long but good info...
anti-lag system
i was in rally info b4, and i think i want to share an info. that i started long time ago when i was still an amateur. i ask these guys the question and they gave me good answers. i was monster my the way
how does misfiring system( bang-bang system/ anti-lag system) work? by monster
First two things I must admit:
I didn't know what you meant with "misfiring system". It is not a misfire in the correct sense, at least the misfire does not happen inside the engine itself. As well a misfire is not the desire of any engineer as it can damage the engine. More like a misfire alike effect happens as a side effect. There is no blame on you as you asked the question and there was probably no better way to describe it. But in future, I would try to avoid the term "misfire system" as it is confusing. Funny enough, it is quite often referred to as the "bang bang system" and everybody knows what is meant. Officially it is called the "ALS", which stands for Anti Lag System.
Next the system has evolved a lot, became hugely technical and there are several ways. So I can't give you more of a technical explaination than Jonkka did. However, am I correct in stating that the system was discovered by coincidence? I.e. the old group B Audi Quattros did a lot of this bang bang but they had no ALS or any form of system. More like when the wastegate opened to stop the turbo spinning, hot exhaust gases bypased the turbo and ignited behind the turbo in the much cooler exhaust. Then at one point engineers were thinking "What would be the side effect if the bangs we have anyway happened before the turbo rather than behind?"
Jonkka, as said I have not the expertise to add much to your description. But one thing desturbs me in your post: "..."turbo lag", a very noticeable delay between the moment when driver pressed the accelerator and the moment when turbo boost gave sudden increase of power. This is because engine revs up much faster than spinning blades of turbo (laws of inertia)..."
Going into the basics of how a turbo operates, the typical delay = turbo lag has nothing to do with inertia. The turbo consists of two turbines (you know that, Jonkka, I am trying to explain to monster and other newcomers to this technology). One turbine accelerates the induction air, compresses this air in the pipe and such forces the induction air/fuel mix into the engine with boost. This means the engine is filled with the gas mixture more efficiently and this also raises the engine compression ratio (which is why turbo engines have a lower compression ratio than atmospheric engines, had they the same compression, then turbo adds compression, engine would blow apart).
But how is the induction air turbine accelerated? It is directly linked via a shaft to another turbine, which is driven by exhaust gases. The beauty in this is, that the turbine is driven by waste, plus, other than i.e. a supercharger, it is not mechanically driven.
A supercharger is mechanically driven, i.e. via a belt from the crankshaft. So the supercharger boost is always in straight relation to the engine revs and reacts directly.
A turbo in comparison is driven by the exhaust gases and its revs are not related to the engine revs! This means in the lower revs, when you have not enough exhaust gases, the turbo might not spin at all, it doesn't force induction, so the engine has lower compression ratio plus an obstruction, the standing turbine, in the way of induction air. Indeed a turbo engine without the turbine spinning will produce much less power than its non-turbo equivalent in the same situation. But once the turbo spins, there is a chain reaction: the turbo presses air into the engine, more exhaust gases come out, accelerate the turbine even more... the turbine is massively, massively efficient in that situation and only stops when the driver lifts off and the wastegate bypasses the turbine.
Indeed, a mechanical supercharger when the engine revs 1,000rpm and the supercharger revs 3,000rpm, that means at engine revs 2,000rpm the supercharger will rev 6,000rpm and at engine revs 6,000rpm the supercharger will rev 18,000rpm. There is a straight relation - in this example charger revs = engine revs x 3.
The turbo is independant meaning it is not directly accelerated by engine revs, but importantly as well not slowed by engine revs. When the engine is at 1,000rpm, the turbo very likely doesn't spin at all. At 2,000rpm the turbo might have 5,000rpm, just started spinning up. But with the engine flat out at 6,000rpm, the turbo may well rev 150,000rpm - and that is not an utopic figure!
All this means quite clearly, once the turbine is spinning, it blows absolutely all alternatives away in efficiency. But as long as it is not spinning, it is effectively an unwanted obstruction. So if you haven't got enough exhaust gases to spin the turbo, you have to find ways of spinning the turbo otherwise. Maybe with artificial exhaust gases created through artificial ignitions, which is what the first anti lag systems did. But there are more ways to it.
