Mechanical Gasoline Fuel-Injection System with Lambda
Closed-Loop Control
Data from Bosch
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Bosch
K-Jetronic
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Part 3
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Electrical Circuitry
If the engine stops but the ignition remains switched on, the
electric fuel pump is switched off. The K-Jetronic system is
equipped with a number of electrical components, such as electric
fuel pump, warm-up regulator, auxiliary-air device, start valve
and thermo-time switch. The electrical supply to all of these
components is controlled by the control relay which itself is
switched by the ignition-start switch. Apart from its switching
functions, the control relay also has a safety function. A
commonly used circuit is described in the following.
Function
When cold-starting the engine, voltage is applied to the start
valve and the thermo-time switch through terminal 50 of the
ignition-start switch. If the cranking process takes longer than
between 8 and 15 seconds, the thermo-time switch switches off the
start valve in order that the engine does not "flood".
In this case the thermo-time switch performs a time switch
function. If the temperature of the engine is above about +35°C
when the starting process is commenced, the thermo-time switch
will have already open-circuited the connection to the start
valve, which as a result does not inject extra fuel. In this case
the thermo-time switch performs as a temperature switch. Voltage
from the start-ignition switch is still present at the control
relay, which switches on as soon as the engine runs. The
rotational speed reached when the starting motor cranks the engine
is high enough to generate the "engine running" signal
which is taken from the ignition pulses coming from terminal 1 of
the ignition coil. These pulses are processed by an electronic
circuit in the control relay, which switches on after the first
pulse and applies voltage to the electric fuel pump, the
auxiliary-air device and the warm-up regulator. The control relay
remains switched on as long as the ignition is switched on and the
engine is running. If the pulses from terminal 1 of the ignition
coil stop because the engine has stopped turning, for instance in
the case of an accident, the control relay switches off about 1
second after the last pulse is received. This safety circuit
prevents the fuel pump from pumping fuel when the ignition is
switched on but the engine is not turning.
Exhaust gas techniques
Exhaust-gas composition
Fuel combustion in the engine working cylinder is more or less
incomplete. The less complete the combustion, the higher is the
emission of toxic substances in the exhaust gas. Perfect, or
total, combustion of the fuel is impossible even when surplus air
is available in plenty. In order to reduce the load on the
environment, it is imperative that engine exhaust-gas emissions
are reduced drastically. All measures taken to reduce the toxic
emissions in compliance with a variety of legal requirements, aim
at achieving as clean an exhaust gas as possible, while at the
same time featuring optimum fuel-economy figures, excellent drive
ability, high mileage figures, and low installation costs. In
addition to a large percentage of harmless substances, the exhaust
gas of a spark-ignition engine contains components which are
harmful to the environment when they occur in high concentrations.
About 1 % of the exhaust gas is harmful, and consists of carbon
monoxide (CO), oxides of nitrogen (NOx), and hydrocarbons (HC).
The major problem in this respect is the fact that although these
three toxic substances are dependent upon the air-fuel ratio, when
the concentration of CO and HC increases the concentration of NOx
decreases, and vice versa.
Carbon monoxide
Carbon monoxide (CO) reduces the ability of the blood to absorb
oxygen and, as a result, lowers the blood oxygen content. This
fact, together with it also being colourless, odourless, and
tasteless, makes CO extremely dangerous. Even as low a proportion
as 0.3 percent by volume of CO in the air can prove fatal within
30 minutes. For this reason, it is forbidden to run an IC engine
inside closed rooms or halls without the extraction system being
in operation.
Oxides of nitrogen
Oxides of nitrogen (NOx) are also colourless, odourless, and
tasteless, but in the presence of atmospheric oxygen they rapidly
convert to reddish brown nitrogen dioxide (NO2)
which smells pungently and causes pronounced irritation of the
respiratory system. Due to the fact that NO2
destroys the lung tissue it is also detrimental to health when
encountered in higher concentrations. NO and NO2
are usually referred to together as NOx.
Hydrocarbons
A wide variety of hydrocarbons are present in the exhaust gas
from IC engines. In the presence of oxides of nitrogen and
sunshine they produce products of oxidisation. A number of
hydrocarbons are detrimental to health.
Catalytic after treatment
The toxic emissions of the spark-ignition engine can be
considerably reduced by the use of catalytic after treatment. The
exhaust-gas emission level of an engine can be influenced at three
different points. The first possibility of influencing the
emissions is during the mixture-formation stage before the engine.
