Anti-lag system

An anti-lag system (ALS) or misfiring system is a system used on turbocharged engines to minimize turbo lag on racing cars. It works by arranging for fuel and air to be in the exhaust duct after the engine, and before the turbocharger. This ignites in the hot ducting and the combustion process that occurs there keeps the turbocharger spinning when the engine is not delivering enough exhaust gas.

ALS was first used in the early days of turbocharged cars in Formula One racing circa mid to late 1980s, until fuel restrictions made its use unsuitable. Later it became a common feature in rally cars because of the increased turbo lag from the mandated restrictors at the intake manifold inlet. Due to the pressure drop across the restriction, the pressure ratio for a given boost level is much higher and the turbocharger must spin much faster to produce the same boost as when the engine operates without restriction. This increases turbo lag significantly compared to unrestricted turbochargers.

An ALS requires an air bypass, generally done in one of two ways. The first method is to use a throttle air bypass; this may be an external bypass valve or a solenoid valve which opens the throttle 12-20 degrees. This allows air to bypass the closed throttle and to reach the engine. The second method is to use a bypass valve which feeds charge air directly to the exhaust manifold.

The throttle bypass/throttle solenoid system is combined with ignition retardation and slight fuel enrichment (mainly to provide cooling), typically ignition occurs at 35-45° ATDC. This late ignition causes very little expansion of the gas in the cylinder; hence the pressure and temperature will still be very high when the exhaust valve opens. At the same time, the amount of torque delivered to the crankshaft will be very small (just enough to keep the engine running). The higher exhaust pressure and temperature combined with the increased mass flow is enough to keep the turbocharger spinning at high speed thus reducing lag. When the throttle is opened up again the ignition and fuel injection goes back to normal operation. Since many engine components are exposed to very high temperatures during ALS operation and also high pressure pulses, this kind of system is very hard on the engine and turbocharger. For the latter not only the high temperatures are a problem but also the uncontrolled turbo speeds which can quickly destroy the turbocharger. In most applications the ALS is automatically shut down when the coolant reaches a temperature of 110–115 °C to prevent overheating.

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Wikipedia