Active dynos - 2
My previous RET-Monitor article on this subject gave a general overview of active dynamometers, and the benefits they can bring to engine development. This month, I want to look at some of the other tasks they can be used for, over and above basic transient testing.
The advantage of an electric active dyno is clearly its ability to motor, allowing it to drive the engine and simulate conditions such as transmission drag, gearbox
downshifts and so on. However, this capability can also be used when the engine is not running to narrow down the source of losses.
Speaking to one engineer from a well known testing and development company, it became clear that the average testing process for a new engine went well beyond basic drive cycle simulation. The engineers would regularly test the engine with components such as the cylinder heads or pistons and rods removed, to ascertain the losses related directly to these components. Not only does this avoid the need for specific test rigs to be built, but the engine can be motored with all other parameters - coolant temperature, oil temperature, oil flow and so on - at the correct levels, thanks to the ancillaries associated with the test cell.
In recent years, this type of testing has been aided by the development of ever more accurate torque sensors, which are vital to obtaining correct data. After all, if an engine is being powered by an input torque of 100lb-ft, and the frictional losses from the bearings accounts for only 1% of the overall losses, then an accuracy of less than this is required from the instrumentation to measure the effect.
The latest generation of magneto-elastic sensors are capable of providing the level of accuracy required for these tasks. The non-invasive nature of their operation, which does not interrupt the torque flow from the dyno in any way, means they can be installed much closer to the prime driver input than usual. This allows detection of transient torque peaks of significantly higher amplitude than those normally recorded with conventional dyno torque sensors such as telemetric strain gauges.
Moving off at a slight tangent, if accurate testing of this type is to be conducted, the overall installation of the dynamometer is of utmost importance. The often very small nature of the losses being measured means factors such as drive shaft misalignment are more relevant than ever, with any offset translating into additional drag in the system, which will consequently be picked up by the torque sensor.
During my conversation with the development engineer another, slightly unexpected benefit of active dynos came to light - fault finding. Imagine you have hooked everything up in the test cell, yet the engine simply refuses to fire. Enter the ability of the dyno to motor the engine.
Most modern race engines are heavily instrumented from the factory with an array of position sensors, pressure sensors and anything else the ECU needs to run the engine. Provided that the ECU is working, an active dyno allows for the engine to be 'run' without firing and all the sensors interrogated for faults using the test bay instrumentation. It's a far quicker approach than having to measure or replace components individually, giving more time for constructive testing.
Next month I will look at dynamometers designed specifically for assessing and measuring reciprocating components, above and beyond those possible with an active dyno designed for use with running engines.
Fig. 1 - A top of the line active engine dyno cell provides engineers with a vast array of information regarding an engine's operational parameters. (Courtesy of Toyota Motorsport)
Written by Lawrence Butcher