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Different Dyno Types Print

Here I waffle on about running the dyno, accuracy and the differences between an inertial and a brake dyno.


Dynamometers


My Dyno in operation. This is a 4th gear run on a mates GSXR1000 although it doesn't sound like it the bike was taken to 12,000rpm. I forgot to video the TLS on it, Doh. It's a Windows Media file and you'll probably need the latest Codecs to view it. Right click and "Save Target As" then open it. Right click here (658kb).


Links on this page.

What is a dynamometer
Why an Inertia Dyno?
Advantages and disadvantages of each type
Results from different types
Just how accurate is this dyno?
The testing procedure

I've typed this document to share some information about dynamometers. There probably aren't many people who know me that are unaware that I have developed and built my own dyno for measuring the rear wheel horsepower of motorbikes. I will try and explain the basics as simply as possible, and I'll also throw in some theory and maths for those that are interested. I'm not a writer, I'm not a programmer, I'm not an engineer either, I'm just a hack that wanted to build a dyno and managed to pull it off. A lot of the information I learned about dyno's and rotational physics I learnt from searching the Internet. The steepest learning curve was teaching myself how to program in Visual Basic, grab the data from my data acquisition module and graph it.

What is a dynamometer.

A dynamometer is used to measure the power produced by an engine. There are basically two types of dynos. Steady-state (brake) dynamometers and inertial dynamometers. Also there are basically two types of each dyno with various configurations of each. These are broadly engine dynos and chassis dynos. With engine dynos the engine is attached directly to the dynamometer. R&D engine workshops would use them, as would manufacturers of engines for testing purposes, i.e. Holden and Ford engine assembly plants. Chassis dynos measure the engine output at the wheel. There are various configurations for chassis dynos; the most common type is where the vehicles wheels spin up a drum or roller(s). Other types involve jacking the vehicle up and removing the wheels; the wheel hub is then mated directly to the dynamometer input shaft.

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Why an Inertia Dyno?

Inertia dynos are quickly becoming the preferred method for obtaining the most accurate "real world" results in dyno testing for racing applications. Inertia dynos more closely simulate the dynamic conditions created when accelerating an engine under load, therefore giving more accurate and repeatable results time after time. The following is a brief and general description some more traditional dynos and their advantages and disadvantages.

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Advantages and disadvantages of each type.

A steady state dyno has the advantage that an engine can be loaded up and held at a constant speed. This can be used to find a miss at a certain speed or tune the engine for maximum power at a certain speed etc. The disadvantage of these types of dynos is that they are expensive (compared to inertial type) and they are more complex. These dynos also need regular calibration of a load cell if they actually use one. If calibration checks aren't performed regularly then the results can be wrong. The braking mechanism of the dyno will generate a lot of heat that has to be dealt with as well. If it is an electrical brake it has to be cooled, water and hydraulic brake will need radiators and may even need cooling towers.

Each type of dyno has its own advantages and disadvantages. Traditional dynamometers are typically known as "pump" or "brake" type dynos, whether they use a hydraulic pump or a water pump they all work on the same principle. The engine being tested is run at a steady speed, load is applied via the pump until the engine can no longer maintain this speed at Wide Open Throttle (WOT), at this point the rotational force or "torque" being applied to the pump housing is measured and converted to "standard" engine output readings. The results are calculated by the values obtained from a load cell or simply converting pump pressure. This method, known as "steady state testing" is normally performed at 200-500 rpm increments across the "power band" of the engine. This type of dyno testing has been used for many years on all types of engines, however for racing applications it has a few inherent problems.

"Pump" type dynos must use some form of fluid to pump, be it water or oil. Everyone knows what happens to oil as it is pumped, it gets hot very quickly and the viscosity or "thickness" of the oil goes down. As this happens the power required to pump it changes dramatically, this will in turn change the power output readings on the dyno. Water has this same tendency, although to a lesser extent. Water and oil pumps also tend to mix air with the fluids, causing them to become aerated, or "foamy", changing it's "thickness" and again changing output readings.

 "Steady State" testing is fine for equipment such as water pumps, generators tractors etc, equipment that operates at a steady load and rpm for long periods of time. How often does your racing engine operate at a steady speed? Almost never, from the start of a race to the finish the engine operates in an almost constant state of acceleration or deceleration. The thermal dynamics of intake and exhaust flow, combustion and mechanical components are much different under these conditions than at a steady state.

"Steady State" testing also requires that an engine be held at WOT at each test increment for a period of time while test readings are taken. The advent of computerized data acquisition systems has helped this considerably, but the engine still spends a relatively long period of time under load at high rpm and WOT.

Over time the internal workings of a pump type dyno wear causing it's power absorption characteristics to change. Unless these changes are carefully monitored and compensated for, the dyno can become very inconsistent and inaccurate.

