Hi everybody. I'm doing a write up on the engine dyno I built for my senior design project last spring. Going to post it on gsrfab.com as we build that site out. The stuff below will be "part 1" of however many.
The kind of things I'm wondering:
Is the technical side appropriate?
Would you like more details or are there enough to give you a headache?
Did you learn anything?
Did you enjoy reading it?
Do you have any feedback I didn't ask about?
And lastly, did you find any technical errors?
With that, thanks and what you think?
Water brake engine dyno
So you think you want an engine dyno, huh? How handy would that be, right?? Have something you can throw an engine on to develop tunes, break in your new engine build, maybe test out some new fuels or something, right?
Well, strap in for some serious thinking. For my senior design project in mechanical engineering, I proposed to the class doing exactly that. After being joined by a few others, we set off on the project.
First, the research. I had pretty much made up my mind already that a water brake was going to be the easiest way to achieve our goals. BUT, that’s not how engineering works, so the team explored and learned about the different types of engine brakes (I.E., engine dyno). Some examples are: gigantic disc or drum brakes, huge electric motors that essentially act like gigantic heater elements (needing liquid or other cooling), one of my teammates even thought it would be acceptable to put an airplane propeller on the output shaft of the engine (it has been done before, after all). All of these options can be viable, but none of them met the “cheap, safe, clean, and cheap” criteria set up at the beginning of the semester. After much debate and learning, a water brake of the toroid type was decided upon. Turns out that toroid is the mathematical term for donut, in case you were wondering. There are patents as far back as the early 1800s for this type of device, the first one designed by William Froud to induce a load on a steam ship (a big one too). The first known toroid water brake was used to absorb 20,000hp, at 90 friggin rpm. Think about that for a second and let it sink in. The equation for hp is:
HP=(Torque∙RPM)/5252
After rearranging the equation to figure out what torque is needed to produce that much power, we get:
(HP∙5252)/RPM=Torque
Dropping what we know into this equation gives us:
(20,000∙5252)/90=1,167,111 ft∙lbs
All by spinning water around in a donut shape…
Second, HOW THE HELL DOES THAT WORK??
And HOW THE HELL DOES THAT WORK??? Well, sit back and prepare to be learned.
So if you remember anything about physics, you might recall that the “definition” of work is a force times a distance. I think they claim this came about from measuring how far a horse could drag a large bag of chips or something, and decided to rate everything from that moment forward based on this finding. So, to minimize your work, you will want to minimize both the force you apply and the distance that force has to be applied for. Take a basic water pump for example, like the kind irrigating the field over yonder. To be an efficient pump, it should take not very much work to run it (to move the water), right? As in, you don’t have to pay a bunch of money to the power company to pump a bunch of water (because it’s not doing much work). Well, if you have ever looked inside of a water pump (or maybe even a turbo?), you might have noticed there are blades in there that spin. The basic idea is that water usually enters in the center and travels in as straight a line as possible to the point of exit. This can be in less than one rotation of the pump even! The key point here though is that it’s traveling in a relatively straight line. That is what makes it efficient. By traveling in basically a straight line from inlet to outlet, in less than one rotation, the force being applied to the water to get it to move has been minimized. This is where blade shapes come into play, and people get PhDs studying that sthi. They are the ones squeezing a hundredth of a percent gain in efficiency out of some pump used to move chemicals in a plant somewhere, so that the company pays $3 less a year in electricity. And god bless em, because we get to use that tech for things like moving water through an engine block at the appropriate pace without creating a bunch of parasitic drag. But I digress…
To recap, a pump moves water (or any fluid really) in as short a distance as possible to reduce the work, needed to drive the device. As a side note, now is the time to introduce the idea of power, which is work times time. Phrased in another way, it’s work done over a period of time. Back to the horse thing about work, now they see how fast a horse can drag the bag of chips some set distance, an magically you have horsepower.
SO, say we wanted to make a device that we hook a big, high horsepower engine up to in order to determine just how much hp and or torque we gain (or lose) from duct taping magnets to the fuel line. If we hook up a plain old water pump to the drive shaft, we are going to pretty much have to be parked next to a lake because we are going to move a literal metric whAt? of water with that thing. That’s because a water pump is designed to move water from point A to point B using as little power as possible.
Are you beginning to see where this is going? If not, that’s ok. Anyway, to make a water brake instead of a water pump, we are basically going to reverse what we are building it for and turn it into the worlds worst water pump. Something that takes so much work to turn, that it can actually stall the engine/motor that is making it turn. Any guesses on how we might do that? If not, that’s ok, we are all learning here…
To make the worlds worst water pump, we are going to play with the distance thing and make the water travel as far as it possibly can from point A to point B. To visualize this, now would be a good time to go grab a donut. Seriously. You shouldn’t have to go far though, as there is probably one on the desk somewhere. Now grab something long and skinny, preferably not attached to anything (dental floss, shoestring, a long strand of hair, etc) and drop one end of it through the hole in the donut. Now, round and round we go, keeping the wraps relatively close together. I promise you will run out of your chosen long skinny stuff before you get halfway around the donut (unless you grabbed the wifes knitting or something, which won’t fare well with the glaze on the donut). At this point, you should be able to make the logical leap that the distance traveled in this manner will lead to the theoretical maximum possible distance traveled without the fluid (string/hair/etc) changing directions (which we don’t really want to do inside of the device, supposedly). You might also guess that this maximum possible distance is much, much, much greater than that of a normal pump.
NOW, we have talked about two ends of the spectrum with the distance aspect of things. Why haven’t we talked about the force aspect of things? Well, the forces involved in both situations are going to be close enough to the same that we can pretend that they are actually the same. A handy little trick used throughout an engineering education. We are going to assume the forces are the same for the worst pump and the best pump because they are both moving water through them, and the distance traveled is so much different that spending time on the forces would be like trying to figure out why some people by more shoes than others.