Lean chart




The goal of a leaning test is to determine the peak EGT operating condition (from which the best economy and best power mixtures can be determined) and to measure how well the fuel flows are balanced between the cylinders. I've enabled only the fuel flow trace and the six EGT traces since those are the most relevant for this test. I've used Fsaver's annotation feature to label the beginning and the end of the leaning operation.

This works best with a slow but steady decrease in fuel flow. I was interested in how fast I was leaning so I used the delta cursors to determine this. First I cursored the fuel flow trace at the start point and hit the delta cursor button (Δ) and then moved the cursor to the end point. The auxiliary x-cursor edit box now shows that the test lasted 6.62 minutes. Then I clicked on the Cursor ID tag to select the slope mode (Y/X). The auxiliary y-cursor edit box now shows that the mixture was reduced by 1.653 gph every minute (or .138 gph for every 5 second sample time). It turns out that this is about as fast as you want to go to achieve accurate results. Note that you can use this Y/X delta cursor trick to compute all kinds of useful information from the graphs. In addition to the mixture change rate, some other examples are:

Once you have a better idea of where the EGTs will peak, you can speed up the test considerably since we really just need about a gph on either side of the peak.

Here I've expanded the display further to include only about 1.8 minutes of the test since the leaning chart comes out best if it's only based on enough data to have a clearly defined maximum for every EGT trace.

Finally I hit the Lean tag, which produced the leaning chart above. The circular spots are individual measurements from the engine analyzer (every 5 seconds) and are represented in the trace selection box as the lower case egt1 thru egt6. Fsaver computes a least squares polynomial fit to these points to create the solid traces represented in the trace selection box as the upper case EGT1 thru EGT6. (You can enable just the individual samples or just the least squares fit from the trace selection box). Then Fsaver computes fuel flow that maximizes the EGT for each of the six polynomial fits and lists these fuel flows in the box (labeled Peaks) just below the trace selection box. They are arranged according to the cylinder number with EGT1 at the top and are also color coded to correspond with the plot colors. That table is the one that you would send to GAMI so they could fine tune your fuel injectors for perfect balance. The least squares polynomial fit technique improves the accuracy of the results especially when the measurements are somewhat noisy. (The measurements in this example are relatively clean). It also provides good results with faster leaning rates than would otherwise be practical.

Note that I used the peak cursor button to move the cursor to the peak of the EGT3 trace. The x-cursor edit box shows that this happens at 16.077 gph which agrees with the value in the Peaks table. Fsaver also computes the difference between the richest and leanest injectors and displays this figure of merit in the graph title. In this example the cylinder mismatch is .33 gph which is small enough you might refrain from making any further adjustments. You can create these charts for carbureted engines as well, although there isn't much you can do to correct the fuel imbalances. However it will help you understand why lean of peak operation is not likely to work well.