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Law & Order: Criminal Intent - Season 7
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Fuel Injector Duty Cycle Calculator
The
fuel injector duty cycle (IDC) is the percentage of time the
injector is supplied with power. The time during which the injector is
powered (or activated) is called the injector pulse width (IPW). During
normal engine operation, the fuel injector fires once during the four
strokes of the Otto cycle, which last for 2 revolutions of the engine.
As an example, at 3000 rpm it takes 0.040 seconds or 40 milliseconds
(ms) for the engine to complete 2 revolutions (3000 rpm divided by 60
equals 50 revs per second; invert to get 0.02 sec per rev or 0.04
second for 2 revs). At 6000 rpm it takes 20 ms for two revolutions. If
a fuel injector is activated for 15 ms (the IPW) at 3000 rpm the duty
cycle is 37.5% (15 ms/40 ms), or rpm times IPW divided by 1200 equals
IDC in percent. If an injector is powered for 15 ms at 6000 rpm, then
IDC is 75% (15 ms/20 ms). If you know the engine speed (rpm) and the
IPW (dataloggers can provide this information), then it is easy to
calculate the IDC.
NOTE: These calculators may not work
correctly in Netscape, Opera, or another browser besides MS Internet
Explorer.
For those of you with IE 7 (or beyond), you may get
a warning
about my web site using ActiveX controls. It does not. I do use
JavaScript for my calculators. If you want the functionality of the
calculators, allow "ActiveX" controls (see instructions by clicking on
the IE bar above my web page, if it is there).
You can use the calculator below to
determine injector duty
cycle when engine speed and injector pulse width are known. You can
enter numbers in the white boxes. The yellow box shows the result and
is read-only. The "Reset fields" button writes the default values into
the white boxes. Press "Calculate" to determine IDC. To change the
value in a white box, first click in the box to give it the
mouse/keyboard focus. Then change the number. Letters are not allowed;
neither are negative values. Click outside the box or click the
"Calculate" button to show the IDC. This simple calculator is
illustrative but not very interesting. The calculators in the next
section are much more useful.
Rich run
limit Low power, black
smoke Rich best torque at
WOT Safe best power at
WOT Lean best torque at
WOT Chemically
ideal Lean light load, part
throttle Best economy, part
throttle Lean run
limit
As
we modify our engines to produce more power, we install larger
fuel injectors. The calculators and information below can assist in
determining the correct injector size to reach your goals by presenting
the required duty cycles for different sized injectors during various
engine loads (mass air flow divided by engine speed). The boost value
is the major factor determining engine load.
The engine control module (ECM) determines the
injector pulse
width using many engine sensors and a variety of control logic
depending on engine operating conditions (see 2-fuelinjection.htm
for
more details). When engine power is more important than fuel
economy, the most important input is the engine speed (from the crank
angle sensor), the mass air-flow rate (from the volume air-flow,
air-temperature, and air-pressure sensors), and the target air-fuel
ratio (A/F; determined from tables programmed into the ECM).
When we install larger-than-stock fuel injectors,
we usually install
the ability to alter the air-fuel ratio using some sort of controller.
While we would like to keep A/F near 12.5 for best power, the A/F
usually must be lowered (the mixture richened) to reduce the tendency
for detonation (knock). Quite often, this means A/F must be near 11 or
a little lower. The table above shows typical A/F limits for different
engine operating conditions. You can select an A/F in the input
parameters below. While 12 is the default, I suggest lowering this to
about 11 to simulate real fuel demands during WOT engine operation at
high boost levels.
We need to know the A/F here because after the mass
air flow
is determined for a particular engine speed (see below) the A/F and
gasoline density are used to determine the volume of fuel required. The
average value for the density of gasoline is usually stated as 6 pounds
per gallon, equivalent to 719 grams per liter. You should probably
leave the gasoline density at the default value unless you know the
value for the gasoline you use or just want to experiment. Knowing the
required fuel volume and the maximum amount of fuel that the given
number of injectors of the selected flow rate can flow if open
constantly, the required injector duty cycle can be calculated.
While the ECM uses measured air flow, we will have
to estimate
air flow at various engine speeds. To do this we must know the
volumetric efficiency of the engine during wide-open throttle (WOT)
operation. Volumetric efficiency (VE) is the ratio of volume of air
entering the air filter(s) to the engine displacement for each Otto
cycle. For forced induction engines, VE can easily exceed 100%. Again,
because we cannot measure air flow here, I'll use a concept I call
natural capacity
(NC) and the pressure ratio to determine the air flow. Natural capacity
is the percentage of cylinder swept area (or the engine displacement
when considering all cylinders) that is replaced with fresh air-fuel
charge, regardless of the air density (boost or vacuum), during the
Otto cycle. The NC cannot be less than 0 nor more than 1.0 (100%).
After all, you can only fill a cylinder completely and that's it.
Pressure ratio is the absolute pressure in the plenum (ambient air
pressure plus boost pressure) divided by the ambient air pressure
(barometric pressure). Volumetric efficiency, then, is estimated as the
natural capacity times the pressure ratio.
The set of default natural capacities in the
calculator below
are ones I have found to be reasonable for stock and for modified
engines. The values can be changed by clicking the mouse inside one of
the white boxes and modifying the value to be from 0.0 to 1.0. Click
"Re-calculate" to update the IDC tables. You can set all the "Modified"
natural capacities to 1.0 (100%) to simulate a perfect flowing engine
(even if nearly impossible to attain); this would be the very maximum
amount of air an engine could flow for a given pressure ratio and
engine speed. Clicking the "Reset fields" button restores the default
NC values; but the "Re-Calculate" button must be clicked to change the
IDC tables.
Natural
Capacities
IDC for a single user-selected injector
size
As in the first little calculator
above, the yellow boxes are
for display only and cannot be edited. Change values in the white boxes
above and click the "Calculate" button to update these tables. Using
the radio buttons, you can select between the natural capacities for a
stock engine or for a modifed engine.
Many
manufacturers recommend that IDC does not remain above 85-90% for
extended periods. Some injectors may actually flow less above 95% IDC
than below that value. Of course, if the listed IDC is greater than
100%, the fuel injector is inadequate for the application.
IDC for a several
pre-selected injector sizes
As above, the
yellow boxes are for display only and cannot be
edited. Change values in the white boxes above and click the
"Calculate" button to update these tables. Only natural capacities for
modified engines are used here. To change these, change values in the
Natural Capacities form above and press the "Re-calculate" button. The
factory fuel injectors are rated at 360 cc/min. DSM (Eclipse, Laser,
and Talon) turbo models had 450 cc/min injectors. The new Mitsubishi
Lancer Evolution VIII has fuel injectors that are reported to flow
about 580 cc/min and also have the dual port pintle opening. The
aftermarket 660 cc/min and larger injectors are popular for the highest
output 3000GT/Stealth engines.
Selecting the right-size fuel
pump
The required fuel flow values, in the
"lph" column in the
tables above, can be used to determine the "size" fuel pump needed. The
chart below shows flow data for two of the most popular upgrade fuel
pumps for 3000GT/Stealth cars. For additional fuel pump upgrade
information see my web page 2-fuelpumpguide.htm.
To
determine if a fuel pump with a provided voltage can provide the
required fuel, first note the "lph" flow at an rpm and boost level in a
table above. Find this lph flow on the left axis in the chart below. On
the top axis of the chart below find the chosen boost level. Locate the
intersection of these two "lines". If the fuel pump flow curve lies
above this value, the fuel pump should be able to satisfy the engine
demands.
