Mechanical Components of an
Inertia Dyno
Please use some common sense in
the construction of a dyno, your life will depend on it!!
(The following is an example of what not to do.)
Recently...
It has come to our attention that someone has built dyno using a flywheel
made from railroad car wheels. This in itself is not all wrong, assuming
of course they are properly balanced etc.
However, connecting them to an engine in a direct drive configuration
and spinning them to 6000 rpm!!!!! is NOT a good design.
Remember the laws of physics still apply and at this RPM there is a very real chance of the flywheel becoming a bomb by exploding due to centrifugal force. Please see the warning below for more information.
Please use a bit of common sense and if you
have ANY doubts as to the suitability of your design PLEASE contact us
first.
WARNING WARNING WARNING WARNING
WARNING WARNING
Size/weight:
We've done some of the work for you here, the wheel that we are using is 24.5" in diameter 1" thick, weight is approx. 135lbs, made from plain old hot-rolled steel plate. We got lucky and came across a couple of steel disks in the scrap yard that were about the right size. We did all the rest ourselves, cut the center hole, welded in the center hubs with 1.25" keyed holes, and then spent many hours trying to make it round and balanced. We wouldn't recommend it to anyone to do, in fact we would pay someone to do the next one, it was far too much work, and we nearly messed it up a couple of times. Not having the proper tools to do this it really wasn't worth the hassle.
Checking some local prices:
Steel: 26" diameter x 1" approx. $85
Weld in center hub and machine round approx. $300
This was quoted from a local shop that is known for being pricey, if you do some checking you should be able to do better, even at these prices, doing it yourself really isn't an option for most people. Again, call in the favors.
(See the FAQ
page for what size wheel you should use for your dyno)
Safe operation is the responsibility
of the operator/builder. TDKMotorsports will not be held liable for any
damages caused by the use of this information.
If you really need to know more than the guidelines given above, here are the relevant formulas...
The formula for determining the torque is:
Torque = PM * rpm per second / 9.551
where "PM" represents the Polar Moment of Inertia of our inertia dyno's flywheel.
If you don't know the Polar moment of Inertia for the flywheel (and your flywheel has a constant thickness cross-section) we can calculate it with the formula:
PM = (W * r^2) / 32.16 / 2
where "W" represents the flywheel weight in pounds and
r is its radius in feet.
(the formula for weight of a steel disk can be found in the "FAQ" page)
Once you have the torque, it is easy to calculate the horsepower with the standard formula:
Hp = Torque * rpm / 5252
Keep in mind that the rpm in the last formula must be the average rpm during the sampling period.
Say our example uses a 10 pound flywheel, 8" in diameter (thus it would have a Polar Moment of Inertia of .017 foot-pounds-second^2). If the engine was able to accelerate this flywheel from say 4,800 rpm to 5,200 rpm in 2/10 of a second (a rate of 2,000 rpm per second) that would represent a torque of 3.6 pound feet. Since our above example had an average rpm of 5,000, it produced 3.4 Hp during the test.
If you have any questions about using this formula feel free to
contact: TDKMotorsports
Pictures showing heavy duty scatter shield to contain any disasters, which might occur.
Here is one place where a picture is worth a thousand words, or absolutely nothing. The pictures will give you a general idea, but the actual configuration is up to you.
Here are a few considerations:
1. Make it sturdy, weld all joints. 2" x 2" x .095 works great. Single cylinder engines produce an amazing amount of vibration, you don't want it to shake apart.
2. Build the frame in such a way that it will allow easy access to the motor.
3. Make everything as close to a regular kart configuration as possible, this makes it easier to take the motor off the kart and mount it on the dyno.
4. Put some rubber castors on the bottom so it can be easily
parked in the corner when your not using it, also helps to absorb some
of the vibrations.
Use standard kart pieces here as much as possible, makes it easy to
integrate everything together. Otherwise nothing fancy.1.25" kart axle,
sprocket hubs, brake rotor, etc. The only exception is using cast iron
pillow block bearings, just because they were easy to bolt down to the
frame. You may want to get an extra sprocket hub to make it easier to swap
between motor types.