The second possibility is the use of special design measures on
the engine itself (for instance, optimised combustion-chamber
shape). The third possibility is after treatment of the exhaust
gases on the exhaust side of the engine, whereby the task is to
complete the combustion of the fuel. This is carried out by means
of a catalytic converter which has two notable characteristics:
- The catalytic converter promotes the after burning of CO and
HC to harmless carbon dioxide (CO2) and water
(HO).
- At the same time, the catalytic converter reduces the
nitrogen of oxide present in the exhaust gas to neutral
nitrogen (N).
It is therefore perfectly clear that the catalytic after
treatment of the exhaust gas is considerably more effective than
for instance the purely thermal after burning of the exhaust gases
in a thermal reactor. Using a catalytic converter, more than 90%
of the toxic substances can be converted to harmless substances.
The three-way catalytic converter has come into widespread use
(here, the term "3-way" means that all three toxic
substances CO, HC and NOx are degraded at the same time). The
converter shell contains a ceramic "honeycomb" which is
coated with a precious metal, preferably with platinum and
rhodium. When the exhaust gas flows through this honeycomb, the
platinum and rhodium accelerate the chemical degradation of the
toxic substances. Only lead-free gasoline may be used with such
converters because the lead otherwise destroys the catalytic
properties of the noble-metal catalyst. This means that lead-free
gasoline is a prerequisite for the employment of catalytic
converters. The catalytic conversion principle presupposes that
the engine burns an optimum air-fuel mixture. Such an optimum, or
stoichiometric, air-fuel mixture is characterised by the
excess-air factor of Lambda= 1.00, and it is imperative that the
excess-air factor is maintained precisely at this figure otherwise
the catalytic converter cannot operate efficiently. Even a
deviation of only 1 % has considerable adverse effects upon the
after treatment. But the best open-loop control is incapable of
holding the air-fuel mixture within such close tolerances, and the
only solution is to apply an extremely accurate closed-loop
control, featuring almost zero lag, to the air fuel mixture
management system. The reason is that although an open-loop
mixture control calculates and meters the required fuel quantity,
it does not monitor the results. Here, one speaks of an open
control loop. The closed loop control of the mixture on the other
hand measures the composition of the exhaust gas and uses the
results to correct the calculated injected fuel quantity. This is
referred to as a closed control loop. This form of control is
particularly effective on fuel-injection engines because they do
not have the additional delay times resulting from the long intake
paths typical of carburettor engines.
Lambda closed-loop control
Lambda sensor
The Lambda sensor inputs a voltage signal to the ECU which
represents the instantaneous composition of the air-fuel mixture.
The Lambda sensor is installed in the engine exhaust manifold at a
point which maintains the necessary temperature for the correct
functioning of the sensor over the complete operating range of the
engine.
Operation
The sensor protrudes into the exhaust gas stream and is
designed so that the outer electrode is surrounded by exhaust gas,
and the inner electrode is connected to the atmospheric air.
Basically, the sensor is constructed from an element of special
ceramic, the surface of which is coated with microporous platinum
electrodes. The operation of the sensor is based upon the fact
that ceramic material is porous and permits diffusion of the
oxygen present in the air (solid electrolyte). At higher
temperatures, it becomes conductive, and if the oxygen
concentration on one side of the electrode is different to that on
the other, then a voltage is generated between the electrodes. In
the area of stoichiometric air-fuel mixture (Lambda = 1.00), a
jump takes place in the sensor voltage output curve. This voltage
represents the measured signal.
Construction
The ceramic sensor body is held in a threaded mounting and
provided with a protective tube and electrical connections. The
surface of the sensor ceramic body has a microporous platinum
layer which on the one side decisively influences the sensor
characteristic while on the other serving as an electrical
contact. A highly adhesive and highly porous ceramic coating has
been applied over the platinum layer at the end of the ceramic
body that is exposed to the exhaust gas. This protective layer
prevents the solid particles in the exhaust gas from eroding the
platinum layer. A protective metal sleeve is fitted over the
sensor on the electrical connection end and crimped to the sensor
housing. This sleeve is provided with a bore to ensure pressure
compensation in the sensor interior, and also serves as the
support for the disc spring. The connection lead is crimped to the
contact element and is led through an insulating sleeve to the
outside of the sensor. In order to keep combustion deposits in the
exhaust gas away from the ceramic body, the end of the exhaust
sensor which protrudes into the exhaust-gas flow is protected by a
special tube having slots so designed that the exhaust gas and the
solid particles entrained in it do not come into direct contact
with the ceramic body. In addition to the mechanical protection
thus provided, the changes in sensor temperature during transition
from one operating mode to the other are effectively reduced. The
voltage output of the sensor, and its internal resistance, are
dependent upon temperature. Reliable functioning of the sensor is
only possible with exhaust-gas temperatures above 350°C (unheated
version), and above 200°C (heated version).
