How does a wheel dyno calculate HP. Formulas and hypothesis inside

First. Angular momentum=L, angular velocity=V(actually omega), angular acceleration=A, kinetic energy=KE, mass=m, Torque=T, inertia=I, radius=R, joules=J, second=sec

Method one. using F=mA but now in circles. T=I*A so you make a run and accelerate the drums. The computer takes an instantaneuos slope of the V vs T graph giving A. Then multiplies it by the I of the drum (I would equal 1/2mR^2) Thus giving torque, but to get horsepower you need rpm of the tire which would be derived from radians/second of the drum and circumference of the tire. This would seem to be a pain with th extra variable. So method two. and the method I think would be used

Using kinetic energy. KE=.5*I*O^2 which becomes .25m(R+O)^2
Now plot KE vs time. If you take dKE/dt you get the instantaneous change in KE put this over time and you get J/sec which is a watt. get 746watts and you have a HP, know you would have more calculations to get torgue, but I think more people are concerned with HP

I think it would be cool to make a chassis dyno and quite easy with some of the data studio equipment I have played with. Who can get me some 44" drums :D

Originally posted by cjudd

I think it would be cool to make a chassis dyno and quite easy with some of the data studio equipment I have played with. Who can get me some 44" drums :D

Dynojet or Superflow will probably sell you just the drums. About 4500-4800 lbs., so they are not cheap. Of course neither would they be cheap to have you local metal butcher make.

The following was lifted from TDK Motorsports site on building an inertial dyno for Kart engines. The principles are the same.

"An inertia dyno, (sometimes called an acceleration dyno) is a system which measures work (Torque/Horsepower) based on the acceleration of an object of a known mass. This "known mass" can be any number of items, assuming you know the weight of the object, the distance it was moved, and the time needed to move the object. For our purposes a wheeled vehicle is most easy to visualize. Any vehicle takes a certain amount of power to accelerate from one given speed to another. If you know the weight of the vehicle you can calculate how much work (torque) it takes to do the acceleration, and if you know the time it takes to do the acceleration you can figure Horsepower.

Simplified formula: 550ft/lbs of work/second = 1 Horsepower

In this case a flywheel spinning on an axle can replace the vehicle or "mass". If you determine the amount of energy required to accelerate the flywheel from one given rpm to another and the time it takes to do the acceleration you can calculate the torque/horsepower produced. "

Your second method looks about right. Torque is calculated from the measured power on an inertia dyno.

Software is available. Check TDK:

http://www1.cedar-rapids.net/tdkmotor/INDEX.HTM

Is building a 1000 hp inertia dyno a possible DIY project? Sure, but not "easy" and certainly not "cheap".

Reinventing the (dyno) wheel is always fun. ;)

Originally posted by OldSStroker
"An inertia dyno, (sometimes called an acceleration dyno) is a system which measures work (Torque/Horsepower) based on the acceleration of an object of a known mass. This "known mass" can be any number of items, assuming you know the weight of the object, the distance it was moved, and the time needed to move the object. For our purposes a wheeled vehicle is most easy to visualize. Any vehicle takes a certain amount of power to accelerate from one given speed to another. If you know the weight of the vehicle you can calculate how much work (torque) it takes to do the acceleration, and if you know the time it takes to do the acceleration you can figure Horsepower.

Simplified formula: 550ft/lbs of work/second = 1 Horsepower

In this case a flywheel spinning on an axle can replace the vehicle or "mass". If you determine the amount of energy required to accelerate the flywheel from one given rpm to another and the time it takes to do the acceleration you can calculate the torque/horsepower produced. "

Your second method looks about right. Torque is calculated from the measured power on an inertia dyno.

Horsepower is measured, or the torque is measured, with the horsepower calculated from the torque?

I know you know, torque is measured, with hp calculated from it. So, what did I take 'out of context'? :)

I dunno Arnie,

A dynojet is an accelerometer and not a true dynomometer and power (hp) to accelerate the drum is measured and torque is calucalated the way I see it.

-brent

Originally posted by arnie
Horsepower is measured, or the torque is measured, with the horsepower calculated from the torque?

I know you know, torque is measured, with hp calculated from it. So, what did I take 'out of context'? :)

No, on an inertia dyno, the horsepower is measured (or actually calculated) by timing how long it takes to accelerate a known amount of inertia (the drums) from one rpm to another. That's work per time or lb-ft per second or power. Knowing engine rpm torque can then be calculated.

