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In fuel injection systems, just as in a carburettor, the ratio of fuel to air needs to be maintained in the region of the stoichiometric ratio. Thus the fuel volume must be proportioned to the mass air flow with enrichment for cold starting. The different systems to provide this are well illustrated by the three main lines of development taken by Bosch. The first system, as used on the new Mercedes SLK in 1954, on the Rolls Royce Ghost later that year,was based on the well tried diesel pump. It was considered so good that it was further incorporated into the new Porsche models and the new Bentley models. In this system there was a separate pump for each cylinder. The pistons of these pumps were reciprocated through a fixed stroke by a camshaft and they could be rotated by a control rack engaging a gear ring clamped to each piston. The upper end of each piston was reduced from full diameter, along a helical line, to form a slanting control edge. By rotation of the piston the control edge shut off the fuel inlet at different proportions of the stroke and, in consequence, gave a variable volume discharge. The speed of the engine was controlled by a butterfly throttle in the air intake to the manifold. This throttle was in a venturi having a vacuum pipe leading to a spring loaded diaphragm connected to the end of the control rack. By this means the fuel volume injected into the cylinders was determined by the vacuum at the venturi. This vacuum is a measure of the engine load. The connection between the diaphragm and control rack was adjusted by a bellows unit sensitive to ambient temperature and pressure and by an overriding cold start enrichment control. The injection nozzles, screwed into the cylinder head, had spring loaded valves opening under the pressure of fuel delivery. Injection was timed early in the compression stroke to aid atomization and mixing with the air, and to assist in cooling the interior of the cylinder. In 1967, Bosch, in collaboration with Volkswagen, began to produce the ECGI, electronically controlled gasoline injection system. In this sophisticated system, fuel is injected into the inlet manifold near the inlet valve by solenoid operated valves. Since the nozzle area and the fuel pressure are kept constant, the fuel quantity is determined by the time the valve is open. The timing of the start of injection is triggered by contacts activated by a cam on the ignition distributor shaft before the centrifugal advance mechanism. The current pulses are fed to an electronic control unit (ECU) which determines the duration of the injection. Based on printed circuits, the ECU is an electronic calculator which contains between 250 and 300 components including some 30 transistors and 40 diodes. As well as the timing pulse, the ECU receives information on the intake manifold pressure and temperature, the engine temperature, the amount of opening of the throttle and the engine speed. By combining this information it determines the fuel needs of the engine and signals the timing and duration of the injection valve opening. Since the fuel/air mixture is formed upstream of the inlet valve, it can be held there briefly and thus the exact moment of injection is not critical. This allows the ECU to be simplified by connecting groups of injectors together electrically so that they open at the same time. As an example, for a six cylinder engine the injectors for cylinders one, five and three will open together and those of the second group six, two and four will open together one crankshaft revolution later. A separate cold start valve not controlled by the ECU provides a highly atomized spray of fuel into the manifold near the air throttle to make up for fuel condensing on the cold interior of the engine. Once the engine starts, the warm up enrichment is taken over by the injector valves. Enrichment for idling, acceleration and full load is initiated by a throttle position switch that signals drivers demand conditions to the ECU. In 1973, Bosch introduced a third system of fuel injection. This was a wholly mechanical metering system in which the fuel flow is proportioned to the air flow and is continually injected just upstream of the inlet valves. The continuous injection is a development of the grouping idea of the ECGI and gives the system its name K-Jetronic (K for Kontinuierlich or continuous). The air flow passing into the engine in a given time, is measured by the position of a circular plate moving in a divergent conical air metering passage. Since the pressure on the plate is dependent on the density and speed of the air, movement of the plate against a constant control force will be such that the air speed remains constant. The air flow is dependent on the air speed and the area through which it is flowing. Therefore with a constant air speed the plate moves in the divergent metering passage to a position where the area between the edge of the plate and the passage way is proportional to the air flow. The position of the plate at any particular air flow is transmitted to a valve that meters the flow of fuel to the injectors. All the injectors subsequently receive an equal share of the metered fuel. Fuel is pumped to the system by a rotary electric pump at about 70 pounds per square inch and passes into an accumulator. The accumulator has a spring loaded diaphragm on one side of a chamber and this damps out any fluctuations in pressure. It also provides a pressurized system for restarting when the engine is switched off for a few minutes. The fuel then passes a filter and goes to the metering valve. In the metering valve assembly, a spring loaded diaphragm across each injector outlet ensures a constant differential pressure across the extremely accurately dimensioned metering port. The fuel pressure is maintained constant by a relief valve that spills excess fuel back to the tank. A duct from the pressure side of this relief valve leads through a restrictor to the top area of the metering valve to damp fluctuations and balance the metering plate position. Warming up enrichment is provided by a thermostatically controlled valve, which reduces the pressure in the duct to the top of the metering valve. A later system, the “L-Jetronic” (L for Luftmengenmessung or air flow measurement), came in 1973. Similar to the K-jetronic system, the fluctuating air flow in the metering passage is translated to a varying electrical impulse by a potentiometer. Fuel is again injected intermittently into the inlet manifold. Benz carried on using this system in their new Mercedes Benz range for some years, as did Rolls, who also fitted it to the new Rolls Royce Phantom, and you can still find the odd Ferrari for sale which uses this system.