Physics of 'popping' on over-run
Discussion
Driving a Tamora with sports exhaust, I am trying to work out why that combination makes more popping noises on over-run than the standard exhaust - and why it makes those noises at all.
Following a quick search, the extra popping noises seem to be attributed to unburnt fuel burning / detonating in the exhaust manifold.
However, on over-run, shouldn't a fuel-injected engine cut fuel completely? There doesn't seem to be much point in injecting fuel when there is no need for power.
That is what seems to happen on most normal cars I drive. The Speed 6 is a much sportier engine, but I can't see why this would be different.
Or is it something to do with other pressure waves in the exhaust system air-flow where there is a large rear silencer that is lacking silencing material?
Following a quick search, the extra popping noises seem to be attributed to unburnt fuel burning / detonating in the exhaust manifold.
However, on over-run, shouldn't a fuel-injected engine cut fuel completely? There doesn't seem to be much point in injecting fuel when there is no need for power.
That is what seems to happen on most normal cars I drive. The Speed 6 is a much sportier engine, but I can't see why this would be different.
Or is it something to do with other pressure waves in the exhaust system air-flow where there is a large rear silencer that is lacking silencing material?
It can be a combination of things but I understand overlap on the cam durations can mean that the exhaust port is open while the inlet valve is also open so fuel goes directly into the exhaust and burns there and/or ECU maps can be tweaked so that fuel can be injected to give the same effect. A bit like the way the 'blown diffuser' in F1 works.
(Someone will probably be along soon to tell me I'm talking BS but that's what I understand
)
(Someone will probably be along soon to tell me I'm talking BS but that's what I understand
)http://www.pistonheads.com/gassing/topic.asp?h=3&a...
Explanation is somewhere in there, indeed due to the batch injection, that the backfiring occurs...
Explanation is somewhere in there, indeed due to the batch injection, that the backfiring occurs...
The Reason you get the pops and bangs is unburnt fuel and hot exhaust gases exiting the tailpipe and the oxygen ignites it. This happens because the injection system is not sequential, on the S6 engine the injectors fire 3 and 3 when you shut the throttle a certain amount of unburnt fuel enters the exhaust pipe as there is no oxygen to burn it. Hope this explains it.
Thanks for the link to that thread and for the other replies.
It sounds like I need to get my head round batch and sequential injection.
TVRs use batch and, as the throttle is closed, that can allow some extra fuel through for ignition in the exhaust manifold.
I would have still thought that extended periods off the throttle, e.g. slowing in one gear from high speed/revs to low speed/revs should be noise-free other than when the throttle is first closed.
When I collect my Tamora on Sunday I will be sure to do lots of testing...
It sounds like I need to get my head round batch and sequential injection.
TVRs use batch and, as the throttle is closed, that can allow some extra fuel through for ignition in the exhaust manifold.
I would have still thought that extended periods off the throttle, e.g. slowing in one gear from high speed/revs to low speed/revs should be noise-free other than when the throttle is first closed.
When I collect my Tamora on Sunday I will be sure to do lots of testing...
Sevenman said:
I would have still thought that extended periods off the throttle, e.g. slowing in one gear from high speed/revs to low speed/revs should be noise-free other than when the throttle is first closed.
I think you'll still find some pops and bangs, even with fully closed throttle. As long as you stay in batch mode, you'll always have half of the cylinders that have the fuel injected at the wrong time of their cycle, even without throttle being applied.The fun part will be to find out how you can make the pops and bangs last the longest
PascalBuyens said:
I think you'll still find some pops and bangs, even with fully closed throttle. As long as you stay in batch mode, you'll always have half of the cylinders that have the fuel injected at the wrong time of their cycle, even without throttle being applied.
That's what confuses me - this isn't an engine known for its economy, but why is fuel being injected at the wrong part of the cycle? I only have a basic understanding of 4-stroke engine operation, but there is one stroke for the injection of fuel/air, and fuel shouldn't be injected outside that - the intake valves should be closed.
If fuel/air is being injected at the wrong time in the cycle for 1/2 the cylinders, doesn't that mean lots of unburnt fuel is sitting around on the intake side?
Just found some more info here
The other injection strategy is batch fired where several injectors are triggered simultaneously and not timed to the intake valve open period. In effect, the fuel sits in the runner for a short period of time before the valve opens and the airflow carries the mixture into the cylinder.
