Bosch K-Jetronic Fuel Injection System: Mechanical Operation
Bosch K-Jetronic Mechanical Fuel Injection System
The K-Jetronic system from Bosch provides a variable fuel flow, mechanically driven in continuous mode. This system performs three main functions:
- Measures the volume of air drawn into the engine through a special flow meter.
- Supplies fuel using an electric pump that sends fuel to a dispenser-distributor, providing fuel to the injectors.
- Prepares the mixture: the volume of air drawn into the engine, depending on the position of the throttle valve, determines the initial fuel dosage. The volume of air is determined by the flow acting on the feeder-distributor.
Fuel System
The system provides a low-pressure feed with the exact amount of fuel needed for the engine in working condition. The power system consists of the fuel tank (1), the fuel pump (2), the fuel tank (3), the fuel filter (4), the pressure regulator (5), the distributor-dispenser fuel (16), and injection valves (9). A roller pump, electrically driven, sucks fuel from the tank and drives it under pressure through a pressure accumulator and a filter.
Electric Fuel Pump
A centrifugal type pump is located at the edge of the deposit. Inside, there is an eccentric chamber with a disc that contains five cavities where the rollers are located. Due to centrifugal force, the rollers are projected against the walls, increasing the volume of the cavities and inhaling the gas, which drives up the distributor tube.
The pump has a relief valve that limits the system pressure. This prevents a possible blockage from causing the failure of the pump.
When the pump is stopped, a valve at the outlet maintains residual pressure in the circuit.
The pump motor itself is bathed in gasoline, which simultaneously serves as a lubricant and coolant.
Although it may seem that there is a risk of combustion being in contact with gasoline and the electric motor, this is not possible due to the absence of air for combustion.
When the ignition is turned on, the pump starts operating and stays on the entire time the engine is running.
A security system stops the pump when there is no power control.
Fuel Tank
The circuit maintains low fuel pressure after the engine stops to facilitate a new start, especially if the engine is hot.
Thanks to the particular shape of its body, the tank has a buffer action on the impulses in the circuit due to pump action.
The interior of the tank is divided by two chambers separated by a membrane (4). One chamber (5) has the mission to collect fuel, and the other (1) contains a spring.
During operation, the accumulation chamber is filled with fuel, and the curve bends to the stop, opposing the pressure exerted by the spring. The membrane is in this position, which corresponds to the maximum volume until the engine stops working. As the circuit loses fuel pressure, the membrane is displaced to compensate for this lack of fuel.
Air Flow Measurement
The regulator has two functions: to measure the volume of air drawn into the engine and to dispense the amount of fuel to achieve a proper air/fuel ratio. The air flow meter, located in front of the butterfly in the intake system, measures the air flow. It consists of an air funnel (2) with a mobile probe plate positioned at the level of the smallest diameter. When the engine draws air through the funnel, the plate (1) is sucked up or down (depending on each installation) and leaves its position. A lever system transmits the movement of the plate to the slide valve (8), which determines the amount of fuel to be injected. To stop the motor, the probe plate returns to the neutral position and rests on an adjustable spring (3) blade (in the case of food-probe moving upwards). To avoid damaging the probe if called by the intake manifold, the plate-tube may oscillate in the opposite direction, against the leaf spring, to a larger section. A rubber shock absorber limits its travel.
Fuel Inlet
The fuel dispenser-distributor dispenses the required amount of fuel and distributes it to the injectors. The amount of fuel varies depending on the position of the plate-tube flow meter of air, and therefore in terms of air drawn into the engine. A set of levers translates the position of the plate-tube into a corresponding position of the spool valve. The position of the slide valve in the cylindrical chamber of ports determines the amount of fuel to be injected. When the piston rises, it increases the section released in the ports, allowing more fuel to pass to the differential pressure valves (upper chambers) and from these to the injectors. The upward movement of the control piston opposes the force that comes from the pressure control circuit. This control pressure is regulated by the “pressure regulator control” and serves to ensure that the slide valve piston always immediately follows the movement of the plate-tube but remains in position before the plate-probe returns to the idle position. The differential pressure valves in the fuel dispenser-distributor ensure the maintenance of a constant pressure drop between the sides of entry and exit of ports. This means that any variation in the fuel line pressure or any difference in pressure between the nozzle opening cannot affect the fuel flow control.
Operation of the Slide Valve
The piston position of the slide valve itself is determined by the position of the plate-tube; therefore, it is a function of air flow in the funnel of the flow meter. The fuel should be distributed evenly among the engine cylinders. The principle of this deal lies in the control of the flow section of the “slices of strangulation,” machined in the cylinder of the “slide valve.” The cylinder has as many openings (cracks of strangulation) as the engine has cylinders.
A differential pressure valve affected each of the slits has the function of keeping the pressure drop at a constant value. This valve is composed of a lower and an upper chamber separated by a steel membrane. The pressure prevailing in the upper chamber is less than 0.1 bar (a value that represents the differential pressure). This pressure difference is produced by a coil spring embedded in the upper chamber. If the amount of fuel that passes through the upper chamber by throttling slits increases, the pressure increases momentarily on this camera. The steel membrane bends towards the bottom and adjusts the section of the nozzle exit to the extent necessary for the establishment at the crack of strangling a pressure differential of 0.1 bar. The slide valve piston, according to its position, more or less discovers cracks bottleneck.
