There is additional information about injection systems which does not fit neatly into any particular category but is nonetheless useful information. This is detailed below:

Injector position

The position of the injector in the inlet tract has a noticeable affect on the way the engine runs.  It can affect economy, transient throttle and power output.  It is generally accepted that injector positioning close to the inlet port gives good economy, transient throttle and idle, together with good emissions, and that injector positions further back in the inlet tract improve power at the expense of these criteria.  Ultimately for the best power output the injector should be sited as far back as possible, i.e. in the trumpet or air-horn.  Siting the injectors here does give a big problem at low throttle openings and low rpm since the fuel hits the butterfly.  It can also cause fuel to be bounced out of the trumpet by pressure waves in the inlet.

Dual injector systems

Dual injector setup on 4 cyl engineDual injector systems attempt to exploit the benefits of the close-to-port injector while also gaining from the power increase to be had from having the injector in the trumpet.  The way this is done is to fit two injectors, one close to the inlet port and one in the trumpet.  The EMS controls these two injectors using the near injector for part throttle, low rpm and transient and switching to the second trumpet mounted injector when the engine is at WOT (Wide Open Throttle).  Some systems switch from one injector to the other immediately a certain set of conditions is reached, while other system go 50/50 between the injectors or grade one injector's usage down while ramping the other's up.  This system if implemented properly gives the best of both worlds.

Twin injector systems

Twin injector systems are normally used when the size of injector required would be very large and might affect the metering and atomisation capabilities at low rpm and idle, typically on a turbocharged engine where fuelling requirements vary enormously from transient to wide open throttle.  The fuel can be metered through one injector when requirements are low, and through both when requirements grow exponentially, or it can be metered through both at all times.  Often a second set of injectors are fitted by after-market tuners whose modifications may require fuelling beyond the capacity of the current injectors.  This is most likely to happen in turbo or supercharged installations.

Injector duty cycle

In order to inject a fuel into the engine, the injector is opened for a period of time, known as the pulse width, this time is always the same for a given quantity of fuel, regardless of engine speed.  As engine rpm increases, the time available per revolution to fire the injector is less - at 6000rpm the time available is exactly half the time at available at 3000rpm.  As this injection opportunity gets progressively smaller the injectors are required to fire much more frequently.  This can result in the injector being open almost all the time.  When the injection system used is sequential, the requirement is to be able to deliver the fuel at a time when the inlet valve is closed.  This further reduces the injectors' opportunity to fire.

The percentage of time that the injector is open is known as the ‘duty cycle’ and this represents the relationship between the time the injector is closed, measured against the time it is open.  If the duty cycle goes above 90per cent anywhere in the rev band (i.e. the injector is open for more 90per cent of the time) then the injector capacity is being reached and the engine may require larger injectors.  These will discharge more fuel in a given period of time which means the injector times can be decreased, bringing the duty cycle into acceptable limits.  Unfortunately this also means that the engine will need re-mapping to suit the new larger injectors or the mixture will be hopelessly rich.

Some EMS have a scaling factor which represents the relationship between the map figure units and the pulse width.  By varying this, the whole map can be scaled up or down for different sized injectors.  This is not a perfect way of coping with a change of injector size because the time taken to open the injector is the same and the scale factor affects this too.  However it will get 95per cent of the way there when changing injector sizes.

Injector sizing

In order to size injectors for a given engine it is important to know their discharge or flow rate.  From this and an approximation of the engine's potential rpm, potential peak power and torque an estimate can be made and an appropriately sized injector chosen.  It is better to err on the large side just in case you reach the injector capacity while mapping and have to start from scratch.  Larger injectors have a couple of disadvantages in that the granularity of adjustment is larger and the atomisation of fuel is poorer with a larger orifice.