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Benefits of data acquisition for vehicles

Posted: 30 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:data-acquisition systems? eDAQ-lite? V8 engine? engine cranking? engine cranking?

Experienced racers can detect a weak cylinder in an engine from the engine's sound. You can verify that by measuring the engine cranking speed and producing a plot like the one shown in the photo above.

Dead cylinders
To demonstrate a dead cylinder, I made two sets of engine cranking-speed measurements. The first set of measurements came with the engine operating normally. Before making the second set, I removed one spark plug to simulate a dead cylinder. Figure 3 is a plot of these measurements with the two overlaid on one another.

Figure 3: The high pulses in the blue trace indicate a dead cylinder. The red trace shows all cylinders firing.

The red trace shows normal engine operation while the blue trace shows the engine operating with a dead cylinder. When the dead cylinder approaches TDC (top dead centre), the engine speed increases instead of decreasing as normal. The speed increase is caused by a lack of resistance from air compression. Furthermore, the mean cranking speed was approximately 10 rpm higher for the engine with the dead cylinder, which explains why the two traces don't line up.

Another way to analyse the performance of the engine is to perform a frequency analysis of the engine speed signal (figure 4). The most significant frequency is 10Hz, which is equivalent to the firing frequency of an 8-cylinder, 4-stroke engine at 150 rpm. It's called the 4th-order effect because it occurs four times for each crankshaft revolution.

Figure 4: Frequency analysis shows the firing frequency of an engine.

The second most significant frequency component occurs at 20Hz, frequency component is an 8th-order effect, caused by the dynamics of the engine's eight cylinders. 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.

Through my investigations, I found that the average cranking speed of 150 rpm might be too slow to achieve good start-ups. At this point, there are several things you can do to increase cranking speed:

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

Avoid kickbacks
One of my goals was to reduce the possibility of "kickback," which 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 kickback, but doing so can reduce engine power. From experience and data analysis, I've found that if I back 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, I can significantly reduce the number of kickbacks.

About the authors
Ray Thompson has a Masters degree in Mechanical Engineering. He retired from John Deere after 36 years of design and testing of agricultural tractors. Ray also has over 35 years of experience as a drag racer. The drag racing experience has been in 12 second street cars, 9 second chassis cars, and 7 second dragsters. In an effort to combine these two passions, "Thompson Engineering and Racing" was created with the focus of applying the engineering knowledge to drag racing vehicles.


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