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.
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.
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.