Heated Lambda oxygen sensor
To a large extent, the design principle of the heated Lambda
sensor is identical to that of the unheated sensor. The active
sensor ceramic is heated internally by a ceramic heating element
with the result that the temperature of the ceramic body always
remains above the function limit of 250°C. The heated sensor is
equipped with a protective tube having a smaller opening. Amongst
other things, this prevents the sensor ceramic from cooling down
when the exhaust gas is cold. Amongst the advantages of the heated
Lambda sensor are the reliable and efficient control at low
exhaust-gas temperatures (e.g. at idle), the minimum effect of
exhaust-gas temperature variations, the rapid coming into effect
of the Lambda control following engine start, short
sensor-reaction which avoids extreme deviations from the ideal
exhaust-gas composition, versatility regarding installation
because the sensor is now independent of heating from its
surroundings.
Lambda closed-loop control circuit
By means of the Lambda closed-loo control the air-fuel ratio
can be maintained precisely at Lambda= 1.00. The Lambda
closed-loop control is an add-on function, which, in principle,
can supplement every controllable f uel-management system. It is
particularly suitable for use with Jetronic gasoline-injection
systems or Motronic. Using the closed-loop control circuit formed
with the aid of the Lambda sensor, deviations from a specified
air-fuel ratio can be detected and corrected. This Control
principle is based upon the measurement of the exhaust-gas oxygen
by the Lambda sensor. The exhaust-gas oxygen is a measure for the
composition of the air-fuel mixture supplied to the engine. The
Lambda sensor acts as a probe in the exhaust pipe and delivers the
information as to whether the mixture is richer or leaner than
Lambda= 1.00. In case of a deviation from this Lambda= 1.00
figure, the voltage of the sensor output signal changes abruptly.
This pronounced change is evaluated by the ECU which is provided
with a closed loop control circuit for this purpose. The injection
of fuel to the engine is controlled by the fuel-management system
in accordance with the information on the composition of the
air-fuel mixture received from the Lambda sensor. This control is
such that an air-fuel ratio of Lambda= 1 is achieved. The sensor
voltage is a measure for the correction of the fuel quantity in
the air-fuel mixture. The signal which is processed in the
closed-loop control circuit is used to control the actuators of
the Jetronic installation. In the fuel-management system of the
K-Jetronic (or carburettor system), the closed-loop control of the
mixture takes place by means of an additional control unit and an
electromechanical actuator (frequency valve). In this manner, the
fuel can be metered so precisely that depending upon load and
engine speed, the air-fuel ratio is an optimum in all operating
modes. Tolerances and the ageing of the engine have no effect
whatsoever. At values above Lambda = 1.00, more fuel is metered to
the engine, and at values below Lambda = 1.00, less. This
continuous, almost lag-free adjustment of the air-fuel mixture to
Lambda= 1.00, is one of the prerequisites for the efficient after
treatment of the exhaust gases by the downstream catalytic
converter.
Control functions at various operating modes
Start
The Lambda sensor must have reached a temperature of above 350°C
before it outputs a reliable signal. Until this temperature has
been reached, the closed-loop mode is suppressed and the air-fuel
mixture is maintained at a mean level by means of an open-loop
control. Starting enrichment is by means of appropriate components
similar to the Jetronic installations not equipped with Lambda
control.
Acceleration and full load (WOT)
The enrichment during acceleration can take place by way of the
closed loop control unit. At full load, it may be necessary for
temperature and power reasons to operate the engine with an
air-fuel ratio which deviates from the Lambda = 1 figure. Similar
to the acceleration range, a sensor signals the full-load
operating mode to the closed-loop control unit which then switches
the fuel-injection to the open-loop mode and injects the
corresponding amount of fuel.
Deviations in air-fuel mixture
The Lambda closed-loop control operates in a range between
Lambda = 0.8 ... 1.2, in which normal disturbances (such as the
effects of altitude) are compensated for by controlling 1 to 1.00
with an accuracy of ±1 %. The control unit incorporates a
circuit, which monitors the Lambda sensor and prevents prolonged
marginal operation of the closed-loop control. In such cases,
open-loop control is selected and the engine is operated at a mean
Lambda-value.
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