Anti-Lag System

Anti-Lag System, ALS, is a system used on mainly turbocharged racing and rally engines to eliminate turbo lag. It was used in the early days of turbo charging in F1 until fuel restrictions made its use unsuitable. Later it became a common feature in rally cars due to the mandated restrictors on the turbocharger inlet. Because of the pressure drop across the restriction, the pressure ratio for a given boost level is much higher and the turbocharger must spin a lot faster to produce the same boost as before. This increases turbo lag significantly compared to unrestricted turbochargers.

An ALS system requires an air bypass, and generally this is 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 open up 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 feed 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 occur 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 fast can destroy a turbocharger. In most applications the ALS is automatically shut down when the coolant reaches a temperature of 110-115°C, this to prevent overheating the engine.

An ALS system working with a bypass valve which feeds air directly to the exhaust system can be made more refined than the system described above. Some early systems used by Ferrari in F1 followed this approach, so does the anti-lag systems used in WRC today, which are even more refined with advanced computer control. Today this kind of system has reached such a refinement that it’s even possible to use the system in a road car. A recent example is the Prodrive P2 prototype. The system works by bypassing charge air directly to the exhaust manifold which acts as a combustor when fuel rich exhaust from the engine meets up with the fresh air from the bypass. This will provide a continuous combustion limited to the exhaust manifold which significantly reduces the heat and pressure loads on the engine and turbocharger. With the latest anti-lag systems the bypass valve can not only be opened or closed but it can actually control the flow of air to the exhaust manifold very accurately. The turbocharger is fitted with a turbo speed sensor and the engine management system has a map based on throttle position and car speed which is used to find a suitable turbocharger speed and boost pressure for every condition. When the engine alone can’t provide enough exhaust energy to reach the turbo speed/boost demanded by the management system, the bypass valve opens and exhaust manifold combustion begins. This not only reduces turbo lag, but it also allows boost to be produced at very low engine speeds where boost was previously limited by compressor surge or exhaust energy. With relatively high boost at low speeds, this makes the low end torque superior even to large naturally aspirated engines. The system also operates very quietly.