- Over the last decade, we've seen leaps and bounds in terms of the technology applied
to our modern crop of aftermarket fuel injectors.
What we're seeing is larger and larger injectors that can still flow incredibly small amounts of
fuel which is vital in order to get good quality idle as well as cruise conditions or control over
our air/fuel ratio in cruise conditions.
Traditionally as we go larger in the injector flow capacity, while the injector can flow more
fuel which is important at high RPM and high boost, where these larger injectors tend to
fall over is that their control of very small quantities of fuel at very small pulse widths
can suffer so that's when we get problems with our air/fuel ratio control.
The modern crop of Bosch EV14 style injectors has really improved technology here.
There's a number of manufacturers providing injectors based around the Bosch EV6 injector,
including this one here which is an ID1050X injector from Injector Dynamics.
These injectors go up to 1700 cc with larger injectors coming out.
That's all well and good for most modified street cars but when we're starting to build drag
cars, particularly high boost drag cars running on methanol fuel, 1700cc injector still is likely
to be insufficient.
One of the problems is here that obviously as we increase the boost pressure and our target
for engine power increases, we need to provide more fuel but with a lot of our cars
now running on alternative fuel such as E85 or even methanol, the amount of fuel
or mass of fuel that we need to provide in order to keep the engine happy and get good
control over our air/fuel ratio also increases.
As a rough guide, when we move from pump gas through to E85, we need to provide around
about 35-40% more fuel.
When we go to methanol, we need to provide somewhere around about 2-2.5 times the
amount of fuel so as you can see, this has a serious impact on the injectors that we fit
to the engine.
Now when we fit larger injectors and some of these injectors are available in the aftermarket
such as the billet atomiser now go up to 500 pounds per hour which is approximately
a little over 5000 cc or five litres of fuel per minute.
It's actually slightly higher than that but just as a rough conversion there.
That's a massive injector so it's more than capable of keeping the engine happy under
high boost and high RPM but again we get to this situation, trying to make the engine idle
happily on a 5000cc per cylinder injector or 5000cc per minute injector can be
problematic.
One of the technologies that has been around for a long time which we can apply to help
fix this is staged injection where we have two or more injectors per cylinder.
There's a couple of ways we can run this.
Now we've seen this a lot in the older, naturally aspirated racecar development
where two injectors are used per runner.
Now this is traditionally not used in order to provide more fuel volume because with
naturally aspirated engines, typically we could easily buy injectors that are big enough to
support the engine's power requirements.
But these are used in a different way with the staged injection and what happens here is that
one set of injectors will be fitted down near the inlet port near the intake valve so these spray
directly on the intake valve and this is used at idle and low RPM.
Then there'll be a second set of injectors that are placed outside of the intake trumpet.
Sometimes completely out of the trumpet itself.
And these wouldn't work very well at idle an at low engine speeds where the air speed is
quite low, however at high RPM, particularly 7000, 8000 RPM and above at wide open throttle,
using these injectors that are fitted outside of the inlet trumpets can actually provide a small
but real advantage in terms of power.
This comes through better atomisation and mixing of the fuel, giving a more homogenous
fuel/air charge entering the cylinder.
So staged injection is used here where the injectors down by the intake port, down by
the intake valve are used at low RPM and low throttle settings, as the driver increases
the RPM and throttle settings, the staging moves the injection out to the outboard
injectors and in most instances at high RPM and wide open throttle, all of the fuel would
be delivered solely through the outboard injectors so that's one technique.
The other technique which we see more often used in our high boost turbocharged engines
is where the staged injection is there to provide more overall fuel flow.
Here we'd run on one set of injectors at low RPM and low load where the fuel demands
aren't that high.
As the RPM and the boost pressure increases, we'll gradually start staging in the secondary
injectors so that we're using basically an equal pulse width or duty cycle at high
RPM and high boost between both injectors, making sure that we're maximising the flow
through both injectors.
Now the injector size doesn't need to be the same here either.
The injectors can be different sizes and quite often a smallish injector will be used for the
primary injector with a much larger secondary injector.
Maybe somewhere in the region of 1000 cc primary injector and maybe those 5000 cc
per minute secondary injectors.
Now the staging here obviously is quite important in order to get good control
over the air/fuel ratio and I want to just show you how this is done.
Every software has its own options, we're going to have a quick look at how
Adaptronic handle this in their modular series of ECU so let's take a look at this.
This is actually the map for our Mazda FD RX7 and the RX7, the 13B engine in stock form uses
staged injection because the fuel demands are quite high, we've only got two rotors so we only
ran one injector per rotor housing, we'd only have two injectors, really difficult to supply
all of the fuel requirements through one injector.
So we're running a set of ID1300 cc primaries with a set of ID1700 cc secondaries.
Here we're looking at our main fuel map so this is it shown graphically here and below
we've got it numerically.
This is a 3D VE table, we've got RPM on our X axis and we've got manifold pressure
on our vertical axis so nothing particularly unusual here.
