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Message
Rusty,
I'll take a stab at why rpm is not a factor in
calculating injector size.
I presume their calculator bases the injector size
required for a given HP on the specified flow rate of the injector and the
number of injectors. In other words, tell me the HP and how many injectors
and I will tell you the flow rate required of the injector. That is all
the information you really require for determine the max HP that injector set
will support. The reason is this - the open time of the injectors (and
therefore the amount of fuel released per injection ) IS NOT
dependent on the RPM but the loading of the engine (manifold pressure or air
flow rate is normally used to determine this). All the rpm does is
determine how many times the injectors fire per unit time (and over
simplification but essentially correct). The injectors will of course fire
more often at higher rpms (but will not necessarily stay open longer-depends on
manifold pressure)
All their calculations say is that with that number
of those flow rate injectors you could get a maximum of X HP. This assumes
that the injectors are wide open (I think they use 85% of the rate flow of the
injectors as a real number for the actual max flow to account for
opening and closing times where no/little fuel flows). So if you use their
calculator you will determine the maximum HP the that injector set is capable of
producing at maximum on time. Now whether you actually get that HP is of
course dependent on whether those injectors ever flow for their
maximum on time.
For instance, you can produce 180 HP at 6000 rpm on
a 13B NA engine (theoretically). You can also produce 215 HP at 6000 rpm
with 5 psi of boost. Here the rpm is the same, but of course the fuel
injected is not the same because the higher manifold pressure with the 5 psi
boost will cause the injectors to say open for a longer period of
time. But, in both cases the injectors are triggered at the same
rate! In the boost case, the injectors simply stay open longer
each time they are triggered, so more fuel is injected and more power
produced. So RPM is not a factor in determining the flow rate of injectors
required for a specified HP. To achieve the HP the injector set says they
will give, depends on how long the injectors stay open (and their
specified flow rate) not the engine rpm. For a give set of injectors,
That max on-time condition might occur at 2000 rpm with a large displacement
engine or might occur at 6000 rpm with a smaller displacement engine. In
each case it will happen with the same max HP produced (or nearly the
same) by the injectors.
Now if you select a set of injectors that have too
low a flow rate, then the injector system will reach a point where it attempts
to keep them open continuously. If the cycle time between
injector firings decreases (due to higher rpm) to the point they are the same or
less than the max on-time of the injectors then the system has topped out.
You have reach the maximum limit of HP that those injectors can provide. In this
case, further RPM increase will not produce more fuel, even higher boost
pressure will not (in this case) produce more fuel.
Hope this has not further confused the
issue
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Wednesday, May 05, 2004 7:51
PM
Subject: [FlyRotary] Re: surging / rough
idle
1. Do not use injectors that are to large. Go to
the RC Engineering web site (www.rceng.com)
and calculate the size you need based on HP needed and an 80% duty
cycle. (225 HP requires 4 injectors of 38 lb each at 80% duty cycle,
fuel pressure of 38 PSI, and a BSFC of 0.5)
I just spent some time
looking at the RC engineering site, as well as the other fuel injector sites I
could find. In each case, these calculations are made without ever
asking about rpm. I just don't see how they can do that ??? How do
they know if my 225 HP would be at 2000 or 10000 rpm? The same
amount of fuel will have to be delivered in either case, but you'll need
a whole lot more flow rate to deliver the fuel at 10000 than you would at
2000.
The injector flow rate will remain the same - its fixed
by the design of the injectors and does not change (excluding wear and
some other operational factors). The HP calculation is based on the max
fuel they can provide at their wide-open max duration on time. It does
not care whether that conditon happens at 2000 rpm or 10,000 rpm. If
your engine can flow sufficient air to consume that max on time of fuel at
2000 rpm then the max HP will occur at 2000 rpm, if at 10,000 then the max HP
will occur at 10,000 rpm.
If you select injectors with too low a flow rate, you
will run into problems. At higher RPMs you can run into
a condition whereby the rate of injector triggering (caused by the rpm)
decreases to where it is on the same order as the injector max on-time.
In other word the injectors no longer respond to every rpm triggered injection
cycle because it has not completed the last one before the new one
arrives. Then the EFI system simply has to use shorter on-times which
means to get the same fuel flow rate you either need more injectors or higher
flow rate injectors.
Ed
Now, before anyone
says it, I know that Tracy's using those wimpy injectors with his current
system, making 200+ HP at 7000+ rpm, and isn't having any problems, so clearly
they're big enough. I really hate it when he does that
:-)
Rusty (prop and
bolts arriving tomorrow)
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