[Diy_efi] Fixed frequency PWM batch mode FI

[Jorgen Karlsson]

Hi,

If you use a fixed frequency the fuelling will vary on a cylinder to
cylinder basis as long as the switch frequency are not a lot higher then the
induction frequency. An extreme situation would be if the injection
frequency are half of the induction frequency and the duty cycle 50%,
depending on timing every other work stroke could be without fuel and the
next could get twice as much as needed.

Even if we say that we have a frequency that is a lot higher then the
induction frequency we would still have to work harder then if we
syncronised the two. Lets assume that we trigg a 555 timer with the ignition
signal, we tune the pulse length to make the engine run fairly rich at full
throttle and loaded to 2k. Without changing the pulse width the engine will
run fairly well regardless of rpm, we would only have to compensate for the
engines volymetric efficiency curve and if we add a throttle we would also
have to compensate for the throttle position or intake vaccum.

If we do the same thing with a constant frequency injection system the
engine would get twice as much fuel as needed at 1k, at 4k it would get half
as much fuel as needed. To make this work we would need to include rpm as a
parameter in the fuel calculation, that is not done at all in an ordinary
injection system.

Jorgen




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