I have to design a car that fits inside a shoebox and whose purpose is to go between 60 - 100 ft (or meters, need to check) carrying between 0 and 500 g. It’s a chemical engineering contest, so the point is to choose a reaction to power it and get a calibration curve. My group decided on a pressure reaction (thermal decomposition of H2O2), but we’re not sure how to propel it. There can’t be any exhaust. Last year we used a single stroke piston that had a piece of string attached to the rear axle. It didn’t work very well. Any suggestions for another mechanism.
What was the problem with the piston design? It seems to be your best bet with the propulsion you’re proposing. If you can get the compression down right, in other words, proper dimensions and sealing, it should work fine.
Forgot to ask, what kind of assemblies do you have access to? Can you have someone build you a more complex design, or do you have to act as the machinist yourself?
This is the first thing that came to my mind:
A small turbine connected to the drive shaft with the H2O2 on one side of the turbine and an “exhaust chamber” (like an empty soda bottle) on the low pressure side. Maybe throw in a check valve if necessary. For the turbine, you might be able to use the impeller from a hairdryer.
Another idea:
Have the H2O2 reaction expand gas in a chamber contained by a liquid. The expanding gas will drive the liquid out and through a turbine into a waste chamber. Hopefully the attached pic is legible.
Idea #3:
Have the reaction push down a horizontally mounted piston. Attached to the base of the piston is a rack that would drive a pinion gear connected to the drive shaft. On the rack, remove some of the teeth closest to the piston so that the “car” can roll freely after the piston has bottomed out.
[quote]slimjim wrote:
Forgot to ask, what kind of assemblies do you have access to? Can you have someone build you a more complex design, or do you have to act as the machinist yourself?[/quote]
We can submit our designs to the machine shop and they’ll do it for us, but we have to have all the specifications. I tried getting advice from them and it was like getting a defendant to admit they’re guilty.
The problem with the piston design was really just that our piston wasn’t long enough and we couldn’t get the minimum distance. We’d need one roughly three times as long to cover the range we need, and I’m not sure if that would fit.
edgecrusher: Your first and third ideas are what I was thinking of. I wasn’t sure how to implement the third one so I’ll check that out. The second one is definitely interesting, I hadn’t thought of that but it could work better than idea #1 as there wouldn’t be any backpressure to limit the distance it could go. I’ll need to check the logistics of the volume for the car, though.
The big problem is I’m the only senior on the team, there are a couple juniors and sophomores and that’s it. Last semester I couldn’t get anyone to do anything. I’m definitely not wasting my whole senior spring on this, but I’d really like to say I was designed a really good car (because I’m definitely the one making decisions).
not sure how much pressure you can build with the gas from the reaction, but what if you just used the reaction to load a coil spring. Think of those matchbox cars that you roll backwards to “wind up” and then let go.
You could use the gas to wind the spring on the wheels, with a pressure regulated switch to release the catch at a certain point. Only concern would be either having the drive wheels lifted off the ground while the spring is loading (so the car doesn’t roll backwards, or have some sort of gear assembly on the pressure switch to engage the wheels.
Use diet coke and a menthos.
Well for the “no exhaust part” could you not have a reservoir for spent gases?
Can you install in impeller directly connected to a drive axle inline with the flow of pressure?
Think of an impeller in a turbo. An axle could extend out to drive wheels directly connected to the impeller shaft (actually the drive axle and impeller shaft would be one).
The gasses would flow past the impeller, spinning it, driving the drive wheels and into to a low pressure container.
The power would be delivered relatively evenly from the start and for a longer duration than the quick aplication of power and resulting coast.
Minimal engineering involved and very few parts to fail.
Crap! Sorry edgecrusher. I think I just copied your idea!
[quote]veruvius wrote:
slimjim wrote:
Forgot to ask, what kind of assemblies do you have access to? Can you have someone build you a more complex design, or do you have to act as the machinist yourself?
We can submit our designs to the machine shop and they’ll do it for us, but we have to have all the specifications. I tried getting advice from them and it was like getting a defendant to admit they’re guilty.
The problem with the piston design was really just that our piston wasn’t long enough and we couldn’t get the minimum distance. We’d need one roughly three times as long to cover the range we need, and I’m not sure if that would fit.[/quote]
Size would be a problem here, I don’t have any better ideas than what edgecrusher has already given you, but I’ll think about it.
[quote]edgecrusher wrote:
Another idea:
Have the H2O2 reaction expand gas in a chamber contained by a liquid. The expanding gas will drive the liquid out and through a turbine into a waste chamber. Hopefully the attached pic is legible.[/quote]
This would probably be the easiest form to set up, and its generally easier to predict the force from a liquid than a gas (more consistent).
Also, a deflated soda bottle, etc could be used as an exhaust resevoir.
I think the nmost accurate would be using a piston, like edge’s third idea. However, there are a lot of problems to run into using this type of design.
How about an electrolytic reaction and just drive a motor?
this is just a random thought, and feel free to point out any problems with it as my chemistry knowledge is weak at best. Are you limited to the single reaction? What if you were to use the same design as last year, single stroke piston, but instead of driving the piston with the pressure of the gas from the initial reaction, use the gas in another reaction to drive the piston with explosive force.
Unless I’m completely off base, I’m pretty sure H2O2 will decompose into O2 and water. So, if you put a little bit of Sodium in the piston chamber then shoot a bunch of water and oxygen in there, you should end up with a pretty powerful piston. The sodium should react with the water, and burn the oxygen, leaving you with some sodium hydroxide (lye), water, and maybe a bit of hydrogen gas if it doesn’t also get burnt up (although I’d think it would). Since it’s just a single stroke, you don’t have to worry about venting the exhaust from the reaction.
I’ve a strong suspicion that I overlooked something, other than this being rather dangerous. And I’d guess it’s not allowed due to the second reaction, but it’d certainly be fun to try.
[quote]yorik wrote:
How about an electrolytic reaction and just drive a motor?[/quote]
Just load up a couple of potatoes or lemon juice to let it go, will have to butcher one of those clocks though.
derek: no worries
Wressler: if the rack and pinion is what concerns you, this might be a good solution:
Take “idea #2”, but put a piston in between the H2O2 and liquid and remove the “valve” (like #3 but with a hydraulic drive). I think the key in all of these is optimizing the relative cross sectional area of the “piston” where the reaction is occurring to that of the drive device or medium. (I don’t know if actually made any sense)
I’m partial to #2 because you can cheat by using the existing hydrostatic pressure, and the ancient Romans or Greeks(I forget which) used to use a similar device to open temple doors.
Good ideas are being pitched here, but after my meeting today, we decided to see what we can do with a voltaic cell and a motor. It will be a hell of a lot easier to calibrate than anything involving a gas. Remember, the competition is about accuracy.
So revised question: how do we go about doing a voltaic cell car? We set up a voltaic cell and just have that power a motor? Is there anything else involved that I don’t know about? What’s the linkage between the motor and the axle? I’m assuming radio shack will have all our needs, but I’ll have to check that.