You are correct about brake dynos like most engine dynos such as Superflow, DTS, Froude, or any electric dyno. They actually measure torque input to the absorber with a load cell (or fish scale!). Horsepower is then calculated from torque and rpm. As you know, this type of dyno can hold an engine at a given rpm (step test) or allow it to accelerate at a fixed rate like 300, 600 or 1000 rpm per second. Inertia only dynos don't do step tests or controlled accelerations. An 800 hp car accelerates the Dynojet a LOT faster than a 200 ricer.

Of course, you could add an absorber like an eddy-current brake to an inertia chassis dyno and actually do steady state tests or controlled acceleration tests or even program a race track. You don't have to, because Superflow's Autodyn chassis dyno does that. While the inertia only Dynojet might be the official NASCAR chassis dyno, you might find an Autodyn or two in some of the shops.

A few more thoughts: how about attaching a couple of absorber dynos to the drive axle flanges of a car? You then have a way to measure torque at the wheels and do step or aceleration or race course programming wihtout the losses an inconsistancies of tires on drums. Here are two links to these directly coupled dynos:

www.dynapack.com

www.rototest.com

Did you ever notice that hp curves from inertia chassis dynos are very smooth with just a little wobbly "noise", but no real dips or sharper peaks that you often see on engine dyno curves? Why is that?

FWIW, my wife says she married Mr. Right...Mr. ALWAYS right. But she exaggerates a little. :)

I assume the smooth curves from an inertial dyno are due to the "damping" effect of the large mass?

Rich Krause

Originally posted by rskrause
I assume the smooth curves from an inertial dyno are due to the "damping" effect of the large mass?

Rich Krause

Shoot! That wasn't even a challenging question I guess. :)

Yep. A Dynojet doesn't really let you see what the engine is doing throughout it's rev range like an engine dyno does, especially for 1000+ hp engines and their fairly quick pulls. It's really nice to run a combo of both types if you can.

I wonder if Pro Stock guys ever use chassis dynos. My choice would be a directly coupled one. I know WJ, for one, is VERY concerned about driveline losses.

I know dynos put a load on the engine, after all, it isn't good to just rev an engine with no resistence. Its also hard to measure the power output without seeing the engine reaction to a load.
If you apply a little load to the engine, and measure the power, it will be uninterupted. If you apply too much load, the engine will bog. I'm proposing, looking at the grass from the other side. What if you measured the bogging and the engine's resistence to bog at each RPM(and at a given load). The reason I picked bogging, is that, to me, it is the threshold to making a useable amount of power.
With the bog point as the base line, it would measure upward, to a set load (obviously enough to prevent engine damage). (It would be a 3-D plot system and would require multiple dyno runs)
This would [obviously] show the power output for each unit of load (since my understanding is that there isn't a set mathematical function for the decrease in power when a certain amount of load is placed on an engine).
This would be helpful since on the road, a car goes through different amounts of load and would allow the car to be built around the engine's resistance to load, rather than the engine's ability to produce max power for a single range of load.
Can someone correct my logic?

-zeroing at bogging point of engine
-using multiple runs with varying dyno load

edit: oldsstroker, the step test sounds like what I am asking about. Does anyone do this step test with varying loads, to see how each part (ideally all) of the power band responds to different loads? And then making a 3-d dyno graph?

I wonder if Pro Stock guys ever use chassis dynos. My choice would be a directly coupled one. I know WJ, for one, is VERY concerned about driveline losses.

Cup cars wear dynos out around here! A couple of years ago, a friend of mine from DEI told me they put a car with a 750 hp motor on the chassis dyno and it put down 690+ rwhp. :eek: Around 8% loss....must be nice! :)

I've had some rather long disccussion with the guys that run the chasis dyno for hendrix motorsports. Just a plain jane dyno jet and they sounded like they love it.

It was a suprize to me that he said they did almost all the tuning on the dyno and just made fine global changes at the track.

He said to standardize everything they run the car for a good half hour.. do 9 pulls back to back without stopping the drum then on the last they plot negative to see how much drivetrain loss is in the car.

