High-Resolution Data Acquisition System Enables Reliable Engine Parameter Measurements

HBM, Inc., Marlborough, Massachusetts

Data acquisition plays a critical role in improving vehicle component performance and overall system reliability. Measuring engine parameters, including engine cranking speed and the mechanical condition of the engine, requires high-resolution data acquisition equipment.

To record engine speed, the tachometer output signal from the ignition system typically is connected to a data logger. This output signal provides four pulses per crank revolution, which is generally enough resolution for most applications. To check the mechanical condition of the engine, however, more detailed information is required. For this application, a speed pickup sensor is connected to the flywheel. The 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.

The plot in Figure 1 shows the engine cranking speed of the vehicle’s 548-cubicinch, 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.

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

Figure 2 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. 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, 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.

Another way to analyze the performance of the engine is to perform a frequency analysis of the engine speed signal. 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 these investigations, it was 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 installing a more powerful starter motor, increasing battery power, using larger battery cables to avoid voltage drops, and ensuring that there is a solid ground from battery to starter.

Another goal was to reduce the possibility of “kick back.” 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. By backing 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 can be greatly reduced.

This work was done by Thompson Engineering and Racing, using equipment from HBM. For more information, Click Here .

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