How Anti-Lag Systems Work
The first hint should be found in the system's actual name. In short, it is a technique that allows turbocharged engines to minimize the lag of the turbo in certain situations. As you all probably already know, virtually all turbochargers out there have a "condition" which doesn't let them provide their maximum amount of pressure at lower revs.
In other words, the so-called "lag" is the actual time needed for the turbocharger to reach its full potential from an intermediate or low rotational speed. Because at low rpm the turbocharger will provide almost no boost whatsoever, and lot of motorsport disciplines such as rallying use turbocharging, various methods to decrease that lag have been invented over the years.
Since most of these technologies are a bit noisy, to say the least, they were almost exclusively fitted to rally cars. First, there was the so-called "turbocharger dump valve", or "blow off valve", which can be heard going into action every time a rally driver lifts his foot from the accelerator.
In essence, the blow-off valve evacuates some of the pressurized air coming out of the turbo while the inlet manifold is closed, therefore allowing the turbine not to lose its momentum. Since most turbocharged race cars have oversized turbines, they normally also display large amounts of turbo lag, so the blow-off valve is insufficient to overcome it.
This is where the anti-lag systems comes in. When the driver lifts his foot from the gas pedal, such as when approaching a corner, the ignition timing is retarded (no, really) and the fuel and air intake supply is enriched. Next there are two versions:
Either the air inlet is kept slightly open or an extra air injector which bypasses the air manifold is used to keep air supply for the engine. Using either method means that the fuel and air mixture keeps getting inside the engine's cylinders even if the driver is no longer pushing the accelerator pedal. With the ignition being retarded (OK, delayed), this means that the air/fuel mixture gets into the exhaust mostly unburned.
When the spark plugs fire for the first time in this cycle, the exhaust valve is just getting open because of the ignition delay. In contact with the high temperature exhaust, the unburned air/fuel mixture explodes in the actual exhaust tubes, right before the turbocharger. This micro-explosion keeps the turbo spinning to provide and adequate amount of pressure even if the driver no longer presses the accelerator pedal.
Obviously, we're talking only about a single explosion here, but in the course of a few seconds tens of them are actually happening, hence the "bang-bang" noise you sometimes hear on rally cars. The main effect is obviously the almost total absence of turbocharger lag, without the need for the engine to be "manually" revved.
It's all fun, games and bang, until the turbocharger or the exhaust explode with a total lack of delight. This is why the system is only used in racing cars, which have engine rebuilds between almost every race. In most countries, the system is banned from road use for obvious reasons, such as an extraordinary amount of noise pollution and the fact that it can make parts of your car explode.
Let's take a look at the actual advantages and disadvantages now:
- the turbo lag almost completely disappears, since the turbocharger provides large amounts of boosts even with the engine idling;
- the sound ALS makes is awesome if you're either A) popcorn lover or B)rally racing fanatic;
- anti-lag-equipped cars can sometime shoot flames from their exhaust;
- the turbocharger's interior temperature rises at over 1100 degrees celsius every time the system is used;
- the exhaust manifold and the turbocharger itself are pretty well shaken up during the ALS procedure, since they have to cope with an explosion which should have taken place inside the engine itself;
- engine braking is very much reduced during ALS operation