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TDC overlay
Posted to Technical Tips Forum on 3/24/2016 14 Replies

There has been recent discussion on the availability of TDC and FLS type waveform overlays. While working on some of the recent posted waveforms; it occurred to me that once I understood how the engine cycles are represented by the waveform; I could work around not having an overlay program and still get the information. The waveform overlay program converts time on the horizontal scale to degrees of rotation, after the user marks the desired 720 degree segment of the waveform. Then divides the segment into equal parts, according to the number of cylinders. Some also can place degree markers.

Pico-scope software has "Rotation Rulers". The two rulers are moved to locate a segment of the scope pattern defining 720* of crankshaft rotation. If a sync marker is available, that can be used, or two identical points, where the pattern repeats, can to used. The number of integral rulers can be set for 1 thru 10 to match the number of cylinders. The rulers divide the pattern to show the same location in the running cycle, for each cylinder, usually the spark. The degrees of rotation is shown for each ruler.

If the ruler "0*" is set to #1 ignition then the other ignition events are marked, in firing order.

In analyzing vacuum waveforms; the intake stroke is 360* after the power stroke. So the intake pull for #1 starts at the ignition point of the companion cylinder, or 360*, or center of the marked out segment. Counting the "pulls" in firing order from there will identify the intake pull for each cylinder. The synced cylinder's intake will always start at the 360* marker. If the engine is 4,6,8 or 10; the companion cylinder spark event will line up on the start of the pull. If it is a 5 cylinder there is no companion cylinder. The intake events occur half way between adjacent spark events. The synced cylinder's pull is at the center, count in firing order to identify the others.

The other vertical degree rulers can be used to show precise timing of events if needed. A vertical ruler can be set to the degrees of the opening, closing of intake or exhaust to help identify events.

This can also be done on any DSO waveform that has horizontal scale markings, usually in milliseconds(msec). The difference being that the rotation rulers have to be manually marked. I like to print out the waveform then use a ruler and dividers(compass) to mark off the cylinder segments, firing events, valve events etc.

To make the rotation rulers, take the msec at the beginning and the end of the 720* shown by the sync pulses. Subtract beginning from end msec's, divide by the number of cylinders to get msec per cylinder. Use the horizontal scale to set the divider legs space to the msec/cylinder number. The dividers can now be used to "step" across the waveform and mark out the repetition of any event, (spark, intake pull). Label the steps in firing order to identify the associated cylinder. The synced cylinder intake pull will be at the center of the waveform, so it's easy to find.

The same technique can be used with in-cylinder pressure waveforms. The 720* is easily id'd by the compression towers. The intake pull starts just after 360*(when the exhaust valve closes). Calculating the degrees/msec from the horizontal scale will enable placing an event marker as needed to identify timing events.

I am trying to find a source for camshaft degree timing of engines. Often the SI does not give valve timing data so a nominal timing data set is substituted. Marking when the event is supposed to happen on the waveform will help identify timing issues.

Steve Park
Public Safety Radio
Gallatin, Tennessee, USA

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