The rest of the article explains that batch fuel injection is well suited to high power applications, and is also cheaper in terms of electronics and injectors.
That now makes a bit more sense
PascalBuyens said:
The fun part will be to find out how you can make the pops and bangs last the longest
Coasting in gear down a reasonably steep hill... Fireworks all the way down. This is particularly fun if said hill happens to be a couple of miles long and forms part of the French Alps.Zippee said:
PascalBuyens said:
The fun part will be to find out how you can make the pops and bangs last the longest
Coasting in gear down a reasonably steep hill... Fireworks all the way down. This is particularly fun if said hill happens to be a couple of miles long and forms part of the French Alps.Note to self: must find the DVD taken from behind the Elise back some day... 26 seconds of about 4-5 meter long backfire flames in that tunnel
Sevenman said:
PascalBuyens said:
I think you'll still find some pops and bangs, even with fully closed throttle. As long as you stay in batch mode, you'll always have half of the cylinders that have the fuel injected at the wrong time of their cycle, even without throttle being applied.
That's what confuses me - this isn't an engine known for its economy, but why is fuel being injected at the wrong part of the cycle? I only have a basic understanding of 4-stroke engine operation, but there is one stroke for the injection of fuel/air, and fuel shouldn't be injected outside that - the intake valves should be closed.
If fuel/air is being injected at the wrong time in the cycle for 1/2 the cylinders, doesn't that mean lots of unburnt fuel is sitting around on the intake side?
Just found some more info here
The other injection strategy is batch fired where several injectors are triggered simultaneously and not timed to the intake valve open period. In effect, the fuel sits in the runner for a short period of time before the valve opens and the airflow carries the mixture into the cylinder.
The rest of the article explains that batch fuel injection is well suited to high power applications, and is also cheaper in terms of electronics and injectors.
That now makes a bit more sense
Batch will inject a rough amount of fuel into a matched pair of cylinders, so one of those cylinders is in the different part of its cycle and doesn't need the fuel at that specific time. When you are on overrun some of this fuel is sucked out of the exhaust port and into the pipes where the heat and pressure ignite it.
For quite a while modern cars have run sequential. This fires fuel into one cylinder at a time at the exact point that it is required. Now, for a standard car with lazy cams this is alright but with racier cams comes a need for even more accurate management systems to get the right amount of fuel into the cylinder head at the right time. Such a system will be more economical but not magnitudes better. The real reason for the switch from batch to sequential is emissions. Sequential allows the engine to run far more cleanly and accurately, thus allowing a cat converter to run at its optimum levels and thus, overall, reduce emissions. There are also various arguements around the flow dynamics of the fuel and its mixing with the air that see sequential favoured as it means every spark occurs in an equally atomised and blended mix, whereas in batch it is every other spark which has a live mix and the next an old mix (if that makes sense)
However, modern petrol injection technology is all derived from deisel tech and the latest cars are becoming common rail. This means that the fuel is now injected directly into the chamber, not the ports so it is even more accurate, burn rates are far more optimised and the whole car runs more economically and efficiently with less emissions. The downside is that the materials needed to allow an injector to be inserted directly into the chamber and handle the massive heat increase are far more expensive.
So, long story short, modern batch systems tend to be the result of cost savings on management systems etc and has other knock on impacts such as map points etc etc. Switching to sequential will cost a lot more for the computing power of the management system but will also give you extra upsides because of the smarter system, such as knock sensors, closer mapping points, multiple bespoke maps etc etc. But you lose the pops and bangs unless you specifically program a map to dump in fuel to cylinders which don't require it.
DonkeyApple said:
Batch was completely standard way to run injection for a long time. It requires far less computing power as timing doesn't need to be so critical. Thus you can use a much cheaper management system. There's nothing actually wrong with batch in real terms. But, cheap management systems are often cheap in other places. You can see with the batch MBE that it has fewer map points through the rev range as well but that is an aside.
Batch will inject a rough amount of fuel into a matched pair of cylinders, so one of those cylinders is in the different part of its cycle and doesn't need the fuel at that specific time. When you are on overrun some of this fuel is sucked out of the exhaust port and into the pipes where the heat and pressure ignite it.
