A & A Machine Shop

In addition to being used to determine the torque or power characteristics of a machine under test (MUT), Dynamometers are employed in a number of other roles. In standard emissions testing cycles such as those defined by the US Environmental Protection Agency (US EPA), dynamometers are used to provide simulated road loading of either the engine (using an engine dynamometer) or full power train (using a chassis dynamometer). In fact, beyond simple power and torque measurements, dynamometers can be used as part of a test bed for a variety of engine development activities such as the calibration of engine management controllers, detailed investigations into combustion behavior and tribology.

In the medical realm, hand dynamometers are used for routine screening of grip strength and initial and ongoing evaluation of patients with hand trauma and dysfunction.

What Does A Dyno Test Tell Me?

The data acquisition computer corrects for acceleration and deceleration inertia, reads torque, rpm, 12 exhaust gas temperatures, fuel flow, air flow, inlet air temperature, engine oil pressure, engine oil temperature, engine manifold pressure, and engine water temperature channels over 1,000 times a second. Barometric pressure and humidity inputs along with the inlet air temperature channel are used for correcting horsepower and torque to a known standard (SAE J1349, J1228, J607, J245). Also recorded are calculated channels such as volumetric efficiency, brake specific fuel consumption, brake mean effective pressure, air fuel ratio, EGT average, and EGT delta.

Electrical dynamometer setup showing engine, torque measurement arrangement and tachometer

A dynamometer consists of an absorption (or absorber/driver) unit, and usually includes a means for measuring torque and rotational speed. An absorption unit consists of some type of rotor in housing. The rotor is coupled to the engine or other equipment under test and is free to rotate at whatever speed is required for the test. Some means is provided to develop a braking torque between dynamometer's rotor and housing. The means for developing torque can be frictional, hydraulic, electromagnetic etc. according to the type of absorption/driver unit.

One means for measuring torque is to mount the dynamometer housing so that it is free to turn except that it is restrained by a torque arm. The housing can be made free to rotate by using trunnions connected to each end of the housing to support the dyno in pedestal mounted trunnion bearings. The torque arm is connected to the dyno housing and a weighing scale is positioned so that it measures the force exerted by the dyno housing in attempting to rotate. The torque is the force indicated by the scales multiplied by the length of the torque arm measured from the center of the dynamometer. A load cell transducer can be substituted for the scales in order to provide an electrical signal that is proportional to torque.

Another means for measuring torque is to connect the engine to the dynamometer through a torque sensing coupling or torque transducer. A torque transducer provides an electrical signal that is proportional to torque.

With electrical absorption units, it is possible to determine torque by measuring the current drawn (or generated) by the absorber/driver. This is generally a less accurate method and not much practiced in modern time, but it may be adequate for some purposes.

A wide variety of tachometers are available for measuring speed. Some types can provide an electrical signal that is proportional to speed.

When torque and speed signals are available, test data can be transmitted to a data acquisition system rather than being recorded manually. Speed and torque signals can also be recorded by a chart recorder or plotter.