From: John De Armond
Subject: Re: Bosch Motronic Fuel Injection
Date: Tue, 26 Oct 93 17:47:23 GMT
Harvey_Chao@smtp.esl.com (Harvey Chao) writes:
> According to the manuals, the Bosch Motronic FI in my '83 BMW 528e is
>wired to trigger the individual fuel injectors in two sets of 3. That is,
>there appear to be two "trigger" lines from the computer to the fuel
>injectors( as opposed to the anticipated 6) , and that each line serves a
>set of 3 injectors. Does this mean that fuel is injected to 3 cylinders
>simultaneously? How does this match my understanding that each of the 6
>cylinder's intake valve is open one at a time in firing order sequence?
>Does this mean that fuel is being sprayed at two cylinder intake valves
>that are closed? Ich verstehe nicht!
The short answer is, for all practical purposes except perhaps emissions,
there is no difference between sequential (timed to each cylinder)
and simultaneous (what you have) injection. The reason is once the
engine is making any power at all, the injectors are open most of the
entire cycle. Consider the numbers. Assume a 4 stroke engine with
a 6000 RPM redline. One revolution takes 10 milliseconds. A whole
cycle takes 20 milliseconds. That means that there is a maximum
of 20 milliseconds in which to inject sufficient fuel for one cycle.
This same engine must idle at about 600 rpm. That's a turndown ratio
of 10:1 (6000:600 rpm). Ignoring the special mixture needs for idle and
any changes in volumetric efficiency with speed,
that also means the injector must be capable of turning down 10:1 (remember
the injector operates with a constant pressure across the orifce so the
only means of control is open time). A 10:1 turndown means about 2 milli-
seconds at idle. That turns out to be about the limit of modern injectors.
Opening takes between 1.25 and 1.75 milliseconds and closing is similar.
This turns out to be about what idle pulse widths are.
Now we have two bounds to work with. The minimum open interval for idle
(~2 ms) and the total time available at the redline (20 ms). The
injector must operate between these bounds. Something subtle here is
the fact that in order to supply the necessary fuel at redline, the injector
must be open almost all of the cycle. That means during intake, compression,
power and exhaust. Indeed, most OEMs design their system such that at
redline, the injectors are on about 90% of the time. For high performace
applications where it might be necessary to exceed the redline for a bit,
I size injectors to 85%. One doesn't want to suddenly max out the
injectors and therefore lean the motor under these conditions.
There is something even more subtle involved here. That is, the function of
the rev limiter. A major reason for designing a rev limiter into modern
engine management systems is to give the designer more headroom
at idle. If the engine can be hard limited to 6,000 rpm, for example,
the injector can be sized smaller so that it is on 100% of the time at
6,000 rpm. The smaller injector means it must also be open longer
at idle and low speed running. This gives the ECU more control
in this most important region as far as emissions are concerned.
The above fact, confirmed by measurement of a number of different brands of
engines is why I turn a jaundiced eye toward "performance chips" that
push up the redline. Without larger injectors or higher fuel pressure,
this is guaranteed to lean the engine in the higher RPM range. That
motors aren't scattered all over the highway is only because most
people can't run their car WOT for very long at a time.
Back to sequential injection for a moment. The major benefit of
sequential injection is at idle and throttle response. Injecting
onto an open valve achieves a bit better atomization and thus improves
low speed running. It also lets manufacturers play around with injector
angle to put the fuel in the vicinity of the plug and achieve some degree
of stratified charge. The very most effective is the so-called reverse-
synched sequential injection. In this scheme the injector CLOSING is
synched to the intake valve closing. The opening time is back-computed
for the engine speed. I haven't done enough testing to discover which -
if any - production cars are using this scheme but I do know Nissan
and Toyota both use this in their race car ECUs.
Date: Mon Dec 9 23:54:18 1991
Subject: Re: zcar d.a.v.e.-jection
>"Introducing dave-jection(digital analog variable enrichment)
>stand alone cock pit adjustable fuel injection control system. Completely
>replaces factory wiring harness and ECU on injected cars. Also allows
>extremely simple retrofit of injection systems onto carberated cars. There
>are only 8 main wires in the system.
>The dave-jectio unit is triggered by an optical sensor and light source mounted
>on the front crank pulley. It varies the injection pulse by a throttle posi-
>tion sensor and a manifolf pressure sensor. The unit fires the injectors
>directly with no dropping resistors.
>$450 includes control unit, optical sensor/light source, wiring harness, and
>cockpit adjustable mixture control. Non-turbo cars can delete maniforl pressure
>sensor and deduct $100. "
I've not seen this unit but based on what you wrote and on my experimental
work of the last few weeks developing my injecton controller, I have
very serious doubts about this system.
The first red flag is the optical pickup. No No No No. They don't work
in the relatively benign environment inside a distributor. It surely
won't work on a crank. This is the cheap way out. The proper way is to
machine a variable reluctor pickup on the crank pully. A second problem,
not as bad because ignition timing is not involved, is that the action
of the harmonic damper will cause trigger jitter.
More importantly consider this. Here's a possible accounting of the 8
Injector lead 3
Manifold pressure 4
Throttle position 6
Oops, no wires for water temperature or air temperature. Frankly, there
ain't no way a system's gonna run well without these signals.
I'd be interested in playing with the unit but I don't expect much.
From: emory!chaos.lrk.ar.us!dave.williams (Dave Williams)
X-Source: The Hotrod Mailing list
Date: Jun 1993
Subject: Re: Carter 4bbl...model 9410/9510 ?
-> significantly upstream of the intake valve. A wetted intake
-> necessitates acceleration enrichment which harms economy and is the
-> major source of coasting emissions. The latter is particularly
-> difficult to deal with.
FYI, the GM Tuned Port Injection uses a snotload of acceleration
enrichment, based on the delta-time from the throttle position sensor.
As far as I can tell they do it to enhance throttle response. I had an
old Mustang with a 302 and the Autolite 2bbl on it once. I found that I
could disconnect the accelerator pump entirely (very easy on the
Autolite) and the throttle response got a little soggy, but nothing
you'd really complain about. It was a pain to start, though.
[True TPI introduces a bit of acceleration enrichment. The system on my
wife's toyota does the same thing. BUT! It is a matter of degree.
The toyota system fires two extra asynchronous pulses during acceleration.
That amounts to perhaps 1-2 CCs extra. Compare that to the 10 cc stock
Holly accelerator pump or the optional 50 cc pump it required to get rid of
the flat spot on my Z with a 390 CFM carb. There is hidden enrichment in
some systems that is not apparent at first glance. Dumping the vacuum
in the intake volume through a relatively small carb/throttle body such as
with your old Mustang provides momentary extra gas. Ditto with mass flow
FI systems. In the old L-jetronic FI systems with the flapper air flow
meter, there is even more enrichment from the inertia of the flapper.
This is tuned by adjusting the clearance of the damping vane. In
all cases the amount of gas involved is small compared to performance
carb setups. JGD]
The TPI also richens the mixture at idle and during cold running.
According to the service manuals, the AFR can hit 4:1!