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Anti-Lag System (ALS, Misfiring System) is a method designed to reduce a delay of the turbocharger response. Thanks to that, the efficiency of a turbocharged engine is maximized. The system exists since the ’80s, but nowadays it becoming more and more important, especially when engine capacities are constantly reduced.
Anti-lag can be installed as a standalone device or be integrated into the engine.
Turbo delay is a phenomenon that is adversely affected by the throttle response due to the inertia of the rotating components in the turbocharger. There is a delay between pressing the accelerator pedal and the additional increase in power obtained from the turbocharger. This delay occurs in all turbochargers, but the larger the turbocharger, the more delay there is.
The delay is inherent in the design of the turbochargers because they rely on exhaust gases when the turbine rotor is rotated. This turbine, however, has its own weight. As a result, you must wait until the engine revs and builds the exhaust gas pressure before the turbine starts to rotate. Due to such characteristics of turbocharged engines, street or racing cars that produce power on very large turbochargers, they must either suffer from a very poor throttle response or look for alternative solutions. When building a drag racing car, it can be solved with a combination of start and gain controllers that allow the engine to rotate freely at a high enough speed to rotate the turbo before starting. Race cars that constantly brake and accelerate, like rally cars, use other components such as anti-delay systems.
The anti-lag system works by intentionally burning fuel and air in the turbocharger housing. The system controls the dosing of fuel to the exhaust manifold and relies on the hot temperatures of the surrounding components to ignite the mixture. This combustion provides the necessary force to keep the turbine spinning, even when the engine does not produce significant flue gas flows. As a result, the turbocharger always rotates and generates a useful amount of air, regardless of the amount of exhaust flow.
The disadvantage of this is that the combustion forces in the turbine housing cause high stresses on the turbocharger subassemblies and significantly shorten the life of these expensive parts. For example, cars of the World Rally Championship (WRC) usually exchange turbochargers after each race due to the destructive forces of systems that counteract delays. These systems are easily recognizable thanks to the characteristic sounds, somewhat similar to those obtained during the shooting of the pistol.
There are several different methods for adding additional fuel and air to the exhaust manifold, which results in some design changes. Most anti-lag systems are based on a computer (sometimes a stand-alone unit, but it is often an OEM ECU function) that modifies fuel and ignition maps when the throttle is closed. Increasing the fuel-air mixture wealth (more fuel) and ignition delay means that additional fuel will be pushed through the engine without burning. When this unburned mixture reaches the turbocharger, it will burn up and cause 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 the 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.