Help the Turbo stay spooled or get spooled (reach an effective turbo rpm) faster.
The goal is to keep the turbo spinning at it working RPM even when the throttle is closed or the engine RPM is below the normal spooling point.
How it's done:
When ALS (ANTI LAG SYSTEM) mode is on either by a switch or automatically by certain ECU's. The following things happen:
A large minimum ("anti lag idle") throttle opening is used, for example if using a bypass around the main throttle, a ~1" diameter hole is used.
So now when the driver lets off the throttle there is still a lot of airflow going through the engine and also through the turbo. However this
large effetive throttle opening ,if used alone, would of course fight the brakes much too strongly. Basically un-intended acceleration.
At the same time as the large minimum throttle opening is used, the ignition timing is retarded 30-40 degrees from normal, so yes final ignition
timing may be 30 degress ATDC (yes AFTER!).
So now even though we have a lot of air (and fuel) going by the throttle and through the engine , the combustion is taking place so late that not
much pressure is pushing down on the piston because it's mostly burning (and expanding) in the exhaust manifold. So there is no longer any
"push" or unintended accleration. Also most of the fuel / air charge is being combusted in the exhaust manifold. This high volume of gases
powers the turbine wheel. One typically sees about 100 kpa in the manifold with the throttle closed (ALS minimum throttle angle),and then when
the throttle is opened, nearly full boost right away.
As soon as the driver increases the throttle greater than a certain angle the ECU restores the ignition timing to a normal value (no more ALS retard)
Very high EGT's will result and this is usually the limit to how much ALS can be run, when setting it up -if one use a very large throttle opening, then more ignition retard must be used to avoid too much power under ASL mode, but then EGT's climb. Extra fuel is normally added during ALS mode to keep temps down, or water injection is used. Also some systems bypass some "boost air" from the intake into the exhaust manifold. Throttle solenoid "kickers" are used to get the ALS minimum throttle opening, or bypass valves around the throttle can be used.
Some ECU's have a function that allows ALS to be automatically ON all of the time . This means no throttle kicker is needed as the throttle idle opening is always large. So when idling it uses a "ROTATIONAL IDLE" strategy in order to keep the idle RPM speed "normal". without being in fire breathibg ALS mode. The ECU does this by completey eliminating cylinder firings in a turn-by turn- "rotational basis" by defeating a fuel injector firing for one cylinder at a time, but never the same cylinder in sequence. This also helps the engine cool down quickly after using ALS because a lot of fresh air is being processed though the engine. The frequency of injector cut determines the idle speed. This is why alot of rally cars sound funny at idle.
If the engine is equipped with a suitable ECU that is made for ALS it is quite user friendly, If not it is more difficult. Autronic has a very nice ALS sytem built in. I have done it with AEM also, but it is not as nice.
Some people wonder if it is the heat that drives the turbo (turbine), but look at the turbine wheel , it looks like an impeller,"a pinwheel" not a solar panel right? So it's themass flow of gasses that spins it, thermal effects by themselves are small and only apply to the gasses not losing a small amount of energy.
Other people ask (not using ALS), can I run less WOT timing during spool to get the turbo to spool up? Yes, but not much or you will more than pay for it in power loss . Basically if you retard the timing so much that the turbo spools significanly you will have lost too much power from retarding to make it worh it.
For drag racing a "Two-step" system is very good for spooling on the line, this lets the driver floor the throttle, the rev limit is set at a chosen rpm, rev limiting occurs by soft rpm limiting using heavy ignition retard and fuel cut combinations. It is possible to leave the line with full boost this way. Usully a switch enables this mode in 1st gear only, or the driver holds a momentary switch during launch. It is basically like an ALS based rev limiter.