Throttle bypass The throttle bypass or "throttle kick" system is combined with ignition timing retardation and slight fuel enrichment (mainly to provide cooling). Typically, ignition occurs at 35–45° ATDC (after top dead center). This late ignition causes very little expansion of the gas in the cylinder, so that the pressure and temperature are kept high when the exhaust valve opens. At the same time, the amount of torque delivered to the
crankshaft is very small (just enough to keep the engine running). The higher exhaust pressure and temperature, combined with increased mass flow, are enough to keep the turbocharger spinning at high speed. When the throttle is opened up, ignition and fuel injection return to normal. Since many engine components are exposed to extreme temperatures and high-pressure pulses during ALS operation, this kind of system places a large amount of stress on the engine, turbocharger, and exhaust manifold. In addition to temperature issues, uncontrolled turbo speeds can quickly destroy the turbocharger. In most applications, the ALS is automatically deactivated to prevent overheating when the coolant reaches a temperature of 110–115 °C.
Intake bypass An ALS working with an intake bypass valve feeds air directly to the exhaust manifold, where it is mixed with partially combusted gasses from the engine, thus igniting them again and spooling up the turbo. Such a system can be made more refined than the throttle bypass system described above. One of the earliest systems of this type was used by the
Ferrari F1 team in the 1980s. Another well-known application of this type of anti-lag system was in the
World Rally Championship versions of the 1995
Mitsubishi Lancer Evolution III and
Toyota Celica GT-Four (ST205). The system was controlled by two pressure valves, operated by the ECU. Besides the racing version, the plumbing of the anti-lag system was also installed in the street-legal Celica GT-Four WRC
homologation model, though the system itself was disabled, the piping and valves only present for homologation purposes. On later
Japanese-market Mitsubishi Lancer Evolution models (IV through IX), the SAS (Secondary Air System) can be modified to provide anti-lag. The
Prodrive P2 prototype uses a more modern, refined intake bypass system. To almost entirely eliminate turbo lag, electrical energy stored in the car's onboard battery was partially deployed to the MGU-H, which spun the compressor turbine. This allowed the turbo system to reach peak boost pressure almost immediately, effectively negating any turbo lag. During normal race conditions, electric motor input was gradually reduced as engine RPM increased and the exhaust gasses were able to sustain the desired boost pressures. During qualifying laps and sometimes strategically throughout the race, energy could be deployed to the MGU-H on demand, even when the engine was running at high RPM. This allowed for the exhaust gasses to bypass the turbo via the
wastegate. This was said to increase power by 5–10%, although at a cost to stored energy levels. The MGU-H could also be used to generate electrical energy by allowing the electric motor that usually spun the turbine to be spun by the turbo system itself, a process known as "harvesting". This scenario existed when exhaust gasses were being routed through the turbo and the turbo system was operating in a conventional manner. Although harvesting came at a cost to overall power, it allowed for a net gain for reduction in overall lap times, as it was done in sections of the track that did not require peak power levels (for example, at the end of straights or between certain corners), or where calculations had ascertained that the loss in torque in those sections of the track was made up for in sections where the generated power could be deployed. ==References==