Dynamometer - Types of Dynamometer Test Procedures - Types of Sweep Tests

Types of Sweep Tests

1. Inertia Sweep: an inertia dyno system provides a fixed inertial mass flywheel and computes the power required to accelerate the flywheel (the load) from the starting to the ending RPM. The actual rotational mass of the engine (or engine and vehicle in the case of a chassis dyno) is not known, and the variability of even the mass of the tires will skew the power results. The inertia value of the flywheel is "fixed", so low-power engines are under load for a much longer time and internal engine temperatures are usually too high by the end of the test, skewing optimal "dyno" tuning settings away from the optimal tuning settings of the outside world. Conversely, high powered engines commonly complete a "4th gear sweep" test in less than 10 seconds, which is not a reliable load condition as compared to operation in the real world. By not providing enough time under load, internal combustion chamber temperatures are unrealistically low and power readings - especially past the power peak - are skewed to the low side.
2. Loaded Sweep, of the brake dyno type, includes:
   1. Simple Fixed Load Sweep: a fixed load - of somewhat less than the output of the engine - is applied during the test. The engine is allowed to accelerate from its starting RPM to its ending RPM, varying at its own acceleration rate, depending on power output at any particular rotational speed. Power is calculated using (rotational speed x torque x constant) + the power required to accelerate the dyno and engine's/vehicle's rotating mass.
   2. Controlled Acceleration Sweep: similar in basic usage as the (above) Simple Fixed Load Sweep Test, but with the addition of active load control that targets a specific rate of acceleration. Commonly, 20fps/ps is used
3. Controlled Acceleration Rate: the acceleration rate used is controlled from low power to high power engines, and overextension and contraction of "test duration" is avoided, providing more repeatable tests and tuning results.

In every type of sweep test, there remains the issue of potential power reading error due to the variable engine/dyno/vehicle total rotating mass. Many modern computer-controlled brake dyno systems are capable of deriving that "inertial mass" value, so as to eliminate this error.

Interestingly, a "sweep test" will almost always be suspect, as many "sweep" users ignore the rotating mass factor, preferring to use a blanket "factor" on every test on every engine or vehicle. Simple inertia dyno systems aren't capable of deriving "inertial mass", and thus are forced to use the same (assumed) inertial mass on every vehicle tested.

Using Steady State testing eliminates the rotating inertial mass error of a sweep test, as there is no acceleration during this type of test.