OH ps..

does anyone else get a kick out of when someone says a dynapack or mustang dyno is more accurate? I mean I wasn't aware F=ma varies from day to day. ;)

I know dynos put a load on the engine, after all, it isn't good to just rev an engine with no resistence. Its also hard to measure the power output without seeing the engine reaction to a load.
If you apply a little load to the engine, and measure the power, it will be uninterupted. If you apply too much load, the engine will bog. I'm proposing, looking at the grass from the other side. What if you measured the bogging and the engine's resistence to bog at each RPM(and at a given load). The reason I picked bogging, is that, to me, it is the threshold to making a useable amount of power.
With the bog point as the base line, it would measure upward, to a set load (obviously enough to prevent engine damage). (It would be a 3-D plot system and would require multiple dyno runs)
This would [obviously] show the power output for each unit of load (since my understanding is that there isn't a set mathematical function for the decrease in power when a certain amount of load is placed on an engine).
This would be helpful since on the road, a car goes through different amounts of load and would allow the car to be built around the engine's resistance to load, rather than the engine's ability to produce max power for a single range of load.
Can someone correct my logic?

-zeroing at bogging point of engine
-using multiple runs with varying dyno load

edit: oldsstroker, the step test sounds like what I am asking about. Does anyone do this step test with varying loads, to see how each part (ideally all) of the power band responds to different loads? And then making a 3-d dyno graph?

Some chassis dynos can be programmed to apply any amount of load you desire to the engine. That's a great way to perform part-throttle tuning, and/or to check/improve fuel economy and emissions.

As far as "bogging", generally if you tell a dyno to hold a given rpm or given acceleration profile, it loads the engine only enough to achieve that rpm or acceleration, no matter what the throttle position or engine is strapped to the dyno. (OK, the engine has to be producing at least enough power to accelerate to the rpm selected.) Wouldn't that capability do what you suggest?

Most highend race series teams durability test their drivelines for well more than race lengths on dynos. They load the driveline just like the race track does, and they can program the throttle opening to exactly what the driver does during a lap. Think about that for a 24 hour road race like LeMans or Daytona 24 hr. OEM's have been doing this stuff since the 60s, well before computer controls.

I believe one F1 team admitted to more than 60000 km. of chassis dyno testing on their new 2.4L V8 car prior to the beginning of the season.

He said to standardize everything they run the car for a good half hour.. do 9 pulls back to back without stopping the drum then on the last they plot negative to see how much drivetrain loss is in the car.

I hear them doing it from my house ALL the time!

Is building a 1000 hp inertia dyno a possible DIY project? Sure, but not "easy" and certainly not "cheap".
I think it could be quite cheap, all you need is a tach signal and some form of weighted resistance on the rear wheels... I have a very large drum that came to me free of charge, and anyone can get access free of charge anywhere in the world. It is approximately 25,000 miles in diameter since this drum is so much heavier than the car you just use the cars weight for calculating horsepower instead of the weight of the drum (really more of a sphere i guess). IF you need a stationary dyno, what about some 55 gallon barrels opened up, welded together to be long enough then insert a shaft with some bearings and some form of support webbing, fill with lead and get your weight. Its a lot larger than a normal drum but its just an example, lots of other things would be better (and not much harder to find... preassurized gas cylinders...). In fact if someone wants me to write the software for them Il'l do it for quite cheap... or free if you're building a chassis dyno near me and are willing to allow me to use it.

I think it could be quite cheap, all you need is a tach signal and some form of weighted resistance on the rear wheels... I have a very large drum that came to me free of charge, and anyone can get access free of charge anywhere in the world. It is approximately 25,000 miles in diameter since this drum is so much heavier than the car you just use the cars weight for calculating horsepower instead of the weight of the drum (really more of a sphere i guess). IF you need a stationary dyno, what about some 55 gallon barrels opened up, welded together to be long enough then insert a shaft with some bearings and some form of support webbing, fill with lead and get your weight. Its a lot larger than a normal drum but its just an example, lots of other things would be better (and not much harder to find... preassurized gas cylinders...). In fact if someone wants me to write the software for them Il'l do it for quite cheap... or free if you're building a chassis dyno near me and are willing to allow me to use it.

Spinning lead filled 55 gallon drums at 120+ mph will definitely be exciting to watch. Don't bother to invite me to the maiden launch.

On the off-chance that you are serious, get some sound mechanical advice before you start shaping metal. FWIW, the software is readily available. That's the easy part...but you already knew that. ;)

I certainly wasnt planning on doing it, was just a suggestion for a smaller vehicle it would work (motorcycle), preassurized gas cylinders probably would hold up quite nicely if someone is really interested I can get a welding inspector for exon to comment.

Reinventing the wheel is not the lost art folks thought it was.

an inertia dyno,

and a brake dyno,
water brake, airplane propeller brake, electric motor brake,

are two different things

the inertia-brake computer software, needs to make a guess,
about driveline inertia