No matter how good your engine dynamometer may be, it is all but useless without the necessary data logging equipment. There is a plethora of different data logging systems available to engine builders, providing all of the functionality they could wish for, but one key factor needs to be considered when choosing the right equipment, and that is the sample rate at which it can operate.
Sample rate is the measure of the number of readings that can be taken in a set period of time, and is measured in Hertz (Hz), the SI unit for cycles per second. Each input on a logging system will be able to receive and process signals at a particular sample rate, and it is important because it dictates the resolution of the data available for analysis; measuring different aspects of an engine’s operation requires differing levels of data resolution. Ultimately you will only be able to study certain events in detail if you have a sufficiently high sampling rate.
Every conceivable engine parameter can now recorded by a dynamometer’s data logger, covering everything from oil pressure to, if suitably instrumented, cylinder pressures. For some areas, such as oil pressure, the sample rate need not be very high – 50 Hz is more than enough, and even that is probably excessive. However, for detailed engine performance and combustion analysis, much higher rates are needed.
Taking the most basic measurement undertaken on an engine dyno, that of power output, sampling at 100 Hz or preferably 200 Hz is the norm. Between cylinder firings there is an instantaneous drop in torque and crankshaft rpm, which is indiscernible to a racecar’s driver but is important from a performance analysis perspective. Taking an engine rotating at 6000 rpm and a data sampling rate of 50 Hz, you will only be recording torque every other rotation of the crank.
The biggest impact of a low sample rate such as this is inaccuracy in the averaged power curve of the engine produced using the gathered data. Some data samples will be in sync with the firing of the plugs, others won’t. By doubling or even quadrupling the sampling rate, sufficient data is acquired to even out the sample range and provide a more reliable average of the power curve – not to mention providing more data points for in-depth analysis of transient engine behaviour.
Moving on to more complex engine testing, sample rates become even more important. For example, if you want to take four samples per crankshaft rotation, of combustion pressure for example, at the same 6000 rpm then you need a minimum sampling rate of 400 Hz. Now, very few race engines operate at such low revs, and logic dictates that to study the same data at 12,000 rpm, you are going to need a sample rate of 800 Hz.
So if, for example, one was looking at combustion pressures in an engine running at 12,000 rpm, a sample rate of 800 Hz would provide sufficient data over a sample of 500 combustion cycles to give a good average of combustion pressure throughout a combustion cycle. Obviously, the validity of such averaging also depends on the accuracy of the crank position measurements.
Two other factors must be considered when choosing a data logging system – the number of channels and their sampling rates. For example, if you want to log cylinder pressures on a V8, you are going to need at least eight channels capable of recording at a sufficiently high rate. Consequently, you also need the ability to process all these channels at their maximum rate simultaneously. The reason high-end equipment carries a hefty price tag is functionality such as this, but the insight it provides into engine performance is exceptional.
Written by Lawrence Butcher