Building a Crankshaft Position Indicator


In order to create a P/V diagram (indicator card) for an IC or Steam engine, the position of the piston must be known. If the diagram is being developed using suitable software, then piston position can be determined "on-the-fly" from crankshaft position.

The device described here provides an analog voltage proportional to crankshaft position. Specifically, the device provides 0 volts at TDC and increments positive every 3.6 degrees of rotation, until at 356.4 degrees it provides 5 volts.

The device consists of two disks fitted to the crankshaft, and an associated electronics package.

The disks are as follows:

This is a view of the two disks, mounted to the crankshaft of an IC engine. The leftmost disk has a single notch cut into the periphery, while the right most disk has notches every 3.6 degrees, except for one "missing" notch. This means that there are 99 notches on one disk and one on the other.

This is a picture of the 99-notch disk during fabrication. The missing notch is at the top.

Study this one carefully. See the two mounting screws in front, then two resistors standing vertical, and behind them are two "U" shaped pieces. These "U" shaped pieces are optical gaps, that is, on one side of the gap there is an LED and on the other side is a photo-transistor. They sense the intrusion of an opaque material into the gap. A much bigger version can be used to count people walking through a doorway. The two disks are positioned so that the notches of the disks pass through the gap are the disks rotate. The optical gap device(s) then provide a pulse every time a notch passes through the gap. The disk with the single notch is to the left while the disk with 99 notches is to the right (the notches can me seen, with some imagination, as the are blurred in the photo)

The disks are positioned so the disk with the single notch generates a pulse (via the optical gap) when the piston is at TDC, while the disk with 99 notches provides a pulse every 3.6 degrees of crank rotation, EXCEPT WHEN THE PISTON IS AT TDC.

These pulses are connected to an 8-bit counter via individual Schmitt Triggers. The TDC pulse sets (resets) the counter to ZERO, while each of the 99 pulses provided by the other disk increment the counter by ONE. The counter is reset every time the piston reaches TDC, the counter at any time indicates the number of 3.6 degree increments the crank has rotated since the last TDC event.

The counts in binary, and the binary output is used to drive a series of 8 LEDs and a (very) low cost 8-bit to Analog converter. The converter output is span adjusted using an Op-Amp so the a count of 99 provides a 5 volt output. The LEDs can be used during calibration and set-up to indicate crank position and verify counter functionality, but are just a blur at operating speed.

This is the electronics (and of course the little bit that was seen above with the gaps), very simple, about $5, not counting the case. An additional output can be taken from the TDC or position pulses for use as a tachometer drive.

If anyone finds this obscure site and wants to build one of these, E-Mail me and I will explain the electronics and send a schematic J .

Steve Wagner

Bellwood PA




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