CIS perceptions

[Dave <dhaupt@bewellnet.com>]

>> (Do the injector really squirt fuel all the time, without
>> interuption?

>Yes, in CIS systems -- that's what the C stands for, continuous.

Not quite correct.  The "C" does mean continuous, but it really means
continuous pressure to the injectors.  As anybody who's done an injector
test can tell you, the injectors used by the Bosch CIS only allow fuel
to flow if there's a sufficient pressure difference from the fuel side
to the manifold (engine) side.  In fact, that's one of the things you
test for.  If they open with too low a pressure, you may have one
cylinder running too rich compared to the others.  No fuel regulator
changes can compensate for that.  If one requires too much pressure,
you'll have a lean-running cylinder and woe unto you when you inspect
that piston.

In normal operation, a (relatively) steady pressure is applied to the
fuel supply side of the injector.  As the piston drops during the intake
stroke, the pressure on the manifold side drops.  Thanks to time delays,
the pressure appears lowest to the injector physically closest to that
cylinder.  At some point, the injector opens.  You want a richer
mixture?  Increase the pressure.  The fuel distributor varies the
applied pressure based on lots of variables, including air & engine
temp, O2 sensor readings, engine speed, air mass flow sensor data, etc.

There's one BIG advantage to any port fuel injection system.  Gasoline,
like any other volatile liquid, has a vapor pressure.  If fuel and air
is mixed in a carburetor, then the interior walls of the intake manifold
will have a film of gasoline deposited on the walls.  If the pressure
decreases due to engine vacuum, then some of that fuel vaporizes and
finds its way into the engine.  When you "step on it", reducing the
vacuum (increasing manifold pressure), more fuel needs to be deposited
on those walls.  That's the purpose of the accelerator pump in a
carburetor.

Throttle-body injection, as used by a lot of American makes, and some
lower end Japanese makes, doesn't fix that problem.  You still have a
huge volume of intake manifold between where the fuel mixes and the
cylinders.  If you place the injectors adjacent to the intake valves,
you don't have the need for keeping the intake manifold walls moist any
more.  CIS or electronic, you get this advantage.