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Reply to "MTH Engine Gear Ratios"

Step 3 : expectations

Scaling factor 16 bit value - I would expect this to be a number that multiplies or divides the number to be scaled.

Like:   actual value = initial value * scaling factor

or:      actual value = initial value / scaling factor

In his reply above, Ted S, thinks this is not a 16 bit value but rather two 8 bit values. He might be right.

DTO and DOD 32 bit value - This has to be derived from the tach strip. In its simplest form, I would expect it to be a count of tach pulses. I would expect the DTO value to be a lower value with a lot of decimal places. A representation that could measure short distances accurately. I would expect the DOD value to be a higher number with few decimal places. A representation that could measure long distances. Think about the number of digits on the odometer and trip odometer in your car.

So if the DTO and DOD values were just a count of tach pulses, we could expect that the scaling factor embodies the gear ratio and wheel diameter to convert the raw values into linear distance.

Step 4: results

Now, running my engine over the measured 1.5884 Smile loop, I got the scale factor, DTO and DOD values from the engine. I used the remote to get these same values displayed as Smiles.

It took some experimenting and hints from Eric Linz, but I came up with these two equations:

DTO in Smiles =  raw_value / (11 * scaling_factor)

DOD in Smiles = (46.5 * raw_value) / scaling_factor

The constant factors in the equation were not exactly pulled out of thin air but I can't explain their values. I got those values by working backwards using the remote to tell me what DTO/DOD value I should see.

Step 5: tests

Over my fixed loop, the DTO and DOD values using the equations agree to within about 0.25% with the actual length of the loop. I tested this with one diesel engine and one steam engine that had different gear ratios and different wheel diameters. Both equations agree with the remote down to a few decimal places (but don't produce the exact same results as the remote).

I don't yet understand how to take a gear ratio and wheel diameter to produce the scaling factor value.

That's all I have been able to figure out about DTO and DOD.

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