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SMT-FTX Technical Manual - AFR Loop Control

SMT-FTX TECHNICAL MANUAL

Perfect Power Online Manuals.

 

Support for your SMT-FTX Unit.

AFR LOOP CONTROL

The AFR TARGET MAP provides the loop target. A zero entry opens the loop. The target should be 14.7 AFR for cruising, and 13.2 AFR for full throttle.

 

 

IMPLEMENTING A FUEL CONTROL LOOP

 

The purpose of a fuel control loop is to optimize the fuel requirements of an engine. For this purpose the Exhaust gas is measured (AFR, Lambda), compared to a preset target, and the fuel delivery is adjusted until the target is met.


NOTE:  If the OEM ECU has already an 'AFR CONTROL LOOP' then it is advisable NOT to use this feature, except in areas where the 'OEM LOOP' is open!


An ‘open’ control loop means that no control is used. This is sometimes done at full power or above a RPM limit.

 

The SMT-FTX can perform an AFR LOOP for the purpose of controlling the required fuel mixture precisely. The unit has a
SHORT TERM strategy, resulting in a rapid fuel adjustment
LONG TERM strategy, resulting in a memorizes adjustment

 

This requires the following items:

 

  • The measured AFR must be valid. This requires that the NAFRIN is in range.
  • The SYSTEM DEF, Enable AFR LOOP must be ticked
  • The AFR limit for this operating column (See AFR MAP) must be >0
  • The AFR target must be valid
  • The parameters: AFR OPERATION or AFR IDLE TIME must be satisfied


The above items activate the SHORT TERM loop, which will be indicated in the STATUS: AFR LOOP ACTIVE
The fuel adjustment performed by this loop cannot exceed the specified limit!
The LONG TERM LOOP is activated by:

SYSTEM DEF: Enable AFR Trim
Parameter: AFR TRIM TIME must be satisfied
The proper loop operation is indicated in STATUS: AFR TRIM ACTIVE
The unit integrates the short term fuel adjustments and saves them.

If both loop strategies are working then the total fuel adjustment is just about double the limit value at this map point.

 

 

Details:
The OXYGEN LOOP LIMIT (Shift F1) sets the up or down limit of the adjustment. This restricts the loop adjustment. The OXYGEN LOOP ADJUSTMENT can be a positive (richer) or negative (leaner) adjustment. The limit map entry applies to positive and negative adjustments equally. An oxygen limit map entry of zero disables the particular map column.

384 AFR target values can be entered in the map. Valid AFR targets are 7 to
35 AFR. A zero entry disables the loop at this point!


NOTE:  Optimum AFR targets are from around 13.8 to 14.7 under cruising conditions and 12.2 to 13.4 under full load. These AFR targets are a guide only and may vary between normally aspirated and forced induction applications.


The loop regulation speed is settable for two conditions:


For IDLE only = throttle position 0-3
For the rest of the throttle range = operation (ops)


The adjustment speed is highest with small numbers. The IDLE speed adjustment should be slow (1-50) to prevent "RPM hunting". The operational speed adjustment should be fast (1-5) to get the loop benefit within fast throttle movements. The real speed settings are best adjusted by experimentation. The idle motor (if installed) plays a major part in the idle AFR loop time adjustments (and may upset the oxygen loop at idle).

 

The goal is to get to the TARGET AFR as fast as possible with changing throttle conditions. Just making the adjustment speed fast, leads to instabilities and very poor loop performance. The real loop performance is influenced by:

 

  • The lambda sensor (type, age)
  • Distance of the sensor from the cylinder head
  • The limit to fuel map entry ratio.
  • Engine size, make, and transition behavior

 

The best results are achieved with a perfect "FUEL BASE" map, and the loop adjusting for temperature and other engine variations. A perfect loop is indicated when the adjustments are small, and swing from positive to negative equally.

 

 

DETAILS LONG TERM AFR LOOP CONTROL

 

The AFR long-term strategy is a SECONDARY OXYGEN LOOP, which operates very slowly, and remembers the fuel adjustment at each tuning spot. To enable the SECONDARY loop in the LetRipp II Tuning Software:

 

AFR TRIM        : Tick (System definition)
AFR TRIM TIME    : Enter a value of 50 (Config, AFR IDLE ETC)

 

As soon as the loop is enabled, the LONG TERM TRIM indicates the SECONDARY fuel adjustment.

From now on, the Fuel is constituted by:


FUEL MAIN MAP ENTRY
+ OXY COMPENSATION
+ LONG TERM TRIM


The long-term trim value is limited to the same limit as the OXY COMP. This implies that the total loop adjustment is TWICE the specified limit. In reality, the total adjustment is approx. 1.9 times the limit due to some mathematical calculation.

The long-term adjustment map is kept during power down.