HOW THE TURBO ANTI-LAG SYSTEM WORKS
Bang-bang (also known as ALS which stands for Anti-Lag System) is an engine management technique that allows to minimize the turbo lag time.
As you may be aware of, turbochargers display what is known as lag time, which is the time needed for the turbine to reach its full throttle from the intermediate rotational speed state. The duration of a turbocharger's lag depends on many factors, including its inertia, airflow efficiency, back pressure, etc. The issue is partly dealt with by fitting a turbo dump valve, which acts every time the driver lifts his foot from the throttle. The dump valve will evacuate the pressure air coming from the turbocharger while the inlet manifold is closed so that the turbine does not stall and avoid possible damage to its bearings. In racing cars it is very common to fit the oversized turbochargers in order to be able to produce enough boost pressure and ensure sufficient engine output. Big turbochargers exhibit significant amounts of lag due to their increased rotational inertia. In such cases the dump valve is insufficient to allow the turbocharger not to lose too much speed when the driver lifts off. Additionally rally cars are fitted with a turbo restrictor, which is regulated by the FIA. One of the restrictor's effects is to increase lag time. This is why in racing cars, and more specifically in rally cars, where torque and engine availability are critical performance factors, most applications use anti-lag systems.
During lag time the engine is much less responsive and its output well below nominal. To counter the effect of the turbocharger's lag time drivers used to anticipate the engine's reactions by accelerating well before they would have done in a non-turbo car. Others have used a technique introduced by German driver Walter Röhrl, known as "left foot braking" where the driver uses his left foot to brake the car while his right foot accelerates to keep the turbocharger in optimal load. Left foot braking is very hard on the brakes that are put in extreme stress but is very effective in keeping the turbo spinning.
ALS was a simple idea but one that was relatively difficult to implement. Only when electronic engine management systems were advanced enough to allow for consideration of many more parameters, in real time, than in the past it became possible to use them efficiently in handling ALS. Toyota Team Europe was the first to use it in racing (Toyota's implementation is known as the Toyota Combustion Control System while Mitsubishi calls the system Post Combustion Control System).
HOW ALS WORKS
When the driver lifts his foot from the gas pedal, the ignition timing is altered with sometimes 40 ° or more delay (retard), and the intake air and fuel supply mixture is made more richer. The inlet butterfly is kept slightly open or air injector, bypassing the inlet butterfly, is used to maintain air supply to the engine. This results in air / fuel mixture that keeps getting into the combustion chambers when the driver does not accelerate. The ignition is severely delayed, the air / fuel mixture reaches the exhaust tubes mostly unburned. When the spark plug fires, the exhaust valve starts to open due to the ignition delay mentioned above. Additionally, the exhaust temperature is extremely high, the unburned fuel explodes at the contact of the exhaust tubes. Luckily the turbo sits right there and the explosion keeps turning (otherwise it would slow down since its intake, the exhaust gas is cut-off). The effect is vastly lower response times with some downsides:
A quick rise of the turbocharger's temperature (which jumps from ~ 800 ° C to 1100 ° C + region) whenever the system is activated
A huge stress on the exhaust manifold and pipes (mounted on a street car a bang-bang system would destroy the exhaust system within 50-100 km)
The turbo produces significant boost even at engine idle speeds
The explosions that occur in the exhaust tubes generate important flames which can, sometimes, be seen at the end of the exhaust tube
Reduced engine brake
The ALS effect is largely dependent on the amount of air allowed in the engine, the more air supplied the more ALS effect will be noticeable. Consequently ALS systems can be more or less aggressive. A mild ALS will maintain a 0 to 0.3 bar pressure in the inlet manifold when activated while, when inactive, the pressure in the inlet manifold with the throttle closed would be in the region of -1 bar (absolute vacuum). Racing ALS versions can maintain a pressure of up to 1.5 bar in the inlet manifold with the throttle closed.
While the systems mounted in Toyota and Mitsubishi racing cars are relatively smooth and noiseless, those fitted in Ford and Subaru cars are much more noiseful and aggressive.
The bang-bang system owns its name to the loud explosion noises one hears whenever the driver lifts off. Most racing implementations have user-selectable anti-lag settings depending on the terrain, usually three settings can be selected by the driver going from mild to very aggressive.
Note that some regional or national European events prohibit the use of ALS systems while more and more WRC events regulate the levels of noise that are allowed by competitors cars that effectively disable ALS.
Starting in 2002, new anti-lag techniques, such as Exhaust Gas Recirculation (EGR), are slowly overtaking the method described above as they are kinder on the engine's mechanical parts.