For quite a while modern cars have run sequential. This fires fuel into one cylinder at a time at the exact point that it is required. Now, for a standard car with lazy cams this is alright but with racier cams comes a need for even more accurate management systems to get the right amount of fuel into the cylinder head at the right time. Such a system will be more economical but not magnitudes better. The real reason for the switch from batch to sequential is emissions. Sequential allows the engine to run far more cleanly and accurately, thus allowing a cat converter to run at its optimum levels and thus, overall, reduce emissions. There are also various arguements around the flow dynamics of the fuel and its mixing with the air that see sequential favoured as it means every spark occurs in an equally atomised and blended mix, whereas in batch it is every other spark which has a live mix and the next an old mix (if that makes sense)
However, modern petrol injection technology is all derived from deisel tech and the latest cars are becoming common rail. This means that the fuel is now injected directly into the chamber, not the ports so it is even more accurate, burn rates are far more optimised and the whole car runs more economically and efficiently with less emissions. The downside is that the materials needed to allow an injector to be inserted directly into the chamber and handle the massive heat increase are far more expensive.
So, long story short, modern batch systems tend to be the result of cost savings on management systems etc and has other knock on impacts such as map points etc etc. Switching to sequential will cost a lot more for the computing power of the management system but will also give you extra upsides because of the smarter system, such as knock sensors, closer mapping points, multiple bespoke maps etc etc. But you lose the pops and bangs unless you specifically program a map to dump in fuel to cylinders which don't require it, or supercharge the engine...
Kindly corrected that for you Sir. LOL Batch will inject a rough amount of fuel into a matched pair of cylinders, so one of those cylinders is in the different part of its cycle and doesn't need the fuel at that specific time. When you are on overrun some of this fuel is sucked out of the exhaust port and into the pipes where the heat and pressure ignite it.
For quite a while modern cars have run sequential. This fires fuel into one cylinder at a time at the exact point that it is required. Now, for a standard car with lazy cams this is alright but with racier cams comes a need for even more accurate management systems to get the right amount of fuel into the cylinder head at the right time. Such a system will be more economical but not magnitudes better. The real reason for the switch from batch to sequential is emissions. Sequential allows the engine to run far more cleanly and accurately, thus allowing a cat converter to run at its optimum levels and thus, overall, reduce emissions. There are also various arguements around the flow dynamics of the fuel and its mixing with the air that see sequential favoured as it means every spark occurs in an equally atomised and blended mix, whereas in batch it is every other spark which has a live mix and the next an old mix (if that makes sense)
However, modern petrol injection technology is all derived from deisel tech and the latest cars are becoming common rail. This means that the fuel is now injected directly into the chamber, not the ports so it is even more accurate, burn rates are far more optimised and the whole car runs more economically and efficiently with less emissions. The downside is that the materials needed to allow an injector to be inserted directly into the chamber and handle the massive heat increase are far more expensive.
So, long story short, modern batch systems tend to be the result of cost savings on management systems etc and has other knock on impacts such as map points etc etc. Switching to sequential will cost a lot more for the computing power of the management system but will also give you extra upsides because of the smarter system, such as knock sensors, closer mapping points, multiple bespoke maps etc etc. But you lose the pops and bangs unless you specifically program a map to dump in fuel to cylinders which don't require it, or supercharge the engine...
PascalBuyens said:
DonkeyApple said:
Batch was completely standard way to run injection for a long time. It requires far less computing power as timing doesn't need to be so critical. Thus you can use a much cheaper management system. There's nothing actually wrong with batch in real terms. But, cheap management systems are often cheap in other places. You can see with the batch MBE that it has fewer map points through the rev range as well but that is an aside.
Batch will inject a rough amount of fuel into a matched pair of cylinders, so one of those cylinders is in the different part of its cycle and doesn't need the fuel at that specific time. When you are on overrun some of this fuel is sucked out of the exhaust port and into the pipes where the heat and pressure ignite it.
For quite a while modern cars have run sequential. This fires fuel into one cylinder at a time at the exact point that it is required. Now, for a standard car with lazy cams this is alright but with racier cams comes a need for even more accurate management systems to get the right amount of fuel into the cylinder head at the right time. Such a system will be more economical but not magnitudes better. The real reason for the switch from batch to sequential is emissions. Sequential allows the engine to run far more cleanly and accurately, thus allowing a cat converter to run at its optimum levels and thus, overall, reduce emissions. There are also various arguements around the flow dynamics of the fuel and its mixing with the air that see sequential favoured as it means every spark occurs in an equally atomised and blended mix, whereas in batch it is every other spark which has a live mix and the next an old mix (if that makes sense)
However, modern petrol injection technology is all derived from deisel tech and the latest cars are becoming common rail. This means that the fuel is now injected directly into the chamber, not the ports so it is even more accurate, burn rates are far more optimised and the whole car runs more economically and efficiently with less emissions. The downside is that the materials needed to allow an injector to be inserted directly into the chamber and handle the massive heat increase are far more expensive.
