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Old 3rd Apr 2011, 2:54 am   #1
Jason Pero
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Location: Kingston, Ontario, Canada.
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Default *analog* engine injector controller questions.

I have already figured out the firing the spark plugs two at a time in waste fire mode with a mechanical distributor using 2 signals based on rpm and vacuum & digital ICs hybrid on concept in my mind.

Now I need to address the fuel injection fuel mixture curves issue what I'm thinking how on earth analog designers managed to tweak the curve along multiple points with several pots?

The concept is take the RPM as frequency and timing for the fuel injectors, sense the manifold vacuum with a analog sensor, and wide band O2 sensor (already built the kit). I know I have to address the cold start stuff.

Reason my vehicle is 1987 4 cylinder caravan using progressive 2 barrel carb and very old spark control computer, this which is very early and it's not working too well because of leak in the diaphragm and can't source new one.

Alternative way is
megasquirt kit is out there but too expensive for me and I figured why not create a sub in analog form instead because I have loads of analog and many signal transistors, resistors and such at my TV shop to use. I even have several analog convergence boards from old projectors.

What do you think?

Cheers, Wizard
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Old 3rd Apr 2011, 7:28 am   #2
Sean Williams
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Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
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Default Re: *analog* engine injector controller questions.

I think this thread is likely to be closed soon - its not exactly in keeping with the forum...


It would help if we knew which vehicle you were referring to - My guess is Ford based.

I have to ask, why reinvent the wheel? there are several options available over here for controlling ignition, all fully programmable via USB, and easy to retrofit to most vehicles.

1987 is hardly a long time ago - so repairing the original system should not be a problem - parts must be available somewhere.

As for building your own.....

You have identified a number of inputs required to control fuellling accurately - none of these will be easy to replcate in an analogue fashion.

Early fuel injection systems were mechanical, and as such had vacuum compensation applied to the metering head to allow higher fuel rates under acceleration - think Triumph TR6, early Volvo 2 and 7 series, and Ford Escort XR3I.

While these systems worked to an acceptable limit, they were nowhere near as good as a late system that had electronically fired injectors, and a proper computed fuel and ignition curve, running in a close loop system.

So, in a nutshell, either repair what you have - this will be safer, and more reliable., or invest a good chunk of change in a professionally manufactured system.

Engineers make things work and have spare bits when finished
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Old 3rd Apr 2011, 8:05 am   #3
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Location: South Bradford, West Yorkshire, UK.
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Default Re: *analog* engine injector controller questions.

As Sean has said this thread will probably be closed soon but before that happens here's my understanding of how injection systems work.

Earlier systems such as the K Jetronic were mechanical and use a flap to measure the airflow and adjust the amount of fuel to be injected accordingly. These were continuous injection systems and fuel vapour accumulated behind the inlet valve to be sucked in when the valve opened. Fairly crude but it worked and improved power and fuel consumption compared to a carburettor. Ignition,usually electronic, was controlled separately.

Electronic systems such as the Bosch L Jetronic use a fuel pressure regulator to keep a constant pressure across the injector. In this way the amount of fuel injected depends only on the time the injector is open. This time is controlled in simple terms by a monostable multivibrator triggered from the ignition system. There is a minimum on time, to keep the engine running at idle, modified by various inputs such as throttle position, airflow, engine temperature and altitude in some engines. These inputs modify the on time to increase the on time to provide the fuel needed when the engine is under load. The airflow can be measured by a flap in the air inlet connected to a pot with a very non linear characteristic or a hot wire system in later versions. All injectors are fired together with fuel accumulating behind the inlet valve to be sucked in when the valve opens.

With the advent of catalytic converters a closed loop system became necessary and the adoption of digital techniques which use a map, basically a big look up table, to control the injector on time With increased processing power more inputs can be procesed to control the injection and the ignition more accurately resulting in improved fuel consumption and engine performance and the ability to meet increasingly stringent legislation on emissions.

Start up enrichment is done using an extra injector on the K and L jetronic systems but on digital systems the injector on time is increased.

That's a basic description of how they work but it is a major task to design one yourself and get it running reliably and comply with any regulations. Probably better to bite the bullet and get an off the shelf unit.

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Old 3rd Apr 2011, 8:36 am   #4
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Default Re: *analog* engine injector controller questions.

Look at I know how long this guy has been working on his own system, not easy.
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Old 3rd Apr 2011, 9:03 am   #5
Brian R Pateman
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Location: Western Lake District, Cumbria (CA20) - UK
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Default Re: *analog* engine injector controller questions.

The two posts mentioning the likely short dwell time of this thread are quite correct.

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