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stupid fuel injection question


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EDIS (?) "knows" position of piston from crank sensor and as it uses wasted spark it does not matter if the psiton is on the compression or exhaust stroke but what about the fuel injectors? Surely they have to be fired only once in the 4 stroke cycle otherwise fuel is wasted - but how is the time when to fire them determined? (has to be another sensor?)

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Everyone who looks at EFi asks this question at least once :D

Why you don't need to worry

There are two common sorts of injection:

1. Throttle body injection - usually one or two injectors for the whole engine

2. Port injection (aka. Multi-Port) - one injector per cylinder

Then there are three common modes of injection timing:

* batch - all injectors fire at once, but not timed to any specific cylinder event,

* bank - ½ the injectors fire at once, then the other ½, and so on, but not timed to any specific cylinder event,

* sequential - each injector fires at a specific point in the 4-stroke cycle for each cylinder (i.e., 8 independent timing events)

Throttle body injected cars are usually batch or bank fire, simply because of the geometry. Most port injection set-ups before the mid-1990s were bank fire as well (including GM Tuned Port Injection for the 305/350).

Sequential injection requires:

* at least as many injectors as you have cylinders, with one dedicated to each cylinder (i.e., not a 4 injector TBI on a 4 cylinder).

* as many injector drivers as you have cylinders,

* and also requires a camshaft position sensor (a crank sensor is not adequate for a 4-stroke cycle engine).

However, sequential injection does not necessarily mean you are injecting into an open intake valve all the time. The intake valve is only open less than 30% of the time in a typical 4 stroke engine. Once you are trying to produce more than about 25% of maximum HP your injectors are firing for longer than the intake valves are open. If your maximum HP is correctly calibrated to a safe 80% duty cycle, your injectors are injecting well over 50% of the time on closed valves.

MegaSquirt has just two injector drivers (that can handle up to ten injectors each).

The benefits of sequential injection are that:

* you may get slightly better mileage and lower emissions at low engine speeds,

* you can tune each cylinder's fuel amount independently (if you know how).

The effect on maximum horsepower is general negligible.

However, for those applications for which sequential injection is desirable, Bowling and Grippo are working on a 'router board' for MegaSquirt-II which WILL do sequential injection (and can be used to operate COP ignition systems as well). It will likely not be released until mid-2006.

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