This is a proper VE based fuel model.
So the numbers in that fuel table are engine volumetric efficiency.
Now let's have a look over at our injector staging.
So we'll have a look at our injector staging here and we can set up the number of stages.
So in this instance being that it is only running two injectors per rotor we've got
two stages of injection.
Now this particular ECU will support three or four stages of injection
depending how much fuel you actually need to get into the engine.
The next option we've got here is a minimum off time for our stage one injectors.
Now it might not make too much sense but essentially what we're doing it telling the
ECU that the injector must be switched off for a minimum of in this case, two
milliseconds before it can be switched on.
And this is all part of the Adaptronic strategy where the staged injection is actually seamless
and automated if we want to run both injectors as hard as we possibly can.
So essentially what this does is limits the maximum duty cycle that the injector,
the primary injector will operate and the way the Adaptronic strategy works is it will run the
primmary injector up to that limit and then once we reach that limit it'll bring in the
secondary injector and hold that primary injector at that minimum two millisecond
off time and it'll bring in the secondary injector as it needs in order to fulfill the
engine's fuel requirements.
Now a big part of this strategy working properly is giving the ECU good data about
the injectors.
It needs to know exactly what injectors are fitted so let's take a look at that.
We'll come over to our outputs and injector type and this is where we can define the
injectors that are fitted to the engine.
Now our primary injectors here as I mentioned, we've got a set of Injector Dynamics ID1300cc.
And it's important that the ECU has good data here.
It wasn't that long ago that all of the data the ECU really needed was a flow rate as well
as an injector offset or dead time table for the injector.
These days ECUs are getting a little bit more sophisticated, they've taken after the way
OE manufacturers are characterising the injectors and they'll also include details
such as a short pulse width adder so they're properly characterising the injector flow right
through the pulse width range that we're going to be applying to the injector.
The better the data we can provide for our ECU, the more accurate that control.
And with a single set of injectors, if we've got some errors there, that may not be a
big deal because we kind of bake in those errors into our VE table.
With staged injection, if we want seamless integration when we switch from primary to
secondary, it's really critical that this data is accurate.
In this case, the Adaptronic ECU makes this really easy, we can just choose our injector
from a fairly significant list in a drop down menu of all of the injectors that Adaptronic
have already done the hard work for us and have characterised so obviously in this case,
ID1300, we don't need to make any changes there.
If we click on our secondary injector, this of course defines the secondary injector,
exactly the same principle here, you can see that this has been set up as an ID1700.
So in this instance, there's really no more work for us to do.
As I mentioned, the system is automatic and it will just run the primary injector up to our
limit and then bring in the secondary but maybe that's not what we want to do.
It's OK, we've got some more control options there.
We go back to our fuel tuning and we come across to our two stage injection map,
we'll bring up this table here.
Now we've got a table at the moment, it's filled with zeros and this is because we're
letting the Adaptronic do the heavy lifting and decide on the split between primary and
secondary injectors on its own.
However we've got a 3D table here, we've got our manifold pressure on our vertical axis,
RPM on our horizontal, just like our main VE table.
Numbers of zero here mean that all of the fuel where possible will be provided through
the primary injector.
If we were running a naturally aspirated engine, as I mentioned earlier, where we
wanted to take advantage of potentially better atomisation and slightly improved
combustion, more homogenous combustion charge from those outboard injectors at
higher RPM, what we could do at high RPM, obviously we'd only be going up to 100 kPa here.
Maybe from 5500 RPM and above we could set our values in this table to 100%.
Now we're not necessarily going to just put In a big step like this and instead, as the RPM
and the throttle position or manifold pressure increases, we'll gradually stage those
secondaries in and get to a point where above, in this case 5500 RPM, we're 100% using those
secondary injectors.
So a lot of control there as to how we use the secondary injectors.
Now as I've mentioned though, Adaptronic there, automated, not every ECU does this.
One of the keys when we are tuning staged injection, is to do some data logging while
we're dialling in the staged injection and what we want to keep in mind or keep our eye
on there is the injector duty cycle or pulse widths, either way, for the primary and the
secondary injectors, we want to make sure that we're keeping control over our maximum
injector duty cycle for our primary injectors.
So generally I like to keep my maximum injector duty cycle at or below about 85-90% so once
we're starting to get there, we're going to take note of where abouts we are in terms of our
injector staging map, our load and our RPM and start bringing in the secondary injectors.
The other issue we can run into is if we start bringing in the secondary injectors too early
and we're trying to drive them at too low a pulse width, we can also get into some
problems there with good control.
Most aftermarket ECUs with staged injection will have some kind of limiting factor on what
pulse width the secondaries need to be above before they'll come in but that is a
consideration.
Get all of this right when you do a dyno run or a wide open throttle acceleration pull
on the road or the racetrack, the lambda trace is going to be perfect, completely
seamless as we cross over and start staging in those secondary injectors.
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