TDK Motorsports Inertia Dyno Testing

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...

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. 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.

1. "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.

2. "Steady State" testing is fine for equipment such as waterpumps, 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.

3."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.

4. 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.

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 a traditional chain and sprocket.

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 both the engine and the dyno flywheel. After shutting off the engine the data collected by the computer is analyzed 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. (up to 30 times/second)

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 setup, warming the engine etc, typical full throttle run times take from 8-12 sec. 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.

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This is an informational site about the construction of a prototype Inertia dyno. Large metal objects spinning at high speeds are very dangerous and may cause severe injury or death. TDKMotorsports will not be held responsible for any damages to you or your property due to your use of the information provided on this site. By using this information you assume all responsibility for any loss or damages incurred.