts of the heat are going elsewhere then the air inside should be even cooler. Where is that extra heat coming from?

Additional heat could be coming from the brakes. Might be interesting to measure rim temperature, it's no coincidence that F1 rims nowadays have complex surfaces to control tire temperature by transferring heat from the brakes, through the 'air' in the tires and to the inner carcass.

Id guess the calcs are right. Consider that the air inside the tyre is a fixed volume of non moving air which is surrounded by a hot tyre carcass on one side and a hot wheel (due to the brakes) on the other side. So there is a lot of heat soak and very little cooling available for this air.

On the other hand, the tread surface has a very large surface area subjected to an endless supply of fast moving cooling air so its not surprising that the tread surface temp is a lot lower than the internal air temp.

You have to use both absolute temperature and pressure in ideal gas equations - not gauge pressure.

Question, are we talking about F1 cars here or a car that you have ready access to the you race or something similar? In a f1 car the starting tyre pressures are much lower than you give. I believe it was last season that Pirelli had to put minimum tyre pressures allowed because teams were running tyre pressures to low that were destroying them which led to what happened in Silverstone with 3 (maybe) blowouts.

The brakes also transfer a low of excess heat to the rim even in road car applications when used heavily if the brake rotor is close enough to the rim.

Jersey Tom wrote:You have to use both absolute temperature and pressure in ideal gas equations - not gauge pressure.

So I have to add the atmospheric pressure to both pressures? That indeed works better as I get 95°C which is very close to my expected temp of 98.

Good catch tom

Tim.Wright wrote:Consider that the air inside the tyre is a fixed volume of non moving air....

That is an assumption made by all. But why should it be the case? A tyre is not completely rigid and I guess its shape (hence enclosed volume) is likely to vary a little with a temperature.

I was speaking in the context of the cooling effect that this air has in comparison to the external air. In this case, I believe its a fair assumption to assume its of a fixed volume and not moving and as a result it is not going to have much of a cooling effect on the carcass.

But OK, for general pressure, temperature calculations, it would be more precise to account for some change of volume as well.

Other thing to consider is humidity. I suspect that you'll have around 20% humidity in the tyre regardless of how well you purge and how dry the nitrogen/air you use is.

Tyres are hygroscopic and will absorb atmospheric humidity. The only way to eliminate this is hot purging, which was done in F1 (circulate nitrogen continuously overnight with the tyre in a blanket at 60-80 degC) This is now banned.

Anyway, back on topic, if you factor in the vapour pressure of 20% humidity then the temp at which you achieve your hot pressure will be lower than you think.

It wouldn't surprise me if the gas temp is lower than a needle probe temp on a hot tyre.

Ben

Race tires are pressurized with dry nitrogen gas, and there is very little water vapor content in this industrial compressed nitrogen product. However, there may be a small amount of ambient air mixture trapped within the internal tire volume after the tire is mounted, unless this air volume is purged when the tire is inflated with dry nitrogen.

Filling the tyre with dry gas does not influence the adsorbed moisture much and it does not influence water absorbed in the tyre at all. The influence of adsorbed and absorbed water is clearly visible in every vacuum device. I don't know exact numbers in case of tyres but the rubber is quite porous material.