Multec 3.5 Fuel Injector Application Manual
Software
Delphi Energy and Chassis Systems
Revision:
11/05-1
5-13
can result in high emissions and potentially high converter temperatures.
Added benefits of DFCO can be reduced fuel consumption and lower
emissions. Transitions from fuel-off to fuel-on must occur smoothly.
Wall Wetting, Fuel Puddling
Even with careful injector targeting, a certain amount of fuel overlaps the
intake valve and hits the walls, reducing the amount of fuel for that pulse.
Fuel may also accumulate and puddle on the walls during rapid changes in
intake manifold vacuum and airflow. Accumulation of fuel will also occur
at high manifold pressure (low vacuum). When vacuum increases, this
accumulated fuel vaporizes and is drawn into the combustion chamber,
increasing the amount of fuel for each pulse. Algorithms are used to
compensate for these conditions.
5.2.3.13 Engine Starting Conditions
Certain requirements during engine start-up need to be met in order for the
engine to start. For example, fuel does not vaporize well at low
temperatures such as in a cold engine, so more fuel is needed to achieve
combustible air vapor ratios in the cylinder to start the engine. During hot
start, fuel vapor is excessive and may require compensation. Also, oxygen
sensors are inactive at start-up and cannot monitor the air/fuel mixture.
Various corrections are used to address these starting conditions.
Prime Pulse
In some applications, a prime pulse of fuel is injected (based on coolant
temperature) at the start of cranking to start the engine faster.
Crank Fuel
Crank fuel is delivered while cranking before the engine starts to run. The
exact amount is determined by engine parameters.
Crank-to-Run Fuel Decay
Following start-up, the temperature of the engine increases and the air/fuel
ratio is gradually enleaned. Crank-to-run fuel decay is essentially the
transition between crank to run. Properly timed enleanment reduces
emissions without causing engine stalls or acceleration sags.
5.2.3.14 Engine Protection
Engine Speed Limiters
Engine speed limiters shut off the fuel when the engine goes above a
predetermined speed (RPM), to avoid engine damage. Hysteresis is
recommended to minimize driveline backlash.
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