Re: Injection time pulse calculation

--- In opendiag@yahoogroups.com, "artgyum" <artgyum@y...> wrote:
> Hi,
> Can anybody suggest me, how can I calculate injection time?
> On which parameters ii may depend (CTS value, Engine speed, O2S
> voltage, etc..)?
> Any variants are appreciated.
>
> Motronic 1.5, KW71
> Thanks

Finally, I got the time to reply your question.

Disclaimer: bear in mind I'm just an electronic engineer, not an
automotive one ;-)

First, you determine your initial set point: you calculate your basic
pulse width based on injector's flow rate and cylinder displacement
for a stoichiometric mixture, i.e. AFR=14.7 (Air-to-Fuel Ratio for
gasoline). For this calculation, let's asume some normalized
conditions:

VE = 100% (Volumetric Efficiency);
MAP = 1atm = 760torr = 30inHg = 14.7 lb/in2 = 101.3kPa
(Manifold Absolute Pressure);
BP = MAP (Barometric Pressure) (it means no boost)
IAT = 70F (Intake Air Temp);

Let's also asume you know the amount of air filling the cylinders.
Since you know your injectors flow rate, and the amount of fuel
injected is proportional to the pulse width controlling the injection
time, the resulting equation in milliseconds is:

PW_Base = 3,600,000 * Cyl_Air_Mass / (14.7 * Inj_FR * Divide_Pulse)

Where:
3,600,000 = number of milliseconds in an hour.
Cyl_Air_Mass = pounds of air filling the cylinder.
14.7 = Stoichiometric Air-to-Fuel Ratio for gasoline.
Inj_FR = Injector Flow Rate in pounds per hour.
Divide_Pulse = Number of injections per engine cycle

Now, we need to calculate the mass of air filling each cylinder:

Cyl_Air_Mass = Cubic_Inch_Displacement * Air_Density /
Number_of_Cylinders

Using the Ideal Gas Law (which you may remember from high school
chemistry lessons), Air Density can be calculated as follows:

P * V = n * R * T

Dividing both sides of this equation by R*T, you get:

n = (P * V) / (R * T)

Now multiply both sides of the last equation by Mol to get M:

M = n * Mol = (P * V * Mol) / (R * T)

Dividing M by V, you get D:

D = M / V = (P * Mol) / (R * T)
D = (MAP * VE * Mol) / (R * [(IAT - 32) * 5/9 + 273])

Where:
P = Pressure (torr);
V = Volume (liters);
T = Absolute Temperature (Kelvin);
D = Density (grams/liter);
M = Mass (grams);
n = number of mols (mol);
Mol = molar mass (grams/mol);
P = MAP * VE
R = 8.314 J/[mol*K] = 62.36 [L*torr]/[mol*K]
IAT = In-take Air Temperature (Fahrenheit)
T = (IAT - 32) * 5/9 + 273 (converts IAT from Fahrenheit to Kelvin)
Mol = 29 g/mol (average molecular mass of air)

Since we've assumed a set of standard conditions, we can calculate
Air_Density as:

D = (760torr * 1 * 29 g/mol) / (62.36 [L*torr]/[mol*K] * 294.1K)
D = 1.2017 g/l

The kind of adjustment (and hence, the kind of fuel map) depends on
the kind of controller (speed-density, Alpha-N, etc). Bosch Motronic
is an Alpha-N fuel controller, i.e. it uses an AFM (Air Flow Meter)
and RPM as main inputs (to calculate the basic set point).

Then the EFI controller uses other inputs, like IAT (Inlet Air Temp),
engine temp, idle switch, WOT switch, O2 sensor, crank speed sensor,
crank reference sensor and A/C switch, to adjust the basic set point
according to different driving conditions.

Any question? ;-)
Max

 
Received on Fri Feb 27 09:02:21 2004