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Diesel Tuning Fundamentals: Diesel Injector Operation

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Diesel Injector Operation

04.58

00:00 - The fuel injectors used for common rail diesel engines are generally grouped into two methods of operation, piezo, or piezo electric, and inductive or solenoid.
00:09 As a diesel engine tuner it's not strictly essential to have a deep understanding of the specifics of the injector construction and operation, however it's still useful to understand the basic concepts.
00:22 In particular it's important to know that the way piezo and inductive injectors are controlled by the ECU are very different.
00:28 This is more relevant for those of you considering the fitment of a univeral standalone ECU rather than reflashing the factory fitted ECU.
00:38 An inductive injector is similar in operation to a conventional low pressure gasoline port fuel injector, where a magnetic coil inside the injector is responsible for opening and closing the injector to control fuel flow.
00:52 In a common rail diesel injector this presents some challenges however as a result of the incredibly high fuel pressures that the injectors are subject to, and the very fast response required from the injector.
01:04 Part of the solution to this is the very high voltages and currents used to control the injectors.
01:11 For example it's common for peak voltages to exceed 100 volts and peak current to reach in excess of 10 amps.
01:19 A piezo injector on the other hand uses a crystal stack inside the injector which is typically made from quartz.
01:26 The principal of operation is that when voltage is applied to the crystal stack, it expands, albeit only very slightly.
01:34 It's this movement that is in turn used to actuate and open the injector to start fuel flowing.
01:40 The original benefit of the piezo injector design was the very fast operation which was up to five times faster than the inductive injectors of the time.
01:50 Advances however in inductive diesel injector technology have meant that both injector types can now provide similar performance and it's difficult to clearly pick a winner.
02:01 In the OE world, Bosch and Siemens produce piezo injectors while Delphi favour the inductive method.
02:08 Denso have their bases covered, developing both technologies.
02:11 The signals used to drive common rail diesel injectors are far more complex than what we may see for a conventional gasoline port injector, requiring accurate control on the part of the ECU in order to control the current through the injector, through the separate stages of the injector's operation.
02:30 Again this is more of a concern for those tuners dealing with aftermarket standalone ECUs than those reflashing a stock ECU.
02:38 Due to the complex operation of the injectors themselves, coupled with the physical fitment to the engine, swapping to larger injectors from a different engine or even aftermarket injectors is difficult or in some cases impossible.
02:52 This leads to some challenges if we're aiming to increase engine performance by virtue of a significant increase in fuel delivery.
03:01 Fortunately there is a work around here as the injector is designed as a two part system.
03:07 We have the injector body which houses the actuation system which we've just discussed, along with a nozzle which is the part of the injector that locates into the combustion chamber and includes a number of tiny holes through which the fuel is physically sprayed into the cylinder.
03:23 In the aftermarket we often have access to larger or modified nozzles that can supply more fuel for a given injector pulse width.
03:31 As diesel engine development has advanced and we've seen a drive for improved fuel economy and lower emissions, manufacturers have put more emphasis into the design of their injector nozzles.
03:42 In particular we've seen the size of the holes in the nozzle shrink and these may be in the region of 0.4 millimetres or thereabouts.
03:51 These very small holes coupled with increased fuel pressure allow for better atomisation of the fuel.
03:57 In order to supply sufficient fuel in a given pulse width, more holes are used and this also allows the manufacturer to control how and where the fuel is distributed within the combustion chamber.
04:09 One of the problems of modern injector production is accurately replicating the size of these holes across every injector in a mass manufacturing situation.
04:19 It's possible for example to see a variation that may be as great as 20% across a set of production injectors.
04:27 The result is that there can be significant differences in injector flow from one injector to the next.
04:33 Obviously this will have a big impact on the fuel delivery to the engine, so in the OE world this is dealt with by an individual code on each injector which describes its flow characteristics.