Physics – Delphi Automotives Needle Lift

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A Fuel injector is electronically controlled valve, which contains amongst other things a tightly coiled spring and a needle (see diagram: however this is a electromagnetic fuel injection system). A pump connected to the fuel injector pushes highly pressurised fuel down to the bottom of the injector, causing pressure to build up. When the pressure reaches around 1600 bar (1,600,000,000 Nm-2), the needle is forced upwards, creating a hole in the injector. The fuel then, moves from an area of high pressure to an area of low pressure, dispersing. The tiny nozzle is designed to atomise the fuel, which aids combustion (due to greater surface area). This process is completed a vast number of times per second.

Diagram:

For many years the Hall effect was not applied practically due to the generated voltage in the metal was so extremely low. However towards the second half of the twentieth century the mass production of semiconductors came underway. Chips based on the Hall effect inexpensive and so were able to be used in a wide range of instances

The Hall effect Integrated Circuit is a very small chip that is made up of many transistors. It consists of a thin layer of silicon as a Hall generator (which works to a greater efficiency than gold) and several transistor circuits: to amplify the Hall voltage to the level required; to trigger output voltage with its growth; and to provide stable work without depending on the power supply voltage changes.

Semiconductors give a small Hall Voltage (VH). This is due to the mobility of the contained electrons being greater than that of the holes. Metals generally show the same effect only on a smaller scale, with a lesser Hall voltage. Some metals even give a potential difference of the opposite polarity.

The above table shows that even though the metal Copper does have a greater number of charge carriers, the semi-conductor, Geranium still has a greater Hall Voltage. This is due to the other variables of the Hall effect. These are the width of the material, the strength of the magnetic field, and the strength of the current passing through the material, which could all constitute to a higher or lower Hall Voltage. In Delphi Automotives, the Hall effect was used to collect results when testing their fuel injectors.

They would use a ……. o tell whether or not the needle was open or closed at the time as the Hall voltage would be greater when the needle was ……. A vast number of reading could be taken each second to find out specific information about the process of the needle lift. The readings would be drawn as a graph (see below) The above graph shows needle lift against time on the x-axis. The distance labelled by marker D is the height of the needle, in the fuel injector; from its closed (bottom) position to its fully open (top) position, where diesel would be released.

The lines have been drawn as vertical as the gradients are so large it may be neglected, due to the speed of the lift. The time at A shows the amount of time that the valve is open, and diesel is flowing into the engine, on the new common rail systems, this timing can be altered by the on board computer. The valve then closes and remains in this position for the time B, also fully variable with common rail. There is a problem however with the speed of the movement that occurs with the needle.

This is that as the needle touches the bottom, it recoils slightly up and if the pressure is not enough to keep it down (from the coiled spring) the valve may become open once more. This would allow an increased influx of diesel from the pump, which will never be burned as the piston by this time will be on the down stroke and to will be ejected on the exhaust stroke and measured as unwanted hydrocarbons (as discuss above). For this reason Delphi tries it’s utmost to minimise this effect to increase the efficiency of the diesel engine and reduce the level of harmful/noxious gases released from the exhaust.

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