Some general questions about injection

[John Dammeyer]

Hi Nicholas,

Since I went through this myself I'll try and answer some of your questions.

No,  it doesn't matter if it squirts into a closed valve.  In fact if you
analyze a stock engine that meets emissions you'll see that they all do
this.  This is a good opportunity to dust off your high school math and
physics and do some calculations for yourself.

First you need to know what the delivery rate of the injectors are and what
type of injectors you are using.  the Pintle type can take as long as 2ms to
open and close and deliver only about 60% of their rated flow during this
interval.  So if they are rated at 24 lbs/hr they deliver 60% of that or
14.4 lbs/hr.  That's the same as 14.4 lbs/360,000ms or in other words
0.00004 lbs/2ms.

Why pick 2ms?  Because for a smooth polution free idle you still need to
reliably deliver fuel and yet at WOT be able to deliver 24lbs/hr for 80% of
a 2 revolution cycle;  the maximum on time of the injector.  At 7000RPM two
revs take about 17ms and 80% of that is 13.7 ms so you are delivering
0.00004 lbs of fuel for 2ms plus 0.000067 lbs of fuel for 11.7ms which gives
a total of 0.000107 lbs of fuel every intake stroke.

Next calculation involves how much air is sucked in on each intake stroke.
At 14.7:1 stochiometric for perfect combustion you'd want 14.7 times as much
air so if you have 0.000107 lbs of fuel you need about 0.0016 lbs of air at
WOT and at idle you need about 0.00059 lbs of air with 0.00004 lbs of fuel.

Since the cylinder volume is fixed then your high school physics will tell
you that if you're O2 sensor is telling you that there is complete
combustion (14.7:1) the only thing that changes inside the cylinder
(forgetting temperature and humidity for the moment) is pressure.  That's
why the MAP sensor is important.

The MAP sensor provides an approximate representation of what the pressure
is inside the cylinder when the intake valve closes at the bottom of the
intake stroke.  Fluid dynamics plays a roll here because the air is moving
and has mass but we'll ignore that too for now.  You use a factor called
Volumetric efficiency to compensate for this.

So if you know the pressure inside the cylinder and you know the volume of
the cylinder you can calculate the amount of air and given the percentage of
O2 in the air you know how much O2 you have to support combustion.  Better
be enough for our 0.0004 lbs of fuel from our 24 lbs/hr injector or your
engine will run rich.

So in effect that's the numbers game that designers play all the time.   To
get a smooth idle and not create a plug fouling rich mixture the injector
needs to turn on only a short time to provide fuel and at the same time be
large enough to provide a 12:1 mixture at WOT and maximum RPM.  Turbocharge
the engine and the Volumtric Efficiency passes 1.0 because at WOT and
maximum RPM you now have a pressure inside the cylinder greater than
ambient.  To provide enough fuel for this configuration your injector may be
too large to create an optimum mixture at idle;  this is a real quandary.

What Maxda does with their tubocharged Rotary and I think Toyota does this
too is idle with only two cyclinders.  Extra power is required to compress
the non firing cylinders so the two that are firing are working harder than
if all 4 were ticking over at 700RPM and therefore they need more fuel.  So
the larger injectors for the boosted engine easily deliver the correct
amount of fuel for idle and WOT.

In order to determine injector size you actually do all these calculations
in the other direction starting with cylinder volume and sea level pressure
and assume a 90% VE at WOT and try for 12:1 Air:Fuel ratio.  That gives you
a starting point for injector size and more important... initial pulse width
for starting and running the engine.  After that it's tuning.

Sorry for being so long winded but I couldn't find any short way of
explaining this.

Regards.

John Dammeyer


> Date: Thu, 28 Sep 2000 17:24:49 +1200
> From: "Nicholas Parker" <nrparker at ihug.co.nz>
> Subject: Some general questions about injection
>
> Hi List,
>                 If a 4 cylinder vehicle has 4 injectors (one per cylinder)
> wired in pairs, can I correctly assume that each pair pulse once per
engine
> revolution ?  Does it matter at all that fuel will be squirted onto a
closed
> valve?
> Is there any benefit in having a control over a single injector per
cylinder
> (power wise)?  In  single injector control per cylinder applications, does
> each injector fire once,  per cycle?  In this case, are high flow
injectors
> needed to put all the fuel into the cylinder through an open valve?
> I have a 4 cyl  (supercharged) with 4 x 365cc injectors, and am wondering
> how to best taylor a fuel injection system for my car.  I want to decide
the
> pulse control architecture for my FI system ie 2x2inj or 4x1inj.
>
> Thank you,  Nick Parker.
>
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