Data Acquisition on a 7 Second Dragster

You can’t get up much speed on a tractor, at least not enough to satisfy Ray Thompson. 

Thompson has 35 years of experience as an engineer, mostly as a test engineer, developing tests and analyzing failures to improve the reliability and safety of John Deere tractors. His real passion, though, has always been drag racing. He's raced twelve-second street cars, nine-second chassis cars, and seven-second dragsters.  After retiring from John Deere, Thompson started Thompson Engineering and Racing to apply his knowledge of vehicle dynamics and failure analysis to the sport of drag racing.

Thompson knows from his long career as a test engineer that data acquisition plays a critical role in improving component performance and overall system reliability.  And he’s brought that expertise to his “encore career” in racing engineering.  To make the measurements he needs, Thompson makes extensive use of the SoMat eDAQlite data acquisition system from HBM.

One of the reasons Thompson chose the eDAQlite is that it not only provides accurate data, but,  also fits nicely into the limited space available inside the dragster. The data provided by the eDAQlite, coupled with Thompson's knowledge of “how things really work,” enable him to improve the elapsed time (ET) consistency, safety and reliability of his racing vehicles.

In addition, the eDAQlite can do things that the other data acquisition systems commonly used in racing cannot. For example, most of the other data acquisition systems have a maximum sample rate of only 100 Samples/s, while the eDAQlite can make up to 100,000 Samples/s. On top of all that, the HBM eDAQlite is one of only a very few data acquisition systems that have been accepted for use in the Sportsman classes of the National Hot Rod Association (NHRA) Championship Drag Racing Series.

Finally, Thompson chose the eDAQlite because of his long relationship with SoMat from HBM. He has been using SoMat products since the 1980s and has always found SoMat products to be accurate and reliable, and when questions came up, HBM tech support has worked with him to answer them quickly.

Cranking it up

Recently, Thompson started a project to make the engine of his seven-second dragster easier to start and to avoid the occasional “kick back.” To complete this project, Thompson knew that he would have to measure several engine parameters, with the most important being the engine cranking speed.

To record engine speed, you typically connect the tachometer output signal from the ignition system to the data logger.  This output signal provides four pulses per crank revolution, and this is generally enough resolution for most applications. 

Fig. 1

To check the mechanical condition of the engine, however, you need more detailed information. For this application, Thompson used a speed pickup sensor connected to the flywheel. This sensor detects the passing of the teeth on the flywheel and outputs 168 pulses per crank revolution. The sampling rate was set at 200 samples per second. Figure 1 shows a comparison of these two measurement methods.

Fig. 2

The plot in Figure 2 shows the engine cranking speed of the dragster's 548 cubic-inch, V8 engine over a two-second time period. The compression ratio of the engine is 15:1. While the average cranking speed is 150 rpm, it can be as high as 225 rpm during a power stroke and as low as 85 rpm during a compression stroke.

At a cranking speed of 150 rpm, the crankshaft makes 2.5 revolutions per second.  For the 4 cycle, V8 engine, there are ten power strokes during a one second period, as shown in the plot. 

This plot alone can be used to compare cylinder to cylinder variation.  Any mechanical issues that affect the “pumping” performance of the cylinder will change the cranking rpm.  Periodically, recording the engine speed while cranking the engine and then comparing the trace shape from cylinder to cylinder is a quick method to check the mechanical condition of the engine.

Many experienced racers can determine if an engine has a weak cylinder by listening to the sound it produces. Measuring the engine cranking speed and producing a plot like the one shown in Figure 1 verifies what these experienced racers have known all along.

Finding a dead cylinder

To demonstrate this phenomenon, Thompson made two sets of engine cranking speed measurements. He made the first set of measurements with the engine operating normally. Before making the second set of measurements, he removed one spark plug to simulate a dead cylinder. 

Fig. 3

Figure 3 is a plot of these measurements with the two overlaid on one another. The red trace shows normal engine operation, while the blue trace shows the engine operating with a dead cylinder.  Note that when the dead cylinder is approaching TDC the engine speed increases as opposed to the normal decrease in speed.  This is due to the lack of resistance from air compression.  Also, note that the mean cranking speed was approximately 10 rpm higher for the engine with the dead cylinder. This is why the two traces do not line up well.

Fig. 4

Another way to analyze the performance of the engine is to perform a frequency analysis of the engine speed signal.  Figure 4 shows a plot of this analysis. The most significant frequency is 10 Hz. This is equivalent to the firing frequency of an 8 cylinder, 4 stroke engine at 150 rpm. This is called the 4th order effect because it happens four times per crankshaft revolution. 

The second most significant frequency component is 20 Hz. This frequency component is an 8th order effect and is due to the dynamics of the eight cylinders in the engine.  These dynamic effects occur because the crankshaft speed slows during each cylinder compression.  Although these dynamic variations are common, they can possibly be reduced by adding more inertia to the flywheel/torque convertor assembly.

As a result of his investigations, Thompson determined that the average cranking speed of 150 rpm might be too slow to have good startups. At this point, there are several things one can do to increase this speed, including:

  • Install a more powerful starter motor,
  • Increase battery power,
  • Use larger battery cables to avoid voltage drops, and
  • Ensure that there is a solid ground from battery to starter.

Avoiding kick backs

Finally, one of Thompson's goals was to reduce the possibility of “kick back.” Kick back occurs because racing engines have lower cranking speeds (about 150 rpm), larger displacements, higher compression and more advance timing than production engines. When a cylinder fires, there is a potential for the crankshaft to turn backwards, hence kicking the flywheel teeth into the starter motor pinion and possibly damaging the gear teeth.

The parameters listed above can be modified to help prevent kick back, but doing so can reduce engine power. From experience and data analysis, Thompson has found that if he backs off the ignition timing by a couple of degrees, while at the same time, allowing the engine to reach full cranking speed before powering the ignition system, the number of kick backs has been greatly reduced.

It's clear that one of the keys to Thompson's success on the drag strip has been the SoMat eDAQlite. The data he acquires with this compact, powerful data acquisition system is just what he needs in his quest to go faster and faster. 

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