Posted to Technical Tips Forum on 3/24/2016
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.
Public Safety Radio
Gallatin, Tennessee, USA
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