Anti Lag System - I need help on implementation
Posted: February 21st, 2007, 10:45 am
Hello everyone,
EDIT: I added some ALS background, to clarify what and how is achieved by applying such a strategy.
===============================================
As many of us know, the problem with the turbocharged engines is that it takes certain time ('lag') for the turbocharger to 'spool' to certain stabile rotating speed (imposed mainly by moment of inertia of the rotating assembly (this affects shaft acceleration only), available exhaust gasses energy, friction and pumping work compressor is doing). Furthermore, to put it simple, compressor speed has lot to do with the available pressure - there is high level of correlation between the two.
The goal on racing cars is to keep the turbocharger rotating assembly rotating as fast as possible, as long as possible, so that the effect of the 'lag' is minimized. This is accomplished by mechanical design of components and suitable control strategy.
I will go in detail into control strategy optimization. We have the following parts of equation:
moment of inertia of rotating assembly - can not be affected by electronic management system
friction - can not be affected by EMS
energy of exhaust gasses - CAN be affected by EMS
pumping work of the compressor - CAN be affected by the EMS
To explain the typical situation, after a straight section of the track, where full boost is reached, car is approaching a curve, that requires car speed to be reduced. This is done by braking. Driver releases the throttle (more or less) and applies brakes. The moment he lifts his foot from the throttle, energy of exhaust gasses goes down and pumping work of the compressor increases (it has to work against higher resistance in the inlet tract). This imposes reduction of turbocharger speed, until an equilibrium is reached again. Once the driver needs more power, it will take time for the turbocharger to accelerate to the level it can generate full boost. Depending on turbocharger, it can be anything from a few tenths of second to a few seconds.
Anti lag control strategy is counteracting this. I like to speak of ALS as another method of torque control. Instead of throttling intake flow (with the effects as above), we let it flow a lot (big bypass valve opens, or throttle kicker solenoid activates), but we retard ignition significantly. So, there is much less throttling in the intake tract and there is lot of exhaust gasses energy to keep the compressor spinning fast (due to good cylinder filling through unthrottled intake and low combustion energy utilization due to retarded ignition).
However, apart from maintaining the charger spinning), this is putting lot of stress on turbocharger and exhaust system. And it is creating familiar 'pops' out of exhaust.
===============================================
I am programming ALS functionality into my control units for turbocharged applications. I read much about this subject, spoke to some people that used it, did some field tests etc.
So, I will explain briefly how it works at the moment in my system:
Parameters:
Max. ignition retard (self-explanatory)
Min. throttle to activate ALS - the throttle position has to be below this preset level to consider activating ALS
Ignition retard slope - once throttle is lower than the min. ALS threshold (parameter above), ignition retard is introduced with this slope (the more the user releases the throttle, the bigger the retard is, up to the max. ignition retard)
ALS min RPM - ALS will be active only if RPM is higher than this value
ALS timeout - once the ALS is activated, it can stay on not more than tis time
ALS fuel correction - when ALS is activated fueling will be altered by percentage given by this parameter
Functional description:
User activates ALS switch - this is a signal for the control unit to activate ALS when conditions are reached. Once the RPM is high enough and throttle is closed enough (on deceleration for example), primary and where available secondary idle valve opens fully, ignition is retarded and fueling is corrected. ALS timer starts ticking. ALS mode is over if any of the following conditions is met:
1) Throttle position higher than minimum for ALS
2) RPM to low for ALS
3) ALS timeout elapsed
End of ALS activity is abrupt, so suddenly idle valve(s) are closed, ignition and fueling restored to normal.
I would be interested to hear if anyone has some more (hands-on) expirience with ALS (either as user or as developer), its parameters, ideas etc. I am not so interested to hear what magazines write about that, as:
1) I've read many of them already
2) It is not always correct (more often the subject is significantly simplified and made sound logical to typical reader, ignoring "second order of effects")
3) I got no impression that the journalist have actually tried to calibrate the thing themselves, they were more reproducing what the manufacturers were saying.
Many thanks,
Nikola Radenkovic
P.S. Please excuse my English writing skills, hopefully it is understandable enough.
