1991 OPEL ASTRA Electronic Workshop Manual

Downloaded from www.Manualslib.com manuals search engine 7 FUEL INJECTION
It is the function of any fuel injection system to ensure
that the correct mass ratio of air and fuel is delivered to
the engine under all operating conditions. We will concen-
trate on TBi (Throttle Body Injection or alternatively
called single point fuel injection systems) in this docu-
ment.
The availability of powerful low cost microprocessors has
made it possible to fit FI (Fuel Injection) systems to a
larger percentage of vehicles. The ECU (Engine Control Unit)
evaluates input sensor data and calculates the required
output signals to control the engine. The most important
function of a FI system is to measure the air mass entering
the engine and calculate the injector opening duration to
ensure the correct A/F ratio under specific engine operating
conditions. The A/F ratio has a direct effect on the power
output of the engine, fuel consumption and exhaust gas
emissions. It is therefore necessary to exercise precise
control over the opening duration of the injector.
A number of operating conditions exist where the A/F ratio
is deliberately modified and forced to deviate from the
calculated ratio to ensure better drivability and smoother
engine operation. These deviations are classified as A/F
corrections and will later be examined in detail in this
document.

Downloaded from www.Manualslib.com manuals search engine 7.1 AIR MASS TO FUEL MASS RATIO
The theoretical air mass to fuel mass ratio required by an
internal-combustion engine for complete combustion is
14.7:1. This ratio is also called the stoichiometric ratio.
The A/F ratio determines the fuel consumption, maximum
engine power output and exhaust gas emission levels. Unfor-
tunately there is no single A/F ratio that optimises these
three requirements.
The ratio of actual air mass supplied to the engine divided
by the theoretical requirement is defined as lambda.
ë = Air mass supplied/theoretical requirement
Where ë = Lambda
ë = 1
The air mass supplied matches the theoretical amount.
ë < 1
A lack of air resulting in a rich mixture. Increased engine
power outputs are obtained for 0.85 < ë < 1.
0.75 < ë < 0.85
A rich mixture suitable for transient conditions where a
sudden load change is experienced.
ë > 1
An excess of air resulting in a lean mixture together with a
reduction in engine power output. Optimum fuel consumption
takes place with 1 < ë < 1.2.
ë > 1.3
Lean mixture making it impossible to achieve reliable igni-
tion.

Downloaded from www.Manualslib.com manuals search engine 7.2 AIR MASS MEASUREMENT
As we have seen previously the principle of fuel injection
is based on measuring the air mass entering the engine and
calculating the fuel mass required to obtain an A/F ratio of
14.7:1. A number of possible methods exist for measuring
the air mass, but only the speed-density method used for
this TBi system will be described in detail.
Some of the possible methods are:
a) Speed-density
b) Throttle butterfly angle
c) Flap or vane type
d) Hot-wire meter
e) Hot-film meter
f) K rm n vortex ultrasonic meter.

Downloaded from www.Manualslib.com manuals search engine 7.3 SPEED DENSITY CONCEPT
If we know the density of the air inside a container it is
possible to calculate the exact mass of the air inside the
container.
Am = Va*p Where Am = Air mass (g)
Va = Air volume (cc)
p = Air density (g/cc)
In an automotive FI application the quantity of fuel to be
injected can be calculated if the displacement volume of the
cylinder and the density of air within the cylinder at the
onset of the compression stroke are known.
The air density could be determined by measuring the abso-
lute pressure and absolute temperature of the air. In an
automotive application it is, however, not practical to
directly measure the air pressure and temperature inside the
cylinder. To overcome this limitation the air pressure and
temperature are measured in the inlet manifold. This leads
to certain pressure measurement errors for which corrections
have to be made.
To understand and correct these errors the engine is mo-
delled as an air pump. For the purpose of explaining these
problems one has to view the engine as a pump sucking air
from the inlet manifold through a restriction which is
formed by the inlet valve in the open position. The dynamic
properties of the air moving past the inlet valves are
engine speed dependent and to such an extent that the inlet
manifold pressure will tend to be slightly higher than the
in-cylinder pressure at the end of an inlet stroke. This
leads to higher than actual air mass measurements which
have
to be corrected. The air pumping efficiency is called the
volumetric efficiency of the engine.