But indeed, as far as the inertia idea goes, yes a turbine has to accelerate as well. But if you are in the right engine revs = you have masses of exhaust gases, you won't believe how quickly a turbine revs up. If you do a gear change with a turbo at, say 6000rpm engine revs, hit the next gear at 4500rpm engine revs, in the split second you lift off to change gear the turbine probably slows from 150,000rpm to 140,000rpm and is bang on the 150,000rpm again. Indeed, especially if you have a dump valve or similar, turbines don't slow down that quickly. It is not for nothing that you are adviced to leave the engine running at idle for a minute before switching off - it is to let the turbo stop as otherwise it would be running without oil. I have driven many turbo cars and with every one of them, changeing gears in the rev range described above, I have an immediate acceleration force any non-turbo engine could only dream of. No delays whatsoever.
Another example for inertia and even more to prove that turbine speed = turbo boost does not at all work in relation to engine revs: In the group B days you often heard that drivers like Stig Blomqvist, Walter Röhrl, Ari Vatanen were left foot braking, not for oversteer but to keep the turbo boost up! Indeed, as the turbine is driven by exhaust gases, it doesn't matter what revs, it matters how hard the engine is working! If you slow down your turbo car, try to slow it down with your left foot on the brake pedal while the right foot is flat out on the accelerator. Take the engine down to 1,500rpm, look at the boost gauge and you will discover that you are still on max boost, because the engine is working hard! It works with any turbo road car and in a way I am sad that in rallying anti lag systems made this driving technic obsolete - it was very clever driving by Blomqvist and co, the stars today have it easy! There is no room any more for clever driving technics today.
how does misfiring system( bang-bang system/ anti-lag system) work? by monster
First two things I must admit:
I didn't know what you meant with "misfiring system". It is not a misfire in the correct sense, at least the misfire does not happen inside the engine itself. As well a misfire is not the desire of any engineer as it can damage the engine. More like a misfire alike effect happens as a side effect. There is no blame on you as you asked the question and there was probably no better way to describe it. But in future, I would try to avoid the term "misfire system" as it is confusing. Funny enough, it is quite often referred to as the "bang bang system" and everybody knows what is meant. Officially it is called the "ALS", which stands for Anti Lag System.
Next the system has evolved a lot, became hugely technical and there are several ways. So I can't give you more of a technical explaination than Jonkka did. However, am I correct in stating that the system was discovered by coincidence? I.e. the old group B Audi Quattros did a lot of this bang bang but they had no ALS or any form of system. More like when the wastegate opened to stop the turbo spinning, hot exhaust gases bypased the turbo and ignited behind the turbo in the much cooler exhaust. Then at one point engineers were thinking "What would be the side effect if the bangs we have anyway happened before the turbo rather than behind?"
Jonkka, as said I have not the expertise to add much to your description. But one thing desturbs me in your post: "..."turbo lag", a very noticeable delay between the moment when driver pressed the accelerator and the moment when turbo boost gave sudden increase of power. This is because engine revs up much faster than spinning blades of turbo (laws of inertia)..."
Going into the basics of how a turbo operates, the typical delay = turbo lag has nothing to do with inertia. The turbo consists of two turbines (you know that, Jonkka, I am trying to explain to monster and other newcomers to this technology). One turbine accelerates the induction air, compresses this air in the pipe and such forces the induction air/fuel mix into the engine with boost. This means the engine is filled with the gas mixture more efficiently and this also raises the engine compression ratio (which is why turbo engines have a lower compression ratio than atmospheric engines, had they the same compression, then turbo adds compression, engine would blow apart).
But how is the induction air turbine accelerated? It is directly linked via a shaft to another turbine, which is driven by exhaust gases. The beauty in this is, that the turbine is driven by waste, plus, other than i.e. a supercharger, it is not mechanically driven.
A supercharger is mechanically driven, i.e. via a belt from the crankshaft. So the supercharger boost is always in straight relation to the engine revs and reacts directly.
A turbo in comparison is driven by the exhaust gases and its revs are not related to the engine revs! This means in the lower revs, when you have not enough exhaust gases, the turbo might not spin at all, it doesn't force induction, so the engine has lower compression ratio plus an obstruction, the standing turbine, in the way of induction air. Indeed a turbo engine without the turbine spinning will produce much less power than its non-turbo equivalent in the same situation. But once the turbo spins, there is a chain reaction: the turbo presses air into the engine, more exhaust gases come out, accelerate the turbine even more... the turbine is massively, massively efficient in that situation and only stops when the driver lifts off and the wastegate bypasses the turbine.
Indeed, a mechanical supercharger when the engine revs 1,000rpm and the supercharger revs 3,000rpm, that means at engine revs 2,000rpm the supercharger will rev 6,000rpm and at engine revs 6,000rpm the supercharger will rev 18,000rpm. There is a straight relation - in this example charger revs = engine revs x 3.
