Antilag system

In internal combustion engines with turbochargers, an anti-lag system (ALS) is a method of reducing turbo lag in racing or performance applications. It works by retarding ignition timing and adding extra fuel (and sometimes air) to balance an inherent loss in combustion efficiency with increased pressure at the turbine. The excess fuel/air mixture escapes through the exhaust valves and combusts in the hot exhaust manifold, spooling the turbocharger and creating higher usable pressure.

Overview

ALSes were first used in the early days of turbocharging in Formula One in the 1980s until fuel restrictions made their use unsuitable. The technology later became common in rally cars because of increased turbo lag from the mandated restrictor plate at the intake. The restrictor results in a significant intake pressure drop; the pressure ratio for a given boost level is much higher, and the turbocharger must spin much faster to produce the same boost as it would unrestricted, significantly increasing turbo lag.

An ALS requires an air bypass, generally done in one of two ways. The first method is to use a throttle air bypass that circumvents the throttle and feeds air to the engine; this may be an external bypass valve or a solenoid that slightly opens the throttle 12–20 degrees. The second method is to use a bypass that feeds charge air (pressurized intake air between the turbo compressor and intake valves) directly to the exhaust manifold.

Types

Throttle bypass

The throttle bypass or "throttle kick" system is combined with ignition timing retardation and slight fuel enrichment (mainly to provide cooling).