Bang-bang (also known as ALS which stands for Anti-Lag System) is an engine management technique that allows to minimize the turbo lag time.
As you may be aware of, turbochargers display what is known as lag time, which is the time needed for the turbine to reach its full throttle from the intermediate rotational speed state. The duration of a turbocharger's lag depends on many factors, including its inertia, airflow efficiency, back pressure, etc. The issue is partly dealt with by fitting a turbo dump valve, which acts every time the driver lifts his foot from the throttle. The dump valve will evacuate the pressure air coming from the turbocharger while the inlet manifold is closed so that the turbine does not stall and avoid possible damage to its bearings. In racing cars it is very common to fit the oversized turbochargers in order to be able to produce enough boost pressure and ensure sufficient engine output. Big turbochargers exhibit significant amounts of lag due to their increased rotational inertia. In such cases the dump valve is insufficient to allow the turbocharger not to lose too much speed when the driver lifts off. Additionally rally cars are fitted with a turbo restrictor, which is regulated by the FIA. One of the restrictor's effects is to increase lag time. This is why in racing cars, and more specifically in rally cars, where torque and engine availability are critical performance factors, most applications use anti-lag systems.During lag time the engine is much less responsive and its output well below nominal. To counter the effect of the turbocharger's lag time drivers used to anticipate the engine's reactions by accelerating well before they would have done in a non-turbo car. Others have used a technique introduced by German driver Walter Röhrl, known as "left foot braking" where the driver uses his left foot to brake the car while his right foot accelerates to keep the turbocharger in optimal load. Left foot braking is very hard on the brakes that are put in extreme stress but is very effective in keeping the turbo spinning.
ALS was a simple idea but one that was relatively difficult to implement. Only when electronic engine management systems were advanced enough to allow for consideration of many more parameters, in real time, than in the past it became possible to use them efficiently in handling ALS. Toyota Team Europe was the first to use it in racing (Toyota's implementation is known as the Toyota Combustion Control System while Mitsubishi calls the system Post Combustion Control System).
HOW ALS WORKS
When the driver lifts his foot from the gas pedal, the ignition timing is altered with sometimes 40 ° or more delay (retard), and the intake air and fuel supply mixture is made more richer. The inlet butterfly is kept slightly open or air injector, bypassing the inlet butterfly, is used to maintain air supply to the engine. This results in air / fuel mixture that keeps getting into the combustion chambers when the driver does not accelerate. The ignition is severely delayed, the air / fuel mixture reaches the exhaust tubes mostly unburned. When the spark plug fires, the exhaust valve starts to open due to the ignition delay mentioned above. Additionally, the exhaust temperature is extremely high, the unburned fuel explodes at the contact of the exhaust tubes. Luckily the turbo sits right there and the explosion keeps turning (otherwise it would slow down since its intake, the exhaust gas is cut-off). The effect is vastly lower response times with some downsides:
A quick rise of the turbocharger's temperature (which jumps from ~ 800 ° C to 1100 ° C + region) whenever the system is activated
A huge stress on the exhaust manifold and pipes (mounted on a street car a bang-bang system would destroy the exhaust system within 50-100 km)
The turbo produces significant boost even at engine idle speeds
The explosions that occur in the exhaust tubes generate important flames which can, sometimes, be seen at the end of the exhaust tube
Reduced engine brake
 |
The ALS effect is largely dependent on the amount of air allowed in the engine, the more air supplied the more ALS effect will be noticeable. Consequently ALS systems can be more or less aggressive. A mild ALS will maintain a 0 to 0.3 bar pressure in the inlet manifold when activated while, when inactive, the pressure in the inlet manifold with the throttle closed would be in the region of -1 bar (absolute vacuum). Racing ALS versions can maintain a pressure of up to 1.5 bar in the inlet manifold with the throttle closed.While the systems mounted in Toyota and Mitsubishi racing cars are relatively smooth and noiseless, those fitted in Ford and Subaru cars are much more noiseful and aggressive.
The bang-bang system owns its name to the loud explosion noises one hears whenever the driver lifts off. Most racing implementations have user-selectable anti-lag settings depending on the terrain, usually three settings can be selected by the driver going from mild to very aggressive.
Note that some regional or national European events prohibit the use of ALS systems while more and more WRC events regulate the levels of noise that are allowed by competitors cars that effectively disable ALS.
Starting in 2002, new anti-lag techniques, such as Exhaust Gas Recirculation (EGR), are slowly overtaking the method described above as they are kinder on the engine's mechanical parts.
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