The circuit pressure control circuit derives power through an “orifice” located in the dispenser-distributor. The signal pressure is determined by the control pressure regulator. The “bottleneck” that is located above the slide valve serves to cushion the sounder plate movements caused by bursts of air that often manifest themselves at low speed.
Pressure Regulator
A fuel pressure regulator located in the regulatory mix (feeder-distributor) maintains a constant pressure of 5 bar at the bottom of the differential pressure valves, irrespective of the use phase of the motor and flow variations of the feed pump. The pressure regulator returns unused fuel to the tank at atmospheric pressure. Also, the pressure regulator returns to the fuel tank the fuel that reaches the “warm-up phase regulator” through the inlet (8) and through the isolation valve (5).
Cold Start
When starting cold, the engine needs more fuel to compensate for losses due to condensation on the cold walls of the cylinder and intake pipes. To compensate for this loss and to facilitate cold start, a cold start injector (10) was installed in the intake manifold, which injects additional fuel during the starting phase. The cold start injector is opened by activating the coil of an electromagnet which is housed inside. A thermal switch timer limits the time of injection of the cold start valve according to engine temperature. To limit the maximum injection of the cold start injector, the thermal switch timer is provided with a small element that is activated when the starter motor starts. The heatable element heats a bimetal strip that is bent due to heat and opens a pair of contacts, so that current to the cold start injector will be cut.
Enrichment for the Heating Phase
As the engine warms up after a cold start, it is necessary to compensate for the gasoline that condenses on the cold walls of the cylinders and the intake pipes. During the heating phase, the fuel/air mixture is enriched, but this enrichment should be reduced gradually as the engine warms up to avoid an overly rich mixture. To control the mixture during the heating phase, a control pressure regulator (also called: heating phase regulator) is provided, which regulates the control pressure. A reduction in control pressure decreases the antagonistic force in the air flow meter, thus allowing the plate to rise more in the funnel, thereby raising the slide valve, allowing more fuel for the lights. Inside the control pressure regulator, a valve diaphragm (1) is controlled by a coil spring (4), a force which opposes a bimetallic plate (3). If the engine is cold, during heating, the bimetal plate bends downward due to the heating element (2) (which is powered during the engine warm-up phase), counteracting the force of the spring (4) with the membrane (1) moving so that the pressure of the slide valve control on gas decreases, fleeing to the pressure regulator and the deposit, thereby reducing the pressure control. The slide valve rises and increases the richness of the mixture supplied to the engine cylinders.
During the cold start, the signal pressure is approximately 0.5 bar, while in normal conditions, it reaches a value of 3.7 bar.
For engines designed to operate at partial load with a very poor air/fuel mixture, the regulatory environment of the heating stage has been improved, equipped with a splice of depression to the intake manifold. This allows the regulator to exercise a reduced control pressure during the warm-up phase, with the corresponding air/fuel mixture richer when the engine is running at full load. In this state of service, the throttle is fully open, and manifold vacuum is very weak. The combined effect of a second valve membrane and a coil spring is to reduce the effect of the diaphragm pressure control, which in turn reduces the signal pressure that causes cross-fertilization with the engine load. The membrane charge-control (5) acts on the second spring (3) because this is subject at the top to the depression of the intake manifold and at the bottom of the atmospheric pressure.
With a load of depression in the middle engine, the intake manifold is sufficient to compress the spring charge controller so that the membrane of the pressure control valve (1) increases the pressure up on the control slide valve, impoverishing the mixture injected into the cylinders.
Additional Air Valve
The frictional resistance of the cold engine makes it necessary to increase the flow of air/fuel while the engine warms up. This also allows maintaining a stable idle. The extra air valve is responsible for increasing the flow of air into the engine while the throttle continues to idle position. The additional air valve opens a bypass duct with the butterfly, and all the air that enters must go through the air flow meter. The plate rises and misses a proportional amount of fuel through the distributor luminaries dispenser fuel. A bimetal strip controls the operation of the additional air valve to regulate the opening section of the bypass line. During a cold start, a larger section is gradually reduced as the engine temperature increases until it finally closes. Around the bimetal strip is a small element that connects when the engine is operational. This will control the opening time, and the device does not work if the engine is hot because the strip receives the engine temperature.
Injectors
The fuel dispensed by the dispenser-distributor is sent to the injectors, and these are injected into the various inlets before the intake valves of the engine cylinders. The nozzles are isolated from the heat generated by the engine, avoiding the formation of small bubbles of steam in the injection tubes after stopping the engine. The valve (1) responds even to small amounts, which ensures adequate spray, even in idle.
The injectors do not contribute to dosage. The injection valves are automatically opened when the pressure exceeds a set value (3.3 bar) and remain open, injecting gasoline while maintaining the pressure. The needle valve at a high frequency obtains excellent vaporization. After the engine stops, the injectors are closed when the supply pressure is below 3.3 bar. When the engine stops and the pressure in the fuel system drops below the opening pressure, a spring-flush valve makes a tight seal that prevents any drop from coming closer to the intake pipes.
