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In turbocharged internal combustion engines, an anti-lag system ( ALS) is a method of reducing turbo lag in Auto 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.
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.
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.
One of the earliest systems of this type was used by the Scuderia Ferrari 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.
When used with a mass air flow sensor (MAF), the check valve should draw air through the MAF to maintain proper air-fuel ratios. This is not necessary in a speed-density configuration.
When a car is ready for launch and at its launch rpm, some ECUs (via a switch or additional throttle) can be programmed to retard the ignition by quite a few degrees and add significantly more fuel. The combustion event occurs as the engine is driving the air/fuel mixture out of the cylinder, closer to the turbine, causing it to spool up either at a lower rpm or make much more boost at the launch rpm than it would normally.
Some software can also engage this "anti-lag" feature via clutch input with full-throttle shifting, effectively making it work between shifts. Like other types of anti-lag, overuse of this type of anti-lag can cause damage to the turbine wheel, exhaust manifold, and more due to high pressure from the air/fuel mixture spontaneously combusting almost (or completely) outside of the combustion chamber. Afterfire may also result.
This form of "anti-lag" tends to work well because it is only active at wide open throttle, during which more air can enter the engine. Consequently, it does not perform as well (or activate at all) at partial or closed throttle, unless combined with a secondary air system or bypass as described above.
During normal race conditions, electric motor input is gradually reduced as engine rpm increases and the exhaust gasses are able to sustain the desired boost pressures. During qualifying laps and sometimes strategically throughout the race, energy can be deployed to the MGU-H on demand, even when the engine is running at high rpm. This allows for the exhaust gasses to bypass the turbo via the wastegate. This is said to increase power by 5-10%, although at a cost to stored energy levels.
The MGU-H can also be used to generate electrical energy by allowing the electric motor that usually spins the turbine to be spun by the turbo system itself, a process known as "harvesting". This scenario exists when exhaust gasses are being routed through the turbo and the turbo system is operating in a conventional manner. Although harvesting comes at a cost to overall power, it allows for a net gain for reduction in overall lap times, as it is done in sections of the track that do not require peak power levels (for example, at the end of straights or between certain corners), or calculations have ascertained that the loss in torque in those sections of the track is made up for in sections where the generated power can be deployed.
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