The portion of the axle that goes through the flywheel and flywheel
bearings is recommended to be solid.
Brakes
Kart type brakes are normally sufficient unless you intend to pull the motor against the brake in an attempt to build heat. If that is the case you may want ventilated or dual rotors. Slowing the flywheel is the only other purpose here. Listening to the flywheel spin itself down gets really irritating! We once let it do this by itself, would you believe it spun for 8 minutes and hadn't stopped yet! Attempting to stop the flywheel too quickly with a large brake can cause the entire dyno to try to tip over. Remember though, all these components contribute to the "inertia weight" which can mess with the accuracy, especially if you change something mid-stream.
One-way clutch
THIS IS VERY IMPORTANT!! After the flywheel/engine has been accelerated and the throttle is closed the flywheel will want to continue to rotate at high rpm and decelerate very slowly (see above), while the engine will attempt to slow much quicker. When this happens the engine becomes the brake for the flywheel. This "engine braking" is very hard on your engine, the vacuum caused by the throttle being closed places the connecting rod and piston in "tension", they are being "pulled" instead of "pushed", exactly the opposite of what they were designed for. Stresses on these parts can be as much as 200-500% higher than normal during this "engine braking" period. Also remember 2-cycle engines are getting no oil when the throttle is closed. A brake on the flywheel is not fast enough. A centrifugal engine clutch doesn't work very well here either, remember the flywheel is doing the driving, all the pieces in the clutch stay locked as long as they are all spinning at the same speed. (until the clutch assembly slows to below stall speed) You can try it without if you want, but it's not a pretty deal. We did alot of searching here, several options were looked at. There are available, some one-way bearings that would work great for this, just like the ones in an electric starter drive. The problem is, by the time you get a bearing that will handle the torque load and abuse of a single cylinder engines alternating power output (one of the most harsh things you can do according to the "power drive" industry, they step up the size of components by about 30%+ if they are used on a single cylinder engine) you get into some expensive pieces. We found one piece that would have worked great, it had an 11/4" keyed hub inside and a "bolt on" flange for sprockets on both sides, really cool! The price was nearly $500! Not an option as far as we were concerned! We've been told that a kart style axle clutch would work here, but haven't gotten hold of one to try. Looking at the pictures, you'll see a small gold colored deal between the sprocket hub and the brake rotor. This is our solution. It's an adapted piece of farm equipment. (Hey, we're farm boys!) It has some limitations, but it was cheap! It needed a small amount of machine work first but works great.
Info added by Performance Trends, Nov 2, 2017: Check the last page (FAQ page) for this TDK website more info on the one-way clutch.
Picture of one-way clutch after machining........
Picture showing one-way Clutch, Brakes, and axle assembly
An engine clutch is used simply for ease of starting etc. Direct drive was attempted, but just didn't work out real well. The engines were hard to get started because we were trying to "start" the flywheel at the same time, the engine would start and have to accelerate the flywheel while trying to get to an idle. Putting a clutch on works much better. The only thing you have to do is set up a clutch for very low engagement in order to be able to accelerate the motor through the entire power band, before and after the torque peak. A Yamaha clutch set to stall at approx. 6000rpm, and a Briggs clutch at approx.2000rpm are used now and work quite well. Now the engine can be started, warmed up, etc. When doing a "run" the engine is brought up to the clutch rpm, the flywheel is allowed to catch up so everything is locked together, then the throttle is opened form there.
Gauge
The data acquisition system is capable of recording and displaying CHT
and EGT "live" depending on the configuration(optional hardware),however
if you do not have this capability you'll need something.
We use a Digatron off of a kart to monitor these in real time. You
can use one from your kart or used ones can be had a swap meet very reasonable.
That should about do it for the pieces-parts explanation, take a look at the pictures for ideas on throttle controls, fuel tanks, etc.
Sorry the dyno isn't real pretty. Since we got it all working, we've
been USING it, and haven't had time to tidy it up much.