The turbo is independant meaning it is not directly accelerated by engine revs, but importantly as well not slowed by engine revs. When the engine is at 1,000rpm, the turbo very likely doesn't spin at all. At 2,000rpm the turbo might have 5,000rpm, just started spinning up. But with the engine flat out at 6,000rpm, the turbo may well rev 150,000rpm - and that is not an utopic figure!
All this means quite clearly, once the turbine is spinning, it blows absolutely all alternatives away in efficiency. But as long as it is not spinning, it is effectively an unwanted obstruction. So if you haven't got enough exhaust gases to spin the turbo, you have to find ways of spinning the turbo otherwise. Maybe with artificial exhaust gases created through artificial ignitions, which is what the first anti lag systems did. But there are more ways to it.
But indeed, as far as the inertia idea goes, yes a turbine has to accelerate as well. But if you are in the right engine revs = you have masses of exhaust gases, you won't believe how quickly a turbine revs up. If you do a gear change with a turbo at, say 6000rpm engine revs, hit the next gear at 4500rpm engine revs, in the split second you lift off to change gear the turbine probably slows from 150,000rpm to 140,000rpm and is bang on the 150,000rpm again. Indeed, especially if you have a dump valve or similar, turbines don't slow down that quickly. It is not for nothing that you are adviced to leave the engine running at idle for a minute before switching off - it is to let the turbo stop as otherwise it would be running without oil. I have driven many turbo cars and with every one of them, changeing gears in the rev range described above, I have an immediate acceleration force any non-turbo engine could only dream of. No delays whatsoever.
Another example for inertia and even more to prove that turbine speed = turbo boost does not at all work in relation to engine revs: In the group B days you often heard that drivers like Stig Blomqvist, Walter Röhrl, Ari Vatanen were left foot braking, not for oversteer but to keep the turbo boost up! Indeed, as the turbine is driven by exhaust gases, it doesn't matter what revs, it matters how hard the engine is working! If you slow down your turbo car, try to slow it down with your left foot on the brake pedal while the right foot is flat out on the accelerator. Take the engine down to 1,500rpm, look at the boost gauge and you will discover that you are still on max boost, because the engine is working hard! It works with any turbo road car and in a way I am sad that in rallying anti lag systems made this driving technic obsolete - it was very clever driving by Blomqvist and co, the stars today have it easy! There is no room any more for clever driving technics today.
Last edited by J!n K@z@mA; Jan 17, 2006 at 03:28 PM.
here's more info
The original ALS was invented to cure what was known as "turbo lag", a very noticeable delay between the moment when driver pressed the accelerator and the moment when turbo boost gave sudden increase of power. This is because engine revs up much faster than spinning blades of turbo (laws of inertia) and even after that it takes some time to build up the pressure at air intake.
Drivers of early turbo cars quickly learned about turbo lag and adopted manual technique to combat that. Driver accelerated far earlier than was required, often on middle of the corner, and by the time turbo lag was over and full boost on, car was already on exit of the corner and driver had all the power he needed. Of course, this was stressing technique and somewhat difficult to master but it worked.
Very soon engineers began to think ways to handle all this automatically and first development was the ALS known as bang-bang. I believe there are several ways technically to accomplish this, like the retarded ignition where fuel mixture burns partly outside the cylinder. The net effect is the same, by burning the fuel on outlet manifold the pressure there increases, as it would in normal acceleration of the car when gas is burnt in engine itself, and turbo has exhaust gasses to work from so turbo pressure is kept up. But engine does not produce power which would wear out components (remember, this is only used in neutral or decelaration conditions). However, out-of-the-cylinder combustion rapidly increases manifold AND turbo charger temperatures to very high levels indeed and that is partly why these components can be ceramic and hence, expensive.
These old bang-bangs are disappearing, partly because catalytic converters, required in FIA sanctioned rally cars, don't cope very well with such abuse and partly because sophisticated electronic engine management systems in use today allow better use of engine internal dynamics. I am not that familiar with the principle of current ALS versions, the proper address for those would be either the teams themselves or some tech wizard from motorsport magazines or industry.
The original ALS was invented to cure what was known as "turbo lag", a very noticeable delay between the moment when driver pressed the accelerator and the moment when turbo boost gave sudden increase of power. This is because engine revs up much faster than spinning blades of turbo (laws of inertia) and even after that it takes some time to build up the pressure at air intake.
