What is acceleration enrichment and why do we need it?
Acceleration Enrichment is a complex matter to discuss but it must be tuned to provide good engine response in transient situations. Lets say our engine is idling and we quickly stab the throttle without any acceleration enrichment. The engine will stumble and fail to immediately rev up due to insufficient fuel creating a lean condition. So why does this happen?
When an engines injector pulse width is relatively static, a film of fuel is “built” up inside the intake port. The amount of fuel this film holds varies depending on intake pressure, temperature, injector pulse with and etc. When we quickly change one of those variables the fuel film is consumed or built up. In our example above we made a quick change in the intake manifold pressure by opening the throttle quickly. This increase in manifold pressure (from vacuum to atmospheric pressure) means the fuel film can grow larger and as such a portion of the injected fuel goes into building up a larger fuel film instead of going into the cylinder for combustion. This results in a lean condition as not all of the injected fuel made it into the cylinder. To compensate for this we need to inject more fuel than is technically needed for combustion during these transient conditions; this is Acceleration Enrichment.
The factory MSD8X DME’s obviously do not have code/logic for calculating and tuning acceleration enrichment for a port injection engine so we had to add our own. Familiarize yourself with our custom acceleration enrichment tables below.
Acceleration Enrichment Tables
Minimum TPS Gradient Threshold
Throttle position setpoint change gradient for enabling acceleration enrichment (*/second)
Number of Hold Events (720* crk rotation)
Once acceleration enrichment is enabled, how long do we hold the maximum calculated enrichment value for before decaying.
Enrichment Decay Rate
Once the above hold events have passed, the enrichment value is reduced by this value every 720* of engine rotation until it is 0 (at which point acceleration enrichment is disabled)
Base Enrichment
This is the main acceleration enrichment table with axis’s of Throttle position setpoint change gradient x Engine RPM.
Base Enrichment Correction (Temp)
This table is multiplied by the Base Enrichment table to provide compensation for different engine coolant temperatures. A value of 100% is effectively a 1.0 multiplier and therefore doesn’t change the base enrichment. A value of 200% is effectively a 2.0 multiplier so final enrichment would be base enrichment * 2.0 in this example.
Base Enrichment Correction (Manifold Pressure)
This table is multiplied by the Base Enrichment table to provide compensation for different intake manifold pressures when acceleration enrichment is requested. A value of 100% is effectively a 1.0 multiplier and therefore doesn’t change the base enrichment. A value of 200% is effectively a 2.0 multiplier so final enrichment would be base enrichment * 2.0 in this example.
Cooldown Segments (720* crk rotation) Before Next Enrichment Allowed
This table allows acceleration enrichment to be blocked for a specific number of engine cycles (720*) after it has been enabled. This is used to prevent acceleration enrichment from enabling multiple times during the same acceleration event. We recommend using 4 as an initial value.
Hold Time for Disabled STFT due to AE
This table defines how long the short-term fuel trim is disabled during an acceleration enrichment event.
Acceleration Enrichment Threshold to disable STFT
This table defines how much acceleration enrichment must occur to disable the short-term fuel trim.
OEM Acceleration Enrichment Ceiling
This table caps how much acceleration enrichment is contributed by the OEM (slow component) acceleration enrichment system.