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Bosch K-Jetronic Fuel Injection Part 3

Bosch K-Jetronic

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.

Remember to take all necessary safety precautions when working on your car.


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