How an Anti-Lag System (ALS) or "Misfiring System" Works
The purpose of this technical article is to discuss how an anti-lag system works. An anti-lag system, commonly known as ALS, is a system of components designed to maximize the performance of turbocharged engines by reducing something called turbo lag. This technology has been around since the '80s but is increasingly relevant in this age of downsized and boosted engines.
Turbo lag is a phenomena in which throttle response is negatively affected due to the inertia of spinning components within a turbocharger. There is a delay between the depression of the accelerator pedal and the extra boost of power gained from the turbocharger. This delay exists in all turbochargers, but the bigger the turbocharger, the more turbo lag it will have.
The lag is inherent in the design of turbochargers because they rely on exhaust gases to spin the turbine wheel. This turbine, however, has mass. As a result, you must wait for the engine to rev and exhaust gases to build before the turbine begins to spin. Because of such characteristics of turbocharged engines, street or race cars that rely on very large turbochargers to make power must either suffer from very poor throttle response or search for alternative solutions. When building a car for drag racing this can be addressed with simply a combination of launch control and boost controllers which allow the engine to freely rev at an RPM high enough to spin the turbo before launching. Race cars that continually brake and accelerate, like rally cars, use other components such as anti-lag systems.
An anti-lag system operates by intentionally combusting fuel and air within the turbine housing of a turbocharger. The system controls the addition of extra fuel into the housing and relies on the hot temperatures of the surrounding components to ignite the mixture. This combustion provides the force necessary to keep the turbine spinning even when the engine is not creating significant exhaust flows. As a result, the turbocharger is always spinning and creating a useful amount of boost, regardless of the amount of exhaust flow.
The downside to this, as you might imagine, is that the combustion forces within the turbine housing apply a large amount of stress on turbocharger components and greatly reduce the life span of these expensive parts. World Rally Championship (WRC) cars, for instance, generally replace their turbochargers after every race due to the destructive forces of anti-lag systems. These systems are easily recognizable by their backfiring sounds, somewhat similar to that of a gunshot as can be witnessed in the video below.
There a few different methods of adding extra fuel and air into the turbine housing resulting in some variation in designs. Most anti-lag systems rely on a computer (sometimes a standalone unit but often a function of the ECU) which modifies fuel and ignition maps while the throttle is closed. Making the air fuel-mixture richer (more fuel) and retarding the ignition (less spark) means some extra fuel will be pushed through the engine without combusting. When this unburned mixture reaches the turbocharger it will then combust, and result in the effects discussed above.
In addition to the extra fuel required to create explosions within the turbocharger, some systems use different methods of piping air into the exhaust system. This can be done using a combination of pipes, valves, and the inherent vacuums created in some intake systems.
Anti-lag systems tremendously boost performance of turbocharged vehicles, but because of the destructively high pressures and temperatures these systems create, you might want to consider another approach if you're on a tight budget.