Inertial dynos are the simplest and cheapest of all types. An Inertia Dyno operates much differently than a "pump type" dyno. Inertia Dynos consists of one major component, a large flywheel, mounted on an axle and connected to the engine via the wheels. The disadvantage of these over brake dynos is that they can only be used for wide-open throttle tests (WOT).

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Results from each type.

Typically a steady state dyno will give results of up to about 20% less than inertia types. Here is an example of how different results occur. You have a bike that shows a maximum of 100HP on an inertia dyno and xxHP on a steady state dyno. Now we lighten the crankshaft and flywheel, fit a lighter rear wheel, fit a lightweight chain and some alloy sprockets. We run the bike on the steady state dyno and it still shows a maximum of xxHP. We run the bike on the inertia dyno and find it is now producing 105HP. These modifications didn't actually make the engine produce more horsepower just as the brake dyno shows. So why does the inertia dyno now say it is producing more horsepower? This is because the inertia dyno gives a true representation of what the "road" sees. Of course the engine isn't producing more horsepower, but there is more horsepower available to accelerate the bike because less power is needed to accelerate the crankshaft, chain and sprockets and finally the wheel. Because less power is need to accelerate these things more is available to accelerate the bike, and it will accelerate faster on the road. The inertia dynamometer calculates horsepower from how fast its drum is accelerated, therefore in this example the bike engine was able to accelerate the drum more quickly after the modifications so more horse power is available at the rear wheel to accelerate the bike on the road. In my opinion steady state dynos are good for tuning tractors that are going to run at 1500-rpm day in day out. To tune an engine for the type of riding I do, or bush riding or racetrack riding I think an inertial dyno is more than sufficient. How often do you care how much power you're making at a steady throttle? When I'm out having fun, the engine is rarely at a constant speed, it is either constantly accelerating or decelerating.

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Just how accurate is this dyno?

"Accuracy", for many reasons, is a very relative term as far as dyno's go. Just how accurate is anyone's dyno? Take 2 motors, test them on 2 different dynos, one comes out at 100hp, the other comes out at 110hp, but they run the same lap times. Which dyno is "accurate"? Which one is giving you the correct numbers? Horsepower is simply a calculated number. Dyno 10 engines on the same dyno at 100HP, put them all in the same bike one at a time, and they all run the exact same lap times. Is the dyno accurate? No, the dyno is however "repeatable". "Repeatability" is what you want in a dyno, test the same engine time after time and get the same results. Take one of the 10 engines from above, change a pipe or a cam so you get 5% more power, run it against all the others again. It runs 5% faster, you re-dyno it and it still says 5% better, now you have an "accurate" and more importantly, a "repeatable" dyno. A couple hints: Always start your tests at the same engine temps and settings. Pay close attention to weather conditions. Get yourself a good barometer, thermometer, and humidity gauge, and enter the conditions into the program each time you make a run.

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The testing procedure

The test procedure is simple:

  1. Start the engine
  2. Warm to operating temperature
  3. Accelerate from near idle through the power band to max rpm
  4. Close the throttle, and apply the brake to slow the flywheel

During the acceleration of the engine, a computerized data acquisition system is monitoring the speed of the dyno flywheel. After shutting off the engine the data collected by the computer is analysed and processed to produce the appropriate information. The computer "knows" the weight of the flywheel and calculates horsepower and torque values based on the amount of time it took to accelerate the flywheel from start to finish and moment to moment.

Notice that nothing was mentioned about: hot or cold fluids, load control valves, throttle actuators, load sensors, component wear, Etc....

These items do not exist on an Inertia Dyno, the flywheel is always the same size, nothing changes over time and temperature except the engine. The dyno's output is consistent run to run, day to day, and year to year. Output readings are very consistent and reliable. This entire process takes a relatively short amount of time, after set-up, warming the engine etc, typical full throttle run times take from 10 - 20 sec depending on the power produced by the bike. The engine is placed under no more stress than a run up the block or a lap around the track for each "run". HUGE amounts of testing can be performed with less "wear and tear" than a night at the races.

Disclaimer

I received a friendly email, suggesting a disclaimer, I'm too lazy to create one, so I just pasted the email

Good day, Stephen:

I appreciate all the work you did on your site about dynamometers. And, it's very cool that you thought to build your own and succeeded! Truly awesome. I only began reading on your site with the Different Dyno Types page (wotid.com/dyno/content/view/19/38/).

Just a suggestion for adding a disclaimer or other note to inform readers that the information is incomplete, dated, and has many errors. I would not want to see people showing-up at dyno shops and arguing with operators based on what you stated. Or, missing tuning opportunities because they didn't know how it could be done with the type of dyno available to them, or paying for time on, again based on your pages.

I'm not writing to bend your ear or stir the pot. Just a friendly note to ensure others truly benefit from your pages, and avoid problems, issues, or money mis-spent, as I am sure that was not your intention.

Thanks for listening.

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