D.4.d. Volumetric Efficiency Table Corrections
To compensate for engines that stray from thermodynamic linearity, the Volumetric Efficiency
(VE) table comes into play. The VE table can be set to anywhere from 8 x 8 to 16 x 16 data points. The X-
axis is the RPM and the Y-axis is the MAP reading. When the tuning program is first being established,
the numbers in the VE table will be all zeros when the “Offset” view mode is selected (the numbers will be
all 100’s when the “Absolute” view mode is selected). This refers to the fact that there is not yet any
change in place from the raw fuel curve. Once a number is entered into the table, the raw fuel curve is
modified at that particular point. Between cells in the VE table, a 256-point interpolation is occurring to
smooth out the fuel curve. Otherwise, the fuel curve would “step” from cell-to-cell (and wouldn’t be much
of a curve!).
The VE table allows the user to select between
“Offset,” “Absolute,” “Pulse Width,”
and
“Duty
Cycle”
for the display mode. The “Offset” view mode will display zeros when there is no change to the
raw fuel curve at a particular point. The Offset Number can be set to a negative or a positive number.
When a negative number is entered, the fuel curve is leaner than the raw fuel curve at a given point. When
the number is positive, it is richer than the raw fuel curve at that point. The “Absolute” view mode will
display 100% when the fuel curve is unmodified. A number less than 100% will lean out the fuel curve,
while a number greater than 100% will richen up the fuel curve. The “Pulse width” view mode shows the
injector pulse width values that the
TECgt
will use throughout the operating range. The numbers in the
“Duty Cycle” view mode should not exceed the maximum value for an RPM when taking the injector
recovery time into consideration. See the section on fuel injector selection for more details on this topic.
Since the Raw Fuel Curve defined by TOG and IOT is a two-dimensional graph (i.e. Load vs. Pulse
width) and the VE table is a three-dimensional graph (i.e. Load vs. RPM vs. Pulse width), it is easy to
visualize VE table changes at specific RPM points. As an example, if TOG was chosen to be 7ms, IOT
was 0ms, and the engine ran well at peak torque and below, but was overly rich when above the torque
peak, the raw fuel curve might be modified as in
Figure D.4. 4
. The modified fuel curve could be obtained
easily by entering –30% into the VE “Offset” at all RPM/Load points above the torque peak. This type of
modification results in a straight-line modification to the raw fuel curve.
Figure D.4. 4
What if the same engine needs less fuel at around 80-90kPa for a particular RPM point (assuming a
1-Bar sensor)? The modification to the Raw Fuel Curve at the problem RPM may look something like
Figure D.4. 5
.
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TECgt Manual Version 2.0
- Page 64 -
©2008 Electromotive, Inc.