Drivers of early turbo cars quickly learned about turbo lag and adopted manual technique to combat that. Driver accelerated far earlier than was required, often on middle of the corner, and by the time turbo lag was over and full boost on, car was already on exit of the corner and driver had all the power he needed. Of course, this was stressing technique and somewhat difficult to master but it worked.
Very soon engineers began to think ways to handle all this automatically and first development was the ALS known as bang-bang. I believe there are several ways technically to accomplish this, like the retarded ignition where fuel mixture burns partly outside the cylinder. The net effect is the same, by burning the fuel on outlet manifold the pressure there increases, as it would in normal acceleration of the car when gas is burnt in engine itself, and turbo has exhaust gasses to work from so turbo pressure is kept up. But engine does not produce power which would wear out components (remember, this is only used in neutral or decelaration conditions). However, out-of-the-cylinder combustion rapidly increases manifold AND turbo charger temperatures to very high levels indeed and that is partly why these components can be ceramic and hence, expensive.
These old bang-bangs are disappearing, partly because catalytic converters, required in FIA sanctioned rally cars, don't cope very well with such abuse and partly because sophisticated electronic engine management systems in use today allow better use of engine internal dynamics. I am not that familiar with the principle of current ALS versions, the proper address for those would be either the teams themselves or some tech wizard from motorsport magazines or industry.
Yes, engine speed is almost directly related to amount of produced exhaust gasses. But that is not necessarily directly related to turbo spinning speed and hence the produced boost. By using smaller revolving masses on blades and by reducing friction involved even relative modest increase in exhaust gasses can produce big increase in turbo speed and hence, turbo boost. Technology has certainly moved ahead in turbos too. But in principle you're correct, engine revs have relation to exhaust gasses and through that also to turbo boost.
Second question is not that simple. Clutch works perfectly, it does not slip and should not. Slippage takes place in differentials, brakes and tyres. If you wonder what happens when driver presses both pedals at the same the answer is dependant on which rally car he is driving.
On old front-wheel cars the catch was to adjust brake balance rearwards so that rear brakes locked while front brakes did not. Because engine was putting power through the front wheels, even hard braking did not lock the front wheels and driver was able to steer (with locked front wheels he would have just skidded straight ahead) and keep engine revving. At the same time rear wheels were locked and if executed correctly on a corner, the rear swung around while steering functioned perfectly. Effectively, FWD behaved like RWD. Granted, this was somewhat tough treatment to front brakes, pad temps soared.
With 4WD cars and modern diffs the thing is simpler and more friendly to mechanical parts. Computer detects what driver is trying to do and locks diffs as needed, allowing the engine to rev full while braking. When say, centre diff is unlocked, engine can scream as much it wants but no power is transmitted anywhere and full braking power is maintained. Of course, it doesn't happen that simply, modern systems are quite complicated but the the obvious difference between previous situation are the existence of freely unlockable differentials.
If you've ever seen a footage from pedal box you know that drivers quite effectively "dance" on the brake and throttle, they rarely keep either pedal pressed down for a long time. Engine response is needed to turn the car as much (or even more) than the steering wheel.
Second question is not that simple. Clutch works perfectly, it does not slip and should not. Slippage takes place in differentials, brakes and tyres. If you wonder what happens when driver presses both pedals at the same the answer is dependant on which rally car he is driving.
On old front-wheel cars the catch was to adjust brake balance rearwards so that rear brakes locked while front brakes did not. Because engine was putting power through the front wheels, even hard braking did not lock the front wheels and driver was able to steer (with locked front wheels he would have just skidded straight ahead) and keep engine revving. At the same time rear wheels were locked and if executed correctly on a corner, the rear swung around while steering functioned perfectly. Effectively, FWD behaved like RWD. Granted, this was somewhat tough treatment to front brakes, pad temps soared.
With 4WD cars and modern diffs the thing is simpler and more friendly to mechanical parts. Computer detects what driver is trying to do and locks diffs as needed, allowing the engine to rev full while braking. When say, centre diff is unlocked, engine can scream as much it wants but no power is transmitted anywhere and full braking power is maintained. Of course, it doesn't happen that simply, modern systems are quite complicated but the the obvious difference between previous situation are the existence of freely unlockable differentials.
If you've ever seen a footage from pedal box you know that drivers quite effectively "dance" on the brake and throttle, they rarely keep either pedal pressed down for a long time. Engine response is needed to turn the car as much (or even more) than the steering wheel.
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