So, long story short, modern batch systems tend to be the result of cost savings on management systems etc and has other knock on impacts such as map points etc etc. Switching to sequential will cost a lot more for the computing power of the management system but will also give you extra upsides because of the smarter system, such as knock sensors, closer mapping points, multiple bespoke maps etc etc. But you lose the pops and bangs unless you specifically program a map to dump in fuel to cylinders which don't require it, or supercharge the engine...
Kindly corrected that for you Sir. LOL Batch will inject a rough amount of fuel into a matched pair of cylinders, so one of those cylinders is in the different part of its cycle and doesn't need the fuel at that specific time. When you are on overrun some of this fuel is sucked out of the exhaust port and into the pipes where the heat and pressure ignite it.
For quite a while modern cars have run sequential. This fires fuel into one cylinder at a time at the exact point that it is required. Now, for a standard car with lazy cams this is alright but with racier cams comes a need for even more accurate management systems to get the right amount of fuel into the cylinder head at the right time. Such a system will be more economical but not magnitudes better. The real reason for the switch from batch to sequential is emissions. Sequential allows the engine to run far more cleanly and accurately, thus allowing a cat converter to run at its optimum levels and thus, overall, reduce emissions. There are also various arguements around the flow dynamics of the fuel and its mixing with the air that see sequential favoured as it means every spark occurs in an equally atomised and blended mix, whereas in batch it is every other spark which has a live mix and the next an old mix (if that makes sense)
However, modern petrol injection technology is all derived from deisel tech and the latest cars are becoming common rail. This means that the fuel is now injected directly into the chamber, not the ports so it is even more accurate, burn rates are far more optimised and the whole car runs more economically and efficiently with less emissions. The downside is that the materials needed to allow an injector to be inserted directly into the chamber and handle the massive heat increase are far more expensive.
So, long story short, modern batch systems tend to be the result of cost savings on management systems etc and has other knock on impacts such as map points etc etc. Switching to sequential will cost a lot more for the computing power of the management system but will also give you extra upsides because of the smarter system, such as knock sensors, closer mapping points, multiple bespoke maps etc etc. But you lose the pops and bangs unless you specifically program a map to dump in fuel to cylinders which don't require it, or supercharge the engine...
Pursyluv said:
So why do some flame and others don't - is it purely down to mapping and the exhaust set up, or can it be even more random than that, as i've spoken to people with similar cars say Tamora's with the same exhaust set up, one flames, the other doesn't.
I would hazard that to get the flames you need a combination of more unburnt fuel and for this to be ignited further down the exhaust.So, a combination of fueling map and exhaust layout. I would imagine that the positioning and/or existence of a cat would be quite significant.
Donkeyapple, thanks for the detailed response.
I think it finally makes sense
Interesting that the Speed 6 came with Batch, but the cars did represent good value for money when new, and given the performance focus, I can see the cost reasons for not using sequential.
As for emissions - this is a fun car, with a relatively small number built, most of which do relatively few miles. Emissions take a distant 3nd place to power and economy.
Do people change the ignition systems? I would have no intention of doing so, just wondered if some people had.
I think it finally makes sense
Interesting that the Speed 6 came with Batch, but the cars did represent good value for money when new, and given the performance focus, I can see the cost reasons for not using sequential.
As for emissions - this is a fun car, with a relatively small number built, most of which do relatively few miles. Emissions take a distant 3nd place to power and economy.
Do people change the ignition systems? I would have no intention of doing so, just wondered if some people had.
You know, I don't think the standard MBE ECU even has overrun fuel cut? Hence why it continues to pop and bang on long overruns. So when you release the throttle the ECU just goes into idle mode so the injectors are still firing small amounts of fuel with each revolution but the raised speed of the engine valve overlap and exhaust harmonics results in air and fuel being sucked into the exhaust manifold and igniting.
It does it best between 2700-3000rpm
It does it best between 2700-3000rpm
Gassing Station | Speed Six Engine | Top of Page | What's New | My Stuff