Determining Inertia

Learn to use DYNO-MAX and dyno to self-calculate the engine's polar-moment-of-inertia.

Intermediate | 07.17.20

How can I determine my engine’s inertia?

USE YOUR DYNAMOMETER TO DETERMINE THE ENGINE’S INERTIA! Often you will have only a vague idea of what an engine’s rotating inertia entry should be – in DYNO-MAX (for RPM-A). After all, not everyone has access to the OEM’s solid modeling data for their crank trains.

DYNO-MAX’s Run Information tab for Inertia has a (user editable) drop down list with approximate engine inertia’s (by type and displacement). However, these are obviously only approximations. If you plan to perform rapid acceleration rate sweep tests, you may want to verify how close the estimates are – and adjust those values as necessary.

Fortunately, you can use your dynamometer and DYNO-MAX to derive very close inertia values. Here are the steps:

1) Examine a recorded graph for your engine at a low sweep rate. Find a 500 RPM band over which its Hp is relatively constant.

2) Take a special recording for that engine, where you oscillate its speed up and down across that same 500 RPM band – at about 1,000 RPM/second (i.e. a 500 RPM tall 1-Hertz sine wave). About five cycles are enough. Note: You may want to create an Automated Life Cycle test to let the automatic-load servo do this test.

3) Display a Time Averaged (0.1 second) graph showing both a Hp and RPM line and using a “Run Time” X-axis . Note: Dampen both your RPM and Torque channels to the same value (e.g. 0.750 seconds) and verify that you have “#1-Compensation On” set in the Inertia Tab.

4) Ideally, for each oscillation, you should see matching Hp (vs. RPM) on both the rising and falling sides of the curve. If you over-specify the inertia, Hp will erroneously climb as RPM increases (and vice versa).

Tip: Grossly inflate your RPM-C inertia entries to see the effect.

5) As necessary, interactively adjust your inertia entries until the graph shows similar Hp on both sides of each oscillation – and matching your low sweep rate test results.

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