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I posted this in the Turbo vs Supercharging section and thought it may be nice to also have its own thread.
Anti-lag is systems are pretty complex to set up right but easy to explain in technical terms. So the simplified tecky on anit-lag in depth.
Enjoy.
First Engine Power - Engine Power is created by one thing only. BMEP (or Brake Mean Effective Pressure). This is the Dynamic (or actual) pressure created within the cylinder during the combustion process. BMEP is the "be all and end all" of the engine power game. Everything we do to create power in any engine is done in an attempt to raise BMEP in a controlled manner.
During the combustion process many things are happening, but of particular interest to tuners is the raise and fall of BMEP within a complete cycle of the engine. During the intake stroke, air and fuel are drawn or pushed into the cylinder, the intake valve then closes and the upward stroke of the piston compresses the mixture. As the piston approaches Top Dead Center (TDC) a spark ignites the mixture and the combustion process begins.
On a scale of 1-10, the actual compression pressure of an engine is about 1 or 2 when no spark is made. The pressure created by the burning and rapidly expanding air/fuel mixture rates about 7 or 8.
This rapidly increases until such time as two things happen.
1. The piston is forced down by its expansion (power is created) and the space created by the piston moving down in turn, creates a pressure drop within the cylinder.
2. The burning/expanding rate begins to slow down due to air/fuel depletion.
Climbing those peaks - the point at which BMEP is at its highest is called "Peak BMEP". Its position within the firing cycle is studied by engine designers and tuners all over the world. It is generally agreed that the point of peak BMEP should fall within the 14-16 degs after TDC.
If you think of ignition spark as the beginning of the combustion process, and we can establish that it takes 46-48 crankshaft degrees (the usual average) to reach "Peak BMEP", then an assumption can be made that we should set ignition timing to start at 32degs before TDC. (32+14=46). Have a think about this and understand it, because it is important.
Now let's do something rather interesting: Let's retard the ignition timing to something like 10degs after TDC. Suddenly we have a threoetical Peak BMEP at 56degs after TDC. Power has dropped signficantly. In fact, we have virtually no power - Why? Simple: the next step in our engine cycle is the exhaust stroke to vent the spent gases, and this is happening just after our retarded Peak BMEP. So, effectively our precious power producing charge is now burning and rapidly expanding inside the exhaust manifold. This is not much good for making power, but it really makes the turbo spin hard!
So if the turbine is spinning hard it mst be making heaps of boost right? Well yes, but we are not making any power because the Peak BMEP is all going out the exhaust valve.
So what good is? Easy. During a race we want instant boost. - the moment we get back on the throttle we want it there ready to go., just waiting for us. So how do we do it?
Anti-lag should only be operational during off-throttle conditions otherwise power will be lost by retared BMEP.
The first thing we do is keep the throttle open about 10 %, so there is air available for the combustion process. Then we retard the ignition timing by a huge amount which creates retared BMEP, and large turbine driving pressures within the exhaust manifold. This in turn, creates strong boost even though the throttle is closed, but when you open throttle again and the ignition timing returns to normal, the turbine is already spinning hard and making boost - so we have instant power.
Want more? Ok let's now say we want even more exhaust gas pressure to really get that turbine spinning at max revs, because we are off the throttle and not trying to make power. Why not introduce some completely unburnt air/fuel misture into the exhaust for even more rapdi expansion of the turbine driving exhaust gases? Good idea.
The easiest way to do that is to introduce an ignition-based missfire which will dump a complete compression/firing cycle of air/fuel misture into the manifold. There it is rapdily ignited by the next cylinders flame front. Now have got some serious amounts of boost in anticipation of the throttle opening.
This is how anti-lag works. What I have discribed is a very simplified explanation of anti-lag and it can only be said that, in real life, there are many many many more variables that come into play. Such as Actual degrees if retard and throttle opening are very different to the numbers I have described, but I have used these as ease explanation.
Anti-lag is systems are pretty complex to set up right but easy to explain in technical terms. So the simplified tecky on anit-lag in depth.
Enjoy.
First Engine Power - Engine Power is created by one thing only. BMEP (or Brake Mean Effective Pressure). This is the Dynamic (or actual) pressure created within the cylinder during the combustion process. BMEP is the "be all and end all" of the engine power game. Everything we do to create power in any engine is done in an attempt to raise BMEP in a controlled manner.
During the combustion process many things are happening, but of particular interest to tuners is the raise and fall of BMEP within a complete cycle of the engine. During the intake stroke, air and fuel are drawn or pushed into the cylinder, the intake valve then closes and the upward stroke of the piston compresses the mixture. As the piston approaches Top Dead Center (TDC) a spark ignites the mixture and the combustion process begins.
On a scale of 1-10, the actual compression pressure of an engine is about 1 or 2 when no spark is made. The pressure created by the burning and rapidly expanding air/fuel mixture rates about 7 or 8.
This rapidly increases until such time as two things happen.
1. The piston is forced down by its expansion (power is created) and the space created by the piston moving down in turn, creates a pressure drop within the cylinder.
2. The burning/expanding rate begins to slow down due to air/fuel depletion.
Climbing those peaks - the point at which BMEP is at its highest is called "Peak BMEP". Its position within the firing cycle is studied by engine designers and tuners all over the world. It is generally agreed that the point of peak BMEP should fall within the 14-16 degs after TDC.
If you think of ignition spark as the beginning of the combustion process, and we can establish that it takes 46-48 crankshaft degrees (the usual average) to reach "Peak BMEP", then an assumption can be made that we should set ignition timing to start at 32degs before TDC. (32+14=46). Have a think about this and understand it, because it is important.
Now let's do something rather interesting: Let's retard the ignition timing to something like 10degs after TDC. Suddenly we have a threoetical Peak BMEP at 56degs after TDC. Power has dropped signficantly. In fact, we have virtually no power - Why? Simple: the next step in our engine cycle is the exhaust stroke to vent the spent gases, and this is happening just after our retarded Peak BMEP. So, effectively our precious power producing charge is now burning and rapidly expanding inside the exhaust manifold. This is not much good for making power, but it really makes the turbo spin hard!
So if the turbine is spinning hard it mst be making heaps of boost right? Well yes, but we are not making any power because the Peak BMEP is all going out the exhaust valve.
So what good is? Easy. During a race we want instant boost. - the moment we get back on the throttle we want it there ready to go., just waiting for us. So how do we do it?
Anti-lag should only be operational during off-throttle conditions otherwise power will be lost by retared BMEP.
The first thing we do is keep the throttle open about 10 %, so there is air available for the combustion process. Then we retard the ignition timing by a huge amount which creates retared BMEP, and large turbine driving pressures within the exhaust manifold. This in turn, creates strong boost even though the throttle is closed, but when you open throttle again and the ignition timing returns to normal, the turbine is already spinning hard and making boost - so we have instant power.
Want more? Ok let's now say we want even more exhaust gas pressure to really get that turbine spinning at max revs, because we are off the throttle and not trying to make power. Why not introduce some completely unburnt air/fuel misture into the exhaust for even more rapdi expansion of the turbine driving exhaust gases? Good idea.
The easiest way to do that is to introduce an ignition-based missfire which will dump a complete compression/firing cycle of air/fuel misture into the manifold. There it is rapdily ignited by the next cylinders flame front. Now have got some serious amounts of boost in anticipation of the throttle opening.
This is how anti-lag works. What I have discribed is a very simplified explanation of anti-lag and it can only be said that, in real life, there are many many many more variables that come into play. Such as Actual degrees if retard and throttle opening are very different to the numbers I have described, but I have used these as ease explanation.
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