Megasquirt-3 MS3 AFR Safety System

Megasquirt 3 - AFR Safety System

Introduction

The "AFR Safety" System is designed to help you protect your engine from unwanted dangerous lean conditions. Typically this will be used for wide open throttle and/or boosted conditions. It only makes sense to use this feature with a wideband lambda sensor on an already well tuned engine.
The system compares the wideband EGO input against your existing AFR target table and a safety limit difference table.
If this situation persists beyond a time you specify, the engine will be shutdown as a protective measure until you get out of boost, off the throttle and down in revs.
NOTE This system will only work if configured correctly and will not protect your engine against bad tuning or knock! However, it is likely it would detect a failing fuel pump or regulator. A single faulty injector could easily go un-noticed with a single wideband reading the average AFR.
This shutdown mechanism is shared with the EGT over-temperature settings.

Settings

Enable AFR Safety - This enables or disables the system
Warning output - This can be used to turn on an output as soon as an over-lean condition is detected.
Load axis - Typically choose MAP if running speed-density or TPS if running Alpha-N
Check above load - The system only checks the AFR above this load.
Check above RPM - The system only checks the AFR above this rpm
Wait for - This delay allows for a brief lean-spike without tripping the shutdown. Set as low as possible.
Kill spark for - Intially spark is cut for this time and then fuel as well.
TPS below, MAP below, RPM below - normal engine operation resumes when all three conditions are met.
The table on the right is the "safety limit" difference table.

Each install is different and it is up to the installer and tuner to decide on safe limits !
The example data in the screenshot above has the system only active above 95kPa and 2500rpm. Once in this region the safety limit difference table is the controlling factor.
For example if at a certain point you have set your target AFR (AFR table) to be 11.9 and you then decide that a worst case of 12.3:1 AFR is allowable, the safety limit table would have 0.4 in that position. i.e. allowed AFR = target AFR + safety limit = 11.9 + 0.4 = 12.3

If then your wideband EGO sensor reports 12.4 or leaner, the warning output will be activated immediately. (Typically this would connect to a dash mounted warning light.)
If this situation persists beyond the time limit (shown as 0.5 seconds here) then the shutdown mode begins. If the AFR returns to a "safe" level before the time limit elapses no shutdown takes place. Review datalogs before setting this time. The lower it is, then the sooner the shutdown kicks in. This could help protect your engine but setting it too low may cause annoying false alarms.

The first step in the shutdown procedure is to cut spark for a specified time (shown as 0.5 seconds here.) During this period fuel continues with the intention of cooling the engine internals as the lean condition is likely to have generated excess heat. On vehicles fitted with a catalyst, 0.0 should normally be specified as catalysts are not designed to cope with raw fuel.

After the "kill spark for" time, fuel is also cut.

Normal engine operation will resume only when your Throttle, MAP and RPM are less than the limits you set.
In operation, if you trip the shutdown, the engine will suddenly die. While this will be irritating, hopefully it is less so than the engine rebuild you might have needed.

If you have a question, comment, or suggestion for this FAQ please post it on the forum.

No part of this manual may be reproduced or changed without written permission from James Murray and Ken Culver.