With your Smart Stirling Engine running, and with the USB cable connecting the engine to your Windows computer, run the Smart Engine Data Logger software. If you see an error message like this, verify that you performed the Installation correctly.
If the green box to the right of the Grab button is red, or if you do not see waveforms like the ones below, verify that you performed the Installation correctly.
Once the connection with the DAQ hardware is working correctly, you will see a display like the following.
There are five main functions, represented as buttons:
| • | Grab: click this to acquire 1 second of data and view the Pressure, Tachometer, and PV graphs; |
| • | Dyna: click this to acquire 4 seconds of data and view the Tachometer, RPM, and Power/Torque graphs; |
| • | Export: click this to save your current data in a CSV file, for later use by Excel or to re-Import the data back into this program; |
| • | Import: click this to re-load data which you saved previously. The software comes pre-loaded with various input files which you can load and inspect. |
| • | Multiple: clicking this allows you to load a series of import files, compute their figures of merit (RPM, power, etc) and create a single spreadsheet (CSV) file that contains a summary of all of this data. This can be very helpful if you want to see RPM or power trends for differing engine temperatures. |
The Help button summons this help information; the Quit button terminates the program.
The Continuous check box allows you to perform Grab operations repeatedly, if you want to watch the engine's performance change over time.
The numbers below the Quit button show the current hot and cold plate temperatures; the numbers in the white grid show the data that has been acquired using Grab, Dyna, or Import.
The colored box to the right of the Grab button should be green at all times: if it becomes red, that indicates that the USB connection with the DAQ system on the Smart Engine has stopped working correctly.
The Graphs
All of the graphs are automatically scaled to fit as much data as possible on the graphs. If you want to look at the details of some part of a graph, position your cursor at the upper left corner of the data you wish to inspect, press the [left] mouse button down, drag the mouse to the lower right corner of the data, and then release the mouse. The graphs will zoom to show you the data you selected. To view the entire graph again, click and drag in the opposite direction, from the lower right to the upper left. You can also pan the graphs, whether zoomed or not, by dragging the graph with the right mouse button. Here is an example of zoomed Pressure and Tachometer graphs:
You can enable or disable any of the five graphs by clicking on the appropriate check boxes in the Enable Graphs panel. You might choose to do this if you wanted to inspect one of the graphs in detail.Typically, three of the graphs are visible when you do a Grab, and a different three graphs are visible when use the Dyna function.
Grabbing Data
When you click the Grab button, you acquire data from the engine for 1 second at 1000 samples per second. The acquired Pressure data is graphed at the top, and should appear as a sinusoid. The Tachometer signal is also acquired, and graphed below the Pressure data. The Tachometer data consists of 16 pulses for each rotation of the engine drive shaft, with one of the 16 pulses twice as wide as the others. The wide pulse occurs when the piston is at the bottom of its stroke (when the engine volume is at a minimum). The software detects this wide pulse and flags it with a red pulse marker.
The current temperature of the engine's left/cold plate is displayed in the grid below the buttons as "Lo Tmp"; the right/hot plate temperature is displayed as "Hi Tmp"; the power consumed by the four 1 ohm heating resistors is displayed as "Res Pwr"; and the difference between the highest and lowest pressures is displayed as "Press".
When the engine is running very slowly, sometimes only one wide/red pulse appears on the Tachometer graph. When this happens, the software is unable to determine the speed at which the engine is running, and the RPM and Indicated Power values cannot be computed and will not be displayed. If the engine is running quickly enough, more than one wide/red Tachometer pulse is acquired, and both RPM and Indicated Power are computed and displayed in the grid below the buttons, as "RPM" and "Ind Pwr".
Here are graphs of an engine that is running very slowly. Notice that the pressure curve is no longer a nice sine shape, and notice that the green tach dots are no longer evenly spaced.
When you Grab data, the "Power" and "Torque" values are always blank: they are computed with the Dyna function.
RPM is computed by locating two adjacent wide/red Tachometer pulses, computing the time that elapses between the pulses, and computing the rotational speed from that time.
Indicated Power is computed by plotting a Pressure/Volume graph (PV graph) and computing the area inside the enclosed line. Pressure data acquired from the engine (and plotted in the top graph) is used directly to create the PV graph. Volume data is not acquired directly from the engine, but is inferred by assuming that the drive shaft is rotating at a constant speed between each tach pulse, and computing the engine volume over time as the piston moves in and out. The area inside the curve is computed and multiplied by the engine RPM to produce a power value.
The Dynamometer
When you click the Dyna button, you acquire data from the engine for 4 seconds at 1000 samples per second. The acquired Tachometer data is plotted in the top graph. The RPMs are computed for the entire 4 second period, and are displayed below the Tachometer data. Power and Torque data are computed and graphed in the final graph, at the bottom. "Press", "RPM", and "Ind Pwr" will remain blank after you press the Dyna button.
To correctly use the Dynamometer facility, you need to first get the engine up to some test temperature. Slow the power output shaft down with your fingers until the engine is just barely running. With the mouse hovering over the Dyna button, release the drive shaft and then click the Dyna button. The software will acquire data during the period when the engine is spinning the flywheel up to speed. Power and Torque are computed based on how quickly the engine can increase the RPM, and on the moment of inertia of the flywheel.
The grid at the lower left allows you to enter the masses (in grams) and diameters (in mm) of the various components of the crankshaft and the flywheel. The values shown in the screen shot, above, are for (in order) the aluminum flywheel, the large brass disk, the full-diameter portion of the smaller brass disk, the small-diameter portion of the smaller brass disk (the hub of the aluminum flywheel), and the steel drive shaft. The moments of inertia are computed using formulas for solid disks/cylinders. The values shown are 1000 times the computed values (in kg and m), so that they can be read reasonably.
The Power and Torque graphs have both thin/dotted lines and thick lines; the former are the raw data, while the latter are "smoothed" data, averaged over more than one value of the raw data.
In theory, the Power and Torque that are computed in the Dynamometer function are both accurate and calibrated, as is the Indicated Power that is computed using the PV approach when you Grab data. In addition, Indicated Power is the maximum theoretical power of the engine, ignoring power that is subsequently lost due to friction or other losses. That is, one would expect the Indicated Power to always be larger than the Power that is computed using the Dynamometer approach, or the braking method described below.
Export
You can export your data (either Grab or Dyna) at any time, by clicking the Export button. Data is saved in CSV form, for later import into the spreadsheet of your choice, or for re-Import into this program.
Import
You can import and re-analyze any of the data that you previously saved using the Export button. The graphs and figures of merit (RPM, power, etc) are re-computed.
Multiple
This button allows you to perform multiple Imports and to save a summary of the figures of merit in a summary spreadsheet.
You will first be asked where you wish to store the summary data. Create an appropriate file name for the summary data.
You will then be asked to select multiple Import files for analysis. Use <Ctrl><click> to toggle which files you wish to include in your analysis. When you have the correct list of files selected, click Open, and each file will be imported, in turn, and the computed values output to the summary CSV file. The resulting file will look like this in your spreadsheet:
