Regarding CO2: Number of shifts

[Dheeraj Chowdary]

Hey all. We are using a pneumatic gear shift system with a manual clutch. Our specs are as follows:

CBR250R engine. Actuated by a double-acting cyinder of 16mm diameter with 100mm stroke. We assume 9Nm as actuation torque over a lever link length of 100mm. CO2 is used for actuation. The minimum actuation pressure is 4bar, so we are running at 7 bar for high speed shifting.

Using the above parameters, I've used the ideal gas equation using a pressure of 10 bar, to be safe, to calculate the mass of CO2 used per shift. It's around 0.03 grams. So for over 2500 shifts, that's 2.6 ounces. I'm thus planning to buy a 3 oz container.

Is this alright? I have observed that most others in the forum have recommended a 9 oz container. Please advise. (Calculations are accurate, triple checked).

Thanks in advance.

[BeunMan]

I can't check your calculations, but keep in mind the effect of the expanding gas on the temperature of the components. We had (minor) problems with it.

[MCoach]

Keep in mind some efficiency losses of air in the system. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Try setting up the system in a test situation and run an endurance expected shift schedule and see where you end up on actual air used.

Also note that during attachment and detachment of the CO2 air bottles there is some loss, or even a lot of loss if it is not screwed on quickly enough to seal.

[Owen Thomas]

You will want one larger than 3 ounces for two reasons:
1) Testing. Replacing the CO2 container at every pit stop will be a pain in the arse.
2) Insurance. What if you develop a leak somewhere at high temperature? Or cold temperature like BeunMan suggested?

I would leave packaging space for something larger and plan on testing the 3oz bottle with a full endurance run. Basically just don't design yourself into a corner because "the numbers worked out".

[Markus]

I'm not completely positive of the following information so take this with a grain of salt:
-we run 9oz bottle with 10 bars of pressure
-we get around >2000 shifts (could be 2500)
-we would use higher than 10 bars to achieve better shiftings but our solenoid safety valves are set to 10bar
-below 7 bars we encounter shifting problems (about same piston-D but shorter stroke)

Recommendations:
-Most of the problems in the system are temperature induced: the pressure regulator is everything but temperature stabile. Solenoid safety valve leaks were quite common nuisance due to raise of operating pressure
-We've also blown a couple of bottle safety valves because the bottle is in direct sunlight when mounted into the car
-Be prepared to try higher pressures - you might find performance on lower pressures inadequate
-Don't go too small on the bottle: couple hundred of grams on the bottle is not much compared to being stuck in gear during endurance.
-Bigger bottle is convenient during testing
-Bigger bottle also gets you further if you have a small "regulated" leak...
-If you don't want to get 9oz bottle make it 6oz. RF = 2 is not so much. Calculations are always theoretic, i.e. rules to describe what someone has observed.
-If you still want to go 3oz, make sure your mounting allows changing the bottle to a bigger one if necessary

In our perspective it was safer to run a 428-chain with 75kW engine than a smaller bottle... But we also did heavy and long testing days so having more gas was convenient. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Dheeraj Chowdary]

Beunman, MCoach, Owen and Markus, thank you so much for your inputs. Just what I wanted. Practical advice from the experienced. Ordering a 9oz right away. Durability is paramount for us. Much appreciated.

[MCoach]

When there is background information and I can see that you've tried to wade your way into this swamp we know as FSAE, then I am more than happy to help.


And thank you for being very reasonable. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[BluSTi]

Account for leaks. Our air shifters never worked right, always leaked, and we often ran out of air after short periods of testing.

My 0.02, I don't like them.

[Cardriverx]

My advice is to avoid CO2 all together. You have to weigh them to check how much you have left, and sometimes you can have issues with liquid CO2 getting in the lines and flash freezing stuff.

We have used HPA for two years now, never had an issue *knock on wood*.


Originally posted by Dheeraj Chowdary:
Beunman, MCoach, Owen and Markus, thank you so much for your inputs. Just what I wanted. Practical advice from the experienced. Ordering a 9oz right away. Durability is paramount for us. Much appreciated.