Using Sparkscope Mode for Ignition Patterns Observation

The problem

In many situations related to secondary ignition diagnostics, specialists need to have a possibility for a quick overview of the combustion in the individual cylinders and a presence of the sparks with sufficient breakdown voltage. A standard approach in such cases would be connecting the high-voltage clamps to the universal oscilloscope inputs, set the corresponding voltage range and long enough time range so that we can simultaneously observe several complete engine operating cycles. The picture we will see contains needles of the breakdown voltage but with a much unstable intensity, varying widely in the individual operating cycles, and sometimes even missing at all. Generally, this creates the impression of a serious problem associated with some instability in the vehicle's ignition system. Of course, this is not the case. The reason is in the operation of the oscilloscope itself.

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Each oscillogram on the screen is constructed using multiple equidistant sampling points, representing values of the measured voltage. On the other hand, the spark ignition voltage pattern lasts only a few μs and is much shorter than the sampling interval. Therefore, if the needle falls somewhere between the individual points it will remain invisible, and this fact completely explains the ignition instability in the picture described so far.

This problem is related to vehicle ignition diagnostics, and there are different approaches to its solution. Some manufacturers set the oscilloscope to the maximum possible sampling rate, thus ensuring the breakdown voltage needles will not be missed. Of course, this is a working solution but requires a huge amount of data points to be collected in each measurement and stored in the oscilloscope's memory. Then, an intensive software processing is required in order to display the collected points on the screen.

Indeed, since the needle in the ignition pattern takes only a few microseconds, then we need to take data points with a sample rate of at least 1 MS/s, i.e. 500 000 points for an observational interval of 0.5 s. This coresponds to 2 000 000 points in total for a four-channel oscilloscope. With a typical resolution of 12-bit for each point, we will need 2 bytes per point or total of 4 MB. This information must be additionally received from the computer, processed, disseminated and displayed on the screen within 0.5 s. All of this leads to more computational complications and, in general, make diagnostic system more expensive.

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Our solution

In order to deal with similar problems BOSA oscilloscopes includes a special mode called Sparkscope™. It implements another approach. Regardless of the sampling rate currently set to the oscilloscope, any breakdown voltage or so-called needle is treated as an independent trigger event taking a sampling point immediately, no matter where in the sampling interval this event occurs. The measurement then continues with the same sampling rate until the next trigger event occurs, then the picture is repeated again, and so on. This algorithm does not require additional resources from the system and is reliable as a result of its entirely hardware internal implementation.

More information about ignition observation modes you can find in the oscilloscope's User Manual.

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