A small crash course in Ford's torque ratio system.
The overall system is known as "SAI" torque control, for "Spark, A/F Ratio, Injectors", and that's the order it prefers to reduce torque in.
First, a note: The system has existed in some form since EEC-V went out the door. It used to be the only system for torque reduction on EEC-V because of cable throttle. Anything that has DBW has this system, but also has the ability to reduce torque via throttle as well. In these systems, there are two tracked torques: A Base torque, for long term torque, which controls the throttle position, and an Instantaneous Torque, which is used for fast torque control and feeds the torque ratio torque reduction system.
Torque ratio is simply a ratio of current or desired brake torque vs torque at indicated conditions (MBT, Stoich, standard environment, etc.)
So now, how it works is every torque source has a configuration setup that basically tells the SAI system what is allowed for that torque source:
- Is spark torque reduction allowed?
- Can I ignore combustion stability clips (i.e. if you retard spark too far the engine starts to lose combustion stability)?
All sources are assumed to allow fuel/injector cut.
So, lets say we get a reduction request, spark reduction IS allowed.
Our desired torque ratio is say 0.7.
First, the system checks if it is allowed to do a spark cut for that torque source, if it is, it checks to see if its allowed to cut via spark only using the Spark Only Torque Ratio vs. Requester table:
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If our desired torque ratio is GREATER than this table, torque will only be reduced by spark if possible. If its below, we're guaranteed to have fuel intervention. Like previously mentioned however, there are clips to how much spark authority you can have before combustion instability happens, so even having something set to 0.0 doesn't guarantee ONLY spark will be used. As well, things like catalyst overtemp disable spark reduction entirely.
If we are lower, the system will use some form of fuel control.
Exactly what that control is depends on the Fuel Cut Torque Ratio.
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If your desired reduction via fuel methods is GREATER than this value, it attempts to use A/F enleanment to reduce torque, using the vehicles calibrated A/F Ratio to Torque multiplier. Typically the authority you have here is only a few percent so enleanment is rarely enough on its own. Any left over reduction will need to be satisfied by cutting 1 or more injectors. I recently updated the description of this table to clear up something the old description led people to believe: Fuel cut may not be avoidable even if you set this to 0, because enleanment and spark can only do so much.
Most factory calibrations you will notice have the fuel cut torque ratio set to 1.0, meaning any kind of of fuel torque reduction must happen with injector cutout only.
This should make it pretty obvious how somethings like DFSO work. DFSO calculates its own torque ratio that it ramps (using the ramp on rate variable) down. Above 0.75 torque ratio, its using spark only. Below that it starts cutting injectors until ultimately you're at 0 Torque Ratio and all injectors are cut. This is part of what makes DFSO a smooth operation, as the TR continually decreases, the engine pulls power first with just simple spark, and then 1 injector, 2 injectors, etc.
As well, I should note some things do have special bypasses around this logic. I.e. Engine Speed Limit. Spark only set at 0 would lead you to believe it isn't allowed to cut fuel (Spark only!), but it bypasses this requirement. Fuel cut for engine speed limiting for example is always a possibility, no values you put in will make it use just spark or enleanment, etc. If your RPM goes over the fuel cut limit, it happens regardless.
Hopefully that clears up how you can set these tables up to achieve what you want.