Downloaded from www.Manualslib.com manuals search engine Volumetric efficiency depends mainly on the:
a) Inlet valve and camshaft design
b) Inlet manifold design
c) Engine speed
The volumetric efficiency is normally less than one (1) and
has the effect that the actual air mass entering the cylin-
der will be less than the measured value.
Peak volumetric efficiency co-insides with the engine speed
where the maximum torque is developed.
7.4 FUEL METERING
Now that a method has been established to determine the air
mass entering the engine it is possible to calculate the
quantity of fuel to be injected to achieve the required A/F
ratio. An electric fuel pump delivers fuel at a constant
pressure to the injector which is opened for the required
injection duration. The fuel pressure is regulated by a
mechanical pressure regulator.
The fuel mass to be injected is:
Fm = Am/(A/F) Where Fm = Fuel mass
Am = Air mass
Since the fuel is injected in bursts it is necessary to know
the fuel flow rate of the injector, pump and regulator
system, as well as the injector opening and closing delays
at various battery voltages to enable calculation of the
injection duration.

Downloaded from www.Manualslib.com manuals search engine 7.5 CONTINUOUS FUEL FLOW RATE
The continuous fuel flow rate at a constant battery voltage
is dependent on the injector design, fuel pump and fuel
pressure used. Tests have shown that the fuel flow rate,
for the Rochester TBi system, is dependent on the applied
battery voltage.
The fuel flow rate becomes lower with reduced battery volt-
age. The ECU compensates for this effect by lengthening the
injection pulses at low battery voltages.
7.6 INJECTOR OPENING AND CLOSING TIME
The current, through the injector coil, creates a magnetic
field which moves the armature, thus opening the nozzle
allowing fuel to be injected. Short opening times therefore
require a fast build up of current in the injector. This
requires a low inductance or a high applied voltage. Short
closing times require a low holding current which is ob-
tained by reducing the opening current while the injector is
open to a current just large enough to keep the injector
open. Since the available voltage in automotive applications
is restricted to the battery voltage (6 to 13.8V) the injec-
tors are normally designed to have low inductance.
The measured characteristics of the test injector are:Inductance = 4mHResistance = 1.5 ohmStatic opening current = 1.1A *Static closing current = 0.5A ** Static opening and closing current is independent of fuelpressure.

Downloaded from www.Manualslib.com manuals search engine The injector drive circuit allows a pull-in current of 4A
and a holding current of 1A. Once an injector current of 4A
is reached the circuit will automatically reduce it to 1A.
Injector opening and closing times vary with battery voltage
and vary between 0.2 ms at 14 volt and 1.35 ms at 6.5 volt.
The ECU takes this into account by measuring the battery
voltage before calculating the injection times.
The electrical opening duration of the injector consists of
the sum of the opening and closing delays and the true
required injection duration.
7.7 INJECTION TIMING
Under static conditions (constant engine load and speed)
fuel will be injected in synchronisation with the crankshaft
position and speed reference signal, i.e. once per intake
stroke. Fuel injection will commence 0.5 ms after the 10ø
BTDC reference.

Downloaded from www.Manualslib.com manuals search engine 7.8 INJECTION DURATION
The base injection duration for different loads and engine
speeds is calculated and fine tuned by mapping to ensure
that the required A/F ratio is obtained under static operat-
ing conditions. (Lambda = 1) Injection durations vary from 1to 5 milliseconds.
The following operating parameters were assumed for the
purpose of obtaining the base map:
a) Continuous fuel flow rates with battery voltage
at 14 volt.
b) Intake air temperature at 303.7 øKelvin.
c) Zero injector delay.
d) One injection cycle per intake stroke.
7.9 CORRECTION FACTORS
Correction factors are applied to the base map to correct
for injection hardware deficiencies and engine operating
conditions.
7.9.1 Injection hardware deficiencies.
The reduced fuel flow rate at low battery voltages necessi-
tates an increase in the injection duration. The factor by
which the injection durations from the base map has to be
multiplied is obtained by normalising the fuel flow rate at
different battery voltages with respect to 14.0 volt.
In addition to the above multiplication factor the base map
injection times are further increased by the injector delays
to ensure that the correct quantity of fuel is injected.