[KeithsTV]

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.

Keith