EDIT: I added some ALS background, to clarify what and how is achieved by applying such a strategy.
===============================================
As many of us know, the problem with the turbocharged engines is that it takes certain time ('lag') for the turbocharger to 'spool' to certain stabile rotating speed (imposed mainly by moment of inertia of the rotating assembly (this affects shaft acceleration only), available exhaust gasses energy, friction and pumping work compressor is doing). Furthermore, to put it simple, compressor speed has lot to do with the available pressure - there is high level of correlation between the two.
The goal on racing cars is to keep the turbocharger rotating assembly rotating as fast as possible, as long as possible, so that the effect of the 'lag' is minimized. This is accomplished by mechanical design of components and suitable control strategy.
I will go in detail into control strategy optimization. We have the following parts of equation:
moment of inertia of rotating assembly - can not be affected by electronic management system
friction - can not be affected by EMS
energy of exhaust gasses - CAN be affected by EMS
pumping work of the compressor - CAN be affected by the EMS
To explain the typical situation, after a straight section of the track, where full boost is reached, car is approaching a curve, that requires car speed to be reduced. This is done by braking. Driver releases the throttle (more or less) and applies brakes. The moment he lifts his foot from the throttle, energy of exhaust gasses goes down and pumping work of the compressor increases (it has to work against higher resistance in the inlet tract). This imposes reduction of turbocharger speed, until an equilibrium is reached again. Once the driver needs more power, it will take time for the turbocharger to accelerate to the level it can generate full boost. Depending on turbocharger, it can be anything from a few tenths of second to a few seconds.
Anti lag control strategy is counteracting this. I like to speak of ALS as another method of torque control. Instead of throttling intake flow (with the effects as above), we let it flow a lot (big bypass valve opens, or throttle kicker solenoid activates), but we retard ignition significantly. So, there is much less throttling in the intake tract and there is lot of exhaust gasses energy to keep the compressor spinning fast (due to good cylinder filling through unthrottled intake and low combustion energy utilization due to retarded ignition).
However, apart from maintaining the charger spinning), this is putting lot of stress on turbocharger and exhaust system. And it is creating familiar 'pops' out of exhaust.
===============================================
I am programming ALS functionality into my control units for turbocharged applications. I read much about this subject, spoke to some people that used it, did some field tests etc.
So, I will explain briefly how it works at the moment in my system:
Parameters:
Max. ignition retard (self-explanatory)
Min. throttle to activate ALS - the throttle position has to be below this preset level to consider activating ALS
Ignition retard slope - once throttle is lower than the min. ALS threshold (parameter above), ignition retard is introduced with this slope (the more the user releases the throttle, the bigger the retard is, up to the max. ignition retard)
ALS min RPM - ALS will be active only if RPM is higher than this value
ALS timeout - once the ALS is activated, it can stay on not more than tis time
ALS fuel correction - when ALS is activated fueling will be altered by percentage given by this parameter
Functional description:
User activates ALS switch - this is a signal for the control unit to activate ALS when conditions are reached. Once the RPM is high enough and throttle is closed enough (on deceleration for example), primary and where available secondary idle valve opens fully, ignition is retarded and fueling is corrected. ALS timer starts ticking. ALS mode is over if any of the following conditions is met:
1) Throttle position higher than minimum for ALS
2) RPM to low for ALS
3) ALS timeout elapsed
End of ALS activity is abrupt, so suddenly idle valve(s) are closed, ignition and fueling restored to normal.
I would be interested to hear if anyone has some more (hands-on) expirience with ALS (either as user or as developer), its parameters, ideas etc. I am not so interested to hear what magazines write about that, as:
1) I've read many of them already
2) It is not always correct (more often the subject is significantly simplified and made sound logical to typical reader, ignoring "second order of effects")
3) I got no impression that the journalist have actually tried to calibrate the thing themselves, they were more reproducing what the manufacturers were saying.
Many thanks,
Nikola Radenkovic
P.S. Please excuse my English writing skills